Problem 183 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following on heating with aqueous KOH, produces acetaldehyde? [2009] (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}\) (b) \(\mathrm{CH}_{2} \mathrm{ClCH}_{2} \mathrm{Cl}\) (c) \(\mathrm{CH}_{3} \mathrm{CHCl}_{2}\) (d) \(\mathrm{CH}_{3} \mathrm{COCl}\)

Step-by-Step Solution

Answer

Option (c): \( \text{CH}_{3}\text{CHCl}_{2} \) produces acetaldehyde.

Step 1: Understand the Reaction Context

To produce acetaldehyde (\( \text{CH}_{3}\text{CHO} \)), we need to determine which compound can undergo hydrolysis or decomposition to yield it. In the presence of aqueous KOH, some organic compounds can form different types of carbonyl groups.

Step 2: Analyze Each Compound

For each option, consider what functional groups or bonds are present and how they react with KOH:- Option (a) \( \text{CH}_{3}\text{CH}_{2}\text{Cl} \) is ethyl chloride, an alkyl chloride, not directly forming aldehydes upon reaction with KOH.- Option (b) \( \text{CH}_{2}\text{ClCH}_{2}\text{Cl} \) is ethylene chloride, when reacted with KOH, it could potentially form acetaldehyde via a different mechanism, but this is not typical.- Option (c) \( \text{CH}_{3}\text{CHCl}_{2} \) is 1,1-dichloroethane. Upon reacting with KOH, it can potentially form acetaldehyde through the elimination of HCl and the subsequent formation of the carbonyl group.- Option (d) \( \text{CH}_{3}\text{COCl} \) is an acyl chloride, which does not produce acetaldehyde directly in this condition.

Step 3: Conclusion Based on Chemical Behavior

Given the reaction with aqueous KOH, option (c) \( \text{CH}_{3}\text{CHCl}_{2} \) is the most likely candidate. When heated with aqueous KOH, it can undergo hydrolysis where one of the chlorine atoms is replaced with an OH group, followed by the elimination of HCl to form acetaldehyde.

Key Concepts

Aqueous KOH Reaction

When organic compounds are heated with aqueous KOH, a variety of chemical transformations can occur. This reaction often involves nucleophilic substitution or elimination processes, depending on the structure of the organic molecule.

Aqueous KOH, being a strong base, contributes hydroxide ions (\( ext{OH}^-\)) to the reaction. These hydroxide ions play a crucial role in facilitating chemical changes:

• Nucleophilic substitution: The hydroxide ion can replace a leaving group, usually a halogen, in the molecule, transforming it into another functional group like alcohol.
• Elimination reaction: If conditions are suitable, these ions can also result in the removal of a small molecule such as hydrogen chloride (HCl), leading to the formation of a double bond or a carbonyl group.

In the context of the given problem, compound c (\( ext{CH}_3 ext{CHCl}_2\)), when heated with aqueous KOH, undergoes such changes to form acetaldehyde.

Organic Functional Groups

Functional groups are specific groupings of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. Understanding these groups aids in predicting the behavior of organic compounds in reactions.

The compounds in the given exercise feature different functional groups:

• Alkyl Chloride: This group is seen in compound a (\( ext{CH}_3 ext{CH}_2 ext{Cl}\)), where a chlorine is attached to an alkyl chain. Typically, they engage in nucleophilic substitution reactions.
• Dichloro Compound: Compound c (\( ext{CH}_3 ext{CHCl}_2\)) possesses this type of functional group, which, under the presence of aqueous KOH, is reactive and can form aldehydes through elimination and substitution.
• Acyl Chloride: Compound d (\( ext{CH}_3 ext{COCl}\)) features this group. Although reactive, it behaves differently, usually forming carboxylic acids or related compounds instead of acetaldehyde under these conditions.

Thus, understanding these groups helps explain why compound c is the most likely to form acetaldehyde.

Carbonyl Group Formation

The carbonyl group is a crucial feature in organic chemistry, characterized by a carbon atom double-bonded to an oxygen atom. This group is central to many chemical reactions, particularly those involving aldehydes and ketones.

Formation of the carbonyl group involves a few key processes:

• Nucleophilic addition: In some mechanisms, a nucleophile adds to the carbon of the carbonyl, altering its structure.
• Elimination: Often, a leaving group is expelled from a molecule, allowing the formation of the carbon-oxygen double bond.

In the case of compound c (\( ext{CH}_3 ext{CHCl}_2\)), heating with aqueous KOH induces a reaction where one chlorine is replaced by a hydroxyl group, and another undergoes elimination to assist the formation of acetaldehyde's carbonyl group. This is why such compound transformations are essential for creating specific organic compounds like acetaldehyde.