Chemical Reactions Of Alkyl Halides: Elimination Reactions
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Elimination reactions:

Haloalkanes also undergo elimination reactions. In these reactions, a haloalkane with a (beta) β-hydrogen atom on heating with an alcoholic solution of potassium hydroxide, loses a halogen atom from the (alpha) α- carbon and a hydrogen atom from the β- carbon to form an alkene molecule. Since beta hydrogen atoms are involved in the elimination, these reactions are often called "(beta) β- elimination reactions".

According to Saytzeff's rule, in dehydrohalogenation reactions, the preferred product is the alkene that has the greater number of alkyl groups attached to the doubly bonded carbon atoms.

Ex:

An alkyl halide may either undergo substitution (or) elimination reaction, depending on its nature and on the strength and size of the nucleophile.

In the presence of a bulkier nucleophile, elimination is preferred over substitution. Due to the steric factors the abstraction of proton is comparatively easy than approaching the tetravalent carbon for substitution.

Ex: primary alkyl halides have little steric hindrance and hence readily undergo SN2 reactions while tertiary alkyl halides undergo SN1 reactions due to the formation of a stable carbocation.

Reaction with metals:

Most organic chlorides, bromides and iodides will react with certain metals to form compounds containing carbon-metal bonds. Such compounds are called organometallic compounds.

One important class of organometallic compounds are the Grignard reagents.

      R - X             +         Mg       →             RMgX
     Alkyl                    Magnesium               Grignard
    halide                                                    reagent

Grignard reagents are alkyl magnesium halides with the general formula RMgX, where R is an alkyl (or) aryl group and X is a chloride, bromide (or) iodide.

A Grignard reagent can be synthesised by combining a haloalkane with magnesium metal in dry ether.

Ex: Bromoethane reacts with magnesium in dry ether to form methyl magnesium bromide (Grignard reagent).

                                                                           Dry ether
        CH3Br           +             Mg                      →                 CH3MgBr

Bromomethane               Mangnesium                                Methyl magnesium bromide
                                                                                                 (Grignard reagent)

In a Grignard reagent, the carbon-magnesium bond is covalent, but very polar, with the carbon being more electronegative than magnesium. The magnesium-halogen bond is essentially ionic in nature.

Grignard reagents are highly reactive compounds. They react with any source of proton, even with weak proton donors such as water, alcohol (or) amines to give hydrocarbons.

Grignard reagents react with water, alcohol and amines to form alkanes.

Wurtz reaction:

Alkyl halides react with sodium in dry ether to make a new carbon-carbon bond. This reaction works best to make symmetric alkanes. Due to side reactions, it has limited usefulness.
   
                                                                                  Dry ether
R - X              +       2Na          +        X - R          →               R - R         +        2NaX

Summary

Elimination reactions:

Haloalkanes also undergo elimination reactions. In these reactions, a haloalkane with a (beta) β-hydrogen atom on heating with an alcoholic solution of potassium hydroxide, loses a halogen atom from the (alpha) α- carbon and a hydrogen atom from the β- carbon to form an alkene molecule. Since beta hydrogen atoms are involved in the elimination, these reactions are often called "(beta) β- elimination reactions".

According to Saytzeff's rule, in dehydrohalogenation reactions, the preferred product is the alkene that has the greater number of alkyl groups attached to the doubly bonded carbon atoms.

Ex:

An alkyl halide may either undergo substitution (or) elimination reaction, depending on its nature and on the strength and size of the nucleophile.

In the presence of a bulkier nucleophile, elimination is preferred over substitution. Due to the steric factors the abstraction of proton is comparatively easy than approaching the tetravalent carbon for substitution.

Ex: primary alkyl halides have little steric hindrance and hence readily undergo SN2 reactions while tertiary alkyl halides undergo SN1 reactions due to the formation of a stable carbocation.

Reaction with metals:

Most organic chlorides, bromides and iodides will react with certain metals to form compounds containing carbon-metal bonds. Such compounds are called organometallic compounds.

One important class of organometallic compounds are the Grignard reagents.

      R - X             +         Mg       →             RMgX
     Alkyl                    Magnesium               Grignard
    halide                                                    reagent

Grignard reagents are alkyl magnesium halides with the general formula RMgX, where R is an alkyl (or) aryl group and X is a chloride, bromide (or) iodide.

A Grignard reagent can be synthesised by combining a haloalkane with magnesium metal in dry ether.

Ex: Bromoethane reacts with magnesium in dry ether to form methyl magnesium bromide (Grignard reagent).

                                                                           Dry ether
        CH3Br           +             Mg                      →                 CH3MgBr

Bromomethane               Mangnesium                                Methyl magnesium bromide
                                                                                                 (Grignard reagent)

In a Grignard reagent, the carbon-magnesium bond is covalent, but very polar, with the carbon being more electronegative than magnesium. The magnesium-halogen bond is essentially ionic in nature.

Grignard reagents are highly reactive compounds. They react with any source of proton, even with weak proton donors such as water, alcohol (or) amines to give hydrocarbons.

Grignard reagents react with water, alcohol and amines to form alkanes.

Wurtz reaction:

Alkyl halides react with sodium in dry ether to make a new carbon-carbon bond. This reaction works best to make symmetric alkanes. Due to side reactions, it has limited usefulness.
   
                                                                                  Dry ether
R - X              +       2Na          +        X - R          →               R - R         +        2NaX

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