HALOALKANES AND HALOARENES

These are compounds containing halogen atoms attached to an alkyl or aryl group. The general
representation of haloalkanes is R-X and that of haloarenes is Ar-X [where X = F, Cl, Br, I].
Classification
I) On the basis of number of halogen atoms:
Based on this, haloalkanes and haloarenes are classified as mono, di or polyhalogen compounds.
Monohalogen compounds contain only one halogen atom, dihalocompounds contain 2 halogen atoms and
polyhalogen compounds contain more than 2 halogen atoms.
II) Compounds containing sp3 C-X bond: They include:
a) Alkyl halides or haloalkanes (R-X): Here the halogen atom is directly bonded to an sp3
hybridized C atom of an alkyl group. They are further classified as primary, secondary or
tertiary according to the nature of carbon to which halogen atom is attached. Their general
formula may be:
Primary haloalkane: R-CH2-X
Secondary haloalkane: R2CH
X
Tertiary haloalkane: R3C-X
b) Allylic halides: Here the halogen atom is bonded to an sp3 hybridized carbon atom next to a C
= C bond. E.g.: CH2=CH-CH2X
c) Benzylic halides: These are compounds in which the halogen atom is bonded to an sp3
hybridized carbon atom next to an aromatic ring. E.g.: C6H5-CH2-X
III) Compounds having sp2 C-X bond: They include
a) Vinylic halides: Here the halogen atom is directly bonded to an sp2 hybridized carbon atom of
a C=C bond. E.g.: CH2=CH-X
b) Aryl halides: Here the halogen atom is directly bonded to an sp2 hybridized carbon atom of an
aromatic ring. E.g. : C6H5-X

Nomenclature
Common name of alkyl halides is obtained by adding –yl halide to the word root (i.e. word root + yl
halide) and the IUPAC name is obtained by adding the prefix ‘halo’ to the name of the parent alkane (i.e. halo
+ alkane). Some examples are:

Compound Common Name IUPAC Name

CH3-Cl Methyl chloride Chloromethane
CH3-CH2-Br Ethyl bromide Bromoethane
CH3-CH2-CH2-Cl n-Propyl chloride 1-Chloropropane
CH3-CHCl-CH3 Isopropyl chloride 2-Chloropropane
CH3-CH2-CH2- N-Butyl bromide 1-Bromopropane
CH2-Br
(CH3)3C-Cl tert-butyl chloride 2-Chloro-2-methyl propane
(CH3)2CH-CH2-Br Isobutyl bromide 1-Bromo-2-methylpropane
C6H5-Cl Chlorobenzene Chlorobenzene
Methods of preparation
I) From alcohols:
a) By the action of concentrated halogen acids on alcohols in presence of anhydrous ZnCl2 as catalyst.
𝐴𝑛ℎ𝑦𝑑𝑟𝑜𝑢𝑠 𝑍𝑛𝐶𝑙2
R-OH + HX →−−−−−−−−−−−−−→ R-X + H2O
Reactions of primary and secondary alcohols with HI require the presence of anhydrous ZnCl2, while
tertiary alcohols do not require the catalyst.
b) Alkyl chlorides are obtained by the action of PCl3, PCl5 or SOCl2 with
alcohols. 3R-OH + PCl3 →−−−−−→ 3R-Cl + H3PO3
R-OH + PCl5 →−−−−−→ R-Cl + POCl3 +
HCl R-OH + SOCl2 →−−−−−→ R-Cl +
SO2 + HCl
Among these methods, the reaction with thionyl chloride (SOCl2) is preferred, since the
byproducts are gases and are easily escaped from the reaction medium.
For the preparation of alkyl bromides and iodides, alcohols are treated with bromine or iodine
in presence of red phosphorus, since PBr3 and PI3 are unstable.
𝑋2 /𝑅𝑒𝑝 𝑃
R-OH →−−−−−−−−−→ R-X (where X2 = Br2 or I2)
II) From Hydrocarbons
a) Free radical halogenation:
Alkanes react with chlorine or bromine in presence of sunlight; we get a mixture of mono, di and
polyhaloalkanes. For e.g. when methane is chlorinated in presence of sunlight (uv light), we get a mixture
of 4 products namely monochloromethane (methyl chloride, CH3-Cl), dichloromethane (methylene
chloride, CH2Cl2), trichloromethane (chloroform, CHCl3) and tetrachloromethane (carbon tetrachloride,
CCl4).
uv light or heat
CH4 + Cl2 →−−−−−−−−−−−→ CH3Cl + CH2Cl2 + CHCl3 + CCl4

b) Electrophilic substitution:
Benzene or its derivatives when heated with Cl2 or Br2 in presence of iron or Lewis acids like
anhydrous FeCl3 (ferric chloride) or AlCl3, we get aryl chlorides or bromides.

The ortho and meta isomers can be easily separated due to their large difference in melting point.
For the preparation of aryl iodides, arenes are treated with I2 in presence of an oxidising agent like
HNO3 or HIO4 (periodic acid) to oxidise the HI formed during the reaction.
 c) Sandmeyer’s reaction:
Aromatic primary amines when treated with mineral acids like HCl and sodium nitrite (NaNO2) at cold
condition (0 – 50C), an aromatic diazonium salt is formed. This reaction is called Diazotisation.

When a diazonium salt is treated with HX in presence of cuprous halide (Cu2X2), we get a halobenzene.
This reaction is called Sandmeyer’s reaction.

Note: If the cuprous halide is replaced by copper powder, the reaction is called Gattermann’s reaction.
For the preparation of iodobenzene, the diazonium salt is treated with potassium iodide (KI).

d) From alkene:
i) Addition of hydrogen halide (HX): Alkenes add HX (HCl, HBr or HI) to form alkyl halides. In the
case of unsymmetrical alkenes, the addition takes place according to Markownikoff’s rule. [The rule
states that “when an unsymmetrical reagent is added to an unsymmetrical alkene, the negative part of
the addendum (adding molecule) gets attached to the carbon containing lesser number of hydrogen
atoms”].
e.g. CH3-CH=CH2 + HBr →−−−−−→ CH3-CH2-CH2Br + CH3-CHBr-CH3
(minor) (major)
ii) Addition of halogen: Alkenes add halogen to form vicinal dihalides (2 halogen atoms on adjacent C
atoms). For e.g. addition of bromine in CCl4 to an alkene results in the formation of vicinal dibromides
and also in the discharge of the reddish brown colour of Br2 in CCl4. So this is used as a test for
unsaturation.
CCl4
e.g. CH2=CH2 + Br2 →−−−−→ CH2Br – CH2Br
(1,2-dibromoethane)
III) Halogen Exchange Reactions
a) Finkelstein reaction: Alkyl chlorides or bromides when treated with NaI in dry acetone, alkyl
iodides are formed. This reaction is known as Finkelstein reaction.
R-X + NaI →−−−−−→ R-I + NaX (where X = Cl, Br)
CH3-CH2-Br + NaI →−−−−−→ CH3-CH2-I + NaX
b) Swarts reaction: This method is used for the preparation of alkyl fluorides. Here alkyl chloride
or bromide is treated with a metallic fluoride like AgF, Hg2F2, CoF2 or SbF3, to get alkyl
fluoride.
R-X + AgF →−−−−−→ R-F + AgX (where X = Cl or Br)
2 CH3-CH2-Br + CoF2 →−−−−−→ 2 CH3-CH2-F + CoBr2
Physical Properties
Melting and boiling
points:
In haloalkanes, the C-X bond is polar due to the greater electronegativity of halogen atom. Due to
greater polarity and higher molar mass, the inter molecular forces of attraction (dipole-dipole and van der
Waals forces) are strong and so they have higher melting and boiling points than hydrocarbons of comparable
molar mass.
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 van der Waal forces
increases.
The boiling points of isomeric haloalkanes decrease with increase in branching. This is because as
branching increases, the surface area of the molecule decreases. So the van der forces decreases and hence the
b.p.
Among isomeric dihalobenzenes, 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.
Solubility
The haloalkanes are only very slightly soluble in water. This is because they cannot form hydrogen
bonds with water (except alkyl fluorides).
CHEMICAL REACTIONS OF HALOALKANES
i) Nucleophilic Substitution Reactions:
These are reactions in which a weak nucleophile is replaced by a strong nucleophile [Nucleophiles are
electron rich species attacks at electron deficient centre]. In general these reactions can be represented by:

Reaction with KCN: Alkyl halides react with alcoholic KCN to give alkane
nitriles (R-CN). R-X + KCN →−−−−−→ R-CN+ KX
Reaction with Silver cyanide (AgCN): Alkyl halides react with AgCN to give alkyl isocyanides or
carbyl amines (R-NC).
R-X + AgCN →−−−−−→ R-NC + AgX
CN- is an ambident nucleophile. i.e. here both C and N contain lone pair of electrons and can bind
to the carbon atom of the alkyl group either through C or through N. Another e.g. is NO - 2
Reaction with KCN gives alkyl cyanides. This is because KCN is mainly ionic and gives CN-
ions in solution. So both C and N are free to donate electron pairs. But C – C bond is stronger than C –
N bond. So cyanides are formed as the major product. But AgCN is mainly covalent and only N is free
to donate an electron pair. So isocyanides are the main product.
Mechanism of Nucleophilic Substitution Reactions
There are two types of mechanisms: Substitution Nucleophilic bimolecular (SN2) and Substitution
Nucleophilic unimolecular (SN1)
1. Substitution Nucleophilic Bimolecular (SN2) Mechanism:
As the reaction proceeds, the bond between the nucleophile and the carbon atom starts forming and
the bond between carbon atom and leaving group (the halogen atom) weakens. In the case of optically active
alkyl halides, during this process, the configuration of carbon atom inverts and hence this process is called as
inversion of configuration. In the transition state, the carbon atom is simultaneously bonded to five atoms
and therefore is unstable.
An example is the reaction between CH3Cl and hydroxide ion to yield methanol and chloride ion. This
reaction follows a second order kinetics, i.e., the rate depends upon the concentration of both the reactants.
Mechanism of this reaction is:
 Since this mechanism requires the approach of the nucleophile to the carbon bearing the leaving group, the
presence of bulky substituents on or near the carbon atom decreases the rate of this reaction. Thus the order of
reactivity of alkyl halides towards SN2 reaction is: Primary halide > Secondary halide > Tertiary halide.
2. Substitution nucleophilic unimolecular (SN1):
SN1 reactions are generally carried out in polar protic solvents (like water, alcohol, acetic acid, etc.). Here
the reaction occurs in two steps. In the first step, the C—X bond undergoes slow cleavage to produce a
carbocation and a halide ion. In the second step, the carbocation is attacked by the nucleophile to form the
product. Here first step is the slowest and it is the rate determining step. Since this step contains only one
reactant, it follows first order kinetics.
E.g.: The reaction between tert-butyl bromide and hydroxide ion to give tert-butyl alcohol.

This reaction occurs in two steps. In step I, the polarised C—Br bond undergoes slow cleavage to
produce a carbocation and a bromide ion. The carbocation thus formed is then attacked by nucleophile in step II to
form the product.

Thus in SN1 reaction, there is an intermediate called carbocation. The greater the stability of the
carbocation, the greater will be the rate of the reaction. In case of alkyl halides, 30 alkyl halides undergo SN1
reaction very fast because of the high stability of 30 carbocations. So the order of reactivity of alkyl halides
towards SN1 reaction is: 30 > 20 > 10.
Allylic and benzylic halides show high reactivity towards the SN1 reaction. This is because of the higher
stability of the carbocation formed. The allyl and benzyl halides are stabilized through resonance as follows:

Allyl carbocation

Benzyl Carbocation
For both the mechanisms, the reactivity of halides follows the order: R–I> R–Br>R–Cl>>R–F.
Stereochemical Aspects of nucleophilic substitution Reactions
Plane Polarised light
It is a light beam in which the particles vibrate in only one direction. It is produced by passing ordinary
light beam through a Nicol prism. When such a light beam is passed through solutions of certain compounds,
they rotate the plane of polarisation. Such compounds are called optically active compounds. The angle by
which the plane polarised light is rotated is called optical rotation, which is measured by an instrument called
polarimeter. If a compound rotates the plane polarised light towards right (i.e. clock-wise direction), it is called
dextro rotatory or d-form or + form and if it rotates the plane polarised light towards left (i.e. anticlock-
wise direction), it is called laevo rotatory or l-form or – form. The d and l form of a compound are called
optical isomers and the phenomenon is called optical isomerism.
Molecular asymmetry and Optical isomerism
Optical isomerism is due to molecular asymmetry. If all the 4 valencies of a carbon atom are satisfied by 4
different groups, it is called asymmetric carbon or chiral carbon or stereo centre. The resulting molecule is
called asymmetric molecule. Such molecules are non-super imposable to their mirror images and are called
chiral molecules and this property is known as chirality. The molecules which are super imposable to their
mirror images are called achiral molecules.
A chiral carbon is denoted by an asteric (*) mark.
*
e.g.: 2-Chlorobutane [CH3 – CHCl – CH2 – CH3]
Here the 2nd C is chiral, since all the four valencies of this C are satisfied by 4 different groups.
*
Other examples: 2-butanol [CH3 – CHOH –CH2 – CH3]
*
2-bromopropanoic acid [CH3 – CHBr – COOH]
*
Lactic acid [CH3 – CHOH – COOH]
Enantiomers
The stereo isomers related to each other as non-super imposable mirror images are called
enantiomers. They have identical physical properties. They differ only in the direction of rotation of the plane
polarised light. If one of the enantiomers is dextro rotatory, the other will be laevo rotatory.
Racemic mixture
An equimolar mixture of d and l form of a compound has zero optical rotation and such a mixture is
called racemic mixture or racemic modification. It is denoted by dl or (+). Here the rotation due to one isomer
is cancelled by the rotation due to the other isomer. The process of conversion of an enantiomer in to a racemic
mixture is called racemisation.
Retention and Inversion of configuration
If during a chemical reaction, there is no change in the spatial arrangement of bonds to an
asymmetric centre, we can say that the reaction proceeds through retention of configuration. (Or,
preservation of the integrity of configuration of a compound is termed as retention).

In general, if during a chemical reaction, no bond to the stereo centre is broken, the product will have the
same configuration as that of the reactant. Such reactions always proceed through retention of configuration.
E.g. Reaction of 2-Methyl-1-butanol with HCl.
 If during a chemical reaction, the incoming group is attached to a position opposite to that of the leaving
group, the configuration of the resulting product is inverted and we can say that the reaction proceeds through
inversion of configuration.
a a
c Y- Y
X b
b c
Nucleophilic Substitution and Optical Activity
ii) Elimination Reactions
Alkyl halides having β-hydrogen atom when treated with alcoholic solution of KOH, they undergo
elimination of one hydrogen halide molecule (dehydrohalogenation) to form alkenes. Since β-hydrogen atom is
eliminated, the reaction is also called β-elimination.
e.g. : CH3-CH2-Br + KOH (alc) →−−−−−→ CH2 = CH2 + KBr + H2O
[Carbon on which halogen atom is directly attached is called α-carbon and the carbon atom adjacent to this
carbon is called β-carbon.]
If there is possibility of formation of more than one alkene during dehydrohalogenation reaction, the
major product is selected by Zaitsev (Saytzeff) rule. The rule states that “in dehydrohalogenation reactions, if
there is possibility of formation of more than one alkene the preferred product is that alkene which contains
greater number of alkyl groups attached to the doubly bonded carbon atoms.”
So if 2-bromobutane is treated with alcoholic KOH, 2-butene is formed as the major product.
CH3-CH2-CHBr-CH3 + KOH (alc) →−−−−−→ CH3-CH2-CH=CH2 + CH3-CH=CH-CH3
2-Bromobutane 1-butene (minor) 2-butene (major)
iii) Reaction with metals
a) Alkyl halides react with Mg metal in ether medium to form alkyl magnesium halide (an
organometallic compound) commonly called Grignard reagent.
𝑒𝑡ℎ𝑒𝑟
R-X + Mg →−−−−−−−→ RMgX [Grignard reagent]
𝑒𝑡ℎ𝑒𝑟
CH3-CH2-Br + Mg →−−−−−−→ CH3-CH2MgBr
Grignard reagent is an example for organometallic compound. These are compounds in which carbon atom of an
organic compound is directly bonded to metal atom. Other examples are tetraethyl lead, trimethyl aluminium
etc.
In the Grignard reagent, the carbon-magnesium bond is covalent but highly polar and the magnesium
halogen bond is ionic.
b) Wurtz reaction: Alkyl halides react with sodium in dry ether to give alkanes with double
the number of carbon atoms. This reaction is known as Wurtz reaction.
R-X +2 Na +X-R →−−−−−→ R-R + 2NaX
CH3-Br + 2Na + Br-CH3 →−−−−−→ CH3-CH3 + 2NaBr
Reactions of Haloarenes
1. Nucleophilic Substitution Reaction:
Aryl halides are less reactive towards Nucleophilic substitution reactions due to the following reasons:
i) Due to resonance effect: In haloarenes, the electron pairs on halogen atom are in conjugation
with π-electrons of the ring and the following resonating structures are possible.
 Due to resonance, the C—X bond acquires a partial double bond character. Since it is difficult to break
a double bond, the replacement of halogen atom by other atoms is not easy. So haloarenes are less
reactive towards nucleophilic substitution reactions.
ii) Due to the difference in hybridisation of carbon atom in C—X bond: In haloalkane, the
halogen atom is attached to an sp3 hybridised carbon while in haloarene, it is attached to an sp2
hybridised carbon. Due to the greater s-character of sp2 hybridised carbon, it is more electronegative
and can hold the electron pair of C—X bond more tightly than sp3-hybridised carbon in haloalkane. So
the C – X bond in haloarene is shorter than that in haloalkane. Since it is difficult to break a shorter
bond than a longer bond, haloarenes are less reactive than haloalkanes towards Nucleophilic
substitution reaction.
iii) Due to the instability of phenyl cation: In haloarenes, the phenyl cation formed as a result of self-
ionisation will not be stabilized by resonance and therefore, SN1 mechanism does not occur.
iv) Due to the repulsion between nucleophile and electron rich benzene ring: Because of the
possible repulsion, it is less likely for the electron rich nucleophile to approach electron rich arenes.

Replacement by hydroxyl group (Conversion to phenol)

Chlorobenzene when heated with aqueous sodium hydroxide solution at a temperature of 623K
and a pressure of 300 atmospheres followed by acidification, we get phenol.

The presence of an electron withdrawing group (-NO2) at ortho- and para-positions increases the reactivity of
haloarenes.
 The effect is more when -NO2 group is present at ortho and para- positions. However, no effect
on reactivity is observed by the presence of electron withdrawing group at meta-position.
2. Electrophilic substitution reactions:
Haloalkanes are resonance stabilized as follows:

In the resonating structures, the electron density is greater on ortho-para positions. So the
electrophile enters at these positions and hence halo group is an ortho-para directing group. Also,
because of its electron withdrawing Inductive effect, the halogen atom has a tendency to withdraw
electrons from the benzene ring. So it is a deactivating group. Hence the electrophilic substitution
reactions in haloarenes occur slowly and require more vigorous conditions.
i) Halogenation: Haloalkanes react with halogen (Chlorine or bromine) in presence of anhydrous
ferric chloride to form o-dichlorobenzene and p-dichlorobenzene.
 ii) Sulphonation: On sulphonation using Conc. H2SO4, chlorobenzene gives
p- chlorobenzenesulphonic acid as the major product

iii) Friedel – Crafts Alkylation: Chlorobenzene when treated with methyl chloride (CH3-Cl) in
presence of anhydrous AlCl3, we get p-chlorotoluene as the major product.

iv) Friedel – Crafts Acylation: Chlorobenzene when treated with acetyl chloride (CH3-CO-Cl)
in presence of anhydrous AlCl3, we get p-chloroacetophenone as the major product.

Reaction with metals:

a) Wurtz-Fittig reaction: When a mixture of alkyl halide and aryl halide is treated with sodium in dry
ether, an alkyl arene is formed and this reaction is called Wurtz-Fittig reaction.

For e.g. when Chlorobenzene is treated with methyl chloride in presence of metallic sodium in ether
medium, we get toluene.
𝑒𝑡ℎ𝑒𝑟
C6H5-Cl + 2Na + CH3-Cl →−−−−−−−→ C6H5-CH3 + 2NaCl
Chlorobenzene Toluene
b) Fittig reaction: Aryl halides when treated with sodium in dry ether, we get diaryls
(diphenyls). This reaction is called Fittig reaction.

Diphenyl
Polyhalogen compounds
Carbon compounds containing more than one halogen atom are usually referred to as polyhalogen
compounds. Some polyhalogen compounds are:
1. Dichloromethane (Methylene chloride, CH2Cl2): It 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. It is also used as a
metal cleaning and finishing solvent.
2. Trichloromethane (Chloroform, CHCl3): It is used as a solvent for fats, alkaloids, iodine and
other substances. The major use of chloroform is in the production of the freon refrigerant R-22.
Chloroform is slowly oxidised by air in the presence of light to an extremely poisonous gas,
carbonyl chloride (COCl2), also known as phosgene.
𝑙𝑖𝑔ℎ𝑡
2CHCl3 + O2 →−−−−−−→ 2COCl2 + HCl
It is therefore stored in closed dark coloured bottles filled up to the neck in order to avoid air.
3. Tetrachloromethane (Carbon tetrachloride, CCl4): It is used in the manufacture of refrigerants
and propellants for aerosol cans. It is also used as feedstock in the synthesis of chlorofluorocarbons
and other chemicals, pharmaceutical manufacturing, and general solvent use.
4. 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.
5. p,p’-Dichlorodiphenyltrichloroethane(DDT): DDT was the first chlorinated organic insecticide.
The effectiveness of DDT as an insecticide was first invented by Paul Muller. The structure of
DDT is: