Alkenes: structure and electrophilic additionEdexcel A-Level Chemistry: Subtopic test

1. 1
   Margarine is made from vegetable oils. The oil molecules contain several C=C double bonds, which is why the oils are liquid at room temperature. The oil is hydrogenated by reacting it with hydrogen gas in the presence of a catalyst, so that it becomes a spreadable solid.

   (a)

   Which statement correctly describes the C=C double bond in the oil molecules?

   [1 mark]

   • AIt consists of one sigma bond and one pi bond
   • BIt consists of two sigma bonds
   • CIt consists of two pi bonds
   • DIt consists of one sigma bond and two pi bonds

   (b)

   Which catalyst is used for the hydrogenation of vegetable oils?

   [1 mark]

   • AIron
   • BVanadium(V) oxide
   • CPhosphoric acid
   • DNickel

   (c)

   One type of oil molecule has four C=C double bonds. Calculate the volume of hydrogen, at room temperature and pressure, needed to hydrogenate 0.0250 mol of this oil until it has no C=C bonds left. (1 mol of gas occupies 24.0 dm³ at room temperature and pressure.)

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Propene, CH₃CH=CH₂, reacts with hydrogen bromide at room temperature by electrophilic addition. The product is mainly 2-bromopropane.

   (a)

   Which statement defines an electrophile?

   [1 mark]

   • AA species that donates a pair of electrons to form a covalent bond
   • BA species that accepts a pair of electrons to form a covalent bond
   • CA species that has an unpaired electron
   • DA species that always carries a positive charge

   (b)

   What happens to the H–Br bond in the first step of the mechanism?

   [1 mark]

   • AIt breaks by homolytic fission, forming a hydrogen radical and a bromine radical
   • BIt breaks with both electrons going to hydrogen, forming H⁻ and Br⁺
   • CIt breaks by heterolytic fission, with both electrons going to bromine
   • DIt does not break until the second step

   (c)

   Explain why 2-bromopropane is the major product of this reaction.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A technician has two colourless liquids, cyclohexane and cyclohexene, which both contain six carbon atoms. She tests both with bromine water and then plans to react the cyclohexene with liquid bromine.

   (a)

   Describe a chemical test that distinguishes cyclohexene from cyclohexane, including the observation with each liquid.

   [3 marks]

   (b)

   Explain how the non-polar bromine molecule reacts with cyclohexene to form 1,2-dibromocyclohexane. Describe the mechanism in words, including the movement of electrons.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemist studies the addition reactions of two alkenes: propene, CH₃CH=CH₂, and 2-methylpropene, (CH₃)₂C=CH₂.

   (a)

   2-Methylpropene reacts with hydrogen bromide to form mainly 2-bromo-2-methylpropane. Explain, with reference to the mechanism, why this is the major product.

   [6 marks]

   (b)

   Propene is converted into three different compounds: (i) by hydrogen with a nickel catalyst, (ii) by steam with an acid catalyst, and (iii) by acidified potassium manganate(VII). Name the major organic product in each case, state the observation in (iii), and explain why all three are addition reactions.

   [6 marks]

   Total for question 4: 12 marks

End of questions