Esters: Synthesis, Odors, and Saponification

Esters are compounds with the group –COO– (RCOOR′), made from a carboxylic acid and an alcohol by esterification, and hydrolyzed by acids or bases; alkaline hydrolysis, called saponification, is the basis of soap making. In brief: ethanoic acid and ethanol react to give ethyl ethanoate and water with an equilibrium constant of about 4, so equimolar amounts give a yield of about 67 %, while the reaction of a fat with NaOH goes to completion to give glycerol and soap.

Many fruity smells come from esters, and fats and oils are esters of glycerol. Polyesters make up fibers and bottles, and biodiesel is a mixture of esters. This article explains how esters are made and named, why they smell, how to calculate the equilibrium yield, and how saponification works, with worked examples.

How are esters named?

An ester name has two words: the first comes from the alcohol (the alkyl group on the oxygen), and the second from the acid (with the ending -oate). CH₃COOCH₂CH₃ is ethyl ethanoate: the “ethyl” comes from ethanol, and “ethanoate” from ethanoic acid. HCOOCH₃ is methyl methanoate. Esters are isomers of carboxylic acids with the same number of carbon atoms: C₃H₆O₂ can be propanoic acid or methyl ethanoate or ethyl methanoate. Esters have no O–H, so they do not form hydrogen bonds with themselves, and their boiling points are much lower than those of the acids.

How are esters made?

Fischer esterification is the reaction of a carboxylic acid with an alcohol, with a strong acid catalyst (concentrated H₂SO₄) and heating:

RCOOH + R′OH ⇌ RCOOR′ + H2O

The reaction is reversible and slow, and it reaches an equilibrium. To increase the yield, use an excess of one reactant (usually the cheaper alcohol), or remove the water as it forms, for example by distillation or with a drying agent. More reactive routes use acyl chlorides or acid anhydrides with the alcohol, which are fast and go to completion: CH₃COCl + C₂H₅OH → CH₃COOC₂H₅ + HCl. Aspirin is made from salicylic acid and ethanoic anhydride.

Which esters smell of fruit?

Esters are volatile and often have pleasant, fruity aromas. They are found in fruits and flowers, and used in food flavorings, perfumes and air fresheners. Natural flavors are complex mixtures, and a single ester gives only an approximation.

How are esters hydrolyzed?

Acid hydrolysis is the reverse of esterification: RCOOR′ + H₂O ⇌ RCOOH + R′OH, catalyzed by dilute acid, with excess water. It is reversible, so it does not go to completion.

Alkaline hydrolysis uses a hot solution of NaOH or KOH: RCOOR′ + OH⁻ → RCOO⁻ + R′OH. This reaction is irreversible, because the carboxylic acid is converted to its salt, which is no longer attacked by the alcohol. The acid can be recovered by acidifying the mixture. This is why base is preferred in preparative work.

What is saponification?

Fats and oils are triglycerides, esters of glycerol (propane-1,2,3-triol) with three long-chain fatty acids. Boiling a fat with sodium hydroxide gives glycerol and the sodium salts of the fatty acids, which are soap:

(RCOO)3C3H5 + 3NaOH → C3H5(OH)3 + 3RCOONa

The name comes from the Latin sapo, soap. Potassium hydroxide gives softer soaps. The soap molecule has a long nonpolar tail that dissolves in grease, and a charged head that dissolves in water, which allows it to lift grease into the water in the form of micelles. In hard water, the calcium and magnesium salts of the fatty acids precipitate as scum.

Worked examples

Example 1: equilibrium yield with equimolar amounts

For CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O, K_c ≈ 4. Start with 1.00 mol of each reactant. Let x be the amount of ester formed. K = x² / (1 − x)² = 4, so x / (1 − x) = 2 and x = 0.667 mol. The yield is 67 %.

Example 2: using an excess of alcohol

With 1.00 mol of acid and 3.00 mol of ethanol, K = x² / ((1 − x)(3 − x)) = 4. Then x² = 4(3 − 4x + x²), which gives 3x² − 16x + 12 = 0. The root less than 1 is x = (16 − √(256 − 144)) / 6 = (16 − 10.58) / 6 = 0.903. The yield rises to 90 % by tripling the amount of ethanol.

Example 3: naming from the parts

The ester formed from propanoic acid and methanol is methyl propanoate, CH₃CH₂COOCH₃. The ester CH₃COOCH₂CH₂CH₃ is propyl ethanoate (from ethanoic acid and propan-1-ol).

Example 4: how much alkali for a fat

How much NaOH is needed to saponify 100 g of tripalmitin, (C₁₅H₃₁COO)₃C₃H₅ (M = 807.3 g/mol)? n = 100 / 807.3 = 0.1239 mol, and 3 mol of NaOH are needed per mole of fat: 0.3716 mol × 40.00 g/mol = 14.9 g. In practice the “saponification value”, the mass of KOH in mg per gram of fat, is used to characterize fats.

Where are esters used?

• Flavors and fragrances: artificial fruit flavors and perfumes.
• Solvents: ethyl ethanoate is a common solvent for paints, glues and nail varnish remover.
• Polyesters: PET (polyethylene terephthalate) is made from benzene-1,4-dicarboxylic acid and ethane-1,2-diol, and used in bottles and fibers.
• Biodiesel: vegetable oils are transesterified with methanol to give methyl esters of fatty acids and glycerol.
• Medicine: aspirin, and many prodrugs are esters; local anesthetics such as procaine are esters of an amino benzoic acid.
• Waxes and fats: beeswax is a mixture of esters of long-chain alcohols and acids.

Who explained esterification?

Esters were first described in the early 19th century, and the term ester was coined by Leopold Gmelin in 1848. The preparation of esters with an acid catalyst was developed by Emil Fischer and Arthur Speier in 1895, and it is still called Fischer esterification. The study of the equilibrium of the ethanol–acetic acid reaction by Marcellin Berthelot and Péan de Saint-Gilles in 1862–1863 was one of the earliest quantitative studies of chemical equilibrium and gave the K of about 4 mentioned above. Michel Eugène Chevreul had shown in the 1810s that fats are esters of glycerol, and explained saponification.

Which mistakes are common?

• Reversing the name: the alcohol part comes first (methyl ethanoate comes from methanol).
• Expecting esterification to go to completion: it is an equilibrium.
• Forgetting that alkaline hydrolysis gives a salt: acidify to get the free acid.
• Mixing up esters and acids of the same formula: only the acid has an O–H group.
• Calling soap a detergent of petroleum origin: soap is made from fats and oils.