Flashcards · 7405 · AS Topic 23

Alcohols (AS Level) flashcards

16 cards on AS Topic 23, subtopics 3.3.5.1 to 3.3.5.3. Pick a subtopic or study them all.

Study mode needs JavaScript. Every card is listed below, so you can still cover the answers and test yourself.

All 16 cards

For a quick read-through or printing. Cover the right-hand side to test yourself.

Hydration of ethene: equation and conditions: CH₂=CH₂ + H₂O → CH₃CH₂OH. Steam, about 300 °C, high pressure, acid catalyst such as phosphoric acid on a solid support.
Fermentation of glucose: equation and conditions: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. Yeast enzymes, around 35 °C, anaerobic, aqueous.
Fermentation temperature: Too cold → slow; too hot → yeast enzymes denature. About 35 °C is a compromise.
Why must fermentation be done without air?: Oxygen would oxidise ethanol to ethanoic acid, and yeast would respire aerobically to CO₂ and water.
Why is bioethanol only "carbon neutral" on paper?: 6 CO₂ are absorbed in photosynthesis and 6 released by fermentation (2) and combustion (4). But fossil fuels used for farm machinery, fertiliser, distillation and transport release more CO₂.

Primary, secondary and tertiary alcohols: The carbon bonded to –OH is bonded to one, two or three other carbons respectively (methanol counts as primary).
Oxidising agent for alcohols and its colour change: Acidified potassium dichromate(VI), K₂Cr₂O₇ / H₂SO₄: orange → green (Cr₂O₇²⁻ → Cr³⁺).
Primary alcohol: how do you get the aldehyde rather than the carboxylic acid?: Aldehyde: distil it off as it forms (it has no hydrogen bonding, so it boils lowest). Carboxylic acid: heat under reflux with excess dichromate.
Products of oxidising a secondary and a tertiary alcohol: Secondary: a ketone. Tertiary: no reaction (stays orange), because there is no H on the carbon bonded to –OH.
Equation for the oxidation of propan-1-ol to propanal: CH₃CH₂CH₂OH + [O] → CH₃CH₂CHO + H₂O
Tollens’ and Fehling’s results with an aldehyde: Tollens’: silver mirror. Fehling’s: blue → red precipitate. A ketone gives no change with either. Warm in a water bath.

Dehydration of an alcohol: reagent and conditions: Heat with concentrated sulfuric acid or concentrated phosphoric acid. An alkene and water form.
Outline the three stages of the mechanism for alcohol elimination: A lone pair on O takes H⁺, forming R–OH₂⁺; the C–O bond breaks and H₂O leaves, giving a carbocation; electrons from an adjacent C–H bond form C=C and H⁺ is released.
Which alkenes form when 2-methylbutan-2-ol is dehydrated?: 2-methylbut-1-ene and 2-methylbut-2-ene. Neither shows E–Z isomerism.
Why is making ethene from ethanol useful?: Ethanol from fermentation → ethene → poly(ethene), so addition polymers can be made without crude oil.
Required practical 5: purifying cyclohexene from the crude distillate: Shake with sodium carbonate solution in a separating funnel (release CO₂), discard the aqueous layer, wash with water, dry with anhydrous CaCl₂ or MgSO₄, then redistil.