Question

On the basis of Saytzeff’s rule, explain the ease of dehydrohalogenation of different types of haloalkanes.

Answer 1

Ease of Dehydrohalogenation of Different Haloalkanes Based on Saytzeff’s Rule:

1. Primary Haloalkanes (1°): 
   1. Dehydrohalogenation is most difficult in primary haloalkanes.
   2. The elimination produces a less substituted and less stable alkene following Saytzeff’s rule.
   3. The formation of the double bond adjacent to a primary carbon is less favored.
2. Secondary Haloalkanes (2°):
   1. Dehydrohalogenation occurs with moderate difficulty.
   2. The alkene formed is moderately substituted and less stable than tertiary alkenes.
   3. Stability and ease of elimination are intermediate.
3. Tertiary Haloalkanes (3°):
   1. Dehydrohalogenation is easiest in tertiary haloalkanes.
   2. The elimination yields a highly substituted and stable alkene as the major product, consistent with Saytzeff’s rule.
   3. Tertiary carbons stabilize the intermediate (carbocation or transition state) better due to the inductive and hyperconjugative effects of alkyl groups.

Hence, the ease of dehydrohalogenation increases in the order:

Primary < Secondary < Tertiary haloalkanes

This order correlates with the stability of the more substituted alkene formed according to Saytzeff’s rule. The reaction favors elimination that yields the more substituted (thus more stable) alkene.