CHAPTER - 04 HALOALKANES AND HALOARENES 1.0 INTRODUCTION Haloalkanes and haloarenes are two classes of orga ing one ormore halogen atoms attached to carbon atoms of alkyl and aryl groups respectively. They differ widely in chemical reactivity depending ‘onthe halogen and the class to which they belong and may even differ within aclass. They have applications in industry as well as in day-to-day life. They are used as solvents for relatively non-polar compounds and as starting materials for the synthesis of organic compounds. Organochlorides are the most common industrially used organohalides, although the other organohalides are used mainly in organic synthesis. Many organohalogen compounds occur in nature. Some of these are clinically useful. Chloramphenicol, a chlorine containing antibiotic, is very effective in the treatment of typhoid fever. The human body produces thyroxine, an iodine containing hormone, the deficiency of which causes goiter. Chloroquine, a synthetic halogen ‘compound is used in the treatment of malaria, Halothane is used as anaesthetic, Hexachloroethane isa substitute for camphor. Certain fully luorinated compounds are being considered as potential blood substitutes in surgery. Many synthetic organic halogen compounds re used in agriculture Halogenated compounds persist in the environment due to resistance to breakdown by soil microorganisms, therefore, halocarbons, including those that might not be hazardous can cause waste disposal issues. 1.1 CLASSIFICATION OFHALOGEN COMPOUNDS ‘The halogen derivatives of hydrocarbons can be broadly classified into (i aliphatic halogen compounds and (ii) aromatic halogen compounds. 1.1.1Classification based on the Number of Halogen Atoms On the basis of number of halogen atoms, these may be classified as mono-, di- or polyhalogen compounds, depending on whether they contain one, two or more halogen atoms. HX CHX P GHX he ae HX i cH,Xx Monohaloalkane Dihaloalkane ‘Tuhaloalkane i co . Mouolualaat ese Disha ease ‘Tualuanense XIN] Chemistry (Class XI) 1.1.2 Classification based on the type of hybridization of the carbon bonded to the halogen atom On the basis of type of hybridization of the carbon to which the halogen is attached, halogen derivatives of hydrocarbons are classified into three types (a) Compounds containing C,,» —X bond (i) Haloalkanes or alkyl halides (R-X). Alkyl halides form a homologous series with the general formula, C\II,,., X. They are further classified as primary, secondary or tertiary according as the halogens linked to primary, secondary or tertiary carbon atom. x B R —§ —x R—C—X Ke Re Primary (1%) Secondary (2°) Tertiary (3°) When R isalicyclic, R-X is called halocycloalkane or cycloalkyl halide. These may be classified as 1°, 2° or 3°. mae Cx ‘Primary (1°) ‘Secondary (2°) Tertiary (3°) ic halides. In these halides, the halogen is attached to an sp’-hybridised carbon atom next to a carbon-carbon double bond, i.e., an allylic carbon. These may be further classified as 1°, 2° or 3° 2 4CHy x CH. ox 2 y J mal oN id, Ka ae Y cy Cr 3-Haloprop-I-ene(I°) 3-Halobut-l-ene(2*)3-Halocyclobex-L-ene(2°) _3-Helo-3-methyleyclohex-1-ene(3") (i) Benzylic halides. In these compounds, the halogen is attached to an sp*-hybridized carbon atom next to an aromatic ring, ie., toa benzylic carbon. These may be further classified as 1°, 2°or 3. . x I I. a wey ‘eo ee Q 7 a aN} A R" Benzyl halide (1°) A benzylic halide (2°) A henzylic halide (2°) A benzylic halide (3°) (b) Compounds containing C,,; —X bond (i) Vinylic halides. In these halides, the halogen is attached to one of the carbon atoms of the carbon-carbon double bond. For example, wes ax CY Haloethene (Finyl halide) 1-Halocyelohex-I-ene(viny! halide) (ii) Aryl halides. In these halides, the halogen is directly attached to the aromatic ring. cay Halobenzene p-Halotoluene(©) Compounds containing C,, —X bond (X =F, Cl, Br or I) Inthese halides, halogens directly attached to one of the carbon atoms of the triple bond. CH, - 1-Chioropro Chloroethyne 1.2 NOMENCLATURE OF HALOGEN COMPOUNDS 1.2.1 Nomenclature of Haloalkanes (a) Monohalogen derivatives. In the common or trivial system, the monohalogen derivatives of alkanes are named as alkyl halides. Their names are derived by naming the alkyl group followed by the name of the halogen as halide. The name of an alkyl halide is written as two separate words, e.g., CH,Br as methyl bromide. Inthe IUPAC system, alkyl halides are considered as derivatives of the corresponding alkanes and are named as haloalkanes. The name is written as one word, e.g., CH,Br as bromomethane. a ie CH,CH,CH, —Br CH, —CH— CH, CH,—CH—CH, Br 1-Rromaprapane 2.Choropropane 1-Rrome7-ethytpopane (o-Propy bromide) (soprpyl chloride) (Isobuy bride) (Wherever two names are given under the formula of a compound, the one outside brackets is the IUPAC name and that given within brackets is the common or trivial name.) cH, cH, r +1 sl CH,CH,CH,—Br cH,cH—1 cu,c—c1 | -Bromopropane 2Todopropane I (Primary) (Secondary) CH, 2-Chloro-2-methylpropane (Tertiary) (b) Dihalogen derivatives (@ Alkylidene dihalides. In the common system, dihaloalkanes in which halogen atoms of the same type are present on the same carbon atom are called alkylidene dihalides (geminal dihalides or gem-dihalides). Inthe TUPAC system, they are named as dihaloalkanes. The position or the locant for the halogen, being repeated twice, is prefixed to the name of the dihaloalkane (for dihalomethanes locants are not used). CHCl, CH;CHBr, CH;CH,CHCL CH;CCI,CH; Dichloromethane 1, L-Dibromoethane 1, L-Dichioropropane 2, 2-Diehloropropane (Methylene dichloride) __(Elbylidene dibromide) _—_(Propylidene dichloride) _(sopropslidene dichloride) Gi) Alkylene dihalides. In the common system, dihalogen derivatives of alkanes in which two halogen atoms of the same type are present on adjacent carbon atoms are called alkylene dihalides (vicinal dihalides or vie-dihalides). In the IUPAC system, the locants for the two halogen atoms are prefixed to the name of the dihaloalkane. CICH,CH,C1 CH;CHBr, CH, -CHCI-CH,CI 1,2-Dichlooethane smaethane 1, 2-Dichloropropane (Exiylene dichloride) (Einytidene dibromile) (Propylene dichloride)IN] Chemistry (Class XI) - Volum 4 ‘Table -01 Common and IUPAC Names of Haloalkanes, ‘Structural Formula ‘Common or Trivial Name TUPAC Name CH; — CL Methyl chloride Chloromethane CH — CH) —Br Ethyl bromide Bromoethane a al eu, —CHy—CH,, 1n Propyl fluoride 1-Fluoropropane a) (iat | CHy cn Isopropyl iodide 2elodopropane I ae Oe ae Le CH, —CH,—CH,—CH,—Cl| Butyl chloride L-Chiorobutane | ee at | CH,—CH—CH,—CH, sec-Butyl chloride 2-Chlorobutane I Isobutyl chloride 1-Chluro-2-methylpropane tert-Butyl chloride 2-Chloro-2-methylpropane 54 32 CH,CH, CH, CH, CH,Cl n-Pentyl chloride or n-Amyl chloride 1-Chloropentane a CH,—CH — CH,CH,C1 cH, Isopentyl chloride or Isoamyl chloride | 1-Chloro-3-methylbutane 3 cu, on, 26 cH, cH, tert-Pentyl chloride or tert-Amyl chloride }2-Chloro-2-methylbutane & cu, a, 70 —Cn,0 Neopentyl chloride or Neoamyl chloride | 1-Chloro-2, 2-dimethyipropane cH, (iii) Polymethylene dihalides. In the common system, dihalogen derivatives of alkanes in which the same halogen atoms are present on the terminal carbon atoms, (ie., oc, «positions of the carbon chain) are called polymethylene dihalides.In the JUPAC system, the locants for the halogen atoms are prefixed to the name of the dihaloalkane. CICH,CH;CH;C1 — BrCHjCH,CH,CH,Br ICH CH,CH,CH,CH,I 1,3 Dichloropropane 1, 4-Dibromobutene 1, $-Di-iodopentane (Trimethylene dichloride) (Tetramethyene dtromide) (Pentamethslene diode) (©) Polyhaloalkanes. /n the common system, trihalo methanes and tetrahalo methanes are called haloforms and carbon tetrahalides respectively CHCI, CHB CHI cel, Trichloromethane Tribromomethane Triodomethane eyachlorometbane (Chloroform) (Bromatorm) Uodotorm) (Carbon teteachloride) Perhalohydrocarbons. Fully halogenated hydrocarbons are called perhalohydrocarbons (per = thoroughly). CR, CR, -CR, Octafloropropane (Perfluropropane) 2.2.2 Nomenclature of Haloarenes (Aryl Halides) i, The commonas well as TUPAC name of ary] halide is Haloarene, ¢.g., chlorobenzene. ii, For dihalogen derivatives, the preifixes 0, m-, p-are used in the common system. In the IUPAC system, numerical prefixes 1,2; 1,3 and 1,4 are used. iii, Ifthe aromatic hydrocarbon carriesa side chain ora substituent, numbering of the carbon atoms of the ring begins with the substituent and if the hydrocarbon is benzene, numbering begins with the halogen atom. iv. Polyhalogen derivatives do not have common names but only IUPAC names in which the position of, halogens are indicated by arabie numerals, cl CH, CH, CH, Br al oa al 4 o of & 6 c 2 2 Org if 2 Chlorobencsne 1-Chloro-2-methylbenzene I-Chlove-2-methyfbenzene Bromobenzene (Chlorobenzene) or 2-Chlorotoluene or 3-Chlorotoluene cl (Bromobenzene) (0-Chlorotoluene) (m-Chlorotoluene) 1-Chloro- 4 thy tbonsone or 4-Chlorotoluene (p-Chlorotoluene) Br Br Br Br Br Wo Br de ho A oy HC A PLL J 7 er 7 Zc a7 N7N Br 2 Dibromobenzene | 3.pibromobenzene ie 1-Bromo-3- 1,4, S-Tribromobenzene (cotiembocoe) [Romer De ASR. tyme (-Dibromobencene) iv. Aralkyl halides are aromatic halogen compounds in which halogen is attached to one of the carbon atoms of the side chain canrying the aryl group. The following aryl groups have special names d on on Pheay! Benzyl BenzalIN] Chemistry (Class XI) 1.3. ISOMERISM IN HALOALKANES AND HALOARENES: (a) Chain isomerism, Haloalkanes containing four or more carbon atoms show chain isomerism. 4 8 2 1 CH,—CH,—CH,—CH,Cl 1-Chlorobwtane CH, CH, 1 > H—CH,Cl CH, cL methyl propane 3c 3 ‘2-Chloro-2-methylpropane (b) Position Isomerism. Haloalkanes containing three or more carbon atoms exhibit position isomerism, e.g, 1-chloropropane and 2-chloropropane are position isomers. a 2 1 3 2 3 1 CH,—CH,—CH,C1 CH,—CH—CH, 1-Chloropropane 2-Chloropropane Haloarenes containing two halogen atoms on the benzene ring also show position isomerism. (©) Optical Isomerism. Optical isomerism is exhibited by alkyl halides having atleast one chiral (asymmetric) carbon atom, a oe CH,—CH,—CH—CH, 2-Chiorobutane 1.4 NATURE OFTHE CARBON - HALOGEN (C-X) BOND ‘The C-X bond isa polar covalent bond. Due to electronegativity difference between carbon and the halogen, the shared pair of electrons is closer to the halogen atom. Asa result, the halogen becames slightly negative while the carbon becomes slightly positive. Not 8 —c:x As the size of halogen atom inereases from fluorine to iodine, the C-X bond length increases and bond enthalpy decreases from C-F to Cl. ‘Table - 02 Some Physical Data of Halomethanes Halomethane |C-X bond length(pm)|C-X bond enthalpy (kJ mol')| Dipole moment (Debye) CIbF 139 452, 1.847 crBCl 178 351 1.86 CHBr 193 293 1.83 Ctl 214 234 1.636 Electronegativity decreases fiom F to I, therefore, the polarity of the C-X bond and hence dipole moment of the haloalkane should also decrease accordingly. But the dipole moment of CH, F is slightly lower than that of CH,Cl. Thisis due to small size of fluorine atom as compared to chlorine (dipole moment is the product of charge and distance). Thus, the order of dipole moments is: C-F < C-Cl> C-Br> C-L1.5 | METHODS OF PREPARATION OF HALOALKANES 1.5.1 From alcohols (i) By the action of halogen acids. Alcohols are converted into haloalkanes by the action of halogen acids (nucleophilic substitution reaction), R-OH+HX— > R-X +H,0 ‘lc Halothane Asnucleophilicity of the halide ions decreases in the order, ° > Br- > Clr, the reactivity of the halogen acids decreases in the same order: HI > HBr > HC1. Since the stability of the carbocations decreases in the order 3° 5 2° > 1°, the order of reactivity of the alcohols for a given haloacid decreases in the same order, Chloroalkanes or alkyl chlorides (a) Primary and secondary chloroalkanes are prepared by passing hydrochloric acid gas through a suitable alcohol in presence of anhydrous zine chloride (Grove’s Process). CH,CH - OH+HCI(g) A228, CHCH, -Cl+ H,0 kyl alcoho! Chloraethane CH,CH(OH)CH; + HCK(g) 8422, CH,CH(CNCH;+H,0 Propan-2-ol 2-Chloropropane (b) Tertiary alcohols react with cone. HCl even in the absence of zine chloride. Bromoalkanes or Alkyl Bromides. Bromoalkanes are pepared by refluxing alcohols with constant boiling hydrobromic acid (48%) in presence of a little conc. H,SO, as catalyst ora mixture of cone, H,SO, and KBr. ROH+KBr+H,$O,—*5R-Br+KHSO, +H,0 Todoalkanes or alkyl iodides. lodoalkanes are prepared by refluxing alcohols with constant boiling hydroiodic acid (57%). Hi may also be prepared by the action of 95% phosphoric acid on KI. CH,CH,OH + HI—2™*_5 CH,CH,1+H,0 CH,OH+KI+H,P0,—*> CH, -I +KH,PO,+H,0 Methanol Todomithane (ii) Action of phosphorus halides. Phosphorus halides react with alcohols to form haloalkanes. ROH + PCI, +R -Cl+POCI, +HCI 3ROH+PX,—93R-X+H,PO, ; (X=C1,Br,1) Since PBr, and Pl, are not very stable, these are generally prepared in situ by the action of red phosphorus on ‘bromine and iodine respectively. ROH—@2% yR-xX ; (X=Br,)IN] Chemistry (Class XI)) - Volume - I! (iii) Action of thionyl chloride. Chloroalkanes are conveniently prepared by refluxing alcohols with thionyl chloride (SO,C1, in presence of pyridine (C,H,N). ROH +S0,Cl, —>RCI+S0, +HCl ‘The byproducts in this reaction, being gases, escape leaving behind the chloroalkanes in almost pure state. 1.5.2 From Hydrocarbons i. Freeradical halogenation of alkanes. Halogenation of alkanes with Cl, or Br, in presence of heat or light gives a complex mixture of mono-, di-, and polyhaloalkanes. Cl, fav we SHC cH, —S2_> CH;Cl + CHCl + CHC + CCl Methane 5°°67°K Crioromethane Dichloromethane Trichloromethane ‘Tetracloromethane Substitution beyond monohalogenation can be suppressed by controlling the ratio of halogen to alkane. This ‘method gives a mixture ofhaloalkanes. Incase of higher alkanes, even monohalogenation gives a mixture of ll possible isomeric haloalkanes indicating that all types of hydrogens whether 1°,2° or 3° are substituted by halogen. CH; -CH, ~ CH; —S="* > CH, ~ CH, -CH-Cl+ CH; -CHCI-CH; Propane 298K“ -Chiorapropane (45%) 2-Chloropropane (33%) The relative amounts of isomeric haloalkanes formed depends on the nature of the halogen (Br, or Cl,) and the number and type of hydrogen (1°, 2° or 3°) thatare substituted. In general, the ease of substitution of hydrogens follows the sequence : 3°> 2°> 1°, odination is reversible, but may be carried out in presence of an oxidising agent, such as HIO,, HNO, HgO, ete., which destroys the HI formed and thus drives the reaction in the forward direction. CH, +1, —==CH,I+HI; SHI+HIO, > 31, +3H,0 Flourination of rydrocarbons using F, gas occurs explosively. This is due to the low bond dissociation energy of F, molecule (158.8kI mol“) and the strongly exothermic nature (447.7 kJ mol”) of one ofthe termination steps (R* + F* — RF). The energy released is larger than required to break a CC bond (347.3 kJ mol, Fluorination of hydrocarbons may be conveniently carried out by diluting F, with an inert gas such as N, or Ar. ii, Swarts reaction, Alkyl fluorides can be prepared indirectly by heating suitable chloro- or bromoalkanes with inorganic fluorides, such as AsF,, SbF, CoF’,, AgF, Hg,F,, etc. CH;Br +AgF——> CHF +AgBr , 2CH3CH3CI+HgF) —> 2CHyCH3F +Hg9Cl Bromomethane Fluoromethane * “Chloroethane Fluoromethane When the organic halide contains two or three halogen atoms on the same carbon, CoF,, or SbF, is used. 3CH, - CCl; CH, +2SbF, —93CH, — CF, ~CH,+28bCI, 2, 2-Dichloropropane 2, 2-Difluoropropane 1.5.3 From Alkenes and Alkynes (i) Addition of halogen acids. Alkenes react with halogen acids to form haloalkanes. The order of reactivity being HI> HBr> HCI > HF. CH=CH, +HI—> CH,-CH,-1 Fhene (Ethylene) Jodosthane (Eth! iodide)IN] Chemistry (Class XI) - Volume - I! ne . Absence of peroxide on oyt_cHts cue! 1H, + HBr — CHs—CH—CH pene h Propylene) s 2-Bromopropane (sopropy! bromide) Similarly, alkynes add halogen acids first to give mono haloalkanes and then dihaloalkanes. HCI . HCl CH;—C =Cl 5—C=CH, —“ +> CH;—C—CH SC SCH iarkaan NSF OR can, © CHS" CHs Propyne a cr a 2-Chloroprop-I-ene 2,2eDichloropropane ‘The addition of hydrogen halides to unsymmetrical alkenes and alkynes takes place according to Markovnikov’s rule. ‘However, in presence of peroxides like benzoyl peroxide (C,H,;COOOCOC,H,), addition of HBr (not HCI or HI) to unsymmetrical alkenes takes place against Markonikov’s rule. This is known as Kharasch effect. CH,—-CH, =CH, + HBr" 5 cH, —CH, —CH, -Br Gi) Addition of halogens to alkenes. ~Dibromides can be prepared by addition of Br, in CCI, to alkenes, BrCH, -CH Br 2-Dibromoethane (vie~dibromide) The reddish brown colour of bromine is discharged due tothe formation of colourless vie-libromide, therefore, this reaction is used as a test for detection of unsaturation (double or triple bond) in organic molecules. ili) Allylic halogenation, When alkenes except ethene are heated with Cl, or Br, at 773K, the hydrogen of the allylic earbon (carbon atom nextto the double bond) is substituted by halogen forming ally| halides. 73K CH ~CH) = CH +X) 7% 5 x ~CH, - CH (CH, +HX 1.5.4 Halogen exchange (i Finkelstein reaction lodoalkanes can be prepared from chloroalkanes or bromoalkanes by heating with sodium iodide in acetone or methanol. R-Cl+ Nal AS"24 R-1+ NaCl} R-Br+Nal "4 5 R -1+ NaBr CH;CH, — Br+ Nal A224, CHSCH, — 1+ Bromoethane Todosthane jaBr Acetone, A. CHjCH CH ~Cl+ Nal “4 5 CH,CHyCH3 -1+ NaCl }-Chloropropane todopropane 1.6 METHODS OF PREPARATION OF HALOARENES Haloarenes cannot be prepared from phenols because it is difficult to replace the hydroxyl group by halogen ‘Thisis because the carbon-oxygen bond in phenol has partial double bond character due to resonance and is, therefore, stronger than carbon-oxygen single bond in alcohol. 1.6.1 Direct halogenation of aromatic hydrocarbons (electrophilic substitution reaction) Haloarenes (chloroarenes and bromoarenes) can be prepared by direct halogenation of aromatic hydrocarbons at low temperature (310-320 K), absence of sunlight and the presence of Lewis acid suchas anhydrous ferric oraluminium halide as catalyst (halogen carrier), Jodine and iron filings also can be used as halogen carter.IN] Chemistry (Class XI) xX (Or Or. = pial CH; au CH; Duc to large difference in melting points, ortho and para isomers can be separated easily. Reactions with iodine eversible and require the presence of an oxidising agent like HNO,, HIO, etc.,t0 oxidise the HI formed. Fluoro compounds are not prepared by this method due to high reactivity of fluorine. Chlorobenzene is formed when benzene is treated with chlorine in presence of iron. Under the conditions of the reaction, iron first reacts with chlorine to form ferric chloride which acts as catalyst. cl Anhyd. FeCl or +c, as, S\sna 2 “310-320K Benzene Chlorobenzene 1.6.2 From diazonium salts i) Sandmeyer reaction, Chloroarenes and bromoarenes are prepared by treating diazonium salts with CuCl dissolved in HCI or CuBr dissolved in HBr respectively. N=NcI- cl SS CuCVHC, SS 7 l fee | +N) +Cr fA fA Benzenediazonium chloride Chlorobenzene Bi cuBuHtB: Sy — fo) oem tor eS Benzenediazonium chloride Bromobenzene Benzenediazonium chloride is obtained by treating aniline (primary amine) in dil. HCl with aqueous 273-278 K. ee CgHs-NH)+ HONO +Hc173238K _, egHs-N=NCI +2H,0 Aniline Nitrous acid Benzenediazonium chloride17 ii) lodoarenes are prepared by warming diazonium salt solution with aqueous KI solution. Sener I S em, S oO + Ki = ff +Ny+K Cr A A Benzenediaznium chloride ledabenzene ii) Baltz-Schiemann reaction, Fluoroarenes are prepared by heating diazonium tetrafluoroborates obtained by the diazotisation of aromatic primary amine with aq. NaNO, in presence of fluoroboric acid at 273-278 K. NH, N=NBFZ NaNOyHB, 4 . » N+ 213 -278K eee Aniline Benzendiazonium tetrafluoroborate Fluorobenzene ‘trifluoride i) Gattermann reaction, This isa modification of Sandmeyer reaction, A mixture of freshly prepared cop- per powder in presence of corresponding halogen acid (HCI or HB1) is used instead of euprous halide dis- solved in the comesponding halogen acid. N cr cl CwHCI ara a Benzenediazonium chloride Chlorobenzene +N, Br oe hon : Benzenediazonium chloride Bromobenzene PHYSICALPROPERTIES OF HALOALKANES AND HALOARENES 1, Physical state, colour, odour, ete, Alkyl halides are colourless when pure, but bromides and iodides develop colour when exposed to light. Methyl chloride, methyl bromide, ethyl chloride and some chlorofluoromethanes are gases at room temperature. Higher chloro, bromo, iodo compounds are either liquids or solids. Many volatile halogen compounds have sweet smell. 2. Melting points and boiling points (a) Haloalkanes. (i) Due to greater polarity and higher molecular mass, dipole-dipole and van der Waals forces are stronger in halogen compounds. Thus, boiling points of chlorides, bromides and iodides are considerably higher than those of hydrocarbons of comparable molecular mass.IN] Chemistry (Class XI) (Gi) Forthe same alkyl group, the boiling points of haloalkanes decrease in the order : RI> RBr> RC1> RE. This is because van der Waals forces of attraction increases with size and mass of the halogen CH, (IIL K) < CHF (195 K) (CH,),CHCH,Cl (342 K) > CH,CH,CHCICH, (341 K) > (CH,),C-Cl(324K) CH. 5 I CH,CH,CH,CH,Br (375 K) > CH,CH, -CH-CH, (364 K) > H,C—C—CH, (346 K) | i Br Br (v) Generally, the boiling points of chloro, bromo and iodo compounds increase as the number of halogen atoms increases. CH,CI(249 K) C,H,Br(429K) > C,H,Cl (405 K) > C,H,F358K) > C,H353 K) (ii) Forthe same halogen atom, the melting and boiling points increase as the size of the aryl group increases. (iv) The boiling points of isomeric dihalobenzenes are nearly the same, but their melting points are quite different, Generally, the melting point of the p-isomer is higher than that of the o-or the m-isomer. This is because the -isomer ismore symmetrical and hence its molecules pack closely inthe crystal lattice. Asaresul, intermolecular forces of attraction are stronger and hence the p-isomer melts at a higher temperature. cl cl cl cl (S bp = 453K bp = 446K bp =448K 2 mp = 256K cy Pn 289K mp = 323K cl 3. Density. Alkyl fluorides and chlorides are generally lighter than water, whereas alkyl bromides, iodides, and polyhalides are heavier. Their relative densities follow the order: RI> RBr> RCL. Compound] Density (g/mL) | Compound] Density (g/mL) | Compound] Density (g/mL) n-CsHiCl 0.89) CHI 2.279 CibCh 1.336 n-CsFhBr 1.335 Hs 1.933 CHC 1.489 n-CsHrl 1.747 n-CsHhl 1,747 ech 1.595Among the alkyl halides, methyl iodide has the highest density. As the size of the alkyl group increases, the densitiy of alkyl halide decreases: CH,I> CH,CH,I> mC. Density increases with increasing number of halogen atoms : CCI, > CHCI,> CH, Cl, All ary| halides are heavier than water. 4. Dipole moments. (a) Haloalkanes. Dipole moment decreaes as the electronegativity of the halogen decreases from Clto Br to I. Fluorides, however, have lower dipole moment than chlorides due to its very small size which outweighs the effect of greater electronegativity. CH,CI (1.860 D)> CH,F(1.847D)>CH, Br (1.830D)>CH,1(1.636 D). (b) Haloarenes. Fluorobenzene (1.60D), chlorobenzene (1.69 D) and bromobenzene (1.70D) have lower dipole moments than the corresponding methyl halides. lodobenzene has higher dipole moment (1.70D) than CHLI (1.636 D). The low dipole moment of fluorobenzene may be due to its small size. ‘5. Solubility. Even though haloalkanes and haloarenes are polar they are insoluble in water. This is due to the fact that haloalkanes can neither form hydrogen bonds with wa they break the hydrogen bonds existing between water molecules. However, they are quite soluble in organic solvents of low polarity such as petroleum ether, benzene, ether, chloroform, carbon tetrachloride, ete 6. Stability. Since the strength of the C-X bond decreases from F to I, stability of haloalkanes having the same alkyl group decreases in the order: alkyl luorides > alkyl chlorides> alkyl bromides > alkyl iodides. Thus, alkyl iodidies on standing become violet or brown due to decomposition in presence of light. 7.Inflammability. Inflammability of organic halogen compounds decreases with increasing halogen content. Polychloro compounds such as tri and tetrachloroethylenes are used as solvents for dry cleaning, CC as fire extinguisher under the name pyrene. 1.8 CHEMICAL REACTIONS OF HALOALKANES ded into thre ) Reactions with metals. 1.8.1 Nucleophilic substitution Reactions Inthis type of reaction, a nucleophile reacts with haloalkane (substrate) having a partial positive charge on the carbon atom bonded to halogen and the halogen atom (leaving group) departs as a halide ion. Since substitution reaction is initiated by anucleophile, itis called nucleophilic substitution reaction. ‘The leaving group such as X- (Cl, Br Fete.) which leaves with an electron pair is called a nucleophile. The order in which the leaving groups depart follows the sequence : I> Br> Cl-> F. The order of reactivity of haloalkanes is : iodoalkanes > bromoalkanes> chloroalkanes > fluoroalkanesIN] Chemistry (Class XI) Table - 03 Nucleophilic Substitution Reactions of Alkyl Halides (Nu +R-X— R-Nu + X°). Reagent Nucleophile (Nu’) mati Ny a oo NaOH, KOH, most | Ho R-OH ‘Alcohol AgO 130 Ho ROH Alcohol R'ONa RO’ ROR Ether Nal r Alkyl iodide NH: NHs Primary amine R'NHa RINH: Secondary amine R'NHR" R'NHR" RNRR" Tertiary amine KCN CoN: RCN Nitrile (cyanide) AgN Isonitrile(Isocyanide) KNO: Alkyl nitrite AgNO: RNO3 Nitroalkane R'COOAg R'COO" R'COOR Ester NaHS HS" RSH Thioaleohol R'SNa RS RSR’ Thioether LiAlHs ir RH Hydrocarbon RM Re RR Alkane alkanes on treatment with s to form alcohols. (i) Substitution of halogen by hydroxyl group (Formation of alcohols). Hal boiling aqueous alkalies or silver oxide in boiling water (AgOH) undergo hydroly: R-X +K*OH™(aq)——>R -OH+K*X~ Haloalkane Alcohol CH ~Br+ K*OH™ (aq) —> 3H; -OH+ K*Br- Bromoethane Ethenol Substitution of halogen by alkoxy group (Formation of ethers). Haloalkanes when treated with sodium or potassium alkoxides form ethers or alkoxyalkanes. This reaction is called Williamson synthesis. R-X +. R'ONa —*>R-0- Alkyl halide Sodium alkoxide Piher CjHs-Br+ CyHsONa —+C,H; -O-C) Bromoethane Sodium ethoxide Ethoxgethan (iii) Substitution of halogen by cyano group (Formation of alkyl cyanides). Haloalkanes react with alcoholic potassium cyanide to form alkyl cyanides oralkanenitiles along with small amount ofalkyl isocyanide. R-X +KCN(alc)—* >R-C = N+K*X™ Haloatkane Alkancnitile CH, -I +KCN(ale) * >CH, -C=N+K*T lodomethane Edhanenitsile‘The reaction of alky! halides with potassium cyanide is a convenient method for increasing the length of the carbon chain by one atom at a time, i. ¢., for aseending the homologous series Gv) Substitution of halogen by isocyanide group (Formation of alkyl isocyanides). When aqueous ethanolic solution of haloalkane (particularly iodoalkane) is heated with silver cyanide, alkylisocyanide oralkyl isonittile oralkyl carbylamine is produced along with small amount of alkyl cyanide CH.0HH,0 Ne R=X +Ag—CN—SHLOUILO | R_NC +AgX tiaiene a Alkylisoeyanide CH, -1 +AgCN(ale,) AROS 5, CH,-NC +Agl lodomethane Methyl isoeyanide Cyanide ionis a resonance hybrid of the following two resonating structures Either the carbon or the nitrogen atom can act as the electron donor. Such a nucleophile which has more than one site through which the reaction can occurs called ambident nucleophile. (v) Substitution of halogen by nitrite group (Formation of alkyl nitrites). Haloalkanes react with sodium. or potassium nitrite to form alkyl nitrites. I: <—> 3C R-X +K'~O-N=0—45R-0- Haloalkane Potassium nitrite Aly it O+K'X” (C)H; ~Br+K*~O0-N=O0—* 9C,H; -O-N=O+K*Br- Bromomethane E (vi) Substitution of halogen by nitro group (Formation of nitroalkanes), When aqueous ethanolic solution of haloalkane is heated with silver nitrite, nitroalkane is formed. (CoHsOH/H>0 22 Pd or R—X + Ag—O—N=0 ——"+R—NY or RONK + gx a S S Haloalkane Silver nitrite O ° Nitroalkane _Nitroalkane CyHsOHIHZO yo CH3CHy)—Br + Ag—O—N=0 “7+ CHSC “N+ AgBr Bromecthane Silver nitrite oO Nitroethane Nitrite ion CON =) also is an ambident nucleophile. Attack through nitrogen gives nitro compounds while attack through oxygen gives nitrites. (vii) Substitution of halogen by carboxylate group (Formation of esters). Haloalkanes form esters when heated with ethanolic solution of silver salt of fatty acid. ° ° 1 Hg i R—C—O— Ag + X—R —2=—> RX—C—OR + AgX Silver salt ofa fatty acid Haloalkane Ester° CH OH 1 OAg + Br—CH,CH, —=—> CH,—C—OCH,CH, + AgBr Situeraretnte _Rromoethane Ethyl ethanoate (Ethyl bromide) (Ethyl acerate) viii, Substitution of halogen by amino group (Formation of amines). Haloalkanes when heated with ethanolic solution of ammonia ina sealed tube at 373 K,, form a mixture of primary, secondary and tertiary amines and quaternary ammonium salts. CHCH ~ Br+ H-NH, 2 CH,CH, -NH)+ HBr Bromocthane Ammonia Ethanamine (1°) CH,CH,NH - H+ Br-CH,CH; "5 (CH;CH,),NH_+HBr Ethanamine Bromoethane N-Ethylethana mine (2°) (CH;CH))N-H+Br-CH,CH; “2, (CH;CH,);N__+HBr N-Bihylthanamine — Bromocthane * ON, N-Pihylethana mine (3°) (CH;CH,),N+Br—CH,CH,; 42H, (CH3CH))4N*Br7 N-Bthylethanamine Bromoethane ‘Tetraethylammonium bromide (Quatemary salt) This reaction is called Hofmann ammonolysis of alkyl halides. (ix) Substitution by hydrogen or Reduction. Primary and secondary alkyl halides are readily reduced to alkanes by lithium aluminium hydride (LiAIH,) which is a hydride donor. H’+R-X— >R-H+X™ or LiAIH, +4R -X—4R-H+AIX, +LiX However, tertiary alky! halides, undergo dehydrohalogenation to give alkenes. Sodium borohydride (NaBH) reduces secondary and tertiary alkyl halides but not primary whereas triphenyl tin hydride (Ph,SnH) reduces all the three types of alkyl halides. (x) Hydrogenolysis or Reduction. (a) Reduction by hydrogen in the presence of a metal catalyst such as nickel, palladium or pl: (CHCH)Br+H, —“>CH, - CH +HBr Bromoethane Pahane (b) Reduction using hydroiodic acid in the presence of red phosphorus at 420K. (CH;CH, — [+ HI 2242208, CH, — CH, +1, lodoethane Fehane (©) By the action of zinc or tin and hydrochloric acid or by the action of zine-copper couple and alcohol. zones (CH3CH, ~Cl ‘or(Zn—Cu)+alcohol (CHs ~CH3+ HCL Chloroethane thane1.8.2 Mechanism of Nucleophilic Substitution Reactions ‘There are two types of nucleophilic substitution reactions, S,2 (Substitution, nucleophilic, bimolecular), and SL (Substitution, nucleophilic, unimolecular). (a) Substitution nucleophilic bimolecular (S,2). Edward Davies Hughes and Sir Christopher Ingold (1937) proposed amechanism for S,2 reactions. The reaction between methyl chloride (CH,Cl) and hydroxide ion (OED) to yield methanol (CH,OH) and chloride ion follows second order kinetics, ie., the rate of the reaction depends on the concentration of both the reactants. Rate=k [CHCl] [OF] ‘Thisrate law implies that both the alkyl halide (CH,Cl) and the nucleophile (OH?) take part in the rate determining step ofthe reaction. When the incoming nucleophile (OF?) stats interacting with the alkyl halide (CHC), the carbon-halogen bond starts breaking and a carbon -OH bond starts forming. These two processes take place ina single step and no intermediate is formed. Thus, S,2 reactions are concerted reactions. Such reactions occur through a transition state in which both reactants are partially bonded to each other. HO- + + or Incoming nucleophile ouong (Reactant) niycleoohile Meth chioride Transition state Methanol (Leaving group) (Reactant, alyt halide) (Product Figure-01 Formation of transition state in a 8,2 reaction In the transition state, the carbon atom is bonded to five atoms and thus it is unstable and hence cannot be isolated, Being unstable, it decomposes to give the product (CH,OH) and the leaving group (CF ion), In S,2 reactions, the attack of the nucleophile (OH) occurs from the back side and the leaving group (CI) leaves from the front side, Asa result, S,2 reactions are always accompanied by inversion of configuration. (justasan umbrella tums inside out ina strong wind). This inversion of configuration is called Walden inversion. ‘The reaction requites the approach of the nucleophile to the carbon bearing the leaving group, therefore, the presence of bulky substituents on or near the carbon atom inhibits the reaction. Thus, methyl halides are the most reactive while tertiary halides are the least reactive in S,2 reactions, The order of reactivity is: Primary halide > Secondary halide > Tertiary halide. (b) Substitution nucleophilic unimolecular (S,1). S,.1 reactions are generally carried out in polar protic (hydroxylic) solvents such as water, alcohol, acetic acid, ete. The reaction between fert-butyl bromide and OIF ion to yield ert-butyl alcohol follows first order kinetics, ie., the rate of reaction depends on the concentration of fert-butyl bromide only and is independent of the concentration of OH-ions. Rate=k [(CH,),CBr]. CH CHS CH—¢—Br +-OH——> CHF —on +Br Cis CH tert Butyl bromide tert- Butyl alcohol or2-Bromo-2- methylpropane or 2-methylpropan - 2- ol,IN] Chemistry (Class XI) ‘The rate law suggests that the reaction occurs in two steps. Inthe first step, fert-butyl bromide undergoes ionisation to produce tert-butyl carbocation and a bromide ion. The energy needed for the cleavage of the C—Brbondis obtained by the solvation of the bromide ion with proton from protic solvents. Step I. This step is slow and reversible, therefore, the rate-determining step of the reaction, CH CH I, RA Fast | x OH Cy —On \ Nucleophile HC’ “CH; CH, tert-Butyl carbocation tert-Butyl alcohol Step Il. The carbocation is immediately attacked by the nucleophile (OH?) ion to give the substitution product (tert-butyl alcohol). This step is fast and hence does not affect the rate of the reaction. Fi Ionization ois slow Hy (Lt a e +Bro oN i H3C CH3 tert-Butyl bromide tert-Butylearbocation Iethe starting alkyl halide is optically active, the product formed is a racemic mixture, This is because the carbocation formed in the first step isa planar species involving sp” hybridisation. Therefore, the attack of the nucleophile can occur from both sides giving an approximately 50 : 50 mixture of the two enantiomers. (c) Relative reactivi y of haloalkanes towards 8.1 and S,2 reactions. In $,2 reactions, the attack of the nucleophile occurs from the back side on the a-carbon (i., carbon carrying the halogen), Therefore, presence of bulky substituents on ornear the o1-carbon atom tends to hinder the approach of the nucleophile towards the ct -carbon due to steric hindrance and thus makes the reaction difficult o occur. Greater the steric hindrance, slower the reaction. Among alkyl halides, methyl halides are the most reactive in S, 2 reactions because they have only three hydrogen atoms on the a-carbon. Tertiary alkyl halides with three bulky alkyl groups are the least reactive followed by secondary alkyl halides with two and primary alkyl halides with one alkyl group. Thus, the order of reactivity of alkyl halides in S.2 reactions is ‘Methyl halides > Primary halides > Secondary halides > Tertiary halides. ; i “OH, Mey ety leone forty, 2° a) a oh” o igure -02 Steric effects in S,2 reactions (decreasing relative rates).On the contrary, in $1 reactions, earbocations are the intermediates. Greater the stability of the carbocation, easier its formation and faster the rate of reaction. Since the stability of carbocations is in the order : 3°> 2°> 1°> CH] , the reactivity of alkyl halides towards 8.1 reactions decreases in the same order 3° alkyl halides> 2° alkyl halides> 1° alkyl halides > methyl halides. It follows that reactivity of alkyl halides towards S,2 and S,1 reactions follows opposite orders. Sy2 reactivityinereases R I R—C—X R—CH—X RCH,—X CH,—xX I 1 R” R’ 3° Aly! halide 2 Alkyl halide 1° Alkyl halide Methyl halide ‘Sy Ineactivityincreases Due to greater stabilisation of allyl and benzyl carbocation intermediates by resonance, even primary allylic and primary benzylic halides show greater reactivity in S,1 reactions than other simple primary halides. CH, Loin, + Gy cH cH, (Resonance stabilization of allyl carbocation) coe. a on diige ie ogi Z tho KE 7 (Resonance stabilization of benzyl carbocation) Whereas methyl and primary alkyl halides predominantly undergo substitution by S,2 mechanism, tertiary alky| halides undergo substitution by S,1 mechanism, Secondary, primary allylic and primary benzylic halides react either by S,.1 or S,2 mechanism or by both the mechanisms depending on the nature of the nucleophile and the solvent. Relative reactivity of haloalkanes: The reactivity of haloalkanes is due to polarity of the carbon-halogen (C-X) bond, Greater the polarity of the C-X bond, more reactive the haloalkane, Thus, polarity of C-X bond in methyl halides decreases in the order: CH, -C1(1.860 D) > CH, -F(.847 D)> CH, —Br (1.830 D) > CH, -1(1.636 D) ‘Therefore, reactivity of haloalkanes should also decrease in the same order, RCL> RF > RBr> RI, but the actual order is : RI> RBr> RCI> RF. This canbe explained in terms of the bond dissociation enthalpies of the C-X bond : (CHF) 452k3 mol > (CH,-Cl) 351kJ mol > (CH,-Br) 293kJ mol"> (CHI) 234 kJ mol".Table - 04 Differences between S,1 and S,2 mechanisms Syl Mechanism Sy? Mechanism 1 First order kinetics: [Second order kinetics. 2 Two - step reaction JOne -step reaction. 3 Carbocations are intermediates, Carbocations are not intermediates. Reaction Joccurs through a transition state. 4 Both backside and front side attack by the Exclusive backside attack of the nucleophile. nucleophile, 5 Generally racemization with some inversion (partial|Complete inversion of configuration occurs. racemization) is observed, 6 Rearrangements are generally observed, Rearrangements are not observed. 7 Elimination is an important side reaction, Elimination is not an important side reaction. 8 Electronic factors affect the reaction rates greatly tronie factors, aff of the reactions. [The order of reactivity follows the sequence : ° ‘The order of reactivity follows the sequen 2°>1° methyl halides, [Methyl> 1° > 2° > 3° alkyl halides 10 Weak nucleophiles such as water, alcohols, ete., _ [Strong nuckeophiles such as alkoxide ions favour favour these reactions. these reactions. 11 Low concentration of nucleophiles generally favour] High concentration of nucleophiles is favourable. them, 12 Polar protic solvents (water, alcoho, acids) of {Polar aprotic solvents (acetone, acetonitrile, high dielectric constants favour Syl reactions. | dimethyl formamide, dimethyl sulphoxide, etc.) of high dielectric constants favour Sx2 reactions. [Methanol and ethanol are usually used. 1.8.3 Stereochemical Aspects of Nucleophilic Substitution Reactions 4) Optical Isomerism and Optical activity. Substances which rotate plane polarised light (a beam. orl which has vibrations only in one plane) when passed through their solutions are said to be optically active the property is called optical aetivity. A substance that rotates plane polarised light in the clockwise direction (towards right) is said to be dextrorotatory. tis represented by the sign (+) or ). Any substance that rotates plane polarised light in the anticlockwise direction (towards left) is said to be laevorotatory, represented by (-) sign or (0. ‘he (+) and (-) isomers of an optically active compound are called optical isomers and the phenomenon is called optical isomerism. i) Molecular asymmetry, chirality and enantiomers, al the toms or groups attached toa carbon ato] are different, such a carbon is called asymmetric carbon or chiral carbon or stereocentre. A molecule which contains an asymmetric carbon has no elements of symmetry and hence called an asymmetric molecule and asymmetry of the molecule is responsible for optical aetivity in such organic compounds. Objects which are non-superimposable on their mirror images are called chiral and the property is called chirality while those which are superimposable on their mirror images are called achiral. Thus the ultimate test of chirality is the non-superimposability of an object and its mirror image.Enantiomers. Optical isomers which are non-superimposable mirror images of each other are called enantiomersand the phenomenon s called enantiomerism, Enantiomers have identical physical and chemical properties, but rotate the plane of polarized light to the same extent in opposite directions. The enantiomer which rotates the plane of polarized light towards right is called dextrorotatory and is represented as ‘d’ or (+) while the enantiomer which rotates the plane of polarized light towards left is called laevorotatory and is represented as ‘7 or (-). i) Diastereomers. Stereoisomers which are not mirror images of each other are called diastereomers and the phenomenon is called diastereoisomerism. iv) Racemic mixture. A mixture of equal amounts of the two enantiomers is called racemic mixture or racemic modification. A racemic mixture is optically inactive because the rotation caused by one enantiomer is exactly cancelled by equal and opposite rotation caused by the same number of the other enantiomer. Itis represented by prefixing dl or (4. ) before the name. The process of conversion of one enantiomer (+ or) into racemic mixtureis called racemisation inversion, retention and racemisation. Inachemical reaction, when bondis roken atthe sereocente, py three things can happen. For example, consider the replacement of a group X by Y in the following reaction, CoHs CoHs CoHs tome gat amt ag dtl SN eee Hs MoH IL | I un If (isthe only product of the reaction, the reaction proceeds with retention of configuration, I¢(I) is the only product obtained, the reaction is said to proceed with inversion of configuration. 1fa50-50 mixture of I and ITs obtained, the process is called racemisation, The productis optically inactive. (a) Stereochemistry of S,2 reactions InS,2 reactions, the attack of the nucleophile occurs from a direction opposite to the one from where the halogen atom leaves. S,2 reactions of optically a asymmetric centre, For example, when (-)-2-bromooctane is heated with sodium hydroxide, (+ formed in which the -OH group occupies a position opposite to what bromine had occupied. tive alkyl halides are always accompanied by inversion of configuration at the octanol is, Hse Cy (TO Hh Ho +e cw] \ mGeri3 Gers (}28nmeoaane (9) 2-e.na Figure - 03 Inversion of configuration during S,? reactions. ‘The reactant and the product in most S, reactions (except when an alkyl iodide is treated with I" ion) are not enantiomers. Therefore, they may have same or opposite signs of optical rotation even though they are not enantiomers.IN] Chemistry (Class XI) (b) Stereochemistry of S, ' reactions In, reactions, if the alkyl halide is optically active, then the products is a racemic mixture. This is because carbocations are the intermediates in S,' reactions. Since carbocations are sp? -hybridised and planar (achiral), attack of the nucleophile can occur from both the faces (front and near) with almost equal ease givinga 50 : 50 mixture of the two enantiomers. In one enantiomer, the nucleophile occupies the same position as originally occupied by the halogen atom while in the other enantiomer, the nucleophile occupies a position opposite to that of the halogen atom. For example, hydrolysis of optically active 2- bromobutane gives racemic or (:) - butan -2-ol —_— mod + Br we HCH CHsCH, (9 or }2Bromobutane atin CHs i CHs HO lL, on Rear attack Y Frontal attack wr" cae \r fnvewioy al | CHyCH, CHgCHs CHgCHy (+}Butan-2-01 (80%) Carbooation (Fianar) (-Heutan-2-01 (D0%) onaceton (phe sande ee eee eee acon bre Figure -04 Racemization during S,1 reactions 1.8.5 Elimination Reactions (Dehydrohalogenation) When haloalkane with f-hydrogen atom is heated with concentrated aleoholic solution of KOH, hydrogen halide is eliminated and an alkene is formed. Ree i H H a KtoIr 1 oI oe oi ts ie eat + KBr +H,0 (ale) | cl, 1 1 H ir Bromoethane Ethene Since the hydrogen atom is eliminated from the f-carbon, this reaction is classified as B-elimination Orientation of dehydrohalogenation. If the halogen is present on the terminal carbon atom, dehydro- halogenation occur only in one direction to give the terminal alkene, e.g,, l-chlorobutane gives butel-ene, B oe CH,CH,—CH—CH, + KOH (alc.) —*> CH;CH,CH = CH) + KCL+H,0 | I sut-I-ene Sia But-1 1-ChlorobutaneIfthe halogens present on a carbon atom within the chain, dehydrohalogenation can occur in two or more directions depending on the number of different types of 2—hydrogensavailable, According to Alexander Zaitsev (Russian chemist, 1875), the more highly substituted alkene will be the major product. Indehydrohalogenation reactions, the preferred product is that alkene which has the greater number of alkyl groups attached to the doubly bonded carbon atoms. This generalization is known as Saytzeff’s rule and this mode of elimination is called Saytzeff elimination. B o B KOHialc.).4 ——_ CH, —CH,—CH—CHt, CH;—CH=CH—CH, + CH,—CH,—CH=CH, Rnt-2-ene (815) Bot-1-ene (19%) Br 2-Bromobutane Any alkyl halide that gives a more stable alkene undergoes dehydrohalogenation faster than one which gives less stable (i.e. less highly substituted) alkene. Among chloroethane (1°), 2-chloropropane (2°), and 2-chloro- 2-methyl propane (3°), the ease of dehydrohalogenation increases as the number of [B— hydrogens increases, ie. (CH),CCI> (CH,), CHCl > CH,CH,Cl. In general, the ease of dehydrohalogenation of different alkyl halides containing the same alkyl group and different halogens decreases in the order: R-I>R -Br>R-CI>R-F. Cis-trans isomerism of Saytzeff products, Ifthe substituted alkene formed by Saytzetf elimination is capable of exhibiting cis-trans isomerism, the trans-alkene, being more stable, will always be the major product. Br 1 Kowtate.a CH. LH CHL _CH, CH, —CH—CH,CH, — Cad + >e=c< + CH,CH,CH = CH, 3 LCs Se gee Non, aoa CD sCH a 2-Bromobutene nO H 1-Butene (19%) srans-2-Butene (71%) cis-2-Butene (10%) 1.8.6 Reaction with Metals (i) Organometallic compounds. Compounds containing carbon -metal bond are called organometallic compounds. Alkyl magnesium halides (RMgX) were discovered by Victor Grignard (Nobel Prize in Chemisty, 1912), and are now called Grignard reagents. Grignard reagents are prepared by the reaction of haloalkanes with magnesium in dry ether. R-X +Mg—2¥ eter, R-Mex Alkythalide Grignard reagent CH;CH, -Br+Mg—2@te!_, CH,CH - MgBr Ethyl bromide Etylmagnesium bromide ‘The carbon-halogen covalent bond in Grignard reagents is highly polar due to large electronegativity difference ‘between carbon (2.5) and magnesium (1.2). The magnesium - halogen bond is ionic. R-MgX Due to high polarity, Grignard reagents are a potential source of carbanions and hence are very reactive. They react with any source of proton or acidic hydrogen to give hydrocarbons. Thus, water, alcohols, H,S, thioalcohols, NH, I°and 2° amines and terminal alkynes convert them to hydrocarbons.IN] Chemistry (Class XI) H-O-H+R-Mg XR -H+ Mg(OH)X Grignard reagents react with moisture, O, and CO,, therefore, they should be prepared and used in the absence of ar Grignard reagents are very useful forthe preparation of different classes of organic compounds like alcohols, aldehydes, ketones, carboxylic acids, ete (i) Wurtz reaction. Alkyl halides react with metallic sodium in presence of dry ether to form symmetrical alkanes containing double the number of carbon atoms. Dry ether ence, R-X+2Na+X-R R-R+2NaX CH, —X+2Na+X—CH, PS Shr, oH, CH, +2NaX ‘The reaction generally fails with tertiary alkyl halides since under the basic conditions of the reaction, they undergo dehydrohalogenation to form alkenes. When a mixture of two different alkyl halides is used, all the three possible alkanes are formed. 3R'—X + 6Na+3R"-X—2#* 4R’-R'ER" “RFR! “R"GNAX Therefore, this isnot a good method for the preparation of unsymmetrical alkanes, 1.9 CHEMICAL REACTIONS OF HALOARENES, Haloarenes and vinyl halides are extremely less reactive than haloalkanes towards nucleophilic substitution reactions due to the following reasons: (i) Resonance effect. In haloarenes, a lone pair of electrons from the halogen atom can delocalize on the benzene ing ce +A: [ j<> vith i & 1 0 Mm Vv v Figure-05 Resonance structures of chlorobenzene Asaresult, the C-Cl bond acquires some double bond character. Consequently, C-X bonds in aryl halides are stronger than in alkyl halides, and hence cannot be easily broke Vinyl halides can be represented as a resonance hybrid of the following structures : <> 7:CH,— CHSC: Asaresult, CX bond in vinyl halides, like in haloarenes, is stronger than in alkyl halides. Aryl and vinyl halides are stabilised by resonance but alkyl halides are not. The energy of activation for displacement ofhalogen from aryl/vinyl halides is much greater than that for alkyl halides. Thus, aryl/vinyl halides are much less reactive than alkyl halides towards nucleophilic substitution reactions. (ii) Difference in hybridization of carbon atom in C-X bond. In haloalkanes, the halogen is attached to sp*=hybridized carbon while in haloarenes and vinyl halides, the halogen is attached to sp? -hybridized carbon,Since an sp?-hybridized orbital is smaller than an sp*-orbital, the C-Cl bond in chlorobenzene or is shorter and hence stronger than in methyl chloride. yi chloride 2 cl a 5 Pm Tip pn jem, —s S aN a Z fH Thus, in chlorobenzene, C-Cl bond is stronger than in methyl chloride and hence difficult to break. (iii) Polarity of the carbon-halogen bond. Another reason for the low reactivity of aryl and vinyl halides over alkyl halides is their lesser polar character. & x b+ Due to greater s-character, the sp?-hybrid carbon is more electronegative than sp*-hybrid carbon. Therefore, the C-X bond in aryl or vinyl halides has lesser tendency to release electrons to the halogen. Asa result, the C-X bond in aryl or vinyl halide is less polar than in alkyl halide and the halogen atom cannot be easily displaced by nucleophiles. (iv) Instability of phenyl cation. In haloarenes and vinyl halides, the phenyl cation or the vinyl cation formed. by self-ionization is not stabilized by resonance because the sp’-hybridized orbital of carbon having +ve charge is perpendicular to the p-orbitals of the phenyl ring or the vinyl group. Therefore, these cations are not formed and hence aryl and vinyl halides do not undergo nucleophilic substitution reactions easily. OLS se O» vr Chlorobenzene Phenyl cation (9) Electronic repulsions. Since arenes and vinyl halides are electron rich due to the presence of n—bonds, they repel the electron rich nucleophiles fom attacking them. 1.9.1 Replacement by hydroxyl group. Dueto low reactivity, ary! halides undergo nucleophilic substitution reactions only under drastic conditions. For example, chlorobenzene when heated with aqueous NaOH at 623 K and 300 atmospheres, gives sodium phenoxide which on acidification gives phenol. This reaction forms the basis of Dow process for the manufacture of phenol. Cc 4 2NAOH (aq) —222 Ks 2004 OH (aq) ess =H0, NaCl Chlorobenzene Sod. phenoxide Phenol Reactivity of haloarenes in nucleophilic substitution reactions. The displacement of the halogen in haloarenes takes place only under drastic conditions. However, the presence of electron-withdrawing groups suchas—NO,,—CN, ete., at o-and p- positions (but not in m-positions) wrt. the halogen greatly activates the halogen towards nucleophilic displacement. Greater the number of such groups at o- and p- positions wart. the halogen, the more reactive is the haloarene.a OH (0 15% NaOH, 443 K ye (i Dil. HCL 3 ? NO, NO, 1-Chloro-4-nitrobenzene 4-Nitrophenol a OH ! 2-NO2 (yaa, NaOH, 368K 2/NOz Oka NOH RS (DIL HCI > ? NO; NOg 2, 4-Dinitrochlorobenzene 2, 4-Dinitrophenol a on ON. 2-NO; ON. 2NOy Z — 2 qT 4] NO} NO, 2,4, 6-Tinitochlorobenzene 2,4, 6 Tinitrophenol (Clery chloride) (Pierie acid) Mechanism. The presence of NO, groups at o-and p-positions withdraws electrons from the benzene ring and thus facilitates the attack of the nucleophile on haloarenes. The carbanion thus formedis resonance stabilised. (i Attack at p-pos al how = ao, Co i Ji ppctrotobaraane on ay > ase Reverace yen pvoghea However, no effectiis observed by the presence of electron withdrawing group at meta- position.(ii) Attack at o-posi ei ty \ +. | h Y i 7 ON. | OPN OFS ON, 7 wW ia o-Chboronitobenzene u Resonating stutice 9 oH og oH 7 A i a A ee, A ry #O Resonance hybrid -Nirophenl & Gi OH [| eer ° N a | X ° ‘mChloronittobonzene x x 2 Resonating structures a r Lew OO -J 0 a | ol RIN, As i, 7 \, Resonance hybrid No mNitrophenot O™ Incase of o-and p-chloronitrobenzenes, one of the resonating structures (Il in case of p-chloronitrobenzene and IV in case of o-chloronitrobenzene) carries a negative charge on the carbon atom bearing the NO, group. Therefore, these carbanions are stabilized by the NO, group as well as by the n-clectrons of the benzene ring, However, incase of m-chloronitrobenzene, there is no such structure in which the negative charge is present on the carbon atom bearing the -NO, group. Therefore, in case of m-chloronitrobenzene, carbanion is not stabilized by the-NO, group but is only stabilized by the w-electrons of the benzene ring. In other words, carbanions resulting from o- chloronitrobenzene and p-chloronitrobenzene are more stable than that resulting from m-chloronitrobenzene. Although the presence of NO, group at m-position wart. the halogen does not stabilize the resulting carbanion by resonance effect, it does stabilize it somewhat by the inductive effect as compared to the carbanion formed from chlorobenzene. Thus, m-chloronitrobenzene is more reactive than chlorobenzene towards nucleophilic substitution reactions,1.9.2 Electrophilic substitution reactions. Haloarenes undergo electrophilic substitution reactions ofthe benzene ring such as halogenation, nitration, sulphonation, ete. Halogen atom is slightly deactivating and o-,p-directing, therefore, further substitution occurs at o- and p-positions wir. the halogen atom. Inthe resonance struetures Il, I and IV of chlorobenzene (Fig. 05), there is anegative charge either at o- or p-positions, ie, the electron density increases at o- and p-positions. However, due to steric hindrance at the o-position, the p-product usually predominates over the o-product. The halogen atom, because of its -I- effect, has some tendency to withdraw electrons from the benzene ring. Asa result, the ring becomes some ‘what deactivated and hence further electrophilic substitution occurs slowly and under more drastic conditions as compared to benzene. (@ Halogenation ca ca A ee aha fl +% iii aa svischome 1 2.Riohraennene (minor) ca ‘nian (ii) Nitration cl t t fe 1 ~ Cone. HyS04 Qe NOz ~ _ ean l + HNO3 z | i | SH (cone. LA A Chilorobenzene 1-Chloro-2-nitrbenzene qf (minor) NO» [-cinacesamcniie (major) (iii) Sulphonation cl ca a p a A AL-S03H Ke + Hy80, —— ( + ( ‘om Z Z abenaeee 2-Chlerobenzeeslphonic acid ai onary SO3H 4-Chordbenzenesshonic aid (major) (iv) Friedel-Crafts alkylation cl cl cl J t a ee ee sas O foe FC. alkylation | O o Rees 1-Ct-2 wthenzene 7 (rninor) CH; |-Chloro-4-methylbenzene (major)(iv) Friedel-Crafts acvlation a a cl 9° 2 4 cS Il _ambyd acts (COCs S ZA * CR O~ Cl ation fi Z Chlorobenzene Acetyl chloride 2-Chloroacetophenone I (minor) 4-Chiloroseetophenone (ngjor) 3. Reactions with Metals (a) Wurtz-Fitting reaction, Haloarenes when treated with ethereal solution of alkyl halide in presence of sodium, form alkyl derivatives of benzene. This reaction is called Wurtz- Fitig reaction. Br CH,CH, bry eth +2Na + CHCHyBr 7, +2NaBr Ethyl bromide Bromobenzene Ethylbenzene (b) Fittig reaction. When haloarenes are treated with sodium, diaryls are produced. cl Dry ether & A A Chlorobenzene Diphenyl + 2NaCl 1.10 POLYHALOGEN COMPOUNDS Carbon compounds containing more than one halogen atom are called polyhalogen compounds. Many polyhalogen compounds are useful in industry and agriculture 1, Dichloromethane (Methylene Chloride) - CH,Cl, Uses. (i) It is widely used as solvent, paint remover, propellant in aerosols and as process solvent in the ‘manufacture of drugs. (ii) Itis also used as metal cleaning and finishing solvent Physiological effects. Methylene chloride acts on the central nervous system (CNS). (j) Exposure to small levels of methylene chloride in air causes slightly impaired hearing and vision. Higher levels of methylene chloride in air causes dizziness, nausea, tingling and numbness in the fingers and toes. (ii) Direct contact with skin causes intense burning and mild redness of skin, Direct contact with the eyes ean bum the comea. 2. Trichloromethane (Chloroform), CHCI, . tis colourless, sweet-smelling, dense liquid (B.P. 61.15°C). (Chloroform is slowly oxidised by airin presence of light o an extremely poisonous gas, carbonyl chloride (phosgene). 2CHCI; +0, —“#", 2cocl, +2HCI Carbonyl chlorideIN] Chemistry (Class XI) Itis, therefore, stored in dark coloured bottles completely filled to the brim and closed to exclude air: 0.6 1.0% alcohol is also added to destroy traces of phosgene that may be formed on storage. Any Phosgene formed combines with ethyl alcohol to form non -toxic diethyl carbonate 2CjHsOH + COCl, —> (CyHs)CO +2HCI Phosgene Diethyl carbonate (Gi) Itreacts with silver powder to give ethyne (acetylene) 2CHCI; + 6Ag—* > HC = CH +6AgCl Uses. (i) Asa solvent for resins, rubbers, oils and fats, alkaloids, iodine, etc. In the production of freon refrigerant, R-22 (chlorodifluoromethane), Physiological effects. It was used as a general anaesthetic agent in surgery but has been replaced by less toxic and safer anaesthetic agents such as ether. Inhaling chloroform vapours depresses the central nervous system and causes dizziness, fatigue and headache. Chronic exposure may damage the liver (where chloroform is metabolised to phosgene) and the kidneys. Some people develop sores when skin is immersed in chloroform. 3, Triiodomethane (lodoform), CHI,, lodoform was earlier used as an antiseptic for dressing wounds. Its antiseptic property is due to the liberation of iodine when it comes in contact with skin. 4. Tetrachloromethane (Carbon Tetrachloride), CCl, Carbon tetrachloride is produced by chlorination of methane in presence of light. Uses. (i) In the manufacture of refrigerants and aerosol propellants. (ii) Asa feedstock in the synthesis of chlorofluorocarbons ({reons ) and other chemicals. ii) Asa solvent in the manufacture of pharmaceuticals. (iv) Until the mid 1960s, it was widely used as a fire extinguisher under the name pyrene. (v)Ascleaning fluid in industry and as a degreasing agent and spot remover at home. Physiological effects. (i) Exposure to carbon tetrachloride causes liver cancer. The most common effects are dizziness, light headedness, nausea and vomiting, which can cause permanent damage to nerve cells. In severe cases, these effects can lead to stupor (laziness), coma, unconsciousness and death. (i) Exposure to carbon tetrachloride vapours can make heart beat irregular or may even stop. It causes invitation to the eyes. Environmental effects. Carbon tetrachloride released into the air, goes to the upper atmosphere and depletes the ozone layer: Depletion of ozone layer increases human exposure to ultraviolet radiations which may lead to increased skin cancer, eye discase and disorders, and disruption of the immune system. 5. Freons Chlorofluoro compounds of methane and ethane in which all the H- atoms are replaced by halogen atoms are collectively called freons, These are further classified on the basis of the number of fluorine atoms present CE, (Freon - 14), CF,Cl (Freon - 13), CF,CI, (Freon - 12), CFCI, (Freon - 11) Preparation. Freon -12 is manufactured from carbon tetrachloride by Swarts reaction. 3CCly + 2SbF; 2 > 3CClyF, + 28bCh; Freon=12, Freon-12 is an odourless, noncorrosive, nontoxic gas which is stable even at high temperatures and pressures, Ithas lowb.p., and low specific heat and can be easily liquefied by applying pressure at room temperature. xr2.0 2A 2.2 Uses. It is widely used as refrigerant in refrigerators and air conditioners. Its also used as propellant in aerosols and foams (hair sprays, deodorants, shaving creams, cleansers, insecticides, etc). Environmental effects. Freon diffuses unchanged into the stratosphere where it initiates radical chain reactions ‘which can disrupt the natural ozone balance. 6. p,pDichlorodiphenyl Trichloroethane (DDT). The name p, p “dichlorodiphenyltrichloroethane is a misnomer. Its actual name is 2-2-bis(4-chlorophenyl)-1,1,1-trichloroethane. DDT was first prepared in 1873 but its effectivenss as an insecticide was discovered only by 1939 by Paul ‘Maller (Nobel Prize in medicine and physiology in 1948) at Geigy pharmaceutical in Switzerland. Preparation, It is prepared by heating chlorobenzene with chloral (trichloroacetaldehyde) in presence of conc. ISO, , i—CCly ae HCCI; + HO 20K pH + 0: Chior ct Chlorobenzene DDT Uses. DDT is a cheap but powerful insecticide. Itis particularly effective against Anopheles mosquitoes which spread malaria and lice that carry typhus. Through its use, malaria has virtually been eliminated from India and large parts of the world, Side effects. DDT is not completely biodegradable. Many species of insects developed resistance to DDT. Ithas high toxicity towards fish, It gets deposited in fatty tissues of animals, This affects the reproductive system of animals. Because of side effects, the use of DDT was banned in USA in 1973. However, in India and inmany other Asian countries, DDT is still being used due to non-availability of cheaper insecticides. ADDITIONAL INFORMATION Propargyl halides. In these halides, the halogen is attached to sp* hybridised carbon next to carbon-carbon triple bond. These may be further classified as 1°, 2° or 3° 4 cl H3C_ CH; 12 3 12 3f \Z 1IC=C—CIhBr HC=C—CH-CH; Ho=¢—C—Br 3-Bromoprop-I-yne (I) 3-Chlorobut-I-yne (2°) (Proprargyl bromide) Preparation of Alkyl Halides Rena t-mathylt- tyme (32) i, Hunsdiecker reaction, Bromoalkanes can be prepared by refluxing silver salt of fatty acid with bromine in carbon tetrachloride. CO1,, Reflux CH3CH,COOAg+ Br) CREM CH.CH, ~Br+ CO) + AgBr Silver propanoate Bromoethane This is called Borodine Hundsdiecker reaction or Hunsdiecker reaction and is believed to occur by a free radical mechanism, The yield of the alkyl halide is: primary > secondary > tertiary.