The following anhydride is produced from a reaction of an acid chloride and a carboxylic acid in the presence of pyridine. What is the source of the oxygen located in the middle bridge of the anhydride?

The hydroxyl oxygen in carboxylic acid

The correct answer is option 2. The hydroxyl oxygen in carboxylic acid.

Let us delve into the reaction mechanism and how we can trace the source of the oxygen in the middle bridge of the anhydride.

Reaction Mechanism for Anhydride Formation:

Reactants:

Acid Chloride \((RCOCl)\): Contains a carbonyl carbon (C=O) bonded to a chlorine atom (Cl).

Carboxylic Acid (R'COOH): Contains a carbonyl group (C=O) and a hydroxyl group (OH).

Role of Pyridine:

Pyridine is a base that is often used to neutralize the HCl produced in the reaction and also helps to activate the carboxylic acid by deprotonating it.

Mechanism:

Source of the Oxygen:

The hydroxyl oxygen in the carboxylic acid (\(\text{R'COOH}\)) becomes the central oxygen in the anhydride. The acid chloride (\(\text{RCOCl}\)) provides the carbonyl carbon (C=O) on one side of the anhydride.

Tracing the Oxygen: To determine the source of the oxygen, consider the transformation:

Hydroxyl Group of Carboxylic Acid: \(\text{R'COOH}\) has an OH group.  During the nucleophilic attack, this OH group loses its hydrogen and bonds with the carbonyl carbon from the acid chloride. Therefore, the oxygen in the OH group becomes the bridging oxygen in the anhydride.

Conclusion: The oxygen located in the middle bridge of the anhydride is derived from the hydroxyl group of the carboxylic acid. Thus, the correct answer is :The hydroxyl oxygen in the carboxylic acid.