Flashcards · 9620 · AS Topic 23

Alcohols (AS Level) flashcards

11 cards on AS Topic 23, subtopics 3.3.5.1 to 3.3.5.2. Pick a subtopic or study them all.

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Primary, secondary and tertiary alcohols: Classified by the number of carbons bonded to the C–OH carbon: one (primary), two (secondary), three (tertiary). Methanol, with none, counts as primary.
Oxidising agent for alcohols, and the colour change: Potassium dichromate(VI) acidified with dilute sulfuric acid: orange (Cr₂O₇²⁻) to green (Cr³⁺).
Oxidation products of primary, secondary and tertiary alcohols: Primary: aldehyde (distil as it forms), or carboxylic acid (reflux with excess oxidising agent). Secondary: ketone. Tertiary: not oxidised (stays orange).
Why is an aldehyde distilled off as it forms?: Aldehyde molecules cannot hydrogen bond to each other, so it boils below the alcohol and escapes before it is oxidised further to the carboxylic acid.
Fehling’s solution and Tollens’ reagent: results with an aldehyde and a ketone: Warm in a water bath. Fehling’s: aldehyde gives a red precipitate (Cu₂O); ketone stays blue. Tollens’: aldehyde gives a silver mirror; ketone, no change.
Why are tertiary alcohols not oxidised by acidified dichromate(VI)?: The C–OH carbon has no hydrogen atom, so C=O cannot form without breaking a C–C bond.

Reagent and conditions to eliminate water from an alcohol: Heat with a concentrated acid catalyst: concentrated H₂SO₄ or concentrated H₃PO₄. Product: an alkene + H₂O.
Mechanism of acid-catalysed dehydration of an alcohol: Lone pair on O attacks H⁺ (protonated alcohol, R–OH₂⁺); the C–O bond breaks and water leaves (carbocation); an adjacent C–H bond’s electrons form C=C and H⁺ is released.
Hexan-2-ol dehydration: which alkenes form?: Neighbours are C1 and C3. Loss of H from C1 gives hex-1-ene. Loss from C3 gives hex-2-ene, which has E and Z forms: three alkenes in total.
Why does making alkenes from alcohols matter?: The alkenes can be made into addition polymers without monomers derived from crude oil (if the alcohol itself does not come from crude oil).
Required practical 4: why is the thermometer bulb level with the side arm in distillation?: It measures the temperature of the vapour leaving, so only the fraction distilling near the product’s boiling point is collected.