21.5 Chemistry of Acid Anhydrides

Synthesis and Reactivity of Acid Anhydrides

Acid anhydrides sit in the middle of the carboxylic acid derivative reactivity scale. They're less reactive than acid chlorides but more reactive than esters, amides, and carboxylic acids themselves. This balanced reactivity makes them practical choices in synthesis when you need a reagent that's reactive enough to get the job done but not so reactive that it's hard to control.

They also show up in real pharmaceutical manufacturing. Aspirin and acetaminophen are both made using acetic anhydride through nucleophilic acyl substitution reactions.

Preparation of Acid Anhydrides

There are two main ways to make acid anhydrides:

Method 1: Carboxylic acid + acyl chloride

1. The carboxylic acid acts as the nucleophile, attacking the electrophilic carbonyl carbon of the acyl chloride.
2. Chloride ion departs as the leaving group.
3. The product is a mixed or symmetric anhydride, depending on whether the two acyl groups match.

This is a straightforward nucleophilic acyl substitution. It works well because chloride is an excellent leaving group.

Method 2: Carboxylic acid + DCC (dicyclohexylcarbodiimide)

1. DCC reacts with one equivalent of the carboxylic acid to form an O-acylisourea intermediate.
2. A second molecule of carboxylic acid attacks the intermediate as a nucleophile.
3. The anhydride forms, and dicyclohexylurea (DCU) precipitates out as a byproduct.

This is a dehydration reaction, since the net result is two carboxylic acid molecules losing one molecule of water. DCC is commonly used as a coupling reagent in peptide synthesis for the same reason: it activates carboxylic acids toward nucleophilic attack.

Acid Anhydrides vs. Acid Chlorides

The reactivity difference between these two derivatives comes down to leaving group ability.

• Leaving group comparison: Chloride ($\text{Cl}^-$) is a better leaving group than carboxylate ($\text{RCOO}^-$). That's why acid chlorides react faster in nucleophilic acyl substitution.
• Electrophilicity: Acid anhydrides are more electrophilic than plain carboxylic acids because the second carbonyl group is electron-withdrawing, which makes the carbonyl carbon more susceptible to nucleophilic attack.
• Hydrolysis: Anhydrides hydrolyze to give two equivalents of carboxylic acid, but this hydrolysis is slower than that of acid chlorides.
• Ester formation: Anhydrides react with alcohols to form esters, but at a slower rate than acid chlorides do.
• Amide formation: Anhydrides react with amines to form amides. Again, slower than acid chlorides but faster than trying to make amides directly from carboxylic acids.

The general trend to remember: acid chlorides > acid anhydrides > esters > amides in terms of reactivity toward nucleophilic acyl substitution.

Acetic Anhydride in Pharmaceuticals

Two well-known drugs are synthesized by acetylation with acetic anhydride.

Aspirin (acetylsalicylic acid)

1. Salicylic acid is treated with acetic anhydride. The phenolic hydroxyl group ($\text{-OH}$) on salicylic acid acts as the nucleophile.
2. It attacks the electrophilic carbonyl carbon of acetic anhydride in a nucleophilic acyl substitution.
3. The carboxylate portion of the anhydride departs as acetic acid (the byproduct).
4. A weak base like sodium acetate is typically added to catalyze the reaction.

The result is an ester linkage where the phenol has been acetylated.

Acetaminophen (paracetamol)

1. p-Aminophenol is treated with acetic anhydride. Here, the amino group ($\text{-NH}_2$) acts as the nucleophile rather than the hydroxyl group, because nitrogen is more nucleophilic than oxygen.
2. The amine attacks the carbonyl carbon of acetic anhydride.
3. Acetic acid is released as the byproduct.
4. Sodium acetate is again used as a weak base to help drive the reaction and neutralize the acetic acid produced.

The result is an amide bond. Notice the pattern: both reactions are nucleophilic acyl substitutions, but aspirin synthesis forms an ester while acetaminophen synthesis forms an amide.

Reaction Mechanism of Acid Anhydrides

Acid anhydrides have the general structure $\text{RCO-O-COR'}$, with two carbonyl groups bridged by an oxygen atom. All of their reactions with nucleophiles follow the same general mechanism:

1. Nucleophilic addition: The nucleophile (alcohol, amine, or water) attacks the electrophilic carbonyl carbon, breaking the $\text{C=O}$ pi bond and forming a tetrahedral intermediate.
2. Elimination of the leaving group: The tetrahedral intermediate collapses, expelling the carboxylate ($\text{RCOO}^-$) as the leaving group and re-forming the $\text{C=O}$ double bond.
3. Proton transfer: A proton transfer step gives the final product and a carboxylic acid byproduct.

The carboxylate leaving group is stabilized by resonance (the negative charge is delocalized over two oxygen atoms), which is why it can depart, even though it's not as good a leaving group as chloride. This resonance stabilization is also why anhydrides are less reactive than acid chlorides but still reactive enough for most synthetic purposes.