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

Short answer

The products are methanal and 3-methyl-2-butanone; answer is (b).

Step-by-step solution

Understanding Ozonolysis

Ozonolysis is a reaction where alkenes react with ozone (O_3) to cleave the multiple bond and form carbonyl compounds. Depending on the process, it can produce aldehydes, ketones, or carboxylic acids.

Identifying the Reactant Structure

The compound given is 2,3-dimethyl-1-butene. Draw the structure: CH3C(CH3)=CHCH2CH3. It is an alkene with a double bond between carbon 2 and 3.

Applying Ozonolysis

When ozonolysis is applied to 2,3-dimethyl-1-butene, the double bond is broken, and the ozonide intermediate forms two separate parts. The bond cleavage occurs at the C=C double bond between carbon 2 and 3.

Reduction to Specific Carbonyl Compounds

In the reduction step using zinc and water, the ozonide is reduced to aldehydes and/or ketones. For the reactant structure, the breakdown products will be one aldehyde and one ketone from the broken parts.

Identifying Products

From the ozonolysis of 2,3-dimethyl-1-butene and subsequent reduction, the two possible products are methanal (HCHO) and 3-methyl-2-butanone (CH3C(O)CH2CH(CH3)2). The former comes from the end carbon of the cleaved double bond, and the latter forms the remaining ketone.

Key concepts

Alkene reactions

Alkene reactions are transformations involving the unsaturated hydrocarbons known as alkenes, which contain at least one carbon-carbon double bond. These reactions are crucial in organic chemistry because they enable the conversion of alkenes into various types of compounds.
A common alkene reaction is the addition reaction, where atoms are added to the carbon atoms at the double bond, effectively "breaking" the unsaturated site. However, ozonolysis is a slightly different process.

• Ozonolysis: In ozonolysis, an alkene reacts with ozone ( O_3 ), leading to the cleavage of the double bond. This specific reaction can produce aldehydes, ketones, or carboxylic acids, with the outcome depending on subsequent conditions.
• In the case of 2,3-dimethyl-1-butene, the double bond between the second and third carbon atoms is the target site for ozonolysis. The reaction breaks this double bond, a feature of alkene reactions, to form an ozonide intermediate.

This breakdown and the conditions afterwards define the carbonyl compounds formed in the reaction.

Carbonyl compounds

Carbonyl compounds make up a significant class of organic molecules characterized by the presence of a carbon-oxygen double bond (C=O). This functional group is crucial due to its reactivity and ability to act as an electrophile in various reactions.
The products of ozonolysis, when involving alkenes, are often compounds with carbonyl groups. Examples include:

• Aldehydes: These have at least one hydrogen atom attached to the carbonyl carbon. Methanal (formaldehyde) is an example, notable for its simplest structure with the formula HCHO .
• Ketones: Both sides of the carbonyl carbon in ketones are bonded to carbon atoms instead of hydrogen. 3-Methyl-2-butanone, resulting from ozonolysis in certain reactions, exhibits this structural property with its formula CH_3C(O)CH_2CH(CH_3)_2 .

These compounds arise due to the cleavage of the double bond in alkenes, highlighting the significant role of carbonyl compounds in reactions resulting from ozonolysis.

Reduction process

The reduction process in ozonolysis involves converting the unstable ozonide intermediates into more stable carbonyl compounds, such as aldehydes and ketones.
This transformation is often achieved by reacting the ozonide with reducing agents.

• Using Zinc and Water: One of the common methods employs zinc in the presence of water. Zinc acts as a reducing agent, facilitating the conversion of the ozonide to more stable carbonyl compounds without further oxidation to carboxylic acids.
• This step is crucial, especially in reactions like those involving 2,3-dimethyl-1-butene, where the goal is to achieve specific alkenes products such as methanal and 3-methyl-2-butanone.

The reduction thus ensures that the reaction yields useful carbonyl compounds, maintaining their simpler structures rather than overly oxidized forms.