What is an ester and what does the ester bond look like?
An ester is an organic compound with the group –COO– in the middle. On one side of it sits a part from an acid; on the other side, a part from an alcohol. The general formula is R–COO–R′. R comes from the acid. R′ comes from the alcohol.
The link C(=O)–O–C is called the ester bond (or ester link). It has one carbon with a double-bonded oxygen and a second oxygen that bridges to the alcohol part.
Small esters are colourless liquids. They smell fruity, boil at lower temperatures than the acids they come from (they cannot form hydrogen bonds with each other) and do not mix well with water.
Esterification: making an ester
carboxylic acid + alcohol ⇌ ester + water
Example: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O (ethanoic acid + ethanol → ethyl ethanoate + water).
Tests with heavy-oxygen labels show the water's oxygen comes from the acid. So the acid loses –OH and the alcohol loses only H.
Role of concentrated sulfuric acid
- It is a catalyst: it speeds up the reaction but is not used up.
- It is a drying agent: it soaks up water.
Shifting the balance
Esterification is reversible and slow. It reaches an equilibrium (a balance where both directions run at the same speed). To get more ester: remove water (H₂SO₄), use extra alcohol or extra acid, or distil off the ester as it forms. This is Le Chatelier's idea: the balance moves to replace what you take away.
Inorganic acids can form esters too. Nitric acid + glycerol gives glyceryl trinitrate (nitroglycerine, a heart medicine and explosive). Phosphoric acid esters are found in DNA and ATP.
Try it (teacher demo only): warm a few drops of ethanol and ethanoic acid with a drop of conc. H₂SO₄ in a water bath, then pour into a beaker of cold water and waft. A sweet, glue-like smell means an ester formed.
Naming esters
Two words: alcohol part + acid part.
- Alcohol name: change -anol to -yl (ethanol → ethyl; methanol → methyl).
- Acid name: change -oic acid to -oate (ethanoic acid → ethanoate).
So methanol + butanoic acid → methyl butanoate (apple smell). Pentanol + ethanoic acid → pentyl ethanoate (banana/pear). Ethanol + butanoic acid → ethyl butanoate (pineapple).
In a formula, read from the side with C=O for the acid: in CH₃COOCH₃ the acid part is CH₃COO– (ethanoate) and the alcohol part is –CH₃ (methyl), so it is methyl ethanoate.
Hydrolysis, fats and soap
Hydrolysis
Hydrolysis means splitting with water. Acid hydrolysis (water + dilute acid, heat) gives back acid + alcohol, but it is reversible, so it is never complete. Alkaline hydrolysis (NaOH, heat) gives a salt of the acid + alcohol. It goes to completion, because the salt cannot react back. Example: CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH.
Fats and oils
Fats and oils are triglycerides: one glycerol (propane-1,2,3-triol, with three –OH) joined by ester bonds to three fatty acids (long-chain acids). Fats with saturated chains (only single C–C bonds) pack tightly and are solid (butter, ghee). Oils with unsaturated chains (C=C bends) pack loosely and are liquid (sunflower, olive oil). Adding hydrogen to oils (hydrogenation) makes them harder, as in margarine.
In the body, fats store energy (about 37 kJ per gram, twice that of sugar), keep us warm and protect organs. Enzymes called lipases hydrolyse fats during digestion.
Soap (saponification)
Fat + NaOH → glycerol + 3 sodium salts of fatty acids. These salts are soap. A soap particle has a long hydrophobic (water-fearing) tail that dissolves in oil, and an ionic hydrophilic (water-loving) head. Tails grab the grease, heads face the water, and the grease is lifted off in tiny balls called micelles.
Uses of esters
Fruit flavours and perfumes, solvents (nail-polish remover, glues), plasticisers, polyesters (PET bottles and fabric), biodiesel (methyl esters of vegetable oils), and medicines such as aspirin.
Key formulas and definitions
- RCOOH + R′OH ⇌ RCOOR′ + H₂O (conc. H₂SO₄, heat)
- Ester bond: –COO– (C=O and C–O–C)
- Acid hydrolysis: RCOOR′ + H₂O ⇌ RCOOH + R′OH
- Alkaline hydrolysis: RCOOR′ + NaOH → RCOONa + R′OH
- Fat + 3 NaOH → glycerol + 3 RCOONa (soap)
- Name = alcohol part (-yl) + acid part (-oate)
Worked examples
1. Name the ester made from methanol and ethanoic acid and write its formula.
Methanol → methyl; ethanoic acid → ethanoate. Ester: methyl ethanoate, CH₃COOCH₃.
2. Which acid and alcohol make propyl methanoate, HCOOC₃H₇?
The acid part HCOO– is methanoate, from methanoic acid HCOOH. The alcohol part –C₃H₇ is propyl, from propanol C₃H₇OH.
3. Write the products when ethyl ethanoate is boiled with NaOH solution. Why does this reaction go to completion?
CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH (sodium ethanoate + ethanol). The carboxylate salt cannot react with ethanol to remake the ester, so there is no backward reaction.
4. What is the molar mass of ethyl ethanoate, CH₃COOC₂H₅? (C = 12, H = 1, O = 16)
C₄H₈O₂: 4×12 + 8×1 + 2×16 = 48 + 8 + 32 = 88 g/mol.
5. A student mixes 1 mol ethanoic acid with 3 mol ethanol instead of 1 mol. What happens to the amount of ester at equilibrium, and why?
It increases. Extra alcohol pushes the equilibrium to the right to use it up, so more acid turns into ester.
Common mistakes
- Writing the name the wrong way round: CH₃COOC₂H₅ is ethyl ethanoate, not ethanoyl ethyl or ethanoate ethyl.
- Thinking the water's oxygen comes from the alcohol. It comes from the acid's –OH.
- Forgetting that esterification is reversible and writing a single arrow without conditions.
- Saying soap forms in acid hydrolysis. Soap (a salt) only forms in alkaline hydrolysis.