Chemistry · Organic chemistry · Grade 10

Acids, esters and soap

How does vinegar plus alcohol give the smell of banana, and cottonseed oil give soap?

The situation

Table vinegar at 9 %, the skin that stings when an ant bites, the pleasant smell of banana and pineapple, cottonseed oil and a bar of soap — all are members of one chemical family. If ethanoic (acetic) acid is heated with an alcohol, a fruity-smelling ester forms; fats are esters of glycerol with fatty acids, and if we boil them with alkali, soap comes out. How do we get from acid to ester, and from ester to soap, and why does soap wash away dirt?

Topic

Carboxylic acids, esters, fats and soap. From vinegar to soap: a fruity-smelling ester from ethanoic acid and alcohol; soap from cottonseed oil

Key ideas

Carboxylic acids
Esterification ⇌
Fats and saponification

Formulas

R - COOH

Carboxylic acids: the carboxyl group –COOH. Ethanoic acid has p $K_{a}$ = 4.76 — a weak acid; saturated monobasic acids are $C_{n} H_{2 n + 1} COOH$ .

CH_{3} COOH + C_{2} H_{5} OH \overset{\overset{}{H_{2} SO_{4}, t}}{⇌} CH_{3} COOC_{2} H_{5} + H_{2} O

Esterification is a reversible reaction, $K \approx 4$ : in an equimolar mixture the ester yield is ≈ 67 %; with excess alcohol the equilibrium shifts to the right.

\frac{x^{2}}{(1 - x) (r - x)} = K

Equilibrium yield $x$ (fraction of the acid), $r$ — moles of alcohol taken per 1 mol of acid.

(C_{17} H_{35} COO)_{3} C_{3} H_{5} + 3 NaOH \overset{}{\to} C_{3} H_{5} (OH)_{3} + 3 C_{17} H_{35} COONa

Saponification is the alkaline hydrolysis of a fat: glycerol and the sodium salts of fatty acids (soap) are formed.

SS = \frac{(V_{0} - V) c M (KOH)}{m}

Saponification value (mg KOH/g): $V_{0}$ and $V$ — volumes of HCl for the blank and for the sample, $c$ — HCl concentration, $m$ — mass of the sample (University).

Theory

Carboxylic acids: structure and properties (3.12, 3.13)

Carboxylic acids contain the carboxyl group –COOH in the molecule: $HCOOH$ is formic (methanoic) acid, $CH_{3} COOH$ ethanoic (acetic) acid, $C_{3} H_{7} COOH$ butanoic (butyric) acid; fatty acids — palmitic $C_{15} H_{31} COOH$ , stearic $C_{17} H_{35} COOH$ , oleic $C_{17} H_{33} COOH$ ; polybasic ones — oxalic, citric, malic acid. The O–H in the carboxyl group is strongly polarised, and in water the acid is partly dissociated: $CH_{3} COOH \overset{}{⇌} CH_{3} COO^{-} + H^{+}$ (p $K_{a}$ = 4.76; pH ≈ 2.9 in a 0.1 mol/L solution). Properties: they change the colour of indicators, release $H_{2}$ with active metals and $CO_{2}$ with carbonates, are neutralised by alkalis (the salt is an acetate), and form esters with alcohols. Formic acid also has an aldehyde group — it gives a silver mirror. Anhydrous ethanoic acid freezes like ice at 16.6 °C (“glacial ethanoic acid”) and boils at 118 °C. Preparation: oxidation of alcohols and aldehydes; vinegar — fermenting wine with acetic bacteria; industrially, by carbonylation of methanol. Worked example: 0.01 mol/L $CH_{3} COOH$ ( $K_{a} = 1.75 \cdot 10^{- 5}$ ): $[H^{+}] \approx \sqrt{K_{a} c} = 4.2 \cdot 10^{- 4}$ mol/L, pH ≈ 3.4.

Esters and esterification (3.14, 3.15)

Esters $R - COO - R^{'}$ form from an acid and an alcohol with loss of water (esterification); the catalyst is concentrated $H_{2} SO_{4}$ (which also binds the water). In an experiment with an alcohol whose oxygen is “labelled” ( $^{18}$ O), this oxygen stays in the ester: the water forms from the OH of the acid and the H of the alcohol. The reaction is reversible, $K \approx 4$ : 1 mol of acid and 1 mol of alcohol give ≈ 0.67 mol of ester; increasing the alcohol or removing water raises the yield (Le Chatelier's principle). The reverse reaction is hydrolysis: reversible in an acidic medium, and going to completion in an alkaline medium (saponification). Lower esters are volatile, pleasant-smelling liquids that give fruits and flowers their scent: isoamyl acetate — banana, pear; ethyl butanoate — pineapple; methyl butanoate — apple; octyl acetate — orange; ethyl acetate — nail varnish remover and a solvent. Practical 3.15: in a test tube, 1 mL of ethanoic acid, 1–2 mL of alcohol and 3–4 drops of conc. $H_{2} SO_{4}$ are heated in a water bath, then poured into cold table-salt solution — the ester is sparingly soluble in water and light: it rises to the surface as a layer, and its smell can be noticed.

Fats (3.16)

Fats (triglycerides) are esters of the trihydric alcohol glycerol with higher fatty acids. Saturated acids (palmitic, stearic) have straight chains and the molecules pack tightly — solid fats (beef and mutton fat); oleic, linoleic and linolenic acids with cis double bonds bend the chain — liquid oils (cottonseed, sunflower, olive). Cottonseed oil contains ≈ 54 % linoleic acid and ≈ 26 % saturated acids. Stearic acid melts at 69.3 °C, oleic acid at 13.4 °C, linoleic at ≈ −7 °C. Oils decolourise bromine water (addition across C=C). Hydrogenation: if a liquid oil is saturated with hydrogen over a nickel catalyst, a solid fat — salomas (hydrogenated fat) — is obtained, from which margarine is made; the Tashkent Oil and Fats Combine produced margarine and vegetable oil for many years. In partial hydrogenation some of the cis bonds change to the trans form (elaidic acid, melting at 45 °C) — trans fats raise the risk of cardiovascular disease. Fats are the most calorific food (≈ 37 kJ/g) and dissolve vitamins A, D and E. In Uzbekistan vegetable oil is mainly obtained from cotton seed.

Soap and how it cleans (3.17)

Soap consists of the sodium (hard soap) or potassium (liquid soap) salts of higher fatty acids. Practical 3.17: 5 g of fat, 10 mL of alcohol (it mixes the fat and the alkali) and 10 mL of 20 % NaOH are placed in a porcelain basin or beaker and boiled for 15–20 min with stirring; if a drop dissolves completely in hot water, saponification is finished. Then saturated table-salt solution is added: the solubility of the soap falls and it floats to the surface (salting out), while glycerol and salt solution remain below. The soap ion $C_{17} H_{35} COO^{-}$ has two parts: the long hydrophobic tail dissolves in fat, and the hydrophilic –COO⁻ head dissolves in water. In water the ions form micelles: tails inside, heads outside; a droplet of dirt (grease) is “swallowed” into the micelle and carried away with the water. A soap solution is alkaline because of hydrolysis (phenolphthalein turns pink). In hard water the $Ca^{2 +}$ and $Mg^{2 +}$ ions give a precipitate of insoluble calcium stearate — soap is used up for nothing and there is little lather. Safety: NaOH solution burns skin and eyes — wear goggles and gloves, and if it gets into the eyes, rinse with plenty of water; keep the boiling alcoholic mixture away from flames.

University: fat analysis, lipids

The saponification value (SV) is the KOH (mg) needed to saponify all the esters and free acids in 1 g of fat: the sample is boiled with excess 0.5 M alcoholic KOH, and the remaining alkali is titrated with 0.5 M HCl; a high SV means short acid chains (coconut 248–265, cotton 189–207). The iodine value (IV) is the $I_{2}$ (g) that 100 g of fat adds, a measure of unsaturation: Wijs reagent ( $ICl$ ) is added, the excess is converted to $I_{2}$ with KI and titrated with sodium thiosulfate (starch indicator); cotton 100–115, sunflower 110–145, beef fat 42–48, coconut 6–11. The acid value (AV) is the KOH (mg) needed to neutralise the free fatty acids in 1 g of fat, an indicator of freshness; the share of free acids is also expressed as oleic acid. Lipids: saponifiable (fats, waxes, phospholipids) and non-saponifiable (steroids, cholesterol $C_{27} H_{46} O$ ). Phospholipids (lecithin) — glycerol + 2 fatty acids + phosphate + amino alcohol: the bilayer of the cell membrane. Cholesterol strengthens the membrane and is the source of steroid hormones and bile acids; an excess in the blood leads to atherosclerosis.

Limits of the model

The pH of the acids comes from the charge-balance equation (25 °C, ideal solution); the rate of the reaction of magnesium and soda is shown schematically as depending on $[H^{+}]$ . In esterification $K = 4$ for all esters, the rate constant is approximate (it doubles about every 11 °C, and is ≈ 100 times slower without $H_{2} SO_{4}$ ), and time is shortened. A drop of bromine water is 0.05 mL of 0.20 mol/L; the fat sample is 0.1 g in hexane; the amount of bromine comes from the iodine value (1 C=C : 1 $Br_{2}$ ). Saponification: 5 g of fat is taken to be tristearin, and time is sped up ≈ 20 times. The fat figures in the titration are typical values within the published ranges, and the acid values are typical. The micelle is schematic (in reality ≈ 5 nm across, 50–100 ions). The molecules are at true bond lengths (1 Å ≈ 3.4 cm).

Did you know?

The earliest evidence of soap-like substances being made dates to Babylon around 2800 BC; and a Babylonian clay tablet from about 2200 BC records a recipe for making soap from water, alkali and cassia oil.
Isoamyl acetate, which gives the banana smell, is also in the bees' “danger” signal: a stinging bee releases this smell and calls other bees to attack.

Inside the experiment

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