Question

The 1,2 adduct and the 1,4 adduct formed by reaction of HBr with 1,3-butadiene are in equilibrium at 40 °C. Propose a mechanism by which the interconversion of products takes place.

Short answer

Hydrohalogenation of alkene or addition of hydrogen halide H-X, e.g., HCl, HBr, HI on alkene to form the haloalkane.

This reaction is carried out in two ways:

• Direct passing of H-X on alkene
• By using a polar solvent

This reaction formed the carbocation intermediate, and its stability only decides the product alkyl halide

Step-by-step solution

Hydrohalogenation of alkene

Hydrohalogenation of alkene or addition of hydrogen halide H-X, e.g., HCl, HBr, HI on alkene to form the haloalkane.

This reaction is carried out in two ways:

• Direct passing of H-X on alkene
• By using a polar solvent

This reaction formed the carbocation intermediate, and its stability only decides the product alkyl halide

Interconversion of 3-Bromo-1-butene and 1-Bromo-2-butene

Allylic halide 3-Bromo-1-butene and 1-Bromo-2-butene will undergo slow dissociation to form the stabilized allylic carbocation through $S_{N}1$reaction.

Both the allylic halides form the same allylic carbocation on dissociation. The addition of bromide ion (act as a nucleophile) on allylic carbocation formed the mixture of Bromobutene: 3-Bromo-1-butene and 1-Bromo-2-butene.

The reaction is running under equilibrium conditions the stable thermodynamic 1-Bromo-2-butene predominates.