Which of the following is the most reactive substrate in an E1 reaction? Br Br I II III IV ΟΙ ΟΙΙ O III O IV
The Correct Answer and Explanation is:
Correct Answer: III
Explanation
To determine which substrate is the most reactive in an E1 reaction, we need to understand the E1 mechanism.
E1 (Elimination Unimolecular) reactions follow two main steps:
- Formation of a carbocation after the leaving group (like Br or I) leaves.
- Loss of a proton (H⁺) from a β-carbon to form a double bond.
The rate-determining step is the formation of the carbocation, so the stability of the carbocation is crucial. The more stable the carbocation intermediate, the faster and more favorable the E1 reaction.
Key factors influencing E1 reactivity:
- Tertiary carbocations are more stable than secondary, which are more stable than primary.
- Allylic and benzylic carbocations are stabilized by resonance.
- Better leaving groups (like I⁻ is better than Br⁻) also favor E1, but carbocation stability dominates.
Now, consider each compound:
- Compound I: Might form a primary or secondary carbocation (less stable).
- Compound II: Possibly forms a secondary carbocation.
- Compound III: Likely forms a tertiary carbocation, which is the most stable type.
- Compound IV: May form a benzylic or allylic carbocation, which is also stable, but not as much as a well-substituted tertiary one.
Also, though iodine (I⁻) is a better leaving group than bromide (Br⁻), carbocation stability is the dominant factor in E1 reactions.
Therefore, compound III, which forms a tertiary carbocation, is the most reactive substrate in an E1 reaction. Even if another compound has a better leaving group, it won’t surpass the rate advantage provided by a more stable carbocation intermediate.
Conclusion: The most reactive substrate in an E1 reaction is III due to its ability to form a tertiary carbocation, which accelerates the rate-limiting step.
