Esters, fats and soaps

Esters are the products of reactions between carboxylic acids and alcohols — the "children" of an acid and an alcohol. The scents of fruit and flowers, animal fats and vegetable oils, and waxes are all esters. They are used as flavourings in the food industry, as solvents for varnishes and paints, and as raw materials for soap and margarine. Fats are the body’s most important energy store. This topic centres on esterification and its reverse, hydrolysis (especially alkaline hydrolysis, or saponification); calculation problems often ask about the mole ratios of fat, alkali and glycerol.

§1. Esters: structure and naming

In an ester the –OH group of the acid is replaced by the –OR′ group of the alcohol. Naming: the alkyl group of the alcohol + the name of the acid anion. CH₃COOC₂H₅ is ethyl ethanoate (ethyl acetate), HCOOCH₃ methyl methanoate (methyl formate), CH₃COOCH₃ methyl ethanoate.

Isomerism: by how the carbon atoms are shared between the acid and alcohol parts (C₃H₆O₂: methyl ethanoate and ethyl methanoate), by the structure of the groups, and functional-group isomerism with carboxylic acids (C₂H₄O₂: ethanoic acid and methyl methanoate).

Physical properties: the lower esters are volatile, pleasant-smelling liquids, only slightly soluble in water because their molecules cannot hydrogen-bond to each other; so they boil much lower than isomeric acids. Fragrant esters include 3-methylbutyl ethanoate (pear), ethyl butanoate (pineapple) and 3-methylbutyl 3-methylbutanoate (apple). Ethyl ethanoate is a solvent for varnishes and glues.
Do not confuse them with ethers (R–O–R′, for example ethoxyethane C₂H₅–O–C₂H₅), which are made by dehydrating alcohols and are isomeric with alcohols.

§2. Esterification

The reaction is reversible and slow; concentrated sulfuric acid acts as catalyst and dehydrating agent. An experiment with labelled oxygen (¹⁸O) showed that the oxygen of the water released comes from the –OH group of the acid, and the hydrogen from the alcohol’s hydroxyl group; the alcohol’s oxygen stays in the ester.
Ways to shift the equilibrium towards the ester (Le Chatelier’s principle): remove water (conc. H₂SO₄), use an excess of one reactant, distil off the ester as it forms.

Worked example

12 g of ethanoic acid was heated with excess ethanol; the ester yield was 60 %. What mass of ethyl ethanoate was obtained?

1. n(CH₃COOH) = 12/60 = 0.2 mol → theoretical n(ester) = 0.2 mol.
2. Theoretical mass: 0.2·88 = 17.6 g.
3. Actual mass: 17.6·0.6 = 10.56 g.

§3. Hydrolysis of esters

Hydrolysis splits an ester with water into an acid and an alcohol:

• in acid it is reversible (the reverse of esterification) and ends in equilibrium;
• in alkali it is irreversible, because the acid formed is at once converted into its salt, which does not react with the alcohol. This is called saponification:

For an ester of a monocarboxylic acid and a monohydric alcohol, n(ester) = n(NaOH), which gives a quick way to find the ester’s molar mass.

Worked example

Saponifying 7.4 g of a saturated ester used 0.1 mol of NaOH. Find the ester’s formula and name its isomers.

1. n(ester) = n(NaOH) = 0.1 mol → M = 7.4/0.1 = 74 g/mol.
2. CₙH₂ₙO₂: 14n + 32 = 74 → n = 3, C₃H₆O₂.
3. Isomers: methyl ethanoate CH₃COOCH₃ and ethyl methanoate HCOOC₂H₅.

§4. Fats: structure and properties

Every fat molecule has three acid residues. The key fatty acids: stearic C₁₇H₃₅COOH and palmitic C₁₅H₃₁COOH (saturated), oleic C₁₇H₃₃COOH (one double bond), linoleic C₁₇H₃₁COOH (two double bonds).

• Solid fats (animal fats) contain mostly saturated acids: tristearin, tripalmitin.
• Liquid fats (vegetable oils) contain mostly unsaturated acids: triolein, trilinolein. Exceptions: fish oil is liquid, palm oil is solid.

Fats are insoluble in water, less dense than it, and dissolve well in organic solvents. Unsaturated fats decolourise bromine water; the iodine value measures how unsaturated a fat is.

Hydrogenation turns a liquid oil into a solid fat (margarine): triolein + 3H₂ → tristearin (Ni, heat).
In the body, fats are hydrolysed by enzymes (lipases); oxidising 1 g of fat releases about 39 kJ — more than twice as much as protein or carbohydrate.

Functions of fats in the body: energy store, heat insulation, protection of internal organs and a solvent for the fat-soluble vitamins (A, D, E, K). Linoleic and linolenic acids cannot be made by the human body and must come from food. Waxes are also esters, but the fatty acid is bonded to a long-chain monohydric alcohol instead of glycerol (beeswax, for example). Oils kept in air for a long time oxidise and acquire a bitter taste and unpleasant smell — they go rancid.

§5. Hydrolysis of fats, and soap

Sodium salts of fatty acids are hard soap, potassium salts soft (liquid) soap. The long hydrocarbon "tail" of a soap molecule dissolves in grease and the ionic "head" in water, so soap carries dirt into the water. In hard water soap forms insoluble calcium and magnesium stearates with Ca²⁺ and Mg²⁺ ions and does not lather. Synthetic detergents (sulfonates) work well even in hard water. To make soap, fat is boiled with alkali solution, salt is added to separate the soap (salting out), and glycerol is recovered from the remaining solution.

Worked example

89 g of tristearin (M = 890) was completely saponified. Find the masses of soap and glycerol.

1. n(fat) = 89/890 = 0.1 mol → NaOH 0.3 mol, glycerol 0.1 mol, m = 0.1·92 = 9.2 g.
2. Sodium stearate C₁₇H₃₅COONa (M = 306): 0.3·306 = 91.8 g.
3. Check: 89 + 0.3·40 = 101 = 91.8 + 9.2.

Worked example

What volume of H₂ (STP) is needed to hydrogenate 0.1 mol of triolein (M = 884) completely?

1. Each oleic acid residue has 1 double bond, so the molecule has 3.
2. n(H₂) = 3·0.1 = 0.3 mol.
3. V = 0.3·22.4 = 6.72 L.

Check yourself

1. Which acid and alcohol give methyl ethanoate?

2. How many esters have the formula C₄H₈O₂?

3. Is saponification reversible?

4. What mass of tristearin must be saponified to obtain 18.4 g of glycerol?

5. How can a liquid oil be turned into a solid fat?

6. Why does soap not lather in hard water?

7. How many esters have the formula C₃H₆O₂?

8. What mass of NaOH saponifies 8.8 g of ethyl ethanoate completely?

Common mistakes

• Writing saponification as a reversible reaction.
• Assuming a fat has one acid residue: it has three, so three times as much NaOH is used.
• Confusing esters with ethers.
• Saying the oxygen in the water formed during esterification comes from the alcohol: it comes from the acid.
• Mixing up oleic and stearic acids: oleic is C₁₇H₃₃COOH, stearic C₁₇H₃₅COOH.