Most of the organic chlorides, bromides and iodides react with certain metals to form compounds containing carbon metal bond.
Such compounds are called organometallic compounds.
Some important reactions of haloalkanes with metals are discussed below :
Formation of Grignard's reagent
Haloalkanes react with magnesium in dry ether to form alkyl-magnesium halides, also known as Grignard's reagents.
The general reaction is :
Example : Ethylmagnesium bromide is formed when ethyl bromide reacts with magnesium.
Grignard's reagents are very reactive with compounds containing hydrogen (H+ ) and form alkanes.
Because of this reason, it is necessary to avoid presence of moisture in the reactions involving Grignard's reagents.
In Wurtz reaction, alkyl halides react with metallic sodium in dry ether to form symmetrical alkanes with double the number of
carbon atoms present in the alkyl halide.
Limitations of Wurtz reaction
Wurtz reaction is not a good method for the preparation of unsymmetrical alkanes because a mixture of many compounds is formed.
In case of tertiary alkyl halides,
elimination takes place because sodium is basic and the order of reactivity towards elimination reaction is tertiary > secondary >
primary (discussed in
Reaction with lithium
Reduction of haloalkanes
Alkyl halides are reduced by many reducing agents like lithium aluminium hydride (LiAlH4), H2 in
the presence of catalyst etc., to produce alkanes.
Question : Grignard reagents should be prepared under anhydrous
conditions. Explain why?
Answer : Grignard reagents, being very reactive, react with moisture present in the apparatus or
the starting material. Hence, Grignard reagents must be prepared under anhydrous conditions.