Lesson 20 of 34
The carbon bearing OH determines the oxidation product
Distillation removes an aldehyde; reflux keeps it with oxidant long enough to form an acid.
A-Level 9476 (2026-2027)
I can describe alcohol reactions using ethanol.Syllabus
Syllabus 9476, 11.6(a). Describe alcohol reactions using ethanol.
I can distinguish primary, secondary and tertiary alcohols.Syllabus
Syllabus 9476, 11.6(b). Distinguish primary, secondary and tertiary alcohols.
I can interconvert alcohols and carbonyl compounds.Syllabus
Syllabus 9476, 11.7(a). Interconvert alcohols and carbonyl compounds.
I can prepare carboxylic acids by oxidation and hydrolysis.Syllabus
Syllabus 9476, 11.8(a). Prepare carboxylic acids by oxidation and hydrolysis.
The carbon bearing OH determines the oxidation product
Distillation removes an aldehyde; reflux keeps it with oxidant long enough to form an acid.
Make a guess. You are not marked.
Ethanol is heated under reflux with excess acidified potassium dichromate(VI). What is the main organic product?
Show the answer
Ethanoic acid, as the ethanal cannot escape
Under reflux, the ethanal first formed cannot escape, so it stays with the oxidant and is oxidised further to the carboxylic acid.
| Transformation | Reagents and conditions | Representative equation or product |
|---|---|---|
| Complete combustion | Oxygen and ignition. | C2H5OH + 3O2 → 2CO2 + 3H2O. |
| Substitution to halogenoalkane | HX, with suitable heating; primary alcohol with HCl often needs an activating catalyst. PCl5 also replaces OH by Cl. | C2H5OH + PCl5 → C2H5Cl + POCl3 + HCl. |
| Reaction with sodium | Sodium with the alcohol; distinguish from water contamination. | 2C2H5OH + 2Na → 2C2H5ONa + H2. |
| Oxidation to aldehyde | Acidified K2Cr2O7; heat and distil the aldehyde as it forms. | CH3CH2OH + [O] → CH3CHO + H2O. |
| Oxidation to acid | Excess acidified KMnO4 or K2Cr2O7; heat under reflux. | CH3CH2OH + 2[O] → CH3COOH + H2O. |
| Dehydration | Concentrated H3PO4 catalyst and heat. | CH3CH2OH → CH2=CH2 + H2O. |
| Alcohol class | Hydrogen on OH-bearing carbon? | Usual oxidation result |
|---|---|---|
| Primary, RCH2OH | Two. | Aldehyde, then carboxylic acid if oxidation continues. |
| Secondary, R2CHOH | One. | Ketone; for example propan-2-ol → propanone. |
| Tertiary, R3COH | None. | Resists these mild oxidation conditions; oxidising further would require carbon-carbon bond cleavage. |
Acidified dichromate(VI) turns from orange to green as it is reduced. Acidified manganate(VII) loses its purple colour as Mn2+ forms. Either colour change shows that oxidation happened. It does not tell a primary alcohol from a secondary one. To identify the product, use a carbonyl test and, if needed, a test that only aldehydes pass.
In distillation, the aldehyde boils off as it forms and leaves the oxidising mixture, so it is not oxidised further. In reflux, the vapour condenses and drips back into the flask, so you can heat for a long time without losing volatile substances. Neither word names a reagent, so write both the oxidising agent and the apparatus condition.
Aldehydes are reduced to primary alcohols, and ketones to secondary alcohols. Use LiAlH4 in dry ether followed by water, or H2 with a nickel catalyst. For example, CH3COCH3 + 2[H] → CH3CH(OH)CH3. The carbon chain stays the same.
Check your understanding
Butan-1-ol, butan-2-ol and 2-methylpropan-2-ol are each warmed with acidified K2Cr2O7, and any product is distilled off at once. Each distillate is warmed with Tollens' reagent. Which result is correct?
Show the answer
Butan-1-ol: orange to green, and its distillate gives a silver mirror
Butan-1-ol is primary. It is oxidised to butanal, and distilling removes the aldehyde before further oxidation. Butanal reduces Tollens' reagent to silver.