Alkanes (Cambridge (CIE) A Level Chemistry): Exam Questions

A reaction scheme involving cyclohexane is shown in Fig. 1.1.

Fig. 1.1.

Reaction 1 involves a free radical substitution mechanism.

State the essential condition required for reaction 1 to occur.

Complete Table 1.1 to give details of the mechanism in reaction 1. Include curly arrows to show the movement of electrons occurring in the termination step.

Table 1.1

Deduce the type of reaction that occurs in reaction 2.

Hex-3-ene is an isomer of cyclohexane. Hex-3-ene can be converted into propanoic acid.

hex-3-ene  propanoic acid

Deduce the reagents and conditions for reaction 3.

i) Explain what is meant by the term cracking.

[1]

ii) Suggest why cracking is a useful process in the petroleum industry.

[1]

i) State the reagent and conditions required for the cracking of alkanes.

[2]

ii) Construct an equation for the cracking of tetradecane, C₁₄H₃₀, to form heptane, C₇H₁₆, and two other organic products.

[1]

The combustion of octane, C₈H₁₈, in internal combustion engines produces a mixture of pollutants, including carbon monoxide, nitrogen monoxide and unburnt hydrocarbons.

i) State one adverse effect of carbon monoxide on human health.

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ii) State one environmental consequence of releasing nitrogen monoxide / oxides of nitrogen into the atmosphere.

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iii) State one environmental consequence of releasing unburnt hydrocarbons into the atmosphere.

[1]

Construct an equation to demonstrate how a catalytic converter removes carbon monoxide and nitrogen monoxide from exhaust gases.

Methane reacts with chlorine gas under suitable conditions to form chloromethane.

i) Name the mechanism that occurs in this reaction.

[1]

ii) Two test tubes are set up, each containing a small amount of methane and pale green chlorine gas. One test tube is left in the dark, whereas the other is placed in direct sunlight.

State and explain the observations in each test tube.

[3]

iii) Construct an equation for the overall reaction that occurs in the sunlight.

[1]

The mechanism for this reaction involves an initiation step.

i) Name the type of bond breaking that occurs in the initiation step. Explain what happens to the electrons in the bond.

[2]

ii) Explain what is meant by the term free radical.

[1]

iii) Suggest why the Cl–Cl bond is broken in the initiation step rather than the C–H bond.

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The mechanism also involves propagation and termination steps.

i) Construct two equations to show the propagation steps in this mechanism.

[2]

ii) Construct an equation to show one possible termination step.

[1]

1,2-dibromoethane reacts with bromine in the presence of ultraviolet light to produce a mixture of further substituted halogenoalkanes.

i) Construct an equation for the initiation step of this reaction.

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ii) Explain why the bond breaking in the initiation step is described as homolytic fission.

[1]

Construct two equations to show the propagation steps in this mechanism that produce 1,1,2-tribromoethane.

Traces of 1,2,3,4-tetrabromobutane are found in the reaction mixture.

i) Construct an equation for the termination step that forms 1,2,3,4-tetrabromobutane.

[1]

ii) Construct an equation to show the overall reaction for the formation of hexabromoethane from 1,1,2-tribromoethane.

[1]

Suggest two reasons why free-radical substitution is not a good method for producing a single, pure halogenoalkane.