Tuesday, February 5, 2008

Alkyl and Aryl Halides - Introduction, Nomenclature

When hydrogen atom or atoms of alkanes are replaced by the corresponding number of halogen atoms, the compounds are called halogen derivatives of alkanes.

They are classified according to the number of halogen atoms that replace hydrogen atoms in the alkane.

Monohalogen derivatives: They contain only one halogen atom.

e.g. CH-3Cl Methyl chloride
CH-3-CH-CH-3 2-Bromopropane
Monohalogen derivatives of alkane are called alkyl halides

Dihalogen alkanes contain two halogen atoms.
Trihalog alkanes contain three halogen atoms.

Monohalo alkanes

The general formula is RX where is a alkyl group and X is a halogen.

Classification of Haloalkanes

A. Type of halogen atoms; Fluorides, chlorides, bromides, iodides
B. Number of halogena atoms, monohalo, dihalo, trihalo, tetra halo.

Classification of Haloalkanes
1. Compounds containing sp3 hybridization
2. Compounds containing sp2 hybridization

Compounds containing sp3 hybridization
a) halo alkances or alkyl halides
(i)Primary halogenoalkanes

In a primary (1°) halogenoalkane, the carbon which carries the halogen atom is only attached to one other alkyl group.

Secondary halogenoalkanes

In a secondary (2°) halogenoalkane, the carbon with the halogen attached is joined directly to two other alkyl groups, which may be the same or different.

Tertiary halogenoalkanes

In a tertiary (3°) halogenoalkane, the carbon atom holding the halogen is attached directly to three alkyl groups, which may be any combination of same or different.

b) Allylic halides: Halogens attached to alkenes to a carbon atom next to carbon-carbon double bond.

c) Benzylic halide (aralalkyl halide): Halogens attached to alkenes to a carbon atom next to to an aromatic ring (not to a carbon atom in the aromatic ring). It is attached to a carbon atom which is inturn attached to a carbon atom in the aromatic ring.

2. Compounds containing sp2 hybridization

a) Vinylic halides: Halogens attached to alkenes to a carbon atom of one of the carbon atoms of a double bond.

B) Aryl halides: Halogens attached to alkenes to a carbon atom of an aromatic ring

Nomenclature – Alkyl and Aryl halides

Nomenclature of alkyl halides

Common system

The monohalogen derivatives of alkanes are called alkyl halides. These are named by naming the alkyl group attached to halogen and adding the name of the halide.
Ex: Methyl halide, Isobutyl halide
The name of the alkyl group and halide are written as two separate words. The prefixes used to distinguish alkanes like n-, iso-, sec-, tert, etc. are also written.

IUPAC System

The monohalogen derivatives of alkanes are called haloalkanes.
The names of haloalkanes are written by prefixing the word ‘halo’ (bromo or chloro or iodo or fluoro) to the name of the alkane corresponding to the longest continuous carbon chain holding the halogen atom.
Ex: Bromoethane

Rules for naming haloalkanes having branches in carbon chains

1. The longest continuous chain containing the carbon attached to halogen group is selected as the parent alkane (principal chain or parent chain). In naming alkanes all rules that apply to alkane names are to be followed.
2. The carbon atoms are numbered in such a way that the carbon atom carrying the halogen atom gets the lowest number.
3. The position of the halogen atom and other substituents are indicated by numbers 1,2,3… etc.
Ex: 1-Iodo-2-methylpropane

Dihalo derivatives

1. When both the halogen atoms are attached to the same C-atom, these are called geminal dihalides. Alkylidene dihalides or alkylidene halides are also names used for such compounds.
Ex: ethlydine dichloride
2. When the two halogen atoms are in adjacent C-atoms, they are called vicinal dihalides. They are prepared from alkenes and hence they are named as the dihalide of the alkene from which they are prepared.
Ex: ethylene dichloride

Polyhalo derivatives

Ex: Trichloromethane
Fully halogenated hydrocarbons are also called perhalohydrocarbons under common system.

Nomenclature of aryl halides

Aryl halides are termed Haloarenes in IUPAC systems. The prefix ‘halo” ((bromo or chloro or iodo or fluoro) is placed before the name of the aromatic hydrocarbon. In case of disubstituted compounds, the relative positions are indicated by (1,2), (1,3) or (1,4). Ortho, meta and para are also used to indicate the positions.

Ex: Chlorobenzene, Bromobenzene

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