Nomenclature of Haloalkanes and Haloarenes

Common or trivial system

The monohalogen derivatives of alkanes are called alkyl halides. Their names are derived by naming the attached alkyl group to halogen and adding the name of the halide. For example,

The name of the alkyl and halide is written as two separate words. The prefixes n-, iso-, sec-, tert-, etc. are also used for alkyl groups and are written as:

The prefix n- is used for alkyl group having continuous chain of C-atoms with no branching.

The prefix iso- is used for those alkyl groups wherein one methyl group is attached next to the end C-atom.

The prefix neo- is used for those alkyl groups which have two methyl groups attached next to the end C-atom.

IUPAC System

Here, the monohalogen derivatives of alkanes are named as haloalkanes. While writing the names, the word 'halo' is prefixed to the name of the alkane corresponding to the longest continuous carbon chain holding the halogen atom. In case of branched chain alkanes, the following rules are followed:

• The longest continuous chain containing the carbon attached to halogen group is selected as the parent alkane.

• The carbon atom is numbered in a way that the carbon atom carrying the halogen atom gets the lowest number.

• The position of the halogen atom and other substituents (alkyl groups) are indicated by suitable numbers i.e., 1,2,3.....etc. Other common rules of alkanes are followed. For example,

The common and IUPAC names of a few alkyl halides are given below

The halo compounds have other names too based on the type of structural and substituent effects.

Hydrogen atoms

Sometimes hydrogen atoms have special names such as vinylic hydrogen (attached to double bonded C-atom), allylic hydrogen (attached next to double bonded C-atom). The replacement of halogen atom for these types of hydrogen atoms results in alkyl halides having the corresponding common names.

Dihalogen alkanes

These alkanes also have special common names when both the halogen atoms are attached to the same C-atom. These are called 'gem-dihalides', or are also called alkylidene dihalides or simply, alkylidene halides.

Also, when two halogen atoms are at adjacent C-atoms, they are called vicinal dihalides. These are named as the dihalide of the alkene from which they may be prepared by the addition of halogen atoms i.e. alkylene dihalide or simply alkylene halide. For example,

Polyhalogen derivatives are named as per IUPAC system.

Classification of alkyl halides

Depending upon the nature of carbon atom to which the halogen is attached alkyl halides or haloalkanes are classified as :

Primary (1^o) haloalkanes

These are alkyl halides that have one or no alkyl group on the carbon linked to the halogen atom.

Secondary (2^o) haloalkanes

These are alkyl halides that have two alkyl groups on the carbon linked to the halogen atom.

Tertiary (3^o) haloalkanes

These are haloalkanes that have three alkyl groups on the carbon linked to the halogen atom.

Haloarenes or aryl halides are named by adding the prefix 'halo' (fluoro, chloro, bromo, iodo) before the name of the aromatic hydrocarbon. In case of di-substituted compounds, the relative positions of the substituents 1,2; 1,3 and 1,4 are indicated by the prefixes ortho (o-), meta (m-) and para (p-) respectively. For example,