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Chapter 4: Problem 161

Meso - 2, 3-dibromobutane when heated with zinc dust in methanol gives mainly (a) cis but- 2 -ene (b) trans-but-2-ene (c) 1,3 -butadiene (d) 2 -bromobut- 2 -ene

Short Answer

Expert verified
Answer: The major product of this reaction is trans-but-2-ene.

Step by step solution

01

Draw the structure of meso-2,3-dibromobutane

Begin by drawing the structure of meso-2,3-dibromobutane which has a four-carbon chain with two bromine atoms located at the second and third positions of the chain. Since it is a meso compound, the configuration of the two chiral centers must be opposite (one R and one S).
02

Identify the reagent and the leaving group

The reaction involves heating meso-2,3-dibromobutane with zinc dust in methanol. Zinc is the reagent in the reaction and the leaving group will be the bromine atom on the 2nd and 3rd carbon.
03

Write the reaction and predict the product

Write the structure of meso-2,3-dibromobutane and the zinc reagent, and show the pathways of bromine atoms leaving the molecule. Zinc reacts with bromine to form zinc bromide, and a double bond forms between the 2nd and 3rd carbon atoms. There are two possible outcomes: cis-but-2-ene and trans-but-2-ene. We need to determine which of these is the major product.
04

Determine the major product

Meso-2,3-dibromobutane has an opposite configuration of chiral centers. When heating the compound with zinc dust, the configuration of the chiral centers will remain the same. Therefore, the major product of the reaction will be trans-but-2-ene, as the stereochemistry at two carbons favors the formation of trans geometry over cis geometry. So, the correct answer is option (b) trans-but-2-ene.

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