Structural isomerism and E-Z isomerismAQA A-Level Chemistry: Subtopic test

1. 1
   A refinery chemist is analysing the C₅ fraction of a petroleum sample. The fraction is a mixture of alkanes with the molecular formula C₅H₁₂, which are blended into petrol.

   (a)

   How many structural isomers of C₅H₁₂ exist?

   [1 mark]

   • A2
   • B3
   • C4
   • D5

   (b)

   Which name correctly describes the isomer of C₅H₁₂ that has a straight chain of four carbon atoms with one methyl group on the second carbon?

   [1 mark]

   • A3-methylbutane
   • B1-methylbutane
   • C2-methylbutane
   • D2-ethylpropane

   (c)

   Explain why pentane and 2-methylbutane are structural isomers of each other.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student is comparing four alkenes: but-1-ene, CH₂=CHCH₂CH₃; but-2-ene, CH₃CH=CHCH₃; 2-methylpropene, (CH₃)₂C=CH₂; and propene, CH₃CH=CH₂.

   (a)

   Which of these alkenes shows E-Z isomerism?

   [1 mark]

   • ABut-1-ene
   • B2-Methylpropene
   • CPropene
   • DBut-2-ene

   (b)

   Which statement explains why E-Z isomers exist?

   [1 mark]

   • ARotation about the C=C bond is restricted by the π bond
   • BThe C=C bond is polar
   • CThe carbon atoms of the C=C bond are chiral centres
   • DThe C=C bond is longer than a C–C bond

   (c)

   In one isomer of but-2-ene, both methyl groups are on the same side of the double bond. Use the CIP priority rules to explain why this isomer is the Z isomer.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   But-1-ene, CH₂=CHCH₂CH₃, is an alkene used to make polymers. Its molecular formula, C₄H₈, is shared by several other compounds.

   (a)

   Explain why but-1-ene does not show E-Z isomerism but but-2-ene does.

   [3 marks]

   (b)

   Give the structural formula of one chain isomer, one position isomer and one functional group isomer of but-1-ene. State the type of isomer for each, and explain why all four compounds are structural isomers.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   The molecular formula C₄H₁₀O is shared by at least five compounds, including butan-1-ol, butan-2-ol, 2-methylpropan-1-ol, 2-methylpropan-2-ol and ethoxyethane, CH₃CH₂OCH₂CH₃. Separately, compound Q, CH₃(Br)C=C(Cl)CH₂CH₃, is named 2-bromo-3-chloropent-2-ene and exists as two stereoisomers.

   (a)

   Explain, using the isomers of C₄H₁₀O as examples, the three types of structural isomerism.

   [6 marks]

   (b)

   Explain why compound Q exists as two stereoisomers. Use the CIP priority rules to deduce which isomer has the bromine atom and the ethyl group on the same side of the double bond, and justify your answer.

   [6 marks]

   Total for question 4: 12 marks

End of questions