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Six original Cambridge-style questions on SN1 and SN2 mechanisms.

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

Primary SN2; tertiary SN1.

01
Analysis
[4 marks]

Describe the SN2 mechanism for the reaction of bromoethane with hydroxide ions.

  • C–Br is polar: Cδ+, Brδ−. ✓
  • Curly arrow from a lone pair on OH− to the δ+ carbon, approaching opposite Br. ✓
  • Curly arrow from the C–Br bond to Br; transition state [HO···CH2(CH3)···Br]−. ✓
  • Products: CH3CH2OH and Br−. ✓
02
Analysis
[3 marks]

Describe the SN1 mechanism for the hydrolysis of 2-bromo-2-methylpropane.

  • Step 1 (slow): C–Br breaks heterolytically; curly arrow from the bond to Br, giving (CH3)3C+ and Br−. ✓
  • Step 2 (fast): curly arrow from a lone pair on OH− to C+. ✓
  • Product: (CH3)3COH. ✓
03
Explain
[3 marks]

Explain why 2-bromo-2-methylpropane reacts by SN1 but 1-bromobutane reacts by SN2.

  • The tertiary carbocation from 2-bromo-2-methylpropane is stabilised by three electron-releasing alkyl groups. ✓
  • Its crowded carbon also blocks backside attack (steric hindrance). ✓
  • 1-bromobutane would give an unstable primary carbocation, and its carbon is unhindered, so the one-step SN2 route is preferred. ✓
04
Recall
[2 marks]

Write the rate equations for the SN1 and SN2 hydrolysis of a halogenoalkane RBr.

  • SN1: rate = k[RBr]. ✓
  • SN2: rate = k[RBr][OH−]. ✓
05
Explain
[2 marks]

Explain why SN1 hydrolysis of an optically active halogenoalkane gives a racemic product.

  • The carbocation intermediate is planar (trigonal). ✓
  • OH− can attack equally from either face, giving equal amounts of both enantiomers. ✓
06
Analysis
[2 marks]

Predict the order of rate of hydrolysis of 1-bromobutane, 2-bromobutane and 2-bromo-2-methylpropane with water, and give a reason.

  • 2-bromo-2-methylpropane > 2-bromobutane > 1-bromobutane. ✓
  • Tertiary forms the most stable carbocation, so the SN1 pathway is fastest. ✓

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.