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Learn7405Paper 2 · Paper 2: Organic chemistryAlcoholsAlcohols
Alcohols · Subtopic 1 of 1

Alcohols

Primary alcohols (RCH₂OH) are oxidised by acidified K2Cr2O7\mathrm{K_2Cr_2O_7} (orange → green) to aldehydes (distil off immediately) and then to carboxylic acids (heat under reflux). Secondary alcohols are oxidised to ketones. Tertiary alcohols are not oxidised (stays orange). Other reactions: combustion; dehydration with hot concentrated H₂SO₄ or Al₂O₃ → alkene; substitution with HX, PCl₅ or SOCl₂ → halogenoalkane; sodium → alkoxide + H₂; esterification with a carboxylic acid (conc. H₂SO₄ catalyst). The triiodomethane test (I₂ + NaOH, yellow precipitate) shows a CH3CH(OH)−\mathrm{CH_3CH(OH)-} group.

Cambridge 9701 Paper 1Cambridge 9701 Paper 2Edexcel YCH11 WCH12AQA 7405 Paper 2≈ 50 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Classify alcohols and describe their oxidation.
  • Choose reflux or distillation correctly.
  • Describe dehydration, halogenation, reaction with Na and esterification.
  • Use tests to distinguish alcohols.
Pause and think

Why does the same oxidising agent give an aldehyde in one set-up and a carboxylic acid in another?

Section 2 of 14

Core idea

Start hereOrange to green

The first roadside breath tests worked because ethanol turns orange dichromate green. That one colour change is the key to telling the three classes of alcohol apart.

The big idea

What happens to an alcohol depends on how many carbons are attached to the C–OH carbon: primary alcohols oxidise twice, secondary once, tertiary not at all.

1. Oxidation ladder

Oxidation of alcohols

Distillation stops at the aldehyde; reflux goes on to the acid.

CH3CH2OH+[O]→CH3CHO+H2O\mathrm{CH_3CH_2OH+[O]\rightarrow CH_3CHO+H_2O}
where
[O]
oxygen from the oxidising agent (acidified K₂Cr₂O₇)
Worked exampleHydrogen from sodium

0.920.92 g of ethanol (MrM_r 46) reacts with excess sodium. What volume of hydrogen forms at r.t.p.?

Goal
Volume of H₂ in cm³.
Strategy
Moles of ethanol, then the 2 : 1 ratio.
  1. n=0.9246=0.020 moln=\frac{0.92}{46}=0.020\ \text{mol}
    Moles of ethanol.
  2. n(H2)=0.010 moln(\mathrm{H_2})=0.010\ \text{mol}
    2C2H5OH+2Na→2C2H5ONa+H22\mathrm{C_2H_5OH}+2\mathrm{Na}\rightarrow2\mathrm{C_2H_5ONa}+\mathrm{H_2}.
  3. V=0.010×24000=240 cm3V=0.010\times24000=240\ \text{cm}^3
    Molar volume at r.t.p.
Answer240 cm³.
✓ CheckHalf as many moles of gas as alcohol ✓.
Exam tipDistil or reflux? Say "distil off as it forms" for an aldehyde and "heat under reflux" for a carboxylic acid. Examiners check the word.
Pause and think

Why is butan-2-ol dehydrated to three different alkenes?

Summary

  • 1° → aldehyde → acid; 2° → ketone; 3° no.
  • Dehydration → alkene.
  • PCl₅ gives steamy HCl: test for –OH.
  • Iodoform: CH₃CH(OH)–.
Key words
Primary alcohol
OH carbon bonded to one other carbon.
Reflux
Heating with a vertical condenser so vapour returns to the flask.
Dehydration
Removal of water to form an alkene.
HistoryBreathalysers. The first roadside breathalysers (1950s) used the colour change of acidified dichromate from orange to green as ethanol was oxidised.
Fun factAntifreeze. Ethane-1,2-diol, an alcohol with two –OH groups, is used in car antifreeze because it lowers water's freezing point and mixes completely via hydrogen bonds.
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