Alkanes (Cambridge (CIE) AS Chemistry)

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.

Crude oil contains a mixture of hydrocarbons which are separated using fractional distillation. 

Further processes can then be carried out, including cracking. 

i) What is meant by the term 'cracking'. 

[1]

ii) Explain why cracking is carried out.

[1]

i) State two conditions required for the cracking of alkanes. 

[2]

ii) Construct a balanced equation for the formation of heptane, C₇H₁₆ and two other products, by cracking tetradecane, C₁₄H₃₀.

[1]

Another alkane, C₈H₁₈ is used in gasoline. It undergoes combustion to produce a mixture of products including carbon monoxide, oxides of nitrogen and unburnt hydrocarbons. 

i) Explain why carbon monoxide is dangerous if inhaled.

[2]

ii) Describe the environmental effect of oxides of nitrogen and unburnt hydrocarbons.

[2]

Catalytic converters are found in the exhaust system of cars. 

Use equations to show how a catalytic converter reduces the levels of carbon monoxide and oxides of nitrogen released into the atmosphere.

Alkanes are generally unreactive and do not react with acids, bases, or with oxidising or reducing agents. However, they will react with halogens under suitable conditions, to form halogenoalkanes.

Methane reacts with chlorine in this way to form chloromethane.

i) Deduce the type of 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.  

Explain the observations that would be observed in each test tube and give an equation for the reaction that has occurred if applicable.

[4]

The reaction described in part (a) consists of three steps. The first step is the initiation step in which the Cl-Cl bond is broken to form two chlorine free radicals.

i) Explain the type of bond breaking that occurs in the initiation step. 

[2]

 ii) Define the term free radical. 

[1]

iii) Explain why the C-H bond in the alkane does not break in the initiation step instead of the Cl-Cl bond. 

[1]

Name each step of the reaction between methane and chlorine as described in part (a). Include relevant equations for each step.

This question is about free radical substitution. 

1,2-dibromoethane reacts with bromine in UV light to produce a mixture of further substituted haloalkanes. 

i) Write an equation for the initiation step. 

[1]

ii) Explain why this is an example of homolytic fission. 

[1]

Write two equations showing the propagation of this chain reaction to produce 1,1,2-tribromoethane. 

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

i) Write an equation to show how this product is formed. 

[1]

ii) Write a balanced symbol equation to show the overall reaction between 1,1,2-tribromoethane with bromine in UV light to form hexabromoethane. 

[1]

Using the information from this question and your own knowledge, suggest the limitations of free radical substitution.