Bromosuccinimide (NBS), one product dominates. Draw this product and explain why it is the major product. (E)-2,5-dimethyl-3-hexene
In the free radical bromination of (E)-2,5-dimethyl-3-hexene using N-bromosuccinimide (NBS), one product dominates. Draw this product and explain why it is the major product. (E)-2,5-dimethyl-3-hexene
The Correct Answer and Explanation is:
Major Product of the Reaction
The reaction of (E)-2,5-dimethyl-3-hexene with N-bromosuccinimide (NBS) in the presence of light (hν) or a radical initiator results in the selective bromination at the allylic position. The major product formed is (E)-2-bromo-2,5-dimethyl-3-hexene.
Explanation
The reaction between an alkene and N-bromosuccinimide (NBS) is a free-radical substitution known as allylic bromination. This reaction is highly selective for the substitution of a hydrogen atom on a carbon adjacent to a double bond (the allylic position). The mechanism proceeds through a free-radical chain reaction, and the selectivity is determined by the stability of the radical intermediate formed.
- Initiation and Hydrogen Abstraction: The reaction is initiated to form a bromine radical (Br•). This highly reactive radical selectively abstracts the most weakly bound hydrogen atom from the substrate to form the most stable possible carbon radical. In (E)-2,5-dimethyl-3-hexene, the hydrogens at the allylic positions (C2 and C5) are the most susceptible to abstraction. Due to the molecule’s symmetry, abstraction from either C2 or C5 yields the same radical intermediate.
- Formation of a Resonance-Stabilized Radical: Abstraction of an allylic hydrogen from C2 results in the formation of a resonance-stabilized allylic radical. This intermediate is a hybrid of two contributing resonance structures:
- Structure I is a tertiary (3°) allylic radical.
- Structure II is a secondary (2°) allylic radical.
- Kinetic Control and Product Formation: The stability of carbon radicals follows the order: tertiary > secondary > primary. Therefore, the tertiary allylic radical (Structure I) is significantly more stable than the secondary allylic radical (Structure II). It is the major contributor to the resonance hybrid, meaning the unpaired electron spends more time on the tertiary carbon (C2).The subsequent reaction of this radical intermediate with molecular bromine (Br₂, supplied in low concentration by NBS) is very fast. According to the Hammond Postulate, the transition state of this step resembles the radical intermediate. The reaction pathway that proceeds through the more stable tertiary radical has a lower activation energy and is therefore kinetically favored. Consequently, the bromine atom preferentially attacks the tertiary carbon, leading to the formation of (E)-2-bromo-2,5-dimethyl-3-hexene as the dominant product. The original (E) stereochemistry of the double bond is retained because the reaction does not occur at the double bond itself. While some of the minor product, 4-bromo-2,5-dimethyl-2-hexene, is formed from Structure II, its yield is much lower due to the lesser stability of the secondary radical intermediate.
