Esterification with addition of catalysts

Sulfuric and hydrochloric acid are the usual catalysts for esterification, but phosphoric and arenesulfonic acids, acid salts, and acid cation-exchangers are also used. Mineral acids act not merely as catalysts but also have waterbinding properties so they are generally used in greater than catalytic amounts, usually 5-10%. 

When sulfuric acid is used the technique is to heat a mixture of the carboxylic acid with a four- to five-fold amount of the alcohol for 5-7 hours. Pyridine-and quinoline-carboxylic acids801 require for their esterification appreciably more catalyst acid than usual but in general too high concentrations of sulfuric acid lead to decomposition. 

The preparation of ethyl benzoate provides an example of a generally applicable technique for acid-catalysed esterification 802 Benzoic acid (50 g) is boiled under reflux for 3 h with anhydrous ethanol (100 g) containing concentrated sulfuric acid (10 g), then most of the alcohol is distilled off on the water-bath. The residue is treated with five times its amount of water and neutralized with solid sodium carbonate. The oil that separates is removed in ether, and the ethereal solution is dried over potassium carbonate. The ether is distilled off and the residue is fractionated, affording almost 90% (55 g) of ethyl benzoate, b.p. 95°/17 mm. 

Use of hydrogen chloride as catalyst (100 g of anhydrous ethanol containing 3 % of HC1), boiling for 2h, and analogous working up gives 76% of the theoretical amount of ethyl benzoate. 

Azeotropic esterification, applied to the acid-catalysed reaction, permits a considerable saving in mineral acid. Mineral acids can then be replaced by aliphatic or aromatic sulfonic acids, and decomposition is then reduced. Secondary as well as primary alcohols can be converted into esters by azeotropic esterification, but in general not tertiary alcohols. 

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