Question

An alkyl halide with molecular formula C$_5$H$_11$Br on dehydrohalogenation gives two isomeric alkenes X and Y with formula C$_5$H$_10$. On reductive ozonolysis, X and Y give four compounds CH$_3$CHO, (CH$_3$)$_2$CHO, CH$_3$CO, HCHO. The alkyl halide is:

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

Hint:

Ozonolysis of alkenes splits the carbon-carbon double bond, producing two carbonyl compounds (aldehydes or ketones). The structure of the products helps deduce the original alkene and thus the starting alkyl halide.

Answer 1

The Correct Option is B

Given the information about the alkyl halide and its ozonolysis products, let us first analyze the nature of the products formed during ozonolysis.

Ozonolysis of alkenes involves the cleavage of the carbon–carbon double bond, producing carbonyl compounds (aldehydes and/or ketones). The identity of these products provides direct information about the structure of the original alkene.

Observed Ozonolysis Products:

• CH₃CHO (Acetaldehyde):
  Indicates the presence of a terminal alkene fragment where one carbon of the double bond carries a methyl group and the other carries a hydrogen.
• HCHO (Formaldehyde):
  Confirms the presence of a terminal =CH₂ group in the alkene.
• CH₃COCH₃ (Acetone):
  Indicates cleavage of a substituted alkene carbon bonded to two methyl groups, forming a ketone.

These products strongly suggest that the alkyl halide undergoes dehydrohalogenation to form two different alkenes, one terminal and one more substituted.

Step-by-Step Reasoning:

• The alkyl halide must be capable of forming more than one alkene upon elimination (E2 reaction).
• Ozonolysis of one alkene gives acetaldehyde and formaldehyde, while ozonolysis of the other gives acetone.
• Among the given options, 2-bromo-3-methylbutane satisfies these conditions.

On dehydrohalogenation, 2-bromo-3-methylbutane forms two alkenes: 2-methyl-1-butene and 2-methyl-2-butene.

Ozonolysis Reactions:

2-Methyl-1-butene + O₃ → Acetaldehyde (CH₃CHO) + Formaldehyde (HCHO)

2-Methyl-2-butene + O₃ → Acetone (CH₃COCH₃)

Conclusion:
The alkyl halide that produces the given ozonolysis products is 2-bromo-3-methylbutane.