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AS · Practice questions · Electrophilic addition

Draw the arrows.

Six original Cambridge-style questions on electrophilic addition to alkenes.

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

π bond to electrophile, then anion to carbocation.

01
Analysis
[4 marks]

Describe, with curly arrows, the mechanism of the reaction between ethene and bromine.

  • The π electrons induce a dipole in Br2 (Brδ+–Brδ−). ✓
  • Arrow from the C=C to Brδ+; arrow from the Br–Br bond to Brδ−. ✓
  • Carbocation CH2BrCH2+ and Br− form. ✓
  • Arrow from a lone pair on Br− to C+, giving CH2BrCH2Br. ✓
02
Explain
[2 marks]

Explain why a non-polar bromine molecule can act as an electrophile.

  • As Br2 approaches the electron-rich π bond, its electrons are repelled. ✓
  • This induces a dipole, so the nearer Br becomes δ+ and accepts the electron pair. ✓
03
Analysis
[3 marks]

Propene reacts with HBr. Name the major and minor products and explain the difference.

  • Major: 2-bromopropane; minor: 1-bromopropane. ✓
  • The major product forms via the secondary carbocation CH3CH+CH3. ✓
  • Secondary carbocations are more stable than primary because two alkyl groups donate electron density (positive inductive effect). ✓
04
Recall
[1 marks]

Rank primary, secondary and tertiary carbocations in order of increasing stability.

  • primary < secondary < tertiary. ✓
05
Analysis
[2 marks]

Explain why bromine water with ethene gives some 2-bromoethanol as well as 1,2-dibromoethane.

  • Water molecules are present in large excess and have lone pairs. ✓
  • They compete with Br− to attack the carbocation; after loss of H+ this gives BrCH2CH2OH. ✓
06
Analysis
[2 marks]

Predict the major product of but-1-ene with HBr and give a reason.

  • 2-bromobutane. ✓
  • H adds to C1, forming the more stable secondary carbocation on C2. ✓

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.