Question

Rearrange the following in the order of increasing ease of dehydrohalogenation:

CH₃CH₂Cl, CH₃CHClCH₃ and CH₃CCl(CH₃)₂.

Give reasons.

Answer 1

Dehydrohalogenation is the removal of H-X from an alkyl halide to form an alkene. The ease of reaction depends on the stability of the alkene formed. Alkene stability increases with the number of alkyl substituents on the double bond due to hyperconjugation.

1. CH₃CH₂Cl is a primary alkyl halide. On dehydrohalogenation it gives ethene, a less stable alkene.
2. CH₃CHClCH₃ is a secondary alkyl halide. It forms more stable substituted alkenes, so it is more reactive than the primary halide.
3. CH₃CCl(CH₃)₂ is a tertiary alkyl halide. It gives a highly substituted and very stable alkene, so it undergoes dehydrohalogenation most easily.

Thus, the ease of dehydrohalogenation increases in the order:

CH₃CH₂Cl < CH₃CHClCH₃ < CH₃CCl(CH₃)₂