Question

Question: When buta-1,3-diene (CH₂=CH-CH=CH₂) is treated with HBr, two constitutional isomers are formed CH₃CHBr-CH=CH₂and Br-CH₂CH=CHCH₂. Draw a stepwise mechanism that accounts for the formation of both products.

Short answer

Answer

Step-by-step solution

Constitutional isomers

Constitutional isomers are formed when a compound has resonance-stabilized intermediates.

Resonance stabilization occurs when there is a movement of electrons.

Explanation

When the given alkene buta-1,3-diene is treated with HBr, protonation occurs on one of the substituted sites of the double bond to generate a stable carbocation.

The carbocation is attacked by the bromide ion nucleophile to form the constitution isomer CH₃CHBr-CH=CH .

It also forms a resonance structure in which an allyl carbocation (most stable). This carbocation is attacked again by the bromide ion to give another constitutional isomer Br-CH₂CH=CHCH₂

Reaction mechanism