Problem 69 - A Complete Resource Book in Chemistry for JEE Main

Question

In an \(\alpha\)-halogenation of aliphatic acids (HVZ reaction) the catalyst used is (a) \(\mathrm{AlCl}_{3}\) (b) P (c) \(\mathrm{Zn}\) (d) \(\mathrm{FeCl}_{3}\)

Step-by-Step Solution

Answer

The catalyst used in the HVZ reaction is P (Phosphorus).

Step 1: Understanding HVZ Reaction

The Hell-Volhard-Zelinsky (HVZ) reaction is a halogenation reaction involving aliphatic carboxylic acids. In this reaction, a halogen is introduced at the alpha (α) position of the carboxylic acid.

Step 2: Recognizing the Catalyst

The catalyst typically used in an HVZ reaction is a compound that can facilitate the formation of the acyl halide intermediate, which is necessary for the reaction to proceed. The standard catalyst for this reaction involves a halogen carrier or a reagent that can interact with halogens.

Step 3: Identifying the Correct Catalyst

Out of the given options, the one that fits the requirement for interacting specifically with halogens in the HVZ reaction is phosphorus (option b, P). Phosphorus is known to facilitate the formation of acyl halide intermediates needed for the subsequent halogenation step.

Key Concepts

Alpha-halogenation

Alpha-halogenation is a critical reaction in organic chemistry where a halogen is introduced at the alpha position of a carbonyl compound. This position is adjacent to the carbon atom of the carbonyl group. In the context of aliphatic carboxylic acids like those involved in the HVZ reaction, this process allows for the introduction of a halogen atom such as chlorine or bromine into the molecule.
This transformation is facilitated by initially converting the carboxylic acid into an acyl halide, which can then undergo halogenation at the alpha position. The alpha-hydrogen is more reactive in these intermediates, making it readily replaceable by a halogen under the right conditions.
Alpha-halogenation is particularly important because it introduces a functional group that can further react, allowing for the building of complex molecular architectures. This process is routinely used for synthesizing compounds with increased acidity, or to prepare molecules for further chemical modifications.

Aliphatic carboxylic acids

Aliphatic carboxylic acids are a class of organic compounds containing a carboxyl group \(-COOH\) attached to a saturated carbon chain. Unlike aromatic carboxylic acids, the chains in aliphatic acids do not include any aromatic rings.
These acids are characterized by their acidic properties and the ability to form hydrogen bonds, which affect their solubility and boiling point. This makes them suitable for various reactions in organic chemistry, including the HVZ reaction.
In the context of alpha-halogenation, the presence of the aliphatic chain allows these acids to be converted into acyl halides. This conversion opens up the alpha position for halogenation, crucial for modifying the molecular structure of the acid. This can lead to the creation of compounds that have a wide range of useful chemical properties.

Acyl halide formation

Acyl halide formation is a key step in many chemical reactions that require a reactive intermediate. In the HVZ reaction, this involves converting a carboxylic acid into its corresponding acyl halide. This is achieved by reacting the acid with a halogen, often in the presence of phosphorus (P) or another halogen carrier.
The resulting acyl halide is highly reactive due to the presence of the halogen attached to the carbonyl carbon, which makes it a good candidate for further reactions. For alpha-halogenation, this formation is pivotal as it allows the introduction of a halogen at the alpha position.
The reaction typically proceeds with the acid in the presence of a halogen source, creating a more reactive intermediate that can facilitate subsequent transformations. This step is essential for enhancing the chemical reactivity of the carboxylic acid, enabling a variety of downstream synthetic applications.