Problem 127 - A Complete Resource Book in Chemistry for JEE Main

Question

Ozonolysis of 2, 3-dimethyl-1-butene followed by reduction with zinc and water gives (a) methanoic acid and 3-methyl-2-butanone (b) methanal and 3 -methyl-2-butanone (c) methanal and 2-methyl-3-butanone (d) methanoic acid and 2-methyl-3-butanone

Step-by-Step Solution

Answer

The correct answer is (b) methanal and 3-methyl-2-butanone.

Step 1: Understand the Ozonolysis Reaction

Ozonolysis is a chemical reaction where ozone (a) cleaves alkenes into aldehydes or ketones. When a compound undergoes ozonolysis, the double bonds are broken, and an oxidative cleavage typically occurs.

Step 2: Analyze the Structure of 2,3-dimethyl-1-butene

The structure of 2,3-dimethyl-1-butene is crucial. It contains a double bond between the first and second carbons with methyl groups attached to the second and third carbon atoms. Draw this structure to visualize it clearly.

Step 3: Apply Ozonolysis

During ozonolysis, the double bond in 2,3-dimethyl-1-butene will split, resulting in the formation of two fragments. The carbon-carbon double bond gets converted into carbonyl groups (C=O).

Step 4: Identify the Resulting Fragments

After ozonolysis on the double bond between the first and second carbon, the molecule splits, yielding methanal (formaldehyde) from the terminal carbon and 3-methyl-2-butanone from the remaining fragment that contains the two methyl groups.

Step 5: Apply Reduction with Zinc and Water

Reduction with zinc and water is done to convert any ozonides formed intermediates directly into carbonyl compounds without further oxidation. In this exercise, however, since aldehydes and ketones are final products, no changes occur during this step.

Step 6: Compare with Given Options

Now that we have methanal and 3-methyl-2-butanone from the ozonolysis, we compare it with the options: (a) methanoic acid and 3-methyl-2-butanone (b) methanal and 3-methyl-2-butanone (c) methanal and 2-methyl-3-butanone (d) methanoic acid and 2-methyl-3-butanone. Option (b) matches our results.

Key Concepts

Alkene Reactions

Alkene reactions are a fascinating part of organic chemistry because they reveal how versatile and reactive molecules can be transformed into various products. When we talk about alkenes, we refer to hydrocarbons that contain a carbon-carbon double bond, which is a significant site for chemical reactions. This double bond is like a magnet for various reagents, allowing diverse chemical transformations, such as halogenation, hydrogenation, and ozonolysis, to take place.

• In halogenation, halogens like chlorine or bromine add across the double bond.
• In hydrogenation, hydrogen adds across the double bond, saturating it to form alkanes.
• Ozonolysis, as seen in our example, is where ozone cleaves the alkene into smaller molecules with carbonyl groups.

Each reaction type emphasizes different aspects of chemistry but shares the process of modifying the carbon skeleton by adding, removing, or rearranging atoms. Understanding these reactions is key to mastering the synthesis and breakdown of various organic compounds.

Aldehyde Formation

When alkenes undergo reaction conditions like ozonolysis, one possible product is an aldehyde. Aldehydes are organic compounds featuring a carbon atom double-bonded to oxygen, known as a carbonyl group, with at least one hydrogen atom also bound to this carbon. In the context of ozonolysis, the double bond breaks and oxygen inserts between the former double-bonded carbons, leading to the formation of such carbonyl compounds.
When considering our exercise of ozonolysis of 2,3-dimethyl-1-butene, we see the terminal carbon undergoes such a transformation:

• The terminal carbon originally involved in the double bond becomes part of a methanal molecule, which is the simplest aldehyde.
• Methanal, also known as formaldehyde, is thus formed as a direct result of this oxidative cleavage.

Comprehending aldehyde formation through ozonolysis provides insight into how varying conditions and structures influence the final organic compounds formed.

Ketone Formation

Ketones are another possible product of ozonolysis and similar reactions involving alkenes. Like aldehydes, ketones are characterized by a carbonyl group. However, in ketones, this carbon is bonded to other carbon atoms rather than hydrogen atoms. Thus, ketones often arise from substitution in areas deeper within the molecular structure, rather than terminal positions.
During the ozonolysis of 2,3-dimethyl-1-butene, ketone formation occurs on the non-terminal carbon involved in the original double bond.

• The result of this process is the formation of 3-methyl-2-butanone, a ketone.
• The double bond breaks, leading to a carbonyl group at the former double bond's carbon that is part of the main carbon chain.
• This process illustrates how ozonolysis can effectively divide an alkene into molecules, each with varying types of carbonyl compounds like aldehydes and ketones.

Understanding ketone formation provides significant insight into how alkenes can be modified to create a variety of substances used in different chemical contexts.