4.2 Alkanes

Methane reacts with chlorine in the presence of UV light to form several products. 

Two of these products are chloromethane and hydrogen chloride. 

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The mechanism for this reaction is free radical substitution. 

Describe this mechanism for the free radical substitution of methane by chlorine. 

Also suggest why a small amount of ethane is formed in the reaction. 

Explain the effect of increasing the intensity of UV light would have on the rate of reaction between methane and chlorine.

Pentane is another saturated hydrocarbon which can undergo free radical substitution.  

Pentane undergoes free radical substitution to form 3-chloropentane.

This question is about isomers of different alkanes.

Heptane can be produced by the fractional distillation of crude oil.

There are a total of eight branched isomers of heptane, four of which are shown below. 

Explain which of the following isomers will have the highest boiling point.

• 2,3-dimethylpentane
• 3-methylhexane
• 2,4-dimethylpentane

Octane and one of its isomers are shown below:

The two isomers in part (c) can be separated in the lab. 

Name and outline a technique that could be used to separate isooctane from a mixture of octane and isooctane.

Describe the bonding and structure of octane. 

Explain how its bonding and structure affects the following properties:

• • Reactivity
  • Boiling Point 

This question is about free radical substitution. 

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

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. 

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

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.

State and explain the observations that the student would see in each test-tube and give an equation for the reaction that has occurred if applicable.

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.

Give the equations for each step of the reaction between methane and chlorine. 

Label each equation with the name of the correct step. 

You only need to provide one equation for the final step. 

Chlorofluorocarbons (CFCs) are organic compounds consisting of carbon, chlorine and fluorine atoms.

They are believed to destroy the ozone layer in a similar reaction as described in part (a).

Ozone can be broken down by radicals formed in the atmosphere. The ozone layer is important to protect the Earth from harmful exposure to too much ultraviolet light. Without this ozone layer, life on Earth would be very different.

An example of a CFC which can damage the ozone layer is CCl₃F. 

Draw the 3D structure of CCl₃F and name the shape.

Ethane is mainly used to produce ethene, which is then used by the petrochemical industry to produce a range of intermediate products, most of which are converted into plastics.

Explain in terms of bond strength and polarity why ethane is an unreactive molecule.

Excess bromine, Br₂, will react with ethane in the presence of UV light to produce bromoethane, CH₃CH₂Br.

By use of equations, explain how the product will become contaminated with CH₃CHBr₂.

Propane undergoes the same reaction as ethane in order to form 1-bromopropane, suggest two different way this reaction will become contaminated with impurities, other than those from multiple bromine propagation steps.

Isooctane which is added to petrol to increase the octane rating burns smoothly in a carengine. It is a branched chain isomer of octane, with the skeletal formula shown below. 

Write an equation for the complete combustion of isooctane.

When insufficient oxygen for the complete combustion to occur, the toxic gas carbon monoxide can be produced. Explain why carbon monoxide is dangerous if inhaled.

A diesel fuel containing the hydrocarbon tetradecane, C₁₄H₃₀, burns with a sooty yellow flame. 

Write a balanced symbol equation, including state symbols, for the reaction described.

Trichloromethane is one of the products formed when dichloromethane reacts with excess chlorine in UV light.

Write an overall equation for formation of dichloromethane from chloromethane.

A compound was found to have 20.3 % carbon, 48.1 % fluorine, 30.0 % chlorine and 1.6 % hydrogen. Calculate the empirical formula for this compound.

Suggest the structural formula of two bromine containing compounds that are formed during the reaction of bromine and dibromomethane in the presence of UV light .