A-Level — chemistryMedium3 min read

Alcohols and Oxidation

Master alcohol classification, oxidation with acidified dichromate, dehydration, and uses of ethanol for A-Level Chemistry.

Tutor AI Team2026-02-25T12:52:43.617Z

# Alcohols and Oxidation

Alcohols contain the hydroxyl group (−OH) and are among the most versatile organic compounds. Their classification as primary, secondary, or tertiary determines their oxidation products, making this a key concept for A-Level organic chemistry.

1. Classification

Class	Structure	Carbon bonded to OH also bonded to...
Primary (1°)	RCH₂OH	1 other carbon (or 0 for methanol)
Secondary (2°)	R₂CHOH	2 other carbons
Tertiary (3°)	R₃COH	3 other carbons

2. Oxidation of Alcohols

The oxidising agent is acidified potassium dichromate ( $\text{K}_2\text{Cr}_2\text{O}_7$ / $\text{H}_2\text{SO}_4$ ). In equations, the oxidising agent is represented as $[\text{O}]$ .

The dichromate changes colour from orange to green (Cr₂O₇²⁻ → Cr³⁺) when oxidation occurs.

Primary Alcohols

Mild oxidation (distillation — product removed immediately): $\text{RCH}_2\text{OH} \xrightarrow{[O]} \text{RCHO} + \text{H}_2\text{O}$ → Aldehyde

Strong oxidation (reflux — excess oxidising agent): $\text{RCH}_2\text{OH} \xrightarrow{[O]} \text{RCHO} \xrightarrow{[O]} \text{RCOOH}$ → Carboxylic acid

Secondary Alcohols

$\text{R}_2\text{CHOH} \xrightarrow{[O]} \text{R}_2\text{CO} + \text{H}_2\text{O}$ → Ketone (cannot be further oxidised under normal conditions)

Tertiary Alcohols

Not oxidised by acidified dichromate. No colour change observed. (No hydrogen on the carbon bonded to OH, so oxidation cannot occur.)

3. Distinguishing Aldehydes and Ketones

Reagent	Aldehyde	Ketone
Tollens' reagent (AgNO₃/NH₃)	Silver mirror forms	No reaction
Fehling's solution (Cu²⁺/NaOH)	Red precipitate (Cu₂O)	No reaction
Acidified dichromate	Orange → green	No reaction

4. Dehydration of Alcohols

Alcohols can be dehydrated to form alkenes using hot concentrated $\text{H}_2\text{SO}_4$ or $\text{H}_3\text{PO}_4$ or by passing over heated $\text{Al}_2\text{O}_3$ catalyst:

$\text{CH}_3\text{CH}_2\text{OH} \xrightarrow{\text{conc. H}_2\text{SO}_4, \text{heat}} \text{CH}_2\text{=CH}_2 + \text{H}_2\text{O}$

This is an elimination reaction.

5. Combustion and Esterification

Combustion: $\text{C}_2\text{H}_5\text{OH} + 3\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O}$

Esterification: Alcohol + carboxylic acid → ester + water (acid catalyst) $\text{CH}_3\text{COOH} + \text{C}_2\text{H}_5\text{OH} \rightleftharpoons \text{CH}_3\text{COOC}_2\text{H}_5 + \text{H}_2\text{O}$

6. Production of Ethanol

Method	Fermentation	Hydration of Ethene
Equation	C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂	C₂H₄ + H₂O → C₂H₅OH
Conditions	Yeast, 25-37°C, anaerobic	H₃PO₄ catalyst, 300°C, 60 atm
Renewable?	Yes (from crops)	No (from crude oil)
Rate	Slow (batch)	Fast (continuous)
Purity	Needs distillation	High purity
Atom economy	Low (CO₂ by-product)	100%

7. Practice Questions

1. Classify butan-1-ol, butan-2-ol, and 2-methylpropan-2-ol as primary, secondary, or tertiary.
1. Write equations for the oxidation of propan-1-ol to: (a) an aldehyde, (b) a carboxylic acid.
1. How would you distinguish between propanal and propanone experimentally?
1. Write the equation for the dehydration of butan-1-ol.
1. Compare fermentation and hydration of ethene for ethanol production.

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Summary

Primary alcohols → aldehydes (distil) → carboxylic acids (reflux)
Secondary alcohols → ketones
Tertiary alcohols → no oxidation
Oxidising agent: acidified K₂Cr₂O₇ (orange → green)
Aldehydes give positive Tollens'/Fehling's tests; ketones do not
Alcohols can be dehydrated to alkenes (elimination)

Tags:

#a-level#chemistry#organic#alcohols

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