Question

Some alkylhalides 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.

Answer 1

An alklyl halide with β-hydrogen atoms when reacted with a base or a nucleophile has two competing routes substitution (via S_N1 or S_N2 mechanism) and elimination. Which route will be taken up depends upon the nature of alkly halide, strength, size of base\nucleophile and reaction conditions. Thus, a bulkier nucleophile will prefer to act as a base and abstracts a proton rather than approach a tetravalent carbon atom due to steric hindrance and vice-versa.

Commonly a primary alkly halide will prefer a S_N2 reaction, and in case of a secondary halide-either S_N2 or elimination process would occur depending upon the strength of base/nucleophile. A tertiary halide with buliker alkyl group would undergo either 'S_N1 or elimination' based upon the stability of carbonation or the more substituted alkene.

Examples:

 (i) \[\ce{\underset{1-Bromopropons}{CH3 - CH2 - CH2 - Br} ->[KOH(aq)/Δ][(Dehydrohalogenation)] \underset{Propene}{CH3 - CH = CH2}} \]

(ii) If the same reaction is carried out in presence of aq KOH, 1-propanol will be the product.

\[\ce{CH3 - CH2 - CH2 - Br ->[aqueous KOH] CH3 - CH2 - CH2 OH}\]