Tuesday, February 5, 2008

Alkyl and Aryl Halides - CHEMICAL REACTIONS

Haloalkanes are one of the most reactive classes of organic compounds. Many organic compounds are prepared using alkyl halides. So alkyl halides are considered as synthetic tools in the hands of an organic chemist to synthesize various compounds.

The reactions of alkyl halides can be grouped under the following heads for a study of them.

Nucleophilic substitution reactions
Elimination reactions
Reactions with metals

Nucleophilic substitution reactions

1. Substitution by hydroxylgroup (OH) leads to the formation of alcohols
2. Substitution by alkoxy group leads to the formation of ether.
3. Substitution by cyano group leads to the formation of cyanides or nitriles.

4. Substitution by isocyanide group leads to the formation of isocyanides.
When haloalkane is treated with alcoholic silver cyanide (AgCN), isocynaides (R-N≡C) are obtained. They are also called carbyl amines.

RX + AgCN ---> RNC + AGX

C2H5Br + AgCN ---> C2H5NC + Ag Br

5. Substitution by amino group leads to the formation of amines.
6. Substitution by nitrite group leads to the formation of nitrite.
7. Substitution by nitro group leads to the formation of nitro alkanes.
8. Substitution by carboxyl group leads to the formation of esters.
9. substitution by hydrosulphide group leads to the formation of thioalcohols.
10. Substitution by mercaptide group leads to the formation of thioethers.
11. Substitution by alkyl group leads to the formation of alkynes.

Elimination reactions (Dehydrohalogenation)

In this reaction alkenes are formed.
This reaction occus when alkyl halides are boiled iwth alcoholic solution of potassium hydroxide. One hydrogen atom and one halogen atom are removed and a double bond forms.

Reaction with active metals
Active metals like sodium, magnesium, cadmium, lithium combine with alkyl halides to give compounds containing carbon metal bonds (called as organometallic compounds).

i) With magnesium, alkyl magnesium halide is formed. This is called as Grignard reagent.

ii)Two molecules of alkyl halides react with sodium in the presence of ether and form alkanes. The reaction is termed Wurtz reaction and is used to prepare symmetrical alkanes


Haloalkanes can be reduced to alkanes by some reagents. The reagents include
i) Hydrogen in the presence of a metal catalyst such as nickel, palladium or platinum.
ii)zinc copper couple and ethyl alcohol
iii) Hydroiodic acid in the presence of red phosphorus

When a haloalkance is heated at 573 K or at a slightly lower temperature in the presence
of anhydrous aluminium chloride as catalyst, the halide undergoes rearrangement to form isomeric haloalkane. So this reaction is an isomerism reaction.

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