What is Alkyl halide?

Alkyl halides are halogen substituted products of alkanes. They have the general formula C_nH_2n+1X or RX. Where R represents the alkyl group, and X the halogen (usually any of Cl, Br, and I).

Nomenclature of Alkyl Halides

They are named by adding the prefix chloro-, bromo-, or iodo- to the parent hydrocarbon, and indicating the position of attachment of the halogen atom.

E.g.

(2 - bromo propane)

Classification of Alkyl Halides

Alkyl halides are classified in a similar way as alkanols. i.e., primary (1^o), Secondary (2^o) and tertiary (3^o) - depending on the number of alkyl groups attached to the carbon bearing the halide.

Preparation of Alkyl Halides

1. From alkanols - phosphorus halides, hydrogen halides or thionyl chlorides with alkanols give alkyl halides.

E.g.

Properties of Alkyl Halides

Physical Properties

1. Few are gases at room temperature, e.g., bromoethane; few are liquids; while most are solids.

2. They are insoluble in water and soluble in organic solvent.

3. Their boiling points decrease with branches, i.e., the branched isomers are of lower boiling points than their straight chain isomers. Therefore, tertiary alkyl halides are of lower boiling points than their secondary isomers; and secondary isomers are of lower boiling points than their primary isomers.

Chemical Properties

1. Nucleophillic substitution

A nucleophile is a negatively charged species which is looking for a positive centre to attach. Examples of nucleophiles are OH^-, NO₂^-, CH₃CH₂O^-, CN^-, NH₃ and Cl^-.

The reaction between a positive carbon and a nucleophile, with the displacement of a negative species, e.g., halogen element is called nucleophillic substitution reaction.

Examples: (a).

Reactions between alkyl halides and aqueous NaOH produces alkanols.

Bromomethane                                                   Methanol

(b). Alkyl halides react with alkoxide ions in alkanolic solutions to give ethers (the williamson’s ether synthesis).

Ethoxide ion                       Diethylether

(c). Cyanide ions in aqueous alkanolic solutions react with alkyl halides to displace halogens and produce nitriles.

ethanenitrile

(d). Ammonia and amines are nucleophiles because they have lone pair of electrons on their nitrogen atoms. They therefore react with alkyl halides to give substituted ammonium salts.

methyl ammonium bromide

2. Elimination reactions

Elimination reactions may accompany substitution reactions depending on the structure of alkyl halides, temperature, strength of alkalis and polarity of the solvent used.

High concentration of strong alkali, high temperatures, low polarity of the solvent and the use of large molecular alkyl halides will all favor elimination reaction.

For example, bromoethane is attacked by hot alkanolic solution of conc. KOH to produce a high yield of ethene together with low quantity of ethanol (from substitution reaction).

Note:

* With methyl halide, only substitution reaction is possible, i.e., only methanol is produced.

3. Reaction with metals

(a). With sodium or potassium - alkyl halides react with sodium or potassium in dry ether solution to give alkanes (this reaction is known as wurtz reaction).

(b). With magnesium - alkyl halides react with magnesium in anhydrous ether to give compounds called Grignard reagents (these compounds are very useful in the production of many different compounds).

(c). With zinc/copper couple - alkyl halides are reduced by zinc/copper couple (i.e. zinc powder coated with copper) in ethanol.

The product is the corresponding alkane.

Uses of Alkyl Halides

1. As solvent for organic solids.

2. For the preparation of many organic compounds such as alkanols.

3. As refrigerants.

4. As pesticides and fumigating agents.