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Chapter 4: Problem 166

Reaction of meso-1, 2 -dibromo \(-1,2\) -diphenylethane (stilbene dibromide) with alc. \(\mathrm{KOH}\) (or \(\left.\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OK} / \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\right)\) gives (a) cis \(-1\) -bromo \(-1,2\) -diphenyl ethene (b) trans-1-bromo-1, 2-diphenyl ethene (c) \(1: 1\) mixture of (a) and (b) (d) \(1: 2\) mixture of (a) and (b)

Short Answer

Expert verified
The major products formed in the reaction are a 1:1 mixture of cis-1-bromo-1,2-diphenyl ethene and trans-1-bromo-1,2-diphenyl ethene.

Step by step solution

01

Write down the reactants

Write meso-1,2-dibromo-1,2-diphenylethane (stilbene dibromide) reacting with an alcoholic KOH or \(\left. C_{2} H_{5} OK / C_{2} H_{5} OH\right)\). It will look like this: meso-1,2-dibromo-1,2-diphenylethane + \(\left. C_{2} H_{5} OK / C_{2} H_{5} OH\right)\)
02

Identify the functional groups in the reactants

In the meso-1,2-dibromo-1,2-diphenylethane molecule we have a dihalide (two bromine atoms) and two phenyl groups. The alcoholic KOH or \(\left. C_{2} H_{5} OK / C_{2} H_{5} OH\right)\) is a strong base.
03

Determine the reaction mechanism

Since we have a dihalide reacting with a strong base, this reaction will follow the E2 (Elimination Bimolecular) mechanism, where the base abstracts a proton from the substrate and the halides leave the substrate to form a new double bond.
04

Perform the E2 elimination in meso-1,2-dibromo-1,2-diphenylethane

During the E2 elimination, the alcoholic KOH (or \(\left. C_{2} H_{5} OK / C_{2} H_{5} OH\right)\) will act as a strong base, abstracting the proton from the alpha-carbon (the carbon adjacent to the carbon attached to the two bromine atoms), leading to the formation of a double bond and the elimination of one bromine atom: meso-1-bromo-1,2-diphenylethane Since the remaining molecule has symmetry, any cis or trans isomers formed will be in 1:1 ratio.
05

Identify the correct answer

In conclusion, after eliminating one bromine atom, the reaction will yield a 1:1 mixture of cis-1-bromo-1,2-diphenyl ethene and trans-1-bromo-1,2-diphenyl ethene. So, the correct option is: (c) 1:1 mixture of (a) and (b)

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Most popular questions from this chapter

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