Question

The one giving maximum number of isomeric alkenes on dehydrohalogenation reaction is
(excluding rearrangement)

• A. 2-Bromopropane
• B. 2-Bromo-3,3-dimethylpentane
• C. 1-Bromo-2-methylbutane
• D. 2-Bromopentane

Answer 1

The Correct Option is D

To determine which compound gives the maximum number of isomeric alkenes upon dehydrohalogenation, we must consider the structure of each compound and the possible elimination reactions they can undergo.

1. 2-Bromopropane:

   The structure of 2-bromopropane is CH_3-CHBr-CH_3. In this molecule, dehydrohalogenation can occur by removing a hydrogen from either the first or the third carbon atom, establishing double bonds between them. Thus, it forms only one alkene, propene (CH_3-CH=CH_2).

2. 2-Bromo-3,3-dimethylpentane:

   The structure of this compound is (CH_3)_3C-CH_2-CHBr-CH_2-CH_3. Here, the molecule can potentially eliminate a halide from the central carbon (with the bromo group) and form a double bond with neighboring carbons. However, because of symmetry and the branching at carbon 3, potential isomers are limited. Generally, two alkenes may arise here with variations based on the position of the double bond.

3. 1-Bromo-2-methylbutane:

   The structure of this molecule is CH_3-CHBr-CH(CH_3)-CH_3. The bromo group is on the terminal second carbon. Dehydrohalogenation could give multiple alkenes such as 2-methyl-1-butene and 2-methyl-2-butene, but stereoisomers are typically limited to these geometrical arrangements.

4. 2-Bromopentane:

   The structure is CH_3-CHBr-CH_2-CH_2-CH_3. Dehydrohalogenation of this substrate can form three possible alkenes. By removing the bromine and a hydrogen from adjacent carbons, alkenes like pent-1-ene, (E)-pent-2-ene, and (Z)-pent-2-ene can be formed. This results in the maximum number of isomeric alkenes for this list.

Based on the above eliminations and considering the possibility of cis-trans isomerism in alkenes, 2-Bromopentane results in the most isomeric forms without rearranging the carbon skeleton.

Therefore, the correct answer is 2-Bromopentane.