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
1
,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 a 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).
METHODS OF PREPARATIONS
I) From alcohols:
a) By the action of concentrated halogen acids on alcohols in presence of anhydrous ZnCl2 as
catalyst
R-OH + HX anhy. ZnCl2 R-X + H2O
Reactions of primary and secondary alcohols with HI require the presence of an. 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 3 R-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.
2
, 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.
R-OH X2/Red P 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).
CH4 + Cl2 uv light or heat CH3Cl + CH2Cl2 + CHCl3 + CCl4
Direct iodination is not possible with iodine as the reaction is reversible.
Direct Flourination is also not possible due to high reactivity of fluorine.
b) 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).
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.
e.g. CH2=CH2 + Br2 CCl4 CH2Br – CH2Br
(1,2-dibromoethane)
3
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
1
,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 a 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).
METHODS OF PREPARATIONS
I) From alcohols:
a) By the action of concentrated halogen acids on alcohols in presence of anhydrous ZnCl2 as
catalyst
R-OH + HX anhy. ZnCl2 R-X + H2O
Reactions of primary and secondary alcohols with HI require the presence of an. 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 3 R-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.
2
, 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.
R-OH X2/Red P 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).
CH4 + Cl2 uv light or heat CH3Cl + CH2Cl2 + CHCl3 + CCl4
Direct iodination is not possible with iodine as the reaction is reversible.
Direct Flourination is also not possible due to high reactivity of fluorine.
b) 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).
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.
e.g. CH2=CH2 + Br2 CCl4 CH2Br – CH2Br
(1,2-dibromoethane)
3
