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A2 · Practice questions · Alcohols: further reactions

Rank the acids.

Six original Cambridge-style questions on the acidity of alcohols and their reaction with acyl chlorides.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Anion stability decides; acyl chlorides go to completion.

01
Explain
[3 marks]

Explain why ethanol is a weaker acid than water.

  • Both lose H+ from an O–H bond, forming C2H5O− and OH−. ✓
  • The ethyl group is electron-donating, so it increases the negative charge density on the O of the ethoxide ion. ✓
  • This makes C2H5O− less stable (more ready to accept H+ back) than OH−, so ethanol ionises less. ✓
02
Calculation
[2 marks]

The pKa of water is 15.7 and that of phenol is 10.0. Calculate Ka for each and the ratio Ka(phenol) : Ka(water).

  • Ka(water) = 10−15.7 = 2.00 × 10−16 mol dm−3; Ka(phenol) = 10−10.0 = 1.00 × 10−10 mol dm−3. ✓
  • Ratio = 105.7 = 5.01 × 105: phenol is about half a million times more acidic. ✓
03
Recall
[2 marks]

Write an equation for the reaction of propan-2-ol with ethanoyl chloride and name the organic product.

  • CH3COCl + CH3CH(OH)CH3 → CH3COOCH(CH3)2 + HCl. ✓
  • The product is 1-methylethyl ethanoate (isopropyl ethanoate). ✓
04
Analysis
[3 marks]

Give two advantages of making ethyl ethanoate from ethanoyl chloride rather than ethanoic acid, and one disadvantage.

  • Advantage: the reaction is fast at room temperature, with no catalyst or heating. ✓
  • Advantage: it is not reversible, so the yield is much higher. ✓
  • Disadvantage: ethanoyl chloride is expensive, reacts violently with water and releases corrosive HCl fumes, so it must be handled in a fume cupboard. ✓
05
Explain
[2 marks]

Sodium ethoxide solution is a stronger base than sodium hydroxide solution of the same concentration. Explain this using the acidities of ethanol and water.

  • Ethanol is a weaker acid than water, so its conjugate base (ethoxide) holds on to H+ more strongly. ✓
  • So C2H5O− is a stronger base than OH− (the weaker the acid, the stronger its conjugate base). ✓
06
Analysis
[2 marks]

An ester C6H5COOCH3 is needed. Name the acyl chloride and alcohol used, and write the equation.

  • Benzoyl chloride and methanol. ✓
  • C6H5COCl + CH3OH → C6H5COOCH3 + HCl (methyl benzoate). ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.