Topic 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)
The carbon bearing OH determines the oxidation product
Distillation removes an aldehyde; reflux keeps it with oxidant long enough to form an 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) changes from orange to green as it is reduced; acidified manganate(VII) loses its purple colour as Mn2+ forms. The colour change indicates oxidation has occurred but does not alone distinguish a primary from a secondary alcohol. Identify the product using a carbonyl test and, where needed, an aldehyde-specific test.
Distillation allows a volatile aldehyde to leave the oxidising mixture, limiting further oxidation. Reflux condenses vapour back into the flask, allowing prolonged heating without losing volatile reactants. Neither term itself identifies a reagent: write both the oxidant and the apparatus condition.
Aldehydes reduce to primary alcohols and ketones to secondary alcohols using LiAlH4 in dry ether followed by aqueous work-up, or H2/Ni under suitable hydrogenation conditions. For example, CH3COCH3 + 2[H] → CH3CH(OH)CH3. The carbon skeleton is retained.