2019 A Level H2 Chemistry Paper 1 Question 18 - Deduce Free Radical Substitution Products

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Let's take a look at question 18 for 2019 A Level H2 Chemistry Paper 1.

2019 p1 q18 question

Propane undergoes free radical substitution with chlorine and we want to predict the possible products.

We need to be familiar with this mechanism to deduce the products quickly.

I have a previous video lesson on free radical substitution mechanism for those who are interested.

Let's go through each compounds and see if they can be formed.

1. CH3CH2CH2Cl, 1-chloropropane

2019 p1 q18 1 chloropropane

This is a monosubstituted product which can be formed in the propagation step of FRS.

2. CH2ClCH2CH2Cl, 1,3-dichloropropane

2019 p1 q18 13 dichloropropane

This is a disubstituted product which can also be formed in the propagation step.

Remember free radical substitution is a random process and multiple substitution at any carbon is possible.

3. CH3CH2CH2CH2CH2CH3, hexane

2019 p1 q18 hexane

Hexane can be formed when a propyl radical meets another propyl radical and forms a carbon-carbon bond in the termination step.

2019 p1 q18 form c c bond in termination step

Since the probability of termination step is very low, hexane is found in trace amounts.

4. CH3CH2CH2CH2CH3, pentane

2019 p1 q18 pentane

We can only form pentane from propane if a propyl (3C) radical meets an ethyl (2C) radical.

Since carbon-carbon bonds are not broken during free radical substitution, ethyl radicals cannot be formed.

This means it's not possible to form pentane.

In fact the alkane products that can be formed must be a multiple of the starting unit.

If propane is the starting alkane, the alkane products that can be formed will be a multiple of 3-carbon, ie 6 carbon, 9 carbon, 12 carbon and so on.

Finally we can conclude that only compounds 1, 2 and 3 are possible products.

Hence the answer to this question will be option A.

Topic: Alkanes, Organic Chemistry, A Level Chemistry, Singapore

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