Unit 10
Haloalkanes and Haloarenes
Objectives
After studying this Unit, you will be able to
- name haloalkanes and haloarenes according to the IUPAC system of nomenclature from their given structures;
- describe the reactions involved in the preparation of haloalkanes and haloarenes and understand various reactions that they undergo;
- correlate the structures of haloalkanes and haloarenes with various types of reactions;
- use stereochemistry as a tool for understanding the reaction mechanism;
- appreciate the applications of organo-metallic compounds;
- highlight the environmental effects of polyhalogen compounds.
Halogenated compounds persist in the environment due to their resistance to breakdown by soil bacteria.
NEETprep Audio Note:
In this Unit, you will study the important methods of preparation, physical and chemical properties and uses of organohalogen compounds.
10.1 Classification
Haloalkanes and haloarenes may be classified as follows:
10.1.1 On the Basis of Number of Halogen Atoms
These may be classified as mono, di, or polyhalogen (tri-,tetra-, etc.) compounds depending on whether they contain one, two or more halogen atoms in their structures. For example,
Monohalocompounds may further be classified according to the hybridisation of the carbon atom to which the halogen is bonded, as discussed below.
10.1.2 Compounds Containing sp3 C—X Bond (X= F, Cl, Br, I)
This class includes NEETprep Audio Note:
(a) Alkyl halides or haloalkanes (R—X)
In alkyl halides, the halogen atom is bonded to an alkyl group (R). They form a homologous series represented by CnH2n+1X. They are further classified as primary, secondary or tertiary according to the nature of carbon to which halogen is attached. If halogen is attached to a primary carbon atom in an alkyl halide, the alkyl halide is called primary alkyl halide or 1° alkyl halide. Similarly, if halogen is attached to secondary or tertiary carbon atom, the alkyl halide is called secondary alkyl halide (2°) and tertiary (3°) alkyl halide, respectively.
(b) Allylic halides
These are the compounds in which the halogen atom is bonded to an sp3-hybridised carbon atom adjacent to carbon-carbon double bond (C=C) i.e. to an allylic carbon.
NEETprep Audio Note: 
(c) Benzylic halides
These are the compounds in which the halogen atom is bonded to an sp3-hybridised carbon atom attached to an aromatic ring.
NEETprep Audio Note: 
10.1.3 Compounds Containing sp2 C—X Bond
This class includes:
(a) Vinylic halides NEETprep Audio Note:
These are the compounds in which the halogen atom is bonded to a sp2-hybridised carbon atom of a carbon-carbon double bond (C = C).
(b) Aryl halides NEETprep Audio Note:
These are the compounds in which the halogen atom is directly bonded to the sp2-hybridised carbon atom of an aromatic ring.
