Problem 202 - A Complete Resource Book in Chemistry for JEE Main

Question

In the following sequence of reactions, the alkene affords the compound ' \(\mathrm{B}\) ' \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CHCH}_{3} \stackrel{\mathrm{O}_{3}}{\longrightarrow}\) (A) \(\stackrel{\mathrm{H}_{2} \mathrm{O}}{\mathrm{Zn}}\) (B) The compound (B) is (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CHO}\) (b) \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COCH}_{3}\) (d) \(\mathrm{CH}_{3} \mathrm{CHO}\)

Step-by-Step Solution

Answer

Compound (B) is \(\mathrm{CH_3CHO}\), corresponding to option (d).

Step 1: Understand the Reaction Type

The sequence of reactions given is an ozonolysis reaction followed by reduction. Ozonolysis involves the cleavage of double bonds using ozone (\(O_3\)), leading to the formation of carbonyl compounds.

Step 2: Identify the Alkene Structure

Identify the structure of the alkene, \(\mathrm{CH_3CH=CHCH_3}\), which is 2-butene. This is a symmetrical alkene with a double bond between the second and third carbon atoms.

Step 3: Perform Ozonolysis

Ozonolysis of 2-butene will cleave the double bond, resulting in two fragments. Since it is symmetrical, both ends will produce the same product, forming two molecules of acetaldehyde (\(\mathrm{CH_3CHO}\)).

Step 4: Reduction Step

In the subsequent reduction step using \(\mathrm{Zn}\) and \(\mathrm{H_2O}\), the ozonolysis products (aldehydes or ketones) remain unaffected. Thus, the products from ozonolysis directly remain as they are.

Step 5: Identify Compound (B)

Thus, compound \(\mathrm{B}\) is the same as the product of the ozonolysis, which is acetaldehyde \((\mathrm{CH_3CHO})\).

Key Concepts

Alkene reactions

Alkenes are hydrocarbons characterized by having at least one carbon-carbon double bond. The reactions of alkenes are numerous and depend on the nature of the reagents they interact with. One of the most important reactions for alkenes is ozonolysis, where the double bond is cleaved by ozone.
This reaction results in the formation of carbonyl compounds, such as aldehydes or ketones, depending on the specific structure of the alkene.
Alkenes can undergo other significant reactions such as:

• Hydration
• Halogenation
• Hydrogenation

Understanding these reactions is crucial, as they allow for the transformation of alkenes into a wide range of products, which can be used in various chemical syntheses.

Carbonyl compounds

Carbonyl compounds are a significant class of organic compounds that contain the carbonyl group (\(>C=O\)).
This group is highly reactive due to the presence of a polar bond between the carbon and oxygen atoms. During ozonolysis of alkenes, the double bonds are transformed into carbonyl compounds.
The type of carbonyl compound formed—whether it's an aldehyde or a ketone—depends on the substituents attached to the double-bonded carbons.

• Aldehydes are formed when the carbonyl group is at the end of a carbon chain.
• Ketones are produced when the carbonyl group is in the middle of a carbon chain.

These compounds are valuable in many chemical synthesis processes, and their identification and understanding are essential in organic chemistry.

Reduction step

In many chemical reactions involving carbonyl compounds, a reduction step is often employed. This can help transform the carbonyl compound further, although in our ozonolysis example, the role of the reduction step is to stabilize the carbonyl compounds formed during ozonolysis.
The common agents used for reduction steps include zinc in the presence of water (\(Zn + H_2O\)), which serves to remove the ozonide intermediates safely and leave the carbonyl compounds unharmed.
While some reduction reactions could lead to alcohols, in this scenario, the reaction conditions are controlled to preserve the aldehyde or ketone structure without further reduction.

Symmetrical alkenes

Symmetrical alkenes have identical substituents on both ends of the double bond. This feature can simplify reaction outcomes, as both sides of the double bond will yield the same products during reactions such as ozonolysis.
In the exercise involving 2-butene, a symmetrical alkene, the double bond is flanked by identical methyl groups. When the double bond is cleaved through ozonolysis, two identical molecules of acetaldehyde are produced.
Working with symmetrical alkenes can sometimes make predicting reaction products more straightforward due to their uniformity.
Understanding the reactions of symmetrical versus asymmetrical alkenes is key in organic chemistry, helping chemists predict and manipulate product outcomes efficiently.