Structure and electrophilic addition reactions of alkenesAQA A-Level Chemistry: Subtopic test

1. 1
   Propene, CH₃CH=CH₂, is an unsaturated hydrocarbon. A student shakes a sample of propene gas with bromine water, and in a separate experiment reacts propene with bromine in an inert solvent.

   (a)

   Why are alkenes attacked by electrophiles?

   [1 mark]

   • AThe C=C bond is a region of low electron density
   • BThe carbon atoms in the C=C bond carry a partial positive charge
   • CAlkenes contain no electrons available for bonding
   • DThe C=C bond is a centre of high electron density

   (b)

   What is observed when propene is shaken with bromine water?

   [1 mark]

   • AThe solution stays orange
   • BThe orange solution becomes colourless
   • CThe orange solution becomes green
   • DA white precipitate forms

   (c)

   Write an equation for the reaction of propene with bromine, using structural formulae, and name the organic product.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Carbocations are intermediates in electrophilic addition reactions of alkenes. They are classified as primary, secondary or tertiary according to the number of alkyl groups attached to the positively charged carbon atom.

   (a)

   What type of carbocation is (CH₃)₂CH⁺?

   [1 mark]

   • APrimary
   • BTertiary
   • CSecondary
   • DQuaternary

   (b)

   Which carbocation is the most stable?

   [1 mark]

   • A(CH₃)₃C⁺
   • B(CH₃)₂CH⁺
   • CCH₃CH₂CH₂⁺
   • DCH₃⁺

   (c)

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

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Alkenes react with electrophiles such as bromine and concentrated sulfuric acid. A student is asked to describe the mechanisms of these reactions using ethene as the example.

   (a)

   Describe the mechanism for the reaction of ethene with bromine, referring to the movement of electron pairs.

   [3 marks]

   (b)

   Ethene reacts with cold concentrated sulfuric acid. Write an equation using structural formulae and name the organic product. Describe the first step of the mechanism.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemist reacts propene with hydrogen bromide. Two isomeric bromopropanes form, but one is produced in a much greater amount than the other.

   (a)

   Explain, with reference to the mechanism and carbocation intermediates, why 2-bromopropane is formed as the major product.

   [6 marks]

   (b)

   4.20 g of propene reacts with excess hydrogen bromide. The overall yield of bromopropanes is 80%, and 2-bromopropane and 1-bromopropane are formed in a 9 : 1 ratio by moles. Calculate the mass of each product formed. (Ar: C = 12.0, H = 1.0, Br = 79.9.)

   [6 marks]

   Total for question 4: 12 marks

End of questions