Esters, Fats and Oils

This page covers content standard 10.8 of the Form 5 Carbon Compounds chapter: esters, fats and oils. This is where the alcohols and carboxylic acids you have just studied come together in one reaction, and where organic chemistry connects to food and everyday materials. Learn the esterification reaction, how to name an ester, and the saturated-versus-unsaturated difference between fats and oils, and you have this standard covered.

What is an ester?

An ester is an organic compound containing the ester linkage –COO– as its functional group. Esters are made by reacting a carboxylic acid with an alcohol. Most esters have a sweet, fruity smell, which is why they are used as flavourings in food and as scents in perfumes.

Esterification. A carboxylic acid + an alcohol → an ester + water, using a concentrated sulfuric acid catalyst and gentle warming. The reaction is reversible.

Using ethanoic acid and ethanol as the classic example:

CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O

The product, ethyl ethanoate, is a colourless liquid with a sweet smell. The concentrated sulfuric acid acts as a catalyst (and helps remove water so more ester forms).

Naming an ester

An ester’s name has two parts, and the order is the opposite of the way it is made:

1. The first part (alkyl) comes from the alcohol, ethanol gives ethyl, methanol gives methyl.
2. The second part (-oate) comes from the carboxylic acid, ethanoic acid gives ethanoate, propanoic acid gives propanoate.

So ethanol + ethanoic acid → ethyl ethanoate; methanol + propanoic acid → methyl propanoate. Getting the two parts in the right order is a very common exam mark.

Fats and oils

Fats and oils are natural esters. They are esters of glycerol (propane-1,2,3-triol, an alcohol with three –OH groups) and fatty acids (long-chain carboxylic acids). The key difference the syllabus asks for is saturation:

• Fats contain mostly saturated fatty acids (no C=C double bonds). They are usually solid at room temperature, for example, animal fats such as butter.
• Oils contain unsaturated fatty acids (with C=C double bonds). They are usually liquid at room temperature, for example, vegetable oils such as palm oil.

Because oils are unsaturated, they decolourise bromine water, while saturated fats do not, the same test for unsaturation you met with the alkenes. Hardening a liquid oil into a solid fat (as in making margarine) is done by adding hydrogen across the C=C bonds with a nickel catalyst (hydrogenation).

Worked example

Question. Methanol reacts with propanoic acid in the presence of a concentrated sulfuric acid catalyst. Name the ester formed and write the equation.

Step 1, Identify the two parts. The alcohol is methanol (CH₃OH), which gives the methyl part. The acid is propanoic acid (C₂H₅COOH), which gives the propanoate part.

Step 2, Combine in the correct order. Alkyl (from the alcohol) comes first, then -oate (from the acid): the ester is methyl propanoate.

Step 3, Write the equation. Esterification gives the ester plus water:

C₂H₅COOH + CH₃OH → C₂H₅COOCH₃ + H₂O

Step 4, Note the conditions. The mixture is warmed with a concentrated sulfuric acid catalyst, and the reaction is reversible.

Answer. The ester is methyl propanoate, formed by C₂H₅COOH + CH₃OH → C₂H₅COOCH₃ + H₂O, warmed with a concentrated sulfuric acid catalyst.

Practice question

Question. Ethanol and ethanoic acid are warmed together with a few drops of concentrated sulfuric acid. Name the organic product, state the role of the concentrated sulfuric acid, and describe one observation.

Answer. The organic product is ethyl ethanoate (CH₃COOC₂H₅), formed by the equation CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O. The concentrated sulfuric acid acts as a catalyst (it speeds up the reaction and helps remove water). One observation is a sweet, fruity smell, which shows that an ester has been produced.

Exam tip

The most common slip is naming the ester in the wrong order. Remember: the alkyl part (from the alcohol) comes first, the -oate part (from the acid) comes second, even though the acid is written first in the equation. Always state the concentrated sulfuric acid as a catalyst and note that esterification is reversible. For fats and oils, tie the physical state to saturation: fats are saturated and solid, oils are unsaturated and liquid, and oils decolourise bromine water. If a question asks how to change an oil into a fat, the answer is hydrogenation, adding hydrogen with a nickel catalyst, exactly as in the alkenes topic.

Uses of esters and everyday links

Because most esters smell sweet and fruity, they are widely used as artificial flavourings in sweets and drinks and as fragrances in perfumes, soaps and air fresheners. Many esters are also good solvents, ethyl ethanoate, for example, is used in some glues and nail-varnish removers. In nature, esters give many fruits their characteristic scents. Fats and oils, the natural esters, are an important part of the diet and a store of energy in living things; industrially they are turned into soap by reaction with hot sodium hydroxide (saponification) and hardened into margarine by hydrogenation. Being able to give one real use for esters and one for fats or oils shows the examiner you can connect the chemistry to the world, which is exactly the kind of application the syllabus rewards.

Where this fits

This is content standard 10.8 of the Carbon Compounds chapter, and it is the natural finale: it combines the alcohols (10.6) and carboxylic acids (10.7) you have just learned, and it reuses the bromine water test for unsaturation from the alkenes (10.4). Practise writing and naming esters in the chapter worked examples, and check the naming order and the fat/oil comparison on the common mistakes page. In our online 1-to-1 SPM Chemistry lessons, taught in English, from RM50/hr, our teachers make sure you can name any ester and explain the fat-versus-oil difference with confidence. Because esterification and the fat/oil comparison recur across SPM Chemistry, this closing standard is well worth the extra practice.

Quick recap

• An ester has the –COO– linkage; made by esterification: acid + alcohol → ester + water (concentrated H₂SO₄ catalyst, reversible).
• Esters have a sweet, fruity smell; used as flavourings and perfumes.
• Naming: alkyl (from alcohol) first, -oate (from acid) second, e.g. ethyl ethanoate.
• Fats and oils are esters of glycerol + fatty acids.
• Fats = saturated, solid; oils = unsaturated, liquid and decolourise bromine water.