Reactions with Alcohols

Updated Apr 17, 2026

Esters can be made from any of the four carboxylic acid derivatives + an alcohol. The conditions (and speed) depend on the starting material.

Acyl Halide + Alcohol (Best for Clean Ester Synthesis)

R-COCl + R’-OH + pyridine → R-COOR’ + pyridinium chloride.

Fast, room temperature, irreversible. Pyridine neutralizes the HCl byproduct.

Anhydride + Alcohol

R-CO-O-CO-R + R’-OH → R-COOR’ + R-COOH.

Commonly used for phenol acetylation (e.g., aspirin synthesis from salicylic acid + acetic anhydride). Half the anhydride becomes the ester; the other half is the carboxylic acid byproduct.

Ester + Alcohol (Transesterification)

R-COOR’ + R”-OH ⇌ R-COOR” + R’-OH.

Equilibrium, driven by the same techniques as Fischer esterification (excess alcohol or water removal). Used in biodiesel production: triglycerides + methanol + base catalyst → fatty acid methyl esters + glycerol.

Carboxylic Acid + Alcohol (Fischer Esterification, Review from Ch 8.8)

R-COOH + R’-OH + H⁺ cat. ⇌ R-COOR’ + H₂O.

Reversible. Excess alcohol or water removal drives it forward. The classic lab method for making esters from the most accessible starting material.

Amide + Alcohol → Ester (Essentially Does Not Work)

Amides cannot be converted directly to esters under mild conditions because the amide N is a terrible leaving group. To go from amide to ester, you must first hydrolyze the amide to the carboxylic acid (strong acid or base, reflux), then do Fischer esterification. Going up the ladder requires two steps and energy input.

Cyclic Esters: Lactones

When a carboxylic acid and an alcohol are on the same molecule (intramolecular), the internal Fischer esterification gives a lactone (cyclic ester). Five- and six-membered lactones form spontaneously and are often isolable.

Gamma-butyrolactone (GBL): 4-carbon chain with COOH and OH on opposite ends gives a 5-ring lactone.
Delta-valerolactone: 5-carbon chain → 6-ring lactone.
Vitamin C (ascorbic acid): a complex 5-membered lactone with multiple hydroxyls.

Cyclic esters (lactones) are important in natural product chemistry and pharmaceuticals. Many antibiotics (erythromycin, rapamycin) are macrolide lactones.

Biology: Triglycerides and Phospholipids

Triglycerides are triple esters of glycerol (a triol) and three fatty acids. Formation in cells uses acyl-CoA (an activated thioester of fatty acid) + glycerol-3-phosphate + enzymes. Hydrolysis (lipase) runs the reverse: triglyceride + water → glycerol + 3 fatty acids.

Phospholipids have a similar structure: glycerol + 2 fatty acid esters (at C1 and C2) + 1 phosphate ester (at C3). The phosphate carries a polar head group (choline, serine, ethanolamine, inositol). This amphipathic structure is the basis of cell membranes.

Polyester Plastics

Polyethylene terephthalate (PET, used in soda bottles and polyester fabric) is a polymer of ethylene glycol (diol) + terephthalic acid (dicarboxylic acid) linked via ester bonds. Industrial synthesis uses transesterification of dimethyl terephthalate + ethylene glycol with a transesterification catalyst. Billions of tons are produced annually.

A chemist wants to make a large quantity of methyl benzoate. She has two options: (a) Fischer esterification (benzoic acid + methanol + H₂SO₄ cat.) or (b) benzoyl chloride + methanol + pyridine. Which is better, and why?

Click to reveal answer

For small scale or clean product, option B (benzoyl chloride + methanol + pyridine) is better: it is fast, irreversible, and gives high yield at room temperature. For industrial scale where cost matters, option A (Fischer) is preferred: benzoic acid is cheaper than benzoyl chloride, and Fischer can be driven to high yield with excess methanol + water removal (Dean-Stark trap). The choice depends on scale and purity requirements. Acyl chloride route is cleaner but requires activation of the starting acid.