Problem 24 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

In the presence of a small amount of phosphorous, aliphatic carboxylic acids react with chlorine or bromine to yield a compound in which \(\alpha\)-hydrogen has been replaced by halogen. This reaction is known as : (a) Wolff - Kishner reaction (b) Rosenmund reaction (c) Etard reaction (d) Hell - Volhard - Zelinsky reaction

Step-by-Step Solution

Answer

The reaction is the Hell-Volhard-Zelinsky reaction (option d).

Step 1: Understanding the Question

The question is asking us to identify a specific type of chemical reaction involving aliphatic carboxylic acids, phosphorous, and a halogen (like chlorine or bromine), where an \(\alpha\)-hydrogen is replaced by a halogen atom.

Step 2: Reviewing the Reaction Types

Let's review the reaction options provided: (a) Wolff-Kishner reaction is used for reducing carbonyl compounds to hydrocarbons, (b) Rosenmund reaction reduces acyl chlorides to aldehydes, (c) Etard reaction oxidizes aromatic hydrocarbons to aldehydes, and (d) Hell-Volhard-Zelinsky reaction involves halogenation at the \(\alpha\)-position of carboxylic acids.

Step 3: Matching the Reaction

The reaction described involves the replacement of an \(\alpha\)-hydrogen in an aliphatic carboxylic acid with a halogen, which directly matches the description of the Hell-Volhard-Zelinsky reaction.

Key Concepts

Aliphatic Carboxylic Acids

Aliphatic carboxylic acids are a type of organic compound characterized by the presence of a carboxyl group \((-COOH)\) attached to an aliphatic chain. Understanding these acids is crucial because they serve as the starting material in many chemical reactions, including the Hell-Volhard-Zelinsky reaction.
The aliphatic chain may be saturated (alkane), unsaturated (alkene), or even cyclic, but it is always open-chain rather than aromatic.
Common examples include acetic acid \((CH_3COOH)\) and propionic acid \((C_2H_5COOH)\).
They are generally known for:

• Being weak acids due to partial dissociation in water.
• Having higher boiling points than other compounds of similar molecular weight owing to their ability to form hydrogen bonds.
• Being widely used in chemical synthesis and industry.

In the context of the Hell-Volhard-Zelinsky reaction, these acids undergo a transformation where the \(\alpha\)-hydrogen, the hydrogen atom attached to the carbon atom next to the carboxyl group, is replaced by a halogen.

Halogenation

Halogenation is a chemical reaction that involves the introduction of one or more halogens into a compound. In the Hell-Volhard-Zelinsky reaction, this process specifically targets the \(\alpha\)-position of the aliphatic carboxylic acid.
The \(\alpha\)-position is crucial because it affects the acidity and reactivity of the molecules, enabling the halogen to replace the hydrogen atom efficiently.
Here's how it generally works:

• A small amount of phosphorus is used as a catalyst to activate the halogen, usually chlorine or bromine.
• The halogen substitutes the \(\alpha\)-hydrogen, resulting in an \(\alpha\)-halogenated carboxylic acid.
• This reaction doesn't affect the carboxyl group, allowing it to retain its characteristics.

This type of reaction is beneficial because it introduces a highly reactive halogen atom into the molecule, thereby allowing further chemical transformations if needed. It also showcases how selective halogenation can be achieved in complex organic molecules.

Alpha-Hydrogen Replacement

The term "alpha-hydrogen replacement" refers to the substitution of the hydrogen atom located at the \(\alpha\)-position, next to the carboxyl group, with a halogen in carboxylic acids.
This specific location of hydrogen makes it particularly reactive.
Here's why this happens:

• The electron-withdrawing nature of the carboxyl group increases the acidity of the \(\alpha\)-hydrogen, making it more prone to reaction.
• During the Hell-Volhard-Zelinsky reaction, the acidic \(\alpha\)-hydrogen is replaced by a halogen atom, typically chlorine or bromine.
• This is facilitated by catalytic amounts of phosphorus, which helps to activate the halogen.

This replacement is a key step in modifying carboxylic acids to make them more reactive for further chemical synthesis. It creates compounds where the halogen atoms make good leaving groups, thus facilitating nucleophilic substitution reactions. This aspect of chemistry underlines the reactivity and versatility of the \(\alpha\)-position in organic synthesis.