The ester bond

An ester bond is the link that forms when the -OH group of an alcohol reacts with the -COOH group of a carboxylic acid, giving the group -COO- and releasing a molecule of water. In biology, ester bonds join fatty acids to glycerol to make triglycerides and phospholipids. They form by condensation and are broken by hydrolysis.

What is an ester bond?

An ester bond is a C-O-C link next to a C=O group. It is made in a condensation reaction: the OH from the acid and the H from the alcohol's OH group leave together as water, and the oxygen that is left joins the two molecules.

Here R and R′ stand for the rest of each molecule. The product, RCOOR′, is called an ester, and the -COO- group holding it together is the ester bond, also called an ester linkage.

Ester bonds in triglycerides

A triglyceride is a lipid made from one molecule of glycerol and three fatty acids. Glycerol is an alcohol with three -OH groups, one on each carbon. Each fatty acid has a -COOH group at one end and a long hydrocarbon chain, the R group, which may be saturated or unsaturated.

Each -OH on the glycerol reacts with the -COOH of one fatty acid. So making one triglyceride forms three ester bonds and releases three water molecules.

In the diagram the glycerol backbone runs down the left. Each row to the right is a fatty acid, attached through an ester bond. R₁, R₂ and R₃ are the hydrocarbon tails, which can be the same or different.

Example: the formula of a triglyceride

Glycerol is C₃H₈O₃ (Mr 92) and stearic acid, a saturated fatty acid, is C₁₈H₃₆O₂ (Mr 284). One glycerol and three stearic acids condense to form the triglyceride tristearin. What is its formula and Mr?

Add the atoms of the four molecules, then take away three H₂O: C₃H₈O₃ + 3 × C₁₈H₃₆O₂ − 3 × H₂O = C₅₇H₁₁₀O₆. Its Mr is 92 + 3 × 284 − 3 × 18 = 890.

Check your understanding.

Breaking an ester bond: hydrolysis

Hydrolysis is the reverse of condensation. A water molecule is added across the ester bond, which splits it back into an -OH group and a -COOH group. Fully hydrolysing a triglyceride gives back glycerol and three fatty acids.

In the gut, the enzyme lipase catalyses the hydrolysis of ester bonds in lipids, helped by bile salts, which break fat into small droplets so lipase has a larger surface to work on. Hydrolysis also happens when fats are boiled with an alkali such as sodium hydroxide, which is how soap is made.

Ester bonds in phospholipids

A phospholipid is like a triglyceride with one fatty acid swapped for a phosphate-containing group. It still has two ester bonds, joining two fatty acids to glycerol. The fatty acid tails are hydrophobic and the phosphate head is hydrophilic, which is why phospholipids form the bilayer of cell membranes. See the phospholipid structure guide for more.

Ester bond vs glycosidic bond vs peptide bond

All three bonds of biological molecules form by condensation and break by hydrolysis. What differs is which molecules they join:

A phosphodiester bond is a pair of ester bonds: the phosphate group forms an ester bond with a sugar on each side, which is where the name comes from. The alpha and beta glucose guide explains glycosidic bonds.

Ester bonds in chemistry

In chemistry, making an ester from an alcohol and a carboxylic acid is called esterification. It needs an acid catalyst, usually concentrated sulfuric acid, and it is reversible. AQA GCSE Chemistry expects you to know the ester ethyl ethanoate:

Small esters like this smell fruity and are used in flavourings and perfumes. When a molecule with two -OH groups reacts with a molecule with two -COOH groups, ester bonds form at both ends again and again, building a polyester by condensation polymerisation.

Common mistakes

• Saying one ester bond per triglyceride. There are three, one for each fatty acid.
• Forgetting the water. Condensation releases water; hydrolysis uses it up.
• Calling the bond in a lipid a glycosidic or peptide bond. Lipids have ester bonds. Glycosidic bonds join sugars and peptide bonds join amino acids.
• Calling triglycerides polymers. They are not: glycerol and fatty acids are different molecules, not repeating monomers.
• Putting the ester bond in the wrong place in a diagram. It is the C-O-C link between the glycerol carbon and the fatty acid's C=O carbon.

Practice questions

1. Name the type of reaction that forms an ester bond.
2. A phospholipid has how many ester bonds?
3. Name the enzyme that hydrolyses ester bonds in lipids during digestion.
4. Ethanol (Mr 46) reacts with ethanoic acid (Mr 60) to form ethyl ethanoate. Calculate the Mr of ethyl ethanoate.
5. Explain why triglycerides are not polymers.

Answers: 1) Condensation. 2) Two. 3) Lipase. 4) 46 + 60 − 18 = 88. 5) They are made from one glycerol and three fatty acids, which are not repeating units of the same monomer.

Practise with a short timed paper

Lipids and their bonds are part of the first topic of A-level Biology, and esters come up in GCSE Chemistry organic chemistry. The daily A-level Biology: Year 1 and GCSE Chemistry Higher papers are short timed papers of 8 questions, with a new one every day.

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