Question

Reaction of one molecule of HBr with one molecule of 1,3-butadiene at 40℃ gives predominantly
A. 3-bromobutene under kinetically controlled conditions
B. 1-bromo-2-butene under thermodynamically controlled conditions
C. 3-bromobutene under thermodynamically controlled conditions
D. 1-bromo-2-butene under kinetically controlled conditions

Answer 1

Hint: The reaction at which the ratio of formation of the product depends on or is determined by the rate of formation of the product, the product formed due to this is called kinetically controlled product, and when the ratio of formation of product is determined by the formation of more stable product, the product formed due to this is called thermodynamically controlled product.

Complete Step by Step Answer:
The conditions under which a kinetically controlled product is formed are called kinetically controlled conditions. A kinetically controlled product is formed on the basis of a fast reaction, the product which is formed at a fast rate.

The conditions under which a thermodynamically controlled product is formed are called thermodynamically controlled conditions. A thermodynamically controlled product is formed when the product formed is based on stability, a thermodynamic product is the most stable product formed in the reaction.

When one molecule of HBr reacts with one molecule of 1,3-butadiene at 40℃, that means there is only one Bromide ion will be formed in the reaction:
$CH_2=CH-CH=CH_2 + \ \ \ HBr\ \ \ \ \rightarrow CH_2Br-CH=CH-CH_3+{\rm CH}_3-CHBr-CH=CH_2$

Since; 1,3-butadiene is a symmetric molecule, the product formed from this reaction is 1-bromo-2-butene and 2-bromo-1-butene. The thermodynamically stable product is 1-bromo-2-butene because the reaction goes through the process of formation of the most stable carbocation.

The formation of the most stable carbocation:

Hence, the product formed is 1-bromo-2-butene under thermodynamically controlled conditions
Thus, Option (B) is correct

Note: In the given reaction Markovnikov’s rule does not apply because of the formation of the most stable product. And Markovnikov rule says that the Nucleophile will only attack the carbon containing less number of hydrogens.