Reaction Analysis: Electrophilic Addition to Alkenes

This reaction involves an alkene undergoing electrophilic addition with bromine (Br₂) in the presence of water (H₂O). This is a classic example of halohydrin formation.

Step-by-Step Mechanism:

Step 1: Formation of Bromonium Ion
The electron-rich double bond of the alkene acts as a nucleophile and attacks the electrophilic bromine molecule (Br₂). This results in the formation of a cyclic bromonium ion intermediate and a bromide ion (Br^-). $R - CH = CH - R' + Br - Br \to [\begin{matrix} R - CH & CH - R' \\ ⏝ & ⏟ \\ Br & + \end{matrix}] + {Br}^{-}$

Step 2: Nucleophilic Attack by Water
In the presence of water, the nucleophile that opens the bromonium ion is H₂O, not Br^-. Water attacks the more substituted carbon of the bromonium ion (following Markovnikov's rule). This results in a bromo-alcohol product, known as a halohydrin. $[\begin{matrix} R - CH & CH - R' \\ ⏝ & ⏟ \\ Br & + \end{matrix}] + H_{2} O \to R - CH - CH - R' + H^{+} where Br OH$ A proton is lost in the final step to yield the neutral halohydrin.

Applying to the Given Alkene:
The reactant is a symmetric cycloalkene (cyclopentene). Because the molecule is symmetric, both carbons of the double bond are equivalent. Therefore, attack by water at either carbon will yield the same product: trans-2-bromocyclopentanol.

Final Answer: The major organic product is trans-2-bromocyclopentanol. This corresponds to the structure in the first option.
Correct Product: trans-2-bromocyclopentanol

Related Topics & Formulae

Key Concept: Halohydrin Formation
General Reaction: $R - CH = CH - R' + X_{2} + H_{2} O \to R - CH - CH - R' + H X (X = Cl, Br) X OH$

Mechanism: Electrophilic Addition
Regiochemistry: Follows Markovnikov's rule; the OH group adds to the more substituted carbon.
Stereochemistry: Anti addition occurs through the ring-opening of the halonium ion, resulting in trans products for cyclic alkenes.
Why water? Water is a better nucleophile than the halide ion in aqueous conditions, leading to the halohydrin.