What stereoisomers are obtained when 2 -butyne undergoes...

What stereoisomers are obtained when 2 -butyne undergoes each of the following reaction sequences? a. $1 . \mathrm{H}_{2} /$ Lindlar catalyst $2 . \mathrm{Br}_{2} / \mathrm{CH}_{2} \mathrm{Cl}_{2}$ b. $1 . \mathrm{Na} / \mathrm{NH}_{3}(\mathrm{liq}),-78^{\circ} \mathrm{C} 2 . \mathrm{Br}_{2} / \mathrm{CH}_{2} \mathrm{Cl}_{2}$ c. $1 . \mathrm{Cl}_{2} / \mathrm{CH}_{2} \mathrm{Cl}_{2}$ 2. $\mathrm{Br}_{2} / \mathrm{CH}_{2} \mathrm{Cl}_{2}$

What stereoisomers are obtained when 2 -butyne undergoes each of the following reaction sequences?
a. $1 . \mathrm{H}_{2} /$ Lindlar catalyst $2 . \mathrm{Br}_{2} / \mathrm{CH}_{2} \mathrm{Cl}_{2}$
b. $1 . \mathrm{Na} / \mathrm{NH}_{3}(\mathrm{liq}),-78^{\circ} \mathrm{C} 2 . \mathrm{Br}_{2} / \mathrm{CH}_{2} \mathrm{Cl}_{2}$
c. $1 . \mathrm{Cl}_{2} / \mathrm{CH}_{2} \mathrm{Cl}_{2}$ 2. $\mathrm{Br}_{2} / \mathrm{CH}_{2} \mathrm{Cl}_{2}$

Key Concepts

Partial Hydrogenation of Alkynes

Partial hydrogenation of alkynes can be carried out under different conditions to give alkenes with distinct stereochemistry. Using a poisoned catalyst such as Lindlar catalyst leads to syn addition of hydrogen, forming cis (Z) alkenes; in contrast, dissolving metal reductions (e.g., Na/NH3) proceed via an electron transfer mechanism that ultimately delivers the more stable trans (E) alkene. This control over stereochemistry is crucial since the configuration of the intermediate alkene determines the stereochemical outcome of further reactions.

Anti Addition Mechanism in Halogenation

The addition of halogens to alkenes generally proceeds through a cyclic halonium ion intermediate, which enforces an anti addition mechanism. This means that the two halogen atoms add to opposite faces of the double bond. The anti nature of the addition is a key factor in determining whether the product is a meso compound or a pair of enantiomers, as the stereochemical arrangement of the substituents on the alkene influences the final configuration of the vicinal dihalide.

Stereochemical Outcome in Vicinal Dihalide Formation

When a halogen is added to a stereodefined alkene, the result is a vicinal dihalide whose stereochemistry is directly linked to the original alkene geometry. In cases where a cis alkene (resulting from, for example, Lindlar reduction) undergoes anti addition, the product is typically a meso (achiral) compound. Conversely, when a trans alkene (obtained from dissolving metal reduction) undergoes anti addition, the product is a racemic mixture of enantiomers. This correlation between the starting alkene configuration and the stereochemical nature of the dihalide is a central concept in stereoselective synthesis.

Strategic Influence of Reaction Sequences

The overall stereochemical outcome in multi?step synthesis is often controlled by the sequence of reactions and the conditions employed. By carefully choosing reagents and reaction pathways—such as the use of selective partial hydrogenation followed by halogenation—chemists can dictate whether the resulting product will be meso or a racemic mixture. This strategic control of stereoisomers is fundamental in organic synthesis and is applied extensively to construct complex molecules with high stereochemical precision.