Explain why bromination of cyclohexene produces trans 1,2-dibromocyclohexane.
The trans 1,2-dibromocyclohexane is formed as a major product in the bromination of cyclohexene due to the regioselectivity and stereochemistry of the reaction. Let's break it down step by step:
1. Regioselectivity: The reaction between cyclohexene and bromine (Br2) is an electrophilic addition reaction. In the presence of a Lewis acid catalyst, such as FeBr3, the reaction occurs via a cyclic bromonium ion intermediate.
2. Formation of the bromonium ion: The cyclic bromonium ion is formed by the attack of one of the bromine atoms on the double bond of cyclohexene. This reaction is regioselective, primarily favoring the electrophilic addition to the more substituted carbon atom of the double bond.
3. Stereoselectivity: The bromonium ion is strained due to the steric interactions between the two bromine atoms. To alleviate this strain, the bromine atom opposite to the nucleophilic attack undergoes a backside attack by a bromide ion, which opens up the bromonium ion.
4. Formation of trans 1,2-dibromocyclohexane: The backside attack of the bromide ion on the cyclic bromonium ion leads to the formation of trans 1,2-dibromocyclohexane. This occurs because the nucleophilic attack happens from the opposite face of the bromonium ion, resulting in the two bromine atoms being placed on opposite sides (trans) of the cyclohexane ring.
To summarize, the bromination of cyclohexene produces trans 1,2-dibromocyclohexane as the major product due to regioselectivity, which favors addition to the more substituted carbon, and stereoselectivity, which leads to the formation of trans products through backside attack of the nucleophile.