Question

Choose the incorrect option: (A) \(\mathrm{R}-\mathrm{OH}+\mathrm{R}^{\prime} \mathrm{COOH} \stackrel{\mathrm{H}^{\oplus}}{\rightleftharpoons} \mathrm{RCOOR}^{\prime}+\mathrm{H}_{2} \mathrm{O}\) (B) \(\mathrm{R}-\mathrm{OH}+\mathrm{R}^{\prime} \mathrm{COCl} \stackrel{\text { Pyridine }}{\rightleftharpoons} \mathrm{R}-\mathrm{O}-\mathrm{C}-\mathrm{R}^{\prime}+\mathrm{HCl}\) (C) Reaction with acid chloride is carried out in the presence of pyridine as to neutralise HCl. (D) Due to reversibility of acetylation with carboxylic acid and acid anhydride, water is removed as soon as it is formed.

Short answer

The incorrect option is (D) because water is not removed in the acetylation reaction with carboxylic acid; however, the removal of water is true for the reaction of alcohols with acid anhydrides.

Step-by-step solution

Option A: Reaction of Alcohol with Carboxylic acid

The given reaction in option A is the esterification reaction, which involves the reaction of a carboxylic acid with an alcohol in the presence of a catalytic amount of an acid (\(\textrm{H}^{\oplus}\)). This reaction produces an ester and water as the by-products. Since this is a true statement, option A is NOT the incorrect option we are looking for.

Option B: Reaction of Alcohol with Acid Chloride

In option B, the reaction of an alcohol with an acid chloride is given, which is also an esterification reaction. The reaction proceeds in the presence of pyridine, which is a base and acts as the catalyst. Pyridine reacts with the generated hydrogen chloride (HCl) from the reaction to neutralize it and form a salt. The main products of the reaction are an ester and a salt. This statement is also correct, so option B is NOT the incorrect option.

Option C: Role of Pyridine in the Reaction

Option C states that the reaction with an acid chloride is carried out in the presence of pyridine to neutralize the HCl formed. This is true, since pyridine is a base and it reacts with HCl to form a salt, thus neutralizing the acidic hydrogen chloride. So, option C is also NOT the incorrect option we are looking for.

Option D: Reversibility of Acetylation and Water Removal

Option D states that due to the reversibility of the acetylation reaction with carboxylic acid and acid anhydride, water is removed as soon as it is formed. This statement is incorrect because water is not removed in the acetylation reaction with carboxylic acid (the esterification reaction described in Option A); however, the removal of water is true for the reaction of alcohols with acid anhydrides. This option is misleading, so Option D is the INCORRECT option we are looking for.

Key concepts

Acid Chloride

Acid chlorides are highly reactive compounds derived from carboxylic acids through the replacement of the hydroxyl group (-OH) with a chlorine atom (Cl). This makes them more reactive than the parent carboxylic acids. The reactivity of acid chlorides makes them ideal intermediates for forming esters.
During esterification, acid chlorides can readily react with alcohols to form esters along with the release of hydrogen chloride (HCl). The general reaction can be represented as:

• Alcohol (\(\text{R-OH}\) reacts with acid chloride \(\text{R'-COCl}\)
• This forms an ester \(\text{R-O-C-R'}\)
• And hydrogen chloride (HCl) as a by-product

The use of acid chlorides eliminates the need for acidic catalysts, which are typically required when using carboxylic acids directly. The mechanism involves the nucleophilic attack by the alcohol on the carbonyl carbon of the acid chloride, followed by the release of HCl.

Carboxylic Acid Reactions

Carboxylic acids are organic acids characterized by the presence of a carboxyl group (-COOH). They are less reactive compared to their derivatives like acid chlorides but are a staple in organic synthesis.
In esterification, carboxylic acids react with alcohols in the presence of an acid catalyst, usually sulfuric acid (\(\mathrm{H}^{+}\)), to produce esters and water. This reaction is reversible, and hence, eliminating water from the reaction mixture helps push the reaction towards ester formation. The reaction can be summarized as:

• Carboxylic acid (\(\text{R-COOH}\)) reacts with alcohol (\(\text{R'-OH}\))
• This forms an ester (\(\text{R-COOR'}\)) and water

This equilibrium reaction highlights the need to remove water to drive the reaction forward for a favorable product yield.
Additionally, unlike acid chloride reactions, carboxylic acid esterifications require strict control of reaction conditions due to their reversible nature. This process is essential in the synthesis of fragrances, flavorings, and many biological esters.

Pyridine Role in Organic Reactions

Pyridine is a basic, nitrogen-containing heterocyclic compound frequently used as a catalyst or solvent in organic reactions. Due to its basic nature, pyridine can neutralize acids formed during some reactions.
When dealing with acid chlorides, pyridine plays a crucial role by interacting with the by-product HCl, formed during the esterification reaction between acid chlorides and alcohols.
Pyridine's reaction with HCl forms pyridinium chloride, a salt, and thus mitigates the acidic environment. This is important because:

• Pyridine helps prevent further reactions that HCl could promote if left unchecked.
• It facilitates smoother progression of the reaction without side reactions.

Furthermore, pyridine's ability to improve the reaction offers a cleaner, more efficient pathway to obtain the desired ester without unwanted by-products, highlighting its versatility in organic synthesis.