Alkenes

Sample questions

Original practice questions written in the style of the exam.

1. Question 12 marks

   Describe a chemical test to show that a liquid hydrocarbon is an alkene rather than an alkane. Give the reagent and the result.

   Mark scheme

   • Add bromine water (1)
   • Colour changes from orange (or yellow/brown) to colourless with the alkene; no change with the alkane (1)
2. Question 24 marks

   Propene reacts with hydrogen bromide to form 2-bromopropane as the major product.

   Describe the mechanism of this reaction, stating where each curly arrow starts and ends, and explain why 2-bromopropane rather than 1-bromopropane is the major product.

   Mark scheme

   • Curly arrow from the C=C double bond to the H of HBr, and curly arrow from the $\text{H}{-}\text{Br}$ bond to the Br (with $\text{H}^{\delta+}{-}\text{Br}^{\delta-}$ dipole) (1)
   • Secondary carbocation intermediate $\text{CH}_3\text{C}^+\text{HCH}_3$ and $\text{Br}^-$ formed (1)
   • Curly arrow from a lone pair on $\text{Br}^-$ to the positive carbon (1)
   • The secondary carbocation is more stable than the primary one because it has more electron-releasing alkyl groups, so it forms in preference (1)