Alkenes and geometric isomerismEdexcel International A Level Chemistry: Subtopic test

1. 1
   A student studies hydrocarbons with a C=C double bond and builds models of several of them to compare their structures.

   (a)

   Which alkene exists as E and Z isomers?

   [1 mark]

   • APropene
   • BBut-1-ene
   • C2-Methylpropene
   • DPent-2-ene

   (b)

   Which is the molecular formula of a cycloalkene with one C=C bond and six carbon atoms?

   [1 mark]

   • AC₆H₁₂
   • BC₆H₁₀
   • CC₆H₁₄
   • DC₆H₈

   (c)

   Describe how the carbon–carbon double bond in an alkene is made up of a sigma bond and a pi bond.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A chemist is explaining why some alkenes exist as pairs of isomers that cannot be interconverted at room temperature, and how these isomers are named.

   (a)

   Why do the two isomers of but-2-ene not interconvert at room temperature?

   [1 mark]

   • AThe C=C bond is shorter than a C–C bond, so the groups are too close together to move.
   • BThe two isomers have different molecular formulas.
   • CRotation about the C=C bond would need the pi bond to break, which needs more energy than is available at room temperature.
   • DThe sigma bond between the carbon atoms is too weak to hold the groups in place.

   (b)

   Which atom or group has the highest priority when assigning E or Z?

   [1 mark]

   • ABr
   • BCl
   • CCH₃
   • DH

   (c)

   Explain why pent-1-ene does not show geometric isomerism.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In an A Level class, but-2-ene and a halogenated alkene are used to illustrate geometric isomerism and the E-Z naming system.

   (a)

   But-2-ene, CH₃CH=CHCH₃, shows geometric isomerism. Explain why, and name the two isomers using the E-Z system.

   [3 marks]

   (b)

   In one isomer of 2-bromo-1-chloroprop-1-ene, CH₃CBr=CHCl, the bromine and chlorine atoms are on the same side of the double bond. Deduce whether this is the E or Z isomer, giving the priorities, and explain why cis/trans naming is unsuitable for this compound.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A teacher is preparing a lesson on the bonding in alkenes, the alkene isomers of C₅H₁₀ and the naming of geometric isomers.

   (a)

   Explain the bonding in the C=C bond of an alkene and why some alkenes show geometric isomerism.

   [6 marks]

   (b)

   Five alkene isomers of C₅H₁₀ are pent-1-ene, pent-2-ene, 2-methylbut-1-ene, 3-methylbut-1-ene and 2-methylbut-2-ene. Identify the only one that shows geometric isomerism, justify your answer, name its isomers using the E-Z system, and explain why E-Z is more widely applicable than cis/trans.

   [6 marks]

   Total for question 4: 12 marks

End of questions