From what you know about the stereochemistry of alkene...

From what you know about the stereochemistry of alkene addition reactions, predict the configurations of the products that would be obtained from the reaction of 2 -butyne with the following: a. one equivalent of $\mathrm{Br}_{2}$ in $\mathrm{CH}_{2} \mathrm{Cl}_{2}$ b. one equivalent of $\mathrm{HBr}+$ peroxide

From what you know about the stereochemistry of alkene addition reactions, predict the configurations of the products that would be obtained from the reaction of 2 -butyne with the following:
a. one equivalent of $\mathrm{Br}_{2}$ in $\mathrm{CH}_{2} \mathrm{Cl}_{2}$
b. one equivalent of $\mathrm{HBr}+$ peroxide

Key Concepts

Alkyne Addition Reactions

Alkyne addition reactions are unique because alkynes contain two ? bonds, allowing for different extents of addition (mono or di-addition) under various conditions. Understanding these reactions requires knowledge of both the stereochemical and regiochemical aspects, as slight changes in reaction conditions and reagents can lead to distinct and predictable geometrical isomers.

Regioselectivity in Addition Reactions

Regioselectivity describes the preference for forming one constitutional isomer over another in an addition reaction. It depends on both the electronic and steric factors inherent in the substrate and the mechanistic pathway—whether it proceeds via an ionic or radical intermediate—thereby influencing the position at which new bonds are formed.

Bromonium Ion Mechanism

The bromonium ion mechanism involves the initial formation of a three?membered cyclic bromonium intermediate when a halogen like Br2 reacts with an unsaturated compound. This intermediate prevents syn addition and enforces anti attack by the bromide ion, leading to products with defined stereochemistry.

Anti Addition

Anti addition is a stereochemical outcome in which substituents add to opposite faces of a multiple bond. This is a common phenomenon in alkene and alkyne reactions, particularly in halogen additions, where the formation of cyclic intermediates forces the nucleophile to attack from the side opposite the leaving group.

Radical Addition and the Peroxide Effect

In the presence of peroxides, HBr addition to unsaturated bonds follows a radical chain mechanism rather than a typical ionic pathway. This change in mechanism, often referred to as the peroxide effect or anti?Markovnikov addition, alters the regioselectivity and sometimes the stereochemistry of the final addition product.

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