Choose the correct set of reagents for the following conversion trans ($\mathrm{Ph}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{3}$) → cis ($\mathrm{Ph}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{3}$)

(B) $\mathrm{Br}_{2}$, alc $\mathrm{KOH}$, $\mathrm{NaNH}_{2}$, $\mathrm{H}_{2}$ Lindlar Catalyst

(A) $\mathrm{Br}_{2}$, alc $\mathrm{KOH}$, $\mathrm{NaNH}_{2}$, $\mathrm{Na}(\mathrm{LiqNH}_{3})$

(C) $\mathrm{Br}_{2}$, aq $\mathrm{KOH}$, $\mathrm{NaNH}_{2}$, $\mathrm{H}_{2}$ Lindlar Catalyst

(D) $\mathrm{Br}_{2}$, aq $\mathrm{KOH}$, $\mathrm{NaNH}_{2}$, $\mathrm{Na}(\mathrm{Liq} \mathrm{NH}_{3})$

JEE Main · 2023 · 31 Jan 2023 · Shift 1 · Hydrocarbons

Choose the correct set of reagents for the following conversion
trans (

Ph - CH = CH - CH_{3}

) → cis (

Ph - CH = CH - CH_{3}

)

(A) $Br_{2}$ , alc $KOH$ , $NaNH_{2}$ , $Na (LiqNH_{3})$
(B) $Br_{2}$ , alc $KOH$ , $NaNH_{2}$ , $H_{2}$ Lindlar Catalyst✓ Correct
(C) $Br_{2}$ , aq $KOH$ , $NaNH_{2}$ , $H_{2}$ Lindlar Catalyst
(D) $Br_{2}$ , aq $KOH$ , $NaNH_{2}$ , $Na (Liq NH_{3})$

Answer: (B)

Solution

The trans-alkene is first brominated to the vicinal dibromide, which on double dehydrohalogenation with alcoholic potassium hydroxide, aided by sodium amide, gives the internal alkyne 1-phenylpropyne. Hydrogenation of this alkyne over Lindlar's catalyst adds the two hydrogen atoms to the same face, delivering the cis-alkene. The required reagents are bromine, alcoholic

KOH

, sodium amide and hydrogen over Lindlar's catalyst.

JEE Main · 2023 · 31 Jan 2023 · Shift 1 · Single correct option

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JEE Main · 2023 · 31 Jan 2023 · Shift 1 · Hydrocarbons

Solution

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