Question

Why do you suppose 1,4 adducts of 1,3-butadiene are generally more stable than 1,2 adducts?

Short answer

1,4 adduct is more stable than 1,2 adduct as it is more substituted, having more alkyl groups that result in more hyperconjugation.

Step-by-step solution

Hyperconjugation

Hyperconjugation or ‘’no bond resonance” is the delocalization of lone pair electrons or sigma electrons to its adjacent pie orbital (p orbital).

This effect helps to explain the stability of carbocation and alkene.

Stability of 1,4 adduct over 1,2 adduct

1,3 butadiene on reacting with halogen form two products 3,4-dihalo bromobut-1-ene (1,2 product) and 1,4-dihalo bromobut-1-ene (1,4 product)

1,4 product is more stable as it more substituted alkene (disubstituted alkene) having four alpha hydrogen results in more hyperconjugation in the compound, which increases the stability of the 1,4-dihalo bromobut-1-ene compound. 1,2 product is less substituted alkene (monosubstituted alkene) having only two alpha hydrogen results in less hyperconjugation in the compound, which decreases the stability of the 3,4-dihalo bromobut-1-ene compound.

Thus 1,4 adduct is more stable than 1,2 adduct.

Formation of 1,4 adduct and 1,2 adduct