Name the mechanism for the reaction of propene with hydrogen bromide.
[1 mark]
Total for this question: 1
Chemistry
Alkenes
AQA 3.3.4
A-level Chemistry (7405) · exam-style practice, examiner-report intelligence and the tools that drill it.
Board and spec code confirmed against AQA 7405 · registry checked 2026-07-11How this checking works
The topic on one screen
- The C=C double bond is an electron-rich pi system — that is why alkenes attract electrophiles (HBr, Br2, H2SO4).
- Electrophilic addition goes through a carbocation intermediate; stability order tertiary > secondary > primary decides the major product (Markovnikov).
- The carbocation is PLANAR — nucleophile attacks from either face, so a new chiral centre forms as a racemic mixture.
- Bromine water orange → colourless is THE test for C=C. 'Clear' is not a colour and scores nothing.
- Addition polymers: draw the repeat unit with the backbone bonds extending through the brackets.
Where students actually lose marks
On the 2023 levels-of-response mechanism question, most students could draw electrophilic addition but few reached level 3 — the missed idea was that attack on the planar secondary carbocation from above or below the plane gives BOTH optical isomers.
June 2023 Paper 2 examiner report (Q04.2)
Structures beat sentences: examiners noted it was 'much easier' to score by drawing the mechanism than describing it in words. If a question allows a diagram, draw it.
June 2023 Paper 2 examiner report (Q04.2)
Curly-arrow precision costs marks on the highest-discrimination questions: arrows from the pi bond itself (not a carbon), and the intermediate drawn with the + on the correct carbon.
June 2022 Paper 2 examiner report (Q01.8)
Try it — exam-style
Easy
ORIGINAL
Medium
ORIGINAL
But-1-ene reacts with HBr. Name the major product and explain why it forms in preference to the minor product.
[3 marks]
Total for this question: 3
Hard
ORIGINAL · after June 2023 Paper 2 Q04.2
When HBr adds to but-1-ene, the 2-bromobutane produced is a racemic mixture. Explain why.
[3 marks]
Total for this question: 3
Easy
ORIGINAL
Describe the test for a C=C double bond and the observation for a positive result.
[2 marks]
Total for this question: 2
Questions are written in the style of past AQA papers (source shown on each) — never copied from them.
Get the printable question packbrowser print viewDrill it properly
Draw the mechanism — HBr addition
The exact mechanism from the exam, marked arrow by arrow.
Draw the mechanism — bromine addition
Br2's induced dipole, drawn and marked.
Question bank: every 3.3.4 question
Filtered to this topic — quiz mode included.
Synthesis Navigator
Alkenes are the crossroads of Year 1 synthesis routes.
Stuck on alkenes?
The racemic-mixture explanation is a level-3 discriminator — I drill exactly this kind of 'why' answer in lessons. Free intro call, then a free first lesson.