Electrophilic addition mechanismsEdexcel International A Level Chemistry: Subtopic test

1. 1
   A student is learning how electrophiles such as bromine and hydrogen bromide react with the C=C bond of ethene, even though a bromine molecule is non-polar.

   (a)

   Why does a non-polar Br₂ molecule react with ethene?

   [1 mark]

   • ABr₂ is an ionic compound, so it is attracted to the double bond.
   • BThe electron-rich C=C bond induces a dipole in the Br₂ molecule as it approaches.
   • CBr₂ has a permanent dipole because the two atoms are different.
   • DEthene has a permanent positive charge that attracts Br₂.

   (b)

   Which describes the breaking of the Br–Br bond in this mechanism?

   [1 mark]

   • AHomolytic fission, with each atom taking one electron
   • BBreaking of an ionic bond into ions
   • CNothing breaks until the carbocation forms
   • DHeterolytic fission, with both electrons going to one bromine atom

   (c)

   Define the term electrophile, and identify the atom in hydrogen bromide that acts as the electrophile in its reaction with ethene.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   2-Methylpropene, (CH₃)₂C=CH₂, reacts with hydrogen bromide by a mechanism that involves a carbocation intermediate.

   (a)

   Which carbocation is the intermediate in the major pathway for the reaction of 2-methylpropene with hydrogen bromide?

   [1 mark]

   • A(CH₃)₃C⁺
   • B(CH₃)₂CHCH₂⁺
   • C(CH₃)₂CH⁺
   • DCH₃⁺

   (b)

   What is the major product of this reaction?

   [1 mark]

   • A1-Bromo-2-methylpropane
   • B2-Bromobutane
   • C2-Bromo-2-methylpropane
   • D1,2-Dibromo-2-methylpropane

   (c)

   Explain why a tertiary carbocation is more stable than a primary carbocation.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Propene reacts with hydrogen bromide to give 2-bromopropane as the major product and 1-bromopropane as the minor product. Ethene also reacts with hydrogen bromide.

   (a)

   Explain why 2-bromopropane is the major product of the reaction of propene with hydrogen bromide.

   [3 marks]

   (b)

   Describe the mechanism of the reaction of ethene with hydrogen bromide, referring to the movement of electron pairs.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student bubbles ethene into aqueous bromine, and into bromine dissolved in aqueous sodium chloride. In aqueous bromine the products include 1,2-dibromoethane and 2-bromoethanol. When sodium chloride is also present, 1-bromo-2-chloroethane is detected as well.

   (a)

   Explain how these observations provide evidence that the addition of bromine to ethene involves a carbocation intermediate rather than both bromine atoms adding at the same time.

   [6 marks]

   (b)

   Describe the mechanism of the reaction of ethene with bromine, including how the bromine molecule becomes polarised, the movement of electron pairs, the intermediate formed and the name of the mechanism.

   [6 marks]

   Total for question 4: 12 marks

End of questions