What is the catalyst used in the acetylation of alcohols?

Pyridine

The correct answer is option 3. Pyridine.

Acetylation is a chemical reaction where an acetyl group (\( -COCH_3 \)) is introduced into a molecule. For alcohols, the reaction generally involves:

Acetylating Agents: Common acetylating agents are acetic anhydride (\( (CH_3CO)_2O \)) and acetyl chloride (\( CH_3COCl \)).

Reaction Mechanism

1. Using Acetyl Chloride:

The acetyl chloride reacts with the alcohol to form an ester and hydrogen chloride (HCl) as a by-product. The reaction is:

2. Using Acetic Anhydride:

The acetic anhydride reacts with the alcohol to form an ester and acetic acid as a by-product. The reaction is:

Role of the Catalyst

Pyridine: When acetyl chloride is used as the acetylating agent, pyridine is often used as a base. Pyridine neutralizes the hydrogen chloride (\( HCl \)) produced during the reaction, which helps to prevent the formation of side products and shifts the equilibrium towards the formation of the desired ester. Pyridine also helps in activating the acetyl chloride and facilitates the reaction.

Function: Pyridine acts as a base to absorb the \(HCl\), which prevents it from reacting with the acetyl chloride or alcohol, thus driving the reaction forward. This helps ensure the acetylation is efficient and produces the desired ester product.

Why Not the Other Catalysts

1. Palladium: Palladium is used in catalytic hydrogenation, cross-coupling reactions, and other transformations but is not involved in acetylation processes.

2. Hydrogen Peroxide: Hydrogen peroxide is used as an oxidizing agent and is not relevant for the acetylation of alcohols.

3. Aluminium Chloride: Aluminium chloride is a Lewis acid used in Friedel-Crafts reactions to catalyze acylation or alkylation of aromatic rings, not in the acetylation of alcohols.

In summary, pyridine is the effective catalyst for acetylation reactions involving acetyl chloride because it neutralizes the by-product \(HCl\) and helps drive the reaction to completion.