AS Chemistry: Alcohols — Revision Notes

Condensed for the final weeks. For the full explanation, use the Alcohols: Reactions and Oxidation study guide.

Classification decides everything

Class	Carbon bearing OH is attached to	Oxidation product
Primary	1 carbon	Aldehyde → then carboxylic acid
Secondary	2 carbons	Ketone (and no further)
Tertiary	3 carbons	Not oxidised

Identify the class first — every question in this topic follows from it.

Oxidation conditions

Oxidising agent: acidified potassium dichromate(VI), K₂Cr₂O₇/H₂SO₄. Orange → green when oxidation occurs.

PRIMARY, DISTIL immediately     ->  ALDEHYDE
        (removes the aldehyde before it oxidises further)

PRIMARY, REFLUX with excess     ->  CARBOXYLIC ACID
        (keeps it in the flask so oxidation completes)

SECONDARY, DISTIL               ->  KETONE
        (the syllabus method; the ketone is not oxidised
         further even if the mixture is refluxed)

TERTIARY                        ->  no reaction, stays ORANGE

Distil vs reflux is the whole answer to “how would you obtain the aldehyde rather than the acid?” — distil, so the product escapes as soon as it forms.

Distinguishing tests

Test	Aldehyde	Ketone
Tollens’ (ammoniacal AgNO₃)	Silver mirror	No change
Fehling’s / Benedict’s	Blue → brick-red precipitate	No change
Acidified dichromate	Orange → green	No change

Both Tollens’ and Fehling’s are oxidising agents — they work because aldehydes can be oxidised further and ketones cannot.

Tertiary alcohol test: no colour change with dichromate, distinguishing it from primary and secondary.

Six routes to an alcohol

Three connect back to classes already studied:

ALKENE + steam, H3PO4 catalyst        ->  alcohol  (hydration)
ALKENE + cold dilute acidified KMnO4  ->  diol
HALOGENOALKANE + NaOH(aq), reflux     ->  alcohol

The other three belong to functional groups met later in AS:

ALDEHYDE/KETONE + NaBH4 (or LiAlH4)   ->  alcohol  (reduction)
CARBOXYLIC ACID + LiAlH4 only         ->  alcohol  (NaBH4 too weak)
ESTER + hydrolysis                    ->  alcohol + acid

Worked example. Butan-2-ol, CH₃CH(OH)CH₂CH₃, is heated under reflux with excess acidified K₂Cr₂O₇. Identify the product and explain why oxidation stops there.

-OH is on C2, bonded to TWO other carbons (C1, C3) -> SECONDARY

Two H atoms removed from C2 (one O-H, one C-H) -> C=O bond forms
Product: butanone, CH3COCH2CH3

No H remains on the carbonyl carbon -> nothing left to remove
-> oxidation stops at the ketone regardless of reflux time

Acidity of alcohols vs water

Alcohols are weaker acids than water. The alkyl group attached to oxygen is electron-donating (inductive effect), which does two things together: it makes the O–H hydrogen less easily released as H⁺, and it makes the resulting alkoxide ion (RO⁻) less stable than hydroxide (OH⁻), since the electron-donating effect concentrates negative charge onto an already electron-rich oxygen rather than dispersing it. Both effects push the equilibrium away from ionisation, so alcohols are the weaker acid.

Other reactions

+ Na               ->  alkoxide + hydrogen        (effervescence)
+ conc. H2SO4/H3PO4, 170 C  ->  ALKENE + water    (dehydration/elimination)
+ carboxylic acid, conc. H2SO4 catalyst -> ESTER + water (esterification)
+ PCl5             ->  chloroalkane + HCl + POCl3  (misty fumes: test for OH)

Ethanol production — the two routes

Fermentation is prior (IGCSE) knowledge, not itself an AS 9701 syllabus point — 16.1.1 names six production routes for alcohols and fermentation is not one of them. It is included below only as useful context for comparing with hydration of ethene, which is the AS syllabus route; no AS marks are awarded for fermentation recall alone.

	Fermentation (prior knowledge — not AS-examinable)	Hydration of ethene (AS syllabus route)
Feedstock	Sugars (renewable)	Ethene (from crude oil, finite)
Conditions	30–40 °C, yeast, anaerobic	300 °C, 60 atm, H₃PO₄
Rate	Slow, batch	Fast, continuous
Purity	Impure, needs distilling	High
Carbon	Approximately neutral	Not neutral

Exam traps

• Writing “reflux” when the aldehyde is wanted — that gives the acid.
• Saying tertiary alcohols oxidise slowly; they do not oxidise.
• Forgetting the colour change is orange → green (Cr₂O₇²⁻ → Cr³⁺).
• Confusing Tollens’ (silver mirror) with Fehling’s (brick-red).
• Dehydration requires concentrated acid and heat, not dilute.
• Using NaBH₄ to reduce a carboxylic acid — it is not a powerful enough reducing agent; only LiAlH₄ can do this.
• Explaining alcohols’ weaker acidity than water using only “alkyl groups are electron-donating” without going on to link that to both hydrogen release and alkoxide-ion stability.

Self-test

1. How would you obtain an aldehyde rather than a carboxylic acid from a primary alcohol?
2. What colour change confirms oxidation with acidified dichromate?
3. Which test distinguishes an aldehyde from a ketone, and what is seen?
4. Why can tertiary alcohols not be oxidised?
5. Give the conditions for dehydrating an alcohol.
6. Name three of the six routes by which an alcohol can be prepared.
7. Explain why alcohols are weaker acids than water.

Answers: 1. Warm with acidified potassium dichromate and distil off the aldehyde as it forms, before it can be oxidised further. 2. Orange to green. 3. Tollens’ reagent — a silver mirror forms with an aldehyde, nothing with a ketone (or Fehling’s, brick-red precipitate). 4. The carbon bearing the OH group has no hydrogen atom attached, and oxidation requires removal of that C–H hydrogen. 5. Concentrated sulfuric or phosphoric acid at about 170 °C, or pass the alcohol vapour over a heated Al₂O₃ catalyst. 6. Any three: hydration of an alkene (steam, H₃PO₄); cold dilute acidified KMnO₄ with an alkene (diol); a halogenoalkane with aqueous NaOH under reflux; reduction of an aldehyde/ketone (NaBH₄ or LiAlH₄); reduction of a carboxylic acid (LiAlH₄ only); hydrolysis of an ester. 7. The alkyl group is electron-donating, making the O–H hydrogen less easily released as H⁺ and making the resulting alkoxide ion less stable than hydroxide, since the electron-donating effect concentrates negative charge on an already electron-rich oxygen; both effects push the equilibrium away from ionisation.

For the full explanation, including the iodoform test, see the Alcohols: Reactions and Oxidation study guide; for exam-style practice with full worked answers, see the Alcohols practice questions.