Acyl chlorides and acid anhydrides are carboxylic acid derivatives that are highly reactive because of the presence of a carbonyl group (\( \text{C}=\text{O} \)) bonded to a good leaving group. In this lesson, we study their structures, their characteristic nucleophilic addition-elimination mechanisms, and compare their industrial utility in manufacturing processes.
🔑 Key Principle
The reactivity of acyl derivatives is governed by the strength of the partial positive charge (\( \delta^+ \)) on the carbonyl carbon and the stability of the leaving group. Acyl chlorides have a highly electron-deficient carbonyl carbon and an excellent leaving group (\( \text{Cl}^- \)), making them extremely reactive.
Structure and Reactivity Comparison
Both families contain the acyl group (\( \text{R-C}(=\text{O})- \)) but differ in what is attached to it:
A derivative of a carboxylic acid where the hydroxyl group is replaced by a chlorine atom, having the general formula \( \text{RCOCl} \).
A derivative formed by condensation of two carboxylic acid molecules, having the general formula \( \text{RCO-O-COR} \).
Because the chlorine atom in acyl chlorides and the carboxylate oxygen in acid anhydrides are highly electronegative, they withdraw electron density from the carbonyl carbon. This increases its partial positive charge (\( \text{C}^{\delta+} \)), making it highly electrophilic and susceptible to attack by nucleophiles. Since chlorine is a better leaving group than a carboxylate group, acyl chlorides react much more vigorously than acid anhydrides.
The Nucleophilic Addition-Elimination Mechanism
Acyl chlorides and acid anhydrides react with nucleophiles via a two-stage nucleophilic addition-elimination mechanism. The pathway has three steps:
- Nucleophilic Addition: The nucleophile attacks the \( \text{C}^{\delta+} \) carbon, breaking the \( \text{C}=\text{O} \) \( \pi \)-bond and forming a tetrahedral intermediate with a negative charge on the oxygen.
- Elimination: The lone pair on the oxygen reforms the \( \text{C}=\text{O} \) double bond, ejecting the leaving group (\( \text{Cl}^- \) or \( \text{RCOO}^- \)).
- Deprotonation: If the nucleophile was neutral (e.g. water, alcohol, or amine), the positive charge on the nucleophilic atom is removed by losing an \( \text{H}^+ \) ion.
Reactions of Acyl Chlorides and Acid Anhydrides
You must know the four primary reactions for both functional groups, using ethanoyl chloride and ethanoic anhydride as model compounds:
| Nucleophile | Reaction with Ethanoyl Chloride | Reaction with Ethanoic Anhydride | Observations/Products |
|---|---|---|---|
| Water (\( \text{H}_2\text{O} \)) | \[ \text{CH}_3\text{COCl} + \text{H}_2\text{O} \rightarrow \text{CH}_3\text{COOH} + \text{HCl} \] | \[ (\text{CH}_3\text{CO})_2\text{O} + \text{H}_2\text{O} \rightarrow 2\text{CH}_3\text{COOH} \] | Chloride reaction is violent at room temperature; gives white, misty fumes of \( \text{HCl} \). Anhydride reaction is slow. Both yield carboxylic acids. |
| Alcohols (\( \text{R'OH} \)) | \[ \text{CH}_3\text{COCl} + \text{R'OH} \rightarrow \text{CH}_3\text{COOR'} + \text{HCl} \] | \[ (\text{CH}_3\text{CO})_2\text{O} + \text{R'OH} \rightarrow \text{CH}_3\text{COOR'} + \text{CH}_3\text{COOH} \] | Yields esters. Chloride reaction is rapid, yielding misty fumes of HCl. Anhydride requires warming. |
| Ammonia (\( \text{NH}_3 \)) | \[ \text{CH}_3\text{COCl} + 2\text{NH}_3 \rightarrow \text{CH}_3\text{CONH}_2 + \text{NH}_4\text{Cl} \] | \[ (\text{CH}_3\text{CO})_2\text{O} + 2\text{NH}_3 \rightarrow \text{CH}_3\text{CONH}_2 + \text{CH}_3\text{COONH}_4 \] | Yields a primary amide (ethanamide) and a salt. Violent reaction; white smoke of ammonium salt is observed. |
| Primary Amines (\( \text{R'NH}_2 \)) | \[ \text{CH}_3\text{COCl} + 2\text{R'NH}_2 \rightarrow \text{CH}_3\text{CONHR'} + \text{R'NH}_3\text{Cl} \] | \[ (\text{CH}_3\text{CO})_2\text{O} + 2\text{R'NH}_2 \rightarrow \text{CH}_3\text{CONHR'} + \text{CH}_3\text{COONH}_3\text{R'} \] | Yields an N-substituted (secondary) amide and an alkylammonium salt. |
Notice that reactions with ammonia and primary amines require two moles of the nitrogen nucleophile. The first mole acts as the nucleophile to form the amide, while the second mole acts as a base to neutralise the acidic by-product (\( \text{HCl} \) or \( \text{CH}_3\text{COOH} \)), forming a salt.
Industrial Advantages: Ethanoic Anhydride vs Ethanoyl Chloride
In industry, ethanoic anhydride is preferred over ethanoyl chloride for acylating reactions, such as the manufacture of aspirin from salicylic acid:
Solution:
Salicylic acid has a phenolic hydroxyl group (\( \text{-OH} \)) that is acylated by ethanoic anhydride:
\[ \text{HOC}_6\text{H}_4\text{COOH} + (\text{CH}_3\text{CO})_2\text{O} \rightarrow \text{CH}_3\text{COOC}_6\text{H}_4\text{COOH} + \text{CH}_3\text{COOH} \]The organic product is aspirin. The by-product is ethanoic acid.
Industry avoids ethanoyl chloride for several critical reasons:
- Safety: Ethanoyl chloride produces highly corrosive, toxic hydrogen chloride (\( \text{HCl} \)) gas, requiring expensive scrubbing systems. Ethanoic anhydride produces ethanoic acid, which is non-toxic and easily recycled.
- Control: Ethanoyl chloride reactions are extremely exothermic and violent, presenting explosion risks. Ethanoic anhydride reacts at a moderate, manageable rate.
- Moisture Sensitivity: Ethanoyl chloride readily hydrolyses with moisture in the air, meaning it must be stored under anhydrous conditions, which increases cost.
- Cost: Ethanoic anhydride is significantly cheaper to produce and transport.