a. How does the first step in the reaction of ethene with Br2 differ from the first step in the

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A. How does the first step in the reaction of ethene with...

a. How does the first step in the reaction of ethene with $\mathrm{Br}_{2}$ differ from the first step in the reaction of ethene with HBr? b. To understand why $\mathrm{Br}^{-}$ attacks a carbon atom of the bromonium ion rather than the positively charged bromine atom, draw the product that would be obtained if $\mathrm{Br}^{-}$ did attack the bromine atom.

a. How does the first step in the reaction of ethene with $\mathrm{Br}_{2}$ differ from the first step in the reaction of ethene with HBr?
b. To understand why $\mathrm{Br}^{-}$ attacks a carbon atom of the bromonium ion rather than the positively charged bromine atom, draw the product that would be obtained if $\mathrm{Br}^{-}$ did attack the bromine atom.

Key Concepts

Electrophilic Addition Mechanism

This is a fundamental reaction type where an electrophile adds to an unsaturated system, such as an alkene, resulting in the formation of intermediates that lead to the final addition products. In different reactions, the nature of the electrophile and the stability of the intermediates determine the pathway and the stereochemical outcome.

Halonium Ion Intermediate

In the reaction of an alkene with a diatomic halogen like Br2, the first step involves the polarization of the halogen bond and the simultaneous formation of a cyclic halonium ion intermediate. This three-membered ring structure, where the positive charge is distributed over the halogen and the carbons, directs the subsequent nucleophilic attack.

Carbocation Intermediate

When an alkene reacts with a hydrogen halide such as HBr, the initial step is the protonation of the double bond, leading to the formation of a carbocation intermediate. This species, which is high in energy and less stabilized compared to a halonium ion, undergoes further nucleophilic attack to complete the addition process.

Nucleophilic Attack Selectivity

This concept explains why the nucleophile, such as Br?, preferentially attacks the carbon atoms of a cyclic intermediate rather than an already positively charged halogen atom. The cyclic structure of the halonium ion creates a situation where the carbon centers are more electrophilic and accessible, thus leading to regio- and stereospecific product formation, while alternative attack on the halogen would result in less favorable or unstable products.

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