Alcohols: -OH group; names end in -ol
An alcohol has the hydroxyl functional group, -OH (spec 4.29C); general formula . The four to name and draw (4.30C) are methanol, ethanol, propan-1-ol and butan-1-ol ( to ). In a displayed formula show every bond, including the O-H. Consecutive members differ by , have similar chemical properties and a trend in physical properties.
Three ways to oxidise ethanol
Spec 4.31C: three ways ethanol is oxidised. (1) Complete combustion → carbon dioxide + water: (2) Microbial oxidation — oxygen and microbes give ethanoic acid (wine to vinegar). (3) Warming with potassium dichromate(VI) in dilute sulfuric acid also gives ethanoic acid (orange → green). In (2) and (3) name ethanoic acid; "a carboxylic acid" is too vague.
Ethanol: hydration of ethene or fermentation
Two routes. Hydration of ethene - steam over a phosphoric acid () catalyst, ~300 C, 60-70 atm: fast, pure, but uses crude oil. Fermentation - glucose with yeast enzymes, anaerobic, ~30 C: renewable and low-temperature, but slow and impure.
Drawn from real examiner reports.
Wrong products or an unbalanced combustion
For complete combustion of ethanol the products are carbon dioxide and water: Only water (or only carbon dioxide) cannot be balanced and scores 0; the other error is correct products left unbalanced. Balance oxygen last - the -OH oxygen counts (7 O each side).
June 2023 Paper 2C Q3(a): candidates lost marks for giving water instead of carbon dioxide as a product (the equation then cannot be balanced), or wrote the correct products but left the equation unbalanced.
Quoting the wrong catalyst for hydration
The catalyst for hydration of ethene is phosphoric acid, . Two wrong answers recur: (1) silica or alumina - these catalyse cracking, not hydration; (2) potassium dichromate(VI) - an oxidising agent (ethanol → ethanoic acid), not a catalyst. Full conditions: phosphoric acid, about 300 C, 60-70 atm.
June 2023 Paper 2C Q3(b)(i): common wrong catalysts were silica and alumina (confused with cracking) and potassium dichromate (an oxidising agent, not a catalyst); the November 2023 Paper 2C report noted the same errors.
Saying "a carboxylic acid", not ethanoic acid
When air oxidises ethanol (microbial oxidation) the product is specifically ethanoic acid, (or vinegar). Writing only "a carboxylic acid" is too vague and scores nothing, because a carboxylic acid could be any member of that series, whereas only ethanoic acid forms from ethanol. Name the exact product.
June 2023 Paper 2C Q3(c): "carboxylic acid" was rejected as too vague; the answer needed ethanoic acid or vinegar.
"Similar", not "same", chemical properties
Members of a homologous series have the same functional group and general formula, SIMILAR chemical properties and a TREND in physical properties (boiling point rises with chain length). Writing members have the "same chemical properties" is not creditworthy - the spec word is similar.
June 2024 Paper 2C Q5(a): "same chemical properties" was not creditworthy; credited answers were same functional group / similar chemical properties / same general formula.
Not drawing the O-H bond
A displayed formula must show every bond. For an alcohol the classic omission is writing "OH" as a block instead of -O-H with the O-H bond drawn - this loses the mark. Also give each carbon exactly 4 bonds and draw every C-H bond, and use the named isomer (propan-1-ol, butan-1-ol), not a branched one.
June 2024 Paper 2C Q5(b)(iii): displayed-formula marks were lost for not showing every bond, including the O-H bond.
Thinking hotter always speeds fermentation
Fermentation uses yeast enzymes with an optimum near 30 C. Above about 40 C the enzymes are denatured and stop working, so the rate falls - hotter is not simply faster; too cold and it is very slow. It must also be anaerobic: air/oxygen would oxidise the ethanol to ethanoic acid and spoil it.
(common Edexcel chemistry error)
Compare-question scaffold: point + reason
"Compare the two routes" wants explained points - an advantage paired with a disadvantage, each with a reason. E.g. fermentation is renewable and low-temperature but slow and impure; hydration is fast and pure but uses crude oil and high temperature/pressure.
Show every step; round only at the end
In mole, mass and yield calculations show each step - , moles, mole ratio, answer - because method marks are for the working. Keep full figures and round only at the end; rounding early (e.g. moles to 1 s.f.) shifts the answer and loses the accuracy mark.
Explain fermentation conditions with reasons
Asked why fermentation is anaerobic and near 30 C, give reasons: no air because oxygen would oxidise the ethanol to ethanoic acid, spoiling it; ~30 C is the yeast enzyme optimum - too hot and they denature. A bare list of conditions gains no marks.
Alcohols are the homologous series with the hydroxyl functional group, . You must be able to name and draw the first four, know the three ways ethanol is oxidised, and compare the two industrial methods of making ethanol.
| Name | Molecular formula | Structural formula |
|---|---|---|
| Methanol | ||
| Ethanol | ||
| Propan-1-ol | ||
| Butan-1-ol |
What is the functional group of the alcohols?
Ethanol burns completely in oxygen.
Write the balanced equation for the complete combustion of ethanol, .
