Edexcel IAL Chemistry 4.18 Free-radical substitution mechanism

Show ultraviolet light and fish-hook arrows for bond fission, write two charge-balanced propagation equations and check that termination consumes two radicals.

Exam points

Draw initiation by homolytic fission with ultraviolet light and fish-hook arrows.
Write propagation equations that form haloalkane and regenerate a radical.
Identify termination equations in which two radicals combine with no radical product.

[Maximum number: 4]

Many cars use fuels containing ethanol as well as hydrocarbons. The E numbers of these fuels show the percentage by volume of ethanol in the fuel. For example, an E5 fuel contains 5\% ethanol and 95\% hydrocarbons by volume.

(a)

Hexane will react with chlorine in the presence of ultraviolet light to form a number of products.

[ 4 ]

(i)

Describe the role of the ultraviolet light in the initiation step of this reaction.

[ 2 ]

(ii)

The alkane dodecane, $\mathrm{C}_{12} \mathrm{H}_{26}$ , is formed in this reaction.

Write equations for the propagation step and for the termination step for the formation of this product.
Use molecular formulae in your equations.
Propagation step

Termination step

[ 2 ]

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4.18—The mechanism of the free radical substitution reaction between an alkane and a halogen: i using free radicals, which are question 1

Question (a)

Question (i)

Question (ii)