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Haloalkanes and Haloarenes
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Chemistry : Haloalkanes and Haloarenes ®
Pre-Medical

HALOALKANES AND HALOARENES

HALOGEN DERIVATIVES
Compounds derived from hydrocarbons by replacement of one or more H-atoms by corresponding no. of
halogen atoms are known as halogen derivatives.
CLASSIFICATION
On the basis of nature of hydrocarbon from which they are obtained, halogen derivatives can be classified
as:
Halogen Derivatives

Alkyl halides Vinylic halides Allylic halides Aryl haides Benzylic halide
CH2=CH–X CH2=CH–CH2–X Ph–X Ph–CH2–X

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Mono halides Di-halides Tri-halides Tetra halides

Primary halides Secondary halides Tertiary halides

MONOHALIDES
General Methods of Preparation of Monohalides
(1) By direct halogenation of alkanes :
U.V.light
R—H + Cl2  → R—Cl + HCl
(excess)
(2) By the addition of H—X on alkenes :
R—CH = CHR + HX → RCH2—CHXR
eg: CH2 = CH2 + HX → CH3 —CH2X
eg: CH3–CH = CH2 + HX → CH3 CH CH3
X
Isopropyl halide
(3) By Alcohols :
(a) By the action of hydrogen halides :
H−X
R—CH2—OH → RCH2—X
(b) By the action of phosphorous halides :
R—OH + PCl5 → R—Cl + POCl3 + HCl
3ROH + PX3 → 3R–X + H3PO3 (X = Cl, Br)
PBr3 and PI3 are usually generated in site (Produced in reaction mixture) by the reaction of red
phosphorous with bromine and iodine, respectively.
(c) By reaction with thionyl chloride (Darzen's procedure) :
Pyridine
R—OH + SOCl2 
(1mole)→
R—Cl + SO2 + HCl

One mole One mole

Because of less stability of SOBr2 and SOI2, R—Br and RI can not be obtained by this method.

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(4) By halide exchange :
Acetone
R–Cl or R—Br + KI → R–I + KCl or KBr (Finkelstein reaction)
2CH3Cl + Hg2F2 
Water
→ 2CH3–F + Hg2Cl2 (Swart reaction)
Note : Finkelstein reaction can only be used to prepare R–I and swart's reaction can only be used to prepare
R–F
Physical Properties
(a) The lower members CH3F, CH3Cl, CH3Br , C2H5Cl and C2H5F are gases at room temp.
(b) Higher B.P. than parent alkanes.
Decreasing order of B.P. is : R–I > R—Br > R—Cl > R—F

among isomeric R—X, decreasing order of B.P. is : Primary > Secondary > tertiary

(c) R—F and R—Cl → lighter than water

®
R—Br and R—I → heavier than water
Decreasing order of density is : R—I > R—Br > R—Cl > R—F
(d) R—X are polar co-valent compounds but insoluble in water because they can not form
H–bonds. They dissolve in organic solvents.
(e) R—X (except R—F) burns with a green flame when interacted with Cu wire.(Beilstein test)
(f) Dipole moment order–

(i) CH3Cl > CH3F > CH3Br > CH3I (ii) Cl > Cl

Chemical Properties
1. Nucleophilic substitution reaction or NSR (SN)
2. Dehydrohalogenation
1. Nucleophilic substitution reaction or NSR (SN) :
δ+ δ–
Due to electronegativity difference the –C–X bond is polarised bond. –C–X

δ+ δ– 
Thus the C-atom of the C− X bond becomes centre to attack by a nucleophile (Nu) .

X  ion from R–X molecule is substituted by a Nu . i.e. SN reaction are the most common reactions in R–X.

R–X + Nu . → R–Nu + X
Two mechanisms are observed in SN reaction :

(a) SN1 mechanism (b) SN2 mechanism

Mechanism of SN1 and SN2 :

SN1 Mechanism : SN1 stands for uni molecular nucleophilic substitution. The mechanism involves two steps.
Consider the hydrolysis of tert. butyl bromide with aqueous NaOH.
Step 1 : The alkyl halide ionises to give a planar carbonium ion. The carbonium ion is planar because the
central positively charged carbon is sp2 hybridized.

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R R R
Slow
C Br → C⊕ + Br
R R
R
t–alkyl bromide

Step 2 : The nucleophile can attack the planar carbonium ion from either side to give the product.

R
R R R
Fast
C⊕ → C OH + OH C
R R R

®
R
R
OH – t–alkyl alcohol t–alkyl alcohol

(i) Ionisation is the rate determining step because it is the slow step. In other words, the rate at which alcohol
is formed depends upon the concentration of tertiary alkyl halide alone.
∴ Rate = K[R3C—Br]

It is obvious that the reaction follows first order kinetics, therefore reaction is called SN1.

(ii) The reactivity order for SN1 reaction ∝ stability of carbocations formed by halides.

∴ reactivity order of halides (SN1) varies as follows :

Benzyl halide > Allyl 3°halide > Allyl 2° halide > Allyl 1° halide > 3°halide > 2° halide
> 1° halide > methyl halide.

(iii) Remember that in case alkyl halide is optically active, SN1 reactions lead to racemisation.

SN2 mechanism : SN2 stands for bimolecular nucleophilic substitution. In this type of nucleophilic
substitution reaction, bond making and bond breaking process occur simultaneously.

H H H H
Slow
δ– δ–
H C Br → HO C Br → HO C H
H
H H
Walden inversion
Transition state

(i) Reactivity of alkyl halides in SN2 substitution is governed by steric factors. The bulkier the group, that less
reactive it will be.
(ii) Reactivity order of alkyl halide varies as follows :
allyl halide > CH3X > 1°halide > 2°halide > 3° halide
(iii) The order of reactivity among 1° alkyl halides is :CH3X > C2H5X > C3H7X etc.

Remember that in case alkyl halide is optically active, SN1 reactions lead to Walden inversion.

(iv) For a given alkyl group the order of reactivity is - (for SN1 and SN2 both) : RI > RBr > RCl > RF

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(1) nucleophilic substitution reaction ( SN ) in alkyl halide :
Replacement of Product
HOH(Warm)
(a) R—OH (alcohol) + HX

KOH aq.
(b) R—OH (alcohol) + KX

moist Ag2O
(c) R—OH (alcohol) + AgX
R—X →
KSH alc. ∆
(d) R—SH + KX
Alkane thiol
(Mercaptane)
NaSR'
(e) RSR' + NaX
Thioether
R' COOAg ∆
(f) R' COOR + AgX

®
(Alkyl alkanoate) ester
(g) Reaction with KCN and AgCN :
δ+ δ– + –
Alc.
R— X + K CN 
∆ → R—C≡N + KX

Ionic bond cyanide (major)

δ+ δ– ••
Alc. 
∆ → R—N = C + Ag —X
R— X + Ag − CN 
covalent bond isocyanide (major)

(h) Reaction with KNO2 and AgNO2 :

δ+ δ– + –
Alc.
R— X + KO− N =
O 
∆ → R—O—N = O + KX

Ionic bond (Alkyl nitrites)

(major)

δ+ δ– •• O
Alc.
O 
R— X + Ag − O − N = ∆ → R—N + Ag—X
covalent bond O
Nitroalkane
(major)

(i) Reaction with NaOR' (Sodium alkoxide) :

R—X + NaOR' → R—OR' + NaX

(williamson ether synthesis reaction)

Ex. (i) CH3—CH2—Cl + NaOCH3 → CH3—CH2—O—CH3

CH3
(ii) CH3–C–Cl + NaOCH3 → CH3–C=CH2 + NaCl + CH3OH
CH3 CH3
more reactive (Alkene)
towards elimination

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(j) Reaction with NH3 :

Ex. (i) R–X + NH3 R–X R–NH–R R–X R–N–R

R–NH2
R
↓R–X
R
⊕
R–N–R X (Quaternary salt)
R

(ii) R – X + NH3 → R – NH2 + H – X

(excess) (Major)

⊕ 
(iii) R – X + NH3 → R 4 N X (major)

®
(excess)

(iv) CH3–CH2–Cl+ NH3 → CH3–CH2–NH2 + HCl

(v) CH3–CH–CH3 + NH3 → CH3–CH–CH3 +HCl

Cl NH2

CH3 CH2
(vi) CH3–C–Cl + NH3 → CH3–C + NH4Cl
CH3 CH3
more reactive
towards elimination

(k) Reaction with CH≡CNa :

∆
R–X + CH≡CNa → R—C≡CH + NaX

BEGINNER'S BOX-1

1. Which is most reactive for SN1 reaction :-

Cl Cl Cl
(1) Ph Cl (2) (3) (4)
Ph Me Me Et Et Et

2. Which is most reactive for SN1?

CH2–Br CH2–Br

(1) (2)

NO2 OH
CH2–Br CH2–Br

(3) (4)

CH3

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2. Dehydrohalogenation : In addition to substitution reaction alkyl halide also undergo elimination
reactions to form alkene with the removal of a molecule of hydrogen halide (dehydrohalogenation). In
dehydrohalogenation, hydrogen and halogen atoms are eliminated from two adjacent carbon atoms, the
reaction also known as β−elimination it may proceed by E1 or E2 mechanism (analogous to SN1 and SN2
mechanism).

Alkyl halides undergo β - elimination on treatment with KOH (alc.) or NaNH2.

β α
∆→ −
R − C H2 − C H2 − X + KOH (alc.)  R CH = CH2 + HX

∆
CH3—CH2—CH2—CH2—Br + KOH(alc.) → CH3—CH2—CH=CH2 + HBr

H Alc.KOH
Br → CH3 − CH = CH − CH3 + CH3 − CH2 − CH = CH2 + HBr
∆
But − 2 − ene (80%) But − 1 − ene (20%)

®
CH3–CH–CH–CH3

Competition between substitution and elimination reactions

The order of elimination reaction is : 3° halides > 2° halides > 1° halides
Reactivity order of alkyl halides : E1 - Reaction : 1° < 2° < 3°

E2 - Reaction : 1° < 2° < 3°

SN1-Reaction : 1° < 2° < 3°

SN2-Reaction : 1° > 2° > 3°

GOLDEN KEY POINTS

2 2
 SN /E is favoured by high conc. of good neucleophile or strong base. (CH3O, HO)

Rate of Reaction ∝ (Substrate) (Reagent)

 SN1/E1 is favoured by low conc. of poor neucleophile or weak base (CH3OH, H2O)

 If an alkyl halide, undergoes SN2/SN1 then SN2 reaction will be favoured by high conc. of good neucleophile
(negetively charged) in presence of polar aprotic solvent where as SN1 – reaction is favoured by low conc. of
poor neucleophile (neutral) in presence of polar protic solvent.

Polar protic solvent : H2O, CH3OH, HCOOH

Polar aprotic solvent : DMSO, CH3CN, C2H5–O–C2H5, DMF

IMPORTANT NAME REACTIONS AND REAGENT
Dry ether
 Wurtz Reaction : 2RX + 2Na 
→ R – R + 2NaX
When a mixture of different alkyl halides, (R1 - X) and (R2 - X) is used a mixture of alkane is formed -
Dry ether
R1—X + 2Na + X—R2 
→ R1—R2 + R1—R1 + R2—R2 + NaX
 Fittig reaction :

dry ether
Cl + 2Na + Cl ∆ + 2NaCl
(Diphenyl)

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 Wurtz fittig reaction :

dry ether
Cl + 2Na + ClCH2CH2CH3 ∆ CH2CH2CH3 + 2NaCl

 Formation of Organometallic compounds :

Dry ether
(i) R—X + Mg 
→ RMgX (Grignard reagent)
Dry ether
(ii) 2C2H5Br + 2Zn 
→ (C2H5)2 Zn (Frankland reagent) + ZnBr2

 Friedel - Crafts reaction :

CH3
AlCl3(anhyd.)
+ CH3Cl → + HCl

®
Benzene Toluene

HALOARENE

1. NSR (Nucleophilic substitution reaction)

2. ESR (Electrophilic substitution reaction)

1. NSR in halobenzene :-

Cl OH
623K
+ fused NaOH → + NaCl
300atm

Presence of deactivating group at ortho and para position makes the nucleophilic substitution easier.

Reactivity Order : (Towards nucleophilic substitution)

Cl Cl Cl Cl
NO2 NO2 NO2
> > >

NO2 NO2 NO2

Illustrations
Illustration 1. Which of the following undergoes Hydrolysis most easily :
Cl Cl Cl Cl
NO2 NO2 NO2 NO2
(1) (2) (3) (4)

NO2 NO2

Solution. If there is more e– withdrawing groups then there will be more nucleophilic substitution reaction.
Ans. (4)

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BEGINNER'S BOX-2

Cl
O2N NO2 +

NaOH
→ A → B, what is B ?
H
1.
∆ , pressure

NO2

OH OH OH OH
O2N NO2 NO2
(1) (2) (3) (4)
NO2
NO2 NO2

2. Most readily hydrolised halide is

(1) C6H5Cl (2) (C6H5)2CHCl (3) C6H5CH2Cl (4) (C6H5)3CCl

®
GC0207
⊕
3. CH3–CH–ONa+CH3Cl →
CH3

(1) CH3–CH (2) CH3–CH–O–CH3 (3) CH3–CH–O–CH2–CH3 (4) CH3–CH–CH3

CH2 CH3 CH3 CH3

2. Electrophilic Substitution Reaction (ESR) in halobenzene :

Cl Cl Cl
Cl
(i) Anhy. FeCl3
+ Cl2 +

Cl
(Major)
Note : Cl is o– and p– directing group.
Cl Cl Cl
NO2
(ii) conc. H2SO4
+ HNO3 +
∆

NO2
(Major)
Cl Cl Cl
SO3H
(iii) ∆
+ conc. H2SO4 +

SO3H
(Major)
Cl Cl Cl
CH3
anhy. AlCl3
(iv) + CH3Cl +

CH3
(Major)

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DIHALIDES
General formula CnH2nX2 .Two H - atoms of alkanes, replaced by two halogen atoms to form dihalides.
Dihalides are classified as :
(a) Gem dihalide : The term Gem is derived from geminal means - same position.
Two similar halogen atoms are attached to same C - atom
Ex. CH3CHX2 Ethylidene dihalide (1, 1 - Dihalo ethane)
CH3
CX2 Isopropylidene dihalide (2, 2 - Dihalo propane)
CH3

(b) Vic dihalides : Vic term from - Vicinal means adjacent C - atoms
Two halogen atoms are attached on adjacent carbon atom.
H

®
CH2–CH2 H–C–CH–CH2
Ex. Vic and Gem dihalides are position isomers.
X X H X X
Ethylene dihalide Propylene dihalide
(1,2-Dihaloethane) (1,2-Dihaloethane)

Ethylene dihalide Propylene dihalide

(1,2-Dihaloethane) (1,2-Dihalopropane)
(c) α, ω dihalides : Halogen atoms are attached with terminal C - atom. They are separated by 3 or more
C - atoms. They are also known as polymethylene halides.

Ex. CH2–CH2–CH2–CH2 (1,4-Dihalobutane)

Tetramethylene dihalide
X X

Physical Properties
(i) Lower members are colourless, oily liquids with sweet smell. Higher members are solid.
(ii) They are heavier than water.

Chemical Properties
(i) Action of KOH(alc.) : (Dehydrohalogenation)
CH2X CH3 KOH(alc.) CH
or
CHX2 –2HX
CH2X CH

(ii) Action of KOH(aq.) : (Hydrolysis) It is a distinction test for gem - and vic - dihalides.

CH2–Cl CH2–OH
1. 2KOH(aq.) + 2KCl
(a) CH2–Cl 2. H3O⊕ CH2–OH
Glycol
Vic-dihalide 1,2-Ethanediol

CH3 KOH(aq) CH3

KOH (aq.)
(b) and CH3CX2CH3 
→ CH3COCH3 (Ketone)
CHX2 CHO
Gem-dihalides

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(iii) Reaction with KCN :

CH2Cl CH2–CN H2O/H+ CH2–COOH ∆ CH2–CO

+ 2KCN O
–2KCl –H2O
CH2Cl CH2–CN CH2–COOH CH2–CO
Vic-dihalide Succinic acid Succinic anhydride

Cl CN COOH
2KCN H2O/H+ ∆
CH3–CH CH3–CH CH3–CH CH3–CH2COOH
Cl –2KCl CN COOH –CO2
Propanoic acid
Gem-dihalide
(i) – CN group on acid hydrolysis gives - COOH
(ii) Two – COOH group on one C – atom always loose CO2 to form monocarboxylic acid on heating.
(iii) Two – COOH group on vic. C – atom loose H2O to form cyclic anhydride on heating.

®
(iv) Dehalogenation :

CH2Br CH2
Vic. CH2OH
+ Zn + ZnBr2
Heat CH2
CH2Br
Same Carbon Product

BEGINNER'S BOX-3
1. Which of the following is not organometallic compound
(1) RMgX (2) R2Zn
(3) RONa (4) R2Hg
2. Which is Finkelstein reaction ?
acetone →
(1) R–X + NaI  (2) R–X + AgF →
(3) R–X + NaF → (4) R–F + AgCl →

POLYHALOGEN COMPOUNDS
Physical Properties
CHCl3 is colourless and sweet smelling liquid. It's B.P. is 61°C and it is insoluble in H2O and have density
more than H2O. Chloroform is used as Anaesthetic.
Chemical Properties
Air and light
(i) Oxidation : CHCl3 + [O] 
→ COCl2 + HCl
Phosgene gas or
Carbonyl Chloride
(Poisonous gas)
CHCl3 is stored in dark coloured bottles which are filled upto the brim to prevent oxidation of CHCl3 into
COCl2 and 1% ethanol is also added to chloroform

Cl OC2H5
O=C + 2HO – C2H5 O=C
Cl –2HCl OC2H5
[Poisonous] Diethyl carbonate
[Non-Poisonous]

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GOLDEN KEY POINTS
Test of CHCl3
Reagent Pure CHCl3 Impure CHCl3 (COCl2 +HCl)
 Blue litmus No Change turns into red
 AgNO3 No reaction White ppt of AgCl

Cl OH
(ii) Hydrolysis : Aq.KOH
→
—H O KOH
CH OH→ H C OH→ H C OK
CH Cl
2

Cl OH
O O
Unstable
Freons
The chlorofluoro derivatives of methane and ethane are called freons.
CF2Cl2 – (dichloro difluoro methane)

®
C2F2Cl4 – (Tetrachloro difluoroethane)
Most useful is CF2Cl2 (Freon– 12)
Nomenclature of freons :
Freon – cba
c = nc—1
b= nH+1
a = nF
c = n c −1 = 1 − 1 = 0  c = n c −1 = 2 − 1 = 1 
CF2Cl2 → b = n H +1 = 0 + 1 = 1 Freon–12 C2F4Cl2 → b = n H +1 = 0 + 1 = 1 Freon–114
=a n= F 2  =
a n= F 4 
 Excess use of Freons is harmful for Ozone layer (depletion of Ozone layer).
Preparation of D.D.T.

Cl
H SO
CCl3CH= O + 2H Cl →
2 4
CCl3CH
(conc.)
Chloral Cl
Dichloro diphenyl trichloro ethane
(DDT)

ANSWER'S KEY
Que. 1 2
BEGINNER'S BOX-1
Ans. 2 2

Que. 1 2 3
BEGINNER'S BOX-2
Ans. 1 4 2

Que. 1 2
BEGINNER'S BOX-2
Ans. 3 1

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NCERT POINTS
Chlorine containing antibiotic, chloramphenicol, produced by soil microorganisms is very effective for the
treatment of typhoid fever. Our body produces iodine containing hormone, thyroxine, the deficiency of
which causes a disease called goiter. Synthetic halogen compounds, viz. chloroquine is used for the
treatment of malaria; halothane is used as an anaesthetic during surgery.
Compounds containing sp C–X bond (X = F, Cl, Br, I)
3

(A) Allylic halides

These are the compounds in which the halogen atom is bonded to an sp3-hybridised carbon atom next to
carbon-carbon double bond (C=C) i.e. to an allylic carbon.

CH2X

®
(B) Benzylic halides
These are the compounds in which the halogen atom is bonded to an sp3-hybridised carbon atom next to an
aromatic ring.

R' X
R''
CH2X

R' = CH3, R'' = H(2°)

R' = R'' = CH3 (3°)
(1°)

Compounds Containing sp2 C —X Bond

This class includes
(a) Vinylic halides
These are the compounds in which the halogen atom is bonded to an sp2-hybridised carbon atom of a
carbon-carbon double bond (C = C).

X
X

(b) Aryl halides

These are the compounds in which the halogen atom is bonded to the sp2-hybridised carbon atom of an
aromatic ring.

X X

H3C

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The dihalo-compounds having same type of halogen atoms are further classified as geminal halides (halogen
atoms are present on the same carbon atom) and vicinal halides (halogen atoms are present on the adjacent
carbon atoms).
H3C – CHCl2 H2 C − CH2
| |
Cl Cl
Common name : Ethylidene chloride Ethylene dichloride
(gem-dihalide) (vic-dihalide)
IUPAC name : 1, 1-Dichlorethane 1,2- Dichloroethane

Nature of (C-X) Bond

Since halogen atoms are more electronegative than carbon, the carbonhalogen bond of alkyl halide is
polarised; the carbon atom bears a partial positive charge whereas the halogen atom bears a partial negative

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charge.
δ+ δ–
C–X

Since the size of halogen atom increases as we go down the group in the periodic table, fluorine atom is the
smallest and iodine atom, the largest. Consequently the carbon-halogen bond length also increases from C—
F to C—I. Some typical bond lengths, bond enthalpies and dipole moments are given in Table.
Table : Carbon-Halogen (C—X) Bond Lengths, Bond Enthalpies and Dipole Moments

Bond Bond length / pm C-X Bond enthalpies/ kJ mol-1 Dipole moment / Debye

CH3-F 139 452 1.847

CH3–CI 178 351 1.860

CH3–Br 193 293 1.830

CH3–I 214 234 1.636

Physical Properties
 Alkyl halides are colourless when pure. Bromides and iodides develop colour when exposed to light. Many
volatile halogen compounds have sweet smell.
 Molecules of organic halogen compounds are generally polar. Due to greater polarity as well as higher
molecular mass as compared to the parent hydrocarbon, the intermolecular forces of attraction (dipole-
dipole and vander Waals) are stronger in the halogen derivatives. That is why the boiling points of chlorides,
bromides and iodides are considerably higher than those of the hydrocarbons of comparable molecular mass.
The attractions get stronger as the molecules get bigger in size and have more electrons.
 For the same alkyl group, the boiling points of alkyl halides decrease in the order: RI> RBr> RCl> RF. This
is because with the increase in size and mass of halogen atom, the magnitude of vander Waal forces
increases.
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Chemistry : Haloalkanes and Haloarenes
Pre-Medical
 The para-isomers are high melting as compared to their ortho and meta-isomers. It is due to symmetry of
para-isomers that fits in crystal lattice better as compared to ortho- and meta-isomers.

Compounds

b.p /K 453 446 448

m.p/K 256 249 323

 The haloalkanes are only very slightly soluble in water. In order for a haloalkane to dissolve in water, energy
is required to overcome the attractions between the haloalkane molecules and break the hydrogen bonds
between water molecules. Less energy is released when new attractions are set up between the haloalkane
and the water molecules as these are not as strong as the original hydrogen bonds in water. As a result, the

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solubility of haloalkanes in water is low.
 The sign of optical rotation is not necessarily related to the absolute configuration of the molecule.
A mixture containing two enantiomers in equal proportions will have zero optical rotation, as the rotation
due to one isomer will be cancelled by the rotation due to the other isomer. Such a mixture is known as
racemic mixture or racemic modification. A racemic mixture is represented by prefixing dl or (±) before the
name, for example (±) butan-2-ol. The process of conversion of enantiomer into a racemic mixture is known
as racemisation.
 In the Grignard reagent, the carbon-magnesium bond is covalent but highly polar, with carbon pulling
electrons from electropositive magnesium; the magnesium halogen bond is essentially ionic.
δ− δ+ δ−
R–MgX

Dichloromethane (Methylene chloride)

Dichloromethane is widely used as a solvent as a paint remover, as a propellant in aerosols, and as a process
solvent in the manufacture of drugs. Higher levels of methylene chloride in air cause dizziness, nausea,
tingling and numbness in the fingers and toes. In humans, direct skin contact with methylene chloride causes
intense burning and mild redness of the skin. Direct contact with the eyes can burn the cornea.
Trichloromethane (Chloroform)

The major use of chloroform today is in the production of the freon refrigerant R-22. its use as an

anaesthetic, inhaling chloroform vapours depresses the central nervous system. Chloroform is slowly

oxidised by air in the presence of light to an extremely poisonous gas, carbonyl chloride, also known as

phosgene. It is therefore stored in closed dark coloured bottles completely filled so that air is kept out.
light
2CHCl3 + O2  → COCl2 + 2HCl
Phosgene
Triiodomethane (Iodoform)

It was used earlier as an antiseptic but the antiseptic properties are due to the liberation of free iodine and

not due to iodoform itself.

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Chemistry : Haloalkanes and Haloarenes ®
Pre-Medical
Tetrachloromethane (Carbon tetrachloride)
It is produced in large quantities for use in the manufacture of refrigerants and propellants for aerosol
cans.There is some evidence that exposure to carbon tetrachloride causes liver cancer in humans.
When carbon tetrachloride is released into the air, it rises to the atmosphere and depletes the ozone layer.
Depletion of the ozone layer is believed to increase human exposure to ultraviolet rays, leading to increased
skin cancer, eye diseases and disorders, and possible disruption of the immune system.
Freons
The chlorofluorocarbon compounds of methane and ethane are collectively known as freons. They are
extremely stable, unreactive, non-toxic, non-corrosive and easily liquefiable gases. Freon 12 (CCl2F2) is one

of the most common freons in industrial use. It is manufactured from tetrachloromethane by Swarts reaction.
These are usually produced for aerosol propellants, refrigeration and air conditioning purposes. In

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stratosphere, freon is able to initiate radical chain reactions that can upset the natural ozone balance.
p,p’-Dichlorodiphenyltrichloroethane(DDT)
DDT, the first chlorinated organic insecticides, was originally prepared in 1873, but it was not until 1939
that Paul Muller of Geigy Pharmaceuticals in Switzerland discovered the effectiveness of DDT as an
insecticide. Paul Muller was awarded the Nobel Prize in Medicine and Physiology in 1948 for this discovery.
The chemical stability of DDT and its fat solubility compounded the problem. DDT is not metabolised very
rapidly by animals; instead, it is deposited and stored in the fatty tissues. If ingestion continues at a steady
rate, DDT builds up within the animal over time.
Cl

Cl
Cl Cl
Cl
H
DDT

 Chirality has a profound role in understanding the reaction mechanisms of SN1 and SN2 reactions. SN2

reactions of chiral alkyl halides are characterised by the inversion of configuration while SN1 reactions are
characterised by racemisation.

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