Haloalkanes and Haloarenes
Class 12 Chemistry | Complete written notes | NCERT, NEET and JEE preparation
1. Introduction
Organic compounds containing one or more halogen atoms are called organohalogen
compounds.
The four common halogens are:
• Fluorine (F)
• Chlorine (Cl)
• Bromine (Br)
• Iodine (I)
A halogen atom replaces one or more hydrogen atoms in a hydrocarbon.
1.1 Haloalkanes
Haloalkanes are compounds in which a halogen atom is attached to an sp3sp^3sp3-
hybridised carbon atom of an alkyl group.
General representation: R−XR-XR−X
Here, RRR represents an alkyl group and XXX represents a halogen.
Examples:
• CH3ClCH_3ClCH3Cl — chloromethane
• CH3CH2BrCH_3CH_2BrCH3CH2Br — bromoethane
• CH3CH2CH2ClCH_3CH_2CH_2ClCH3CH2CH2Cl — 1-chloropropane
1.2 Haloarenes
Haloarenes are compounds in which a halogen atom is directly attached to an aromatic ring.
General representation: Ar−XAr-XAr−X
Here, ArArAr represents an aryl group.
Examples:
• C6H5ClC_6H_5ClC6H5Cl — chlorobenzene
• C6H5BrC_6H_5BrC6H5Br — bromobenzene
• C6H5IC_6H_5IC6H5I — iodobenzene
Important exam distinction:
, • C6H5ClC_6H_5ClC6H5Cl: chlorobenzene, a haloarene.
• C6H5CH2ClC_6H_5CH_2ClC6H5CH2Cl: benzyl chloride, a benzylic halide, not a
haloarene.
The difference is whether the halogen is attached directly to the benzene ring or to the side
chain.
2. Classification of haloalkanes and haloarenes
2.1 Based on the number of halogen atoms
Monohalo compounds: contain one halogen atom.
Example: CH3ClCH_3ClCH3Cl
Dihalo compounds: contain two halogen atoms.
Example: CH2Cl2CH_2Cl_2CH2Cl2
Trihalo compounds: contain three halogen atoms.
Example: CHCl3CHCl_3CHCl3
Polyhalo compounds: contain several halogen atoms.
Example: CCl4CCl_4CCl4
2.2 Geminal and vicinal dihalides
Geminal dihalides: both halogen atoms are attached to the same carbon atom.
Example:
CH3−CHCl2CH_3-CHCl_2CH3−CHCl2
IUPAC name: 1,1-dichloroethane.
Vicinal dihalides: the two halogen atoms are attached to adjacent carbon atoms.
Example:
ClCH2−CH2ClClCH_2-CH_2ClClCH2−CH2Cl
IUPAC name: 1,2-dichloroethane.
Remember: geminal means same carbon; vicinal means neighbouring carbons.
2.3 Classification according to the carbon attached to the halogen
, Haloalkanes are classified as primary, secondary or tertiary by counting the number of
carbon atoms directly attached to the carbon bearing the halogen.
Primary (1°) haloalkane
The carbon attached to the halogen is connected to only one other carbon atom.
Example:
CH3CH2ClCH_3CH_2ClCH3CH2Cl
This is chloroethane.
Secondary (2°) haloalkane
The carbon attached to the halogen is connected to two other carbon atoms.
Example:
CH3CHClCH3CH_3CHClCH_3CH3CHClCH3
This is 2-chloropropane.
Tertiary (3°) haloalkane
The carbon attached to the halogen is connected to three other carbon atoms.
Example:
(CH3)3CCl(CH_3)_3CCl(CH3)3CCl
This is 2-chloro-2-methylpropane.
A methyl halide, such as CH3ClCH_3ClCH3Cl, is usually classified separately because its
carbon is attached to no other carbon atoms.
2.4 Special types of halides
Vinylic halides: the halogen is attached directly to a carbon of a carbon–carbon double
bond.
Example: CH2=CHClCH_2=CHClCH2=CHCl, vinyl chloride.
Allylic halides: the halogen is attached to a carbon next to a carbon–carbon double bond.
Example: CH2=CHCH2ClCH_2=CHCH_2ClCH2=CHCH2Cl, allyl chloride.
Aryl halides: the halogen is attached directly to an aromatic ring.
Class 12 Chemistry | Complete written notes | NCERT, NEET and JEE preparation
1. Introduction
Organic compounds containing one or more halogen atoms are called organohalogen
compounds.
The four common halogens are:
• Fluorine (F)
• Chlorine (Cl)
• Bromine (Br)
• Iodine (I)
A halogen atom replaces one or more hydrogen atoms in a hydrocarbon.
1.1 Haloalkanes
Haloalkanes are compounds in which a halogen atom is attached to an sp3sp^3sp3-
hybridised carbon atom of an alkyl group.
General representation: R−XR-XR−X
Here, RRR represents an alkyl group and XXX represents a halogen.
Examples:
• CH3ClCH_3ClCH3Cl — chloromethane
• CH3CH2BrCH_3CH_2BrCH3CH2Br — bromoethane
• CH3CH2CH2ClCH_3CH_2CH_2ClCH3CH2CH2Cl — 1-chloropropane
1.2 Haloarenes
Haloarenes are compounds in which a halogen atom is directly attached to an aromatic ring.
General representation: Ar−XAr-XAr−X
Here, ArArAr represents an aryl group.
Examples:
• C6H5ClC_6H_5ClC6H5Cl — chlorobenzene
• C6H5BrC_6H_5BrC6H5Br — bromobenzene
• C6H5IC_6H_5IC6H5I — iodobenzene
Important exam distinction:
, • C6H5ClC_6H_5ClC6H5Cl: chlorobenzene, a haloarene.
• C6H5CH2ClC_6H_5CH_2ClC6H5CH2Cl: benzyl chloride, a benzylic halide, not a
haloarene.
The difference is whether the halogen is attached directly to the benzene ring or to the side
chain.
2. Classification of haloalkanes and haloarenes
2.1 Based on the number of halogen atoms
Monohalo compounds: contain one halogen atom.
Example: CH3ClCH_3ClCH3Cl
Dihalo compounds: contain two halogen atoms.
Example: CH2Cl2CH_2Cl_2CH2Cl2
Trihalo compounds: contain three halogen atoms.
Example: CHCl3CHCl_3CHCl3
Polyhalo compounds: contain several halogen atoms.
Example: CCl4CCl_4CCl4
2.2 Geminal and vicinal dihalides
Geminal dihalides: both halogen atoms are attached to the same carbon atom.
Example:
CH3−CHCl2CH_3-CHCl_2CH3−CHCl2
IUPAC name: 1,1-dichloroethane.
Vicinal dihalides: the two halogen atoms are attached to adjacent carbon atoms.
Example:
ClCH2−CH2ClClCH_2-CH_2ClClCH2−CH2Cl
IUPAC name: 1,2-dichloroethane.
Remember: geminal means same carbon; vicinal means neighbouring carbons.
2.3 Classification according to the carbon attached to the halogen
, Haloalkanes are classified as primary, secondary or tertiary by counting the number of
carbon atoms directly attached to the carbon bearing the halogen.
Primary (1°) haloalkane
The carbon attached to the halogen is connected to only one other carbon atom.
Example:
CH3CH2ClCH_3CH_2ClCH3CH2Cl
This is chloroethane.
Secondary (2°) haloalkane
The carbon attached to the halogen is connected to two other carbon atoms.
Example:
CH3CHClCH3CH_3CHClCH_3CH3CHClCH3
This is 2-chloropropane.
Tertiary (3°) haloalkane
The carbon attached to the halogen is connected to three other carbon atoms.
Example:
(CH3)3CCl(CH_3)_3CCl(CH3)3CCl
This is 2-chloro-2-methylpropane.
A methyl halide, such as CH3ClCH_3ClCH3Cl, is usually classified separately because its
carbon is attached to no other carbon atoms.
2.4 Special types of halides
Vinylic halides: the halogen is attached directly to a carbon of a carbon–carbon double
bond.
Example: CH2=CHClCH_2=CHClCH2=CHCl, vinyl chloride.
Allylic halides: the halogen is attached to a carbon next to a carbon–carbon double bond.
Example: CH2=CHCH2ClCH_2=CHCH_2ClCH2=CHCH2Cl, allyl chloride.
Aryl halides: the halogen is attached directly to an aromatic ring.
