1-butene is subjected to allylic bromination using N-Bromosuccinimide (NBS) in the presence of light — Haloalkanes and Haloarenes Chemistry Question
Question
1-butene is subjected to allylic bromination using N-Bromosuccinimide (NBS) in the presence of light. The reaction yields a mixture of two structurally distinct brominated alkenes. Which of the following represents the thermodynamically major product and its formation rationale?
💡 Solution & Explanation
Abstraction of the allylic hydrogen from 1-butene ($CH_2=CH-CH_2-CH_3$) forms a resonance-stabilized allylic radical: $[\dot{C}H_2-CH=CH-CH_3 \leftrightarrow CH_2=CH-\dot{C}H-CH_3]$. The bromine atom can trap the radical at either carbon. Trapping at the $2^\circ$ carbon yields 3-bromo-1-butene (a monosubstituted alkene). Trapping at the $1^\circ$ carbon yields 1-bromo-2-butene (a disubstituted alkene). Although the $2^\circ$ radical contributor has a slightly lower energy, the reaction is reversible and under thermodynamic control. The major product is determined by the stability of the final alkene. According to Zaitsev's rule, the more highly substituted alkene (1-bromo-2-butene) is thermodynamically more stable and predominates as the major product.