Alcohols are usually the friendliest of the organic families in carbon compounds, because almost everything about them hangs off one small group of atoms. If you can explain what that group is, how it names the compound, and the four reactions ethanol undergoes, you have covered nearly every alcohol question SPM will ask. This guide gives you the explanations in the order that makes them stick.
Start with the functional group
An alcohol is any organic compound whose functional group is the hydroxyl group, –OH, bonded to a carbon chain. The general formula for the series is CₙH₂ₙ₊₁OH. Because every member carries the same –OH group, every member behaves in the same chemical ways, that is the whole logic of a homologous series, and it is why you only need to learn ethanol’s reactions rather than a fresh set for each alcohol.
The first four members are worth writing from memory:
- Methanol, CH₃OH (n = 1)
- Ethanol, C₂H₅OH (n = 2)
- Propanol, C₃H₇OH (n = 3)
- Butanol, C₄H₉OH (n = 4)
Notice how each name uses the carbon-number stem (meth-, eth-, prop-, but-) with the ending -ol that marks the alcohol series. Ethanol is the star of the syllabus, so anchor your explanations to it.
How ethanol is made
SPM expects you to describe two preparations, and to know the conditions for each.
Fermentation. Glucose from sources such as sugar cane is broken down by the enzyme in yeast, in the absence of air, at about 37 °C, to give ethanol and carbon dioxide:
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
State the conditions in full, yeast, warm temperature, no air, because marks are given for them, not just the equation.
Hydration of ethene. Industrially, ethanol is made by reacting ethene with steam over a phosphoric acid catalyst under pressure. This is the reverse idea of the dehydration reaction below, and it is worth mentioning as the large-scale route.
The four reactions to explain
Examiners return again and again to the same four reactions of ethanol. Learn the equation, the conditions, and the observation for each.
1. Combustion. Ethanol burns completely in plenty of air with a clean blue flame, releasing heat:
C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
This is an exothermic reaction, which is why ethanol is used as a fuel. The same pattern holds for the higher members, and the heat released rises as the carbon chain lengthens.
2. Oxidation to a carboxylic acid. Warmed with an oxidising agent, ethanol is oxidised to ethanoic acid. Use acidified potassium dichromate(VI) and the colour changes from orange to green; use acidified potassium manganate(VII) and it changes from purple to colourless. We write the oxidising agent as [O]:
C₂H₅OH + 2[O] → CH₃COOH + H₂O
This is the reaction behind wine turning sour in air. The oxidation of alcohols page and the oxidation of alcohol to carboxylic acid experiment give you the full apparatus and observations to quote.
3. Dehydration to an alkene. Remove water from ethanol and you get ethene. Heat ethanol vapour over hot porous pot or aluminium oxide, or heat it with excess concentrated sulfuric acid at about 180 °C:
C₂H₅OH → C₂H₄ + H₂O
The alkene product decolourises bromine water, which is a neat way to confirm it. The preparation of ethene by dehydration of ethanol experiment is a common Paper 3 context.
4. Esterification. Warmed with a carboxylic acid and a little concentrated sulfuric acid as catalyst, ethanol forms a sweet-smelling ester. With ethanoic acid the product is ethyl ethanoate:
CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O
The concentrated sulfuric acid acts as a catalyst and dehydrating agent, and the fruity smell is the observation that scores the mark.
A quick way to explain the pattern
When you are asked to explain rather than just state, tie the reactions back to the –OH group. Combustion happens because the molecule is a hydrocarbon derivative rich in C and H. Oxidation, dehydration and esterification are all transformations of the –OH group specifically: oxidation converts –OH into –COOH, dehydration removes the –OH together with an H to make a C=C, and esterification swaps the –OH’s hydrogen into an ester link. Framing the reactions as “what happens to the –OH” turns four facts into one story.
Where students lose marks
The commonest slips are forgetting the fermentation conditions, mixing up the two oxidising agents’ colour changes, and leaving the dehydration temperature out. Write the observation next to every equation in your notes so the two travel together. If the reactions still blur, our online one-to-one teachers can build you a single ethanol reaction map and drill it against past-paper wording; lessons run in English from RM50 an hour with a paid one-hour trial. Get ethanol solid and the rest of the alcohol questions follow.
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