Explain why bromination of cyclohexene produces trans 1,2-dibromocyclohexane.

The bromination of cyclohexene produces trans 1,2-dibromocyclohexane because of the concerted mechanism involved in this reaction.

To understand why the reaction proceeds in a trans-specific manner, we need to consider the mechanism of the bromination reaction. Bromination of an alkene involves the addition of a bromine molecule (Br2) across the double bond to form a cyclic bromonium ion intermediate.

First, the π electrons of the double bond in cyclohexene attack one of the bromine atoms of Br2, breaking the π bond and forming a sigma bond between carbon and bromine. This leads to the formation of a cyclic bromonium ion with a positive charge on the carbon atom bonded to the bromine.

Now, the bromide ion (Br-) attacks the carbon atom on the opposite side of the cyclic bromonium ion. This nucleophilic attack leads to the displacement of the bromine atom and the formation of the final product, trans 1,2-dibromocyclohexane.

The key reason for the trans-selectivity in this reaction is the steric hindrance between the two bromine atoms. Due to the cyclohexane ring's shape, the two carbon atoms involved in the reaction are not adjacent to each other. As a result, there is less steric hindrance between the incoming nucleophile (bromide ion) and the cyclic bromonium ion.

The nucleophilic attack can occur from either side of the cyclic bromonium ion, but one side has less steric hindrance, favoring the nucleophile's attack from that side. This leads to the formation of the trans isomer of the product.

In summary, the bromination of cyclohexene produces trans 1,2-dibromocyclohexane due to the concerted mechanism involving the cyclic bromonium ion intermediate and the steric hindrance favoring the nucleophilic attack from the side with less hindrance.