Question

Choose the correct set of reagents for the following conversion trans $\left( Ph - CH = CH - CH _3\right) \rightarrow \operatorname{cis}\left( Ph - CH = CH - CH _3\right)$

• A. $Br _2$, alc $\cdot KOH , NaNH _2, Na \left( Liq NH _3\right)$
• B. $Br _2$, alc $\cdot KOH , NaNH _2, H _2$ Lindlar Catalyst
• C. $Br _2$, aq $\cdot KOH , NaNH _2, H _2$ Lindlar Catalyst
• D. $Br _2, aq \cdot KOH , NaNH _2, Na \left( Liq NH _3\right)$

Hint:

The conversion of trans-alkenes to cis-alkenes can be achieved by using Lindlar's catalyst under hydrogenation conditions, which reduces alkynes to cis-alkenes.

Answer 1

The Correct Option is B

To convert trans-\((\text{Ph} - \text{CH} = \text{CH} - \text{CH}_3)\) to cis-\((\text{Ph} - \text{CH} = \text{CH} - \text{CH}_3)\), we need to consider the reagents that facilitate this stereochemical inversion.

The process involves the following steps:

1. Bromination: The trans-alkene is first treated with \(Br_2\), which adds bromine across the double bond, forming a dibromo compound.
2. Dehydrohalogenation: Using alcoholic \(\text{KOH}\), the dibromo compound undergoes elimination to form an alkyne. The halogens are eliminated, yielding a triple bond.
3. Conversion to Alkyne: The formed alkyne is then isomerized or will be partially reduced.
4. Partial Hydrogenation: The alkyne is then partially hydrogenated using \(H_2\) in the presence of a Lindlar catalyst, which reduces the alkyne to a cis-alkene. The Lindlar catalyst selectively stops the reduction at the alkene stage and ensures that the resulting double bond has the cis configuration.

Therefore, the correct set of reagents which completes this transformation is:

• \(Br_2\) for the addition of bromine.
• Alcoholic \(KOH\) for dehydrohalogenation to form the alkyne.
• \(NaNH_2\) to ensure further control of manipulation, although not directly used here for some transformations similar reactions can employ it.
• \(H_2\) in the presence of a Lindlar catalyst, for turning the formed alkyne to cis-alkene.

Thus, the correct answer is the set of reagents: \(Br _2, \text{alc} \cdot KOH , NaNH _2, H _2\) Lindlar Catalyst.