Describe the stereochemistry of the bromohydrin formed in each reaction (each reaction is stereospecific). (a) cis-3-Hexene \(+\mathrm{Br}_{2} / \mathrm{H}_{2} \mathrm{O}\) (b) trans-3-Hexene \(+\mathrm{Br}_{2} / \mathrm{H}_{2} \mathrm{O}\)

The reaction of alkenes with Br₂/H₂O is the bromohydrin formation reaction. The mechanism consists of two main steps: (1) the addition of bromine to the double bond to form a bromonium ion, and (2) the attack of water on the bromonium ion, leading to the bromohydrin product. Both steps are stereoselective, and the stereochemistry of the initial alkene will determine the stereochemistry of the resulting bromohydrin.

(a) In the reaction of cis-3-Hexene with Br₂/H₂O, the alkene moiety has two bulky groups (ethyl and methyl) on the same side of the double bond. When the Br₂ molecule approaches the double bond, the bromonium ion forms with the less hindered anti configuration. This leads to a bromonium ion with bromine on the opposite side of these two bulky groups. Next, water attacks the more hindered carbon of the bromonium ion, due to its electron density being higher (the other carbon is shielded by the bromine atom). The attack of water from the less hindered side (opposite to the ethyl and methyl groups) results in the formation of a bromohydrin product with the R configuration at the alcohol-bearing carbon and the S configuration at the bromine-bearing carbon.

(b) In the reaction of trans-3-Hexene with Br₂/H₂O, the alkene moiety has its bulky groups (ethyl and methyl) on opposite sides of the double bond. When the Br₂ molecule approaches the double bond, the bromonium ion forms again with the less hindered anti configuration. However, in this case, the bromine atom will be installed on the same side as one of the bulky groups, unlike in cis-3-Hexene. Water will attack the more hindered carbon of the bromonium ion for the same reasons as in the previous case. Since the bromonium ion intermediate has a different arrangement of groups than in the cis-3-Hexene case, the water will attack a carbon that is already next to one of the bulky groups. As a result, the bromohydrin product will have the S configuration at the alcohol-bearing carbon and the R configuration at the bromine-bearing carbon. In conclusion, the stereochemistry of the bromohydrin products formed in the reactions of (a) cis-3-Hexene and (b) trans-3-Hexene with Br₂/H₂O are: (a) R configuration at alcohol-bearing carbon and S configuration at bromine-bearing carbon (b) S configuration at alcohol-bearing carbon and R configuration at bromine-bearing carbon