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  • Some alkyl halides undergo substitution whereas some undergo elimination reaction on treatment with bases. Discuss the structural features of alkyl halides with the help of examples which are responsible for this difference.

Some alkyl halides undergo substitution whereas some undergo elimination reaction on treatment with bases. Discuss the structural features of alkyl halides with the help of examples which are responsible for this difference.

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Primary alkyl halides prefer SN2 mechanism in which a nucleophile attacks at 180? to the halogen atom. A transition state is formed in which carbon is bonded to two nucleophiles and finally halogen atom is pushed out. Below is the mechanism- 

Hence, in SN2  mechanism, substitution takes place. Tertiary alkyl halides follow SN1 mechanism. In this case, tert alkyl halides form 3? carbocations. If the reagent used is a weak base then substitution occur while if it is a strong base than instead of substitution elimination occur.

The reagent used is aq. KOH. It is a weak base so, substitution takes place.

As alc. KOH is a strong base, so elimination competes over substitution and alkene is formed

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