Deduce Ratio of Monosubstituted Products for Free Radical Substitution

In this JC1 webinar we want to learn how to deduce the number of monosubstituted products for FRS and the ratio of these products.

Let's take a look at our example propane, C3H8.

deduce ratio of frs pdt assume equal probability

Notice there are 2 different types of carbons in propane which we have labelled as carbon-1 and carbon-2.

Assuming the probability of substitution at each C-H bond is equal:

- substituting 6 of these hydrogens at both of carbon-1 will give 1-chloropropane
- substituting 2 of these hydrogens at carbon-2 will give 2-chloropropane

Hence the ratio of 1-chloropropane to 2-chloropropane will be 6:2 or 3:1.

Sometimes the reactivity of different types of carbons are given, so we have to take that into consideration as well.

In general the reactivity increases from primary carbon to secondary carbon to tertiary carbon.

Let's say for example the relative reactivity is:

1 for primary carbon
4 for secondary carbon
7 for tertiary carbon

deduce ratio of frs pdt consider reactivity of carbon

We then need to determine the type of carbon and multiply its probability by its reactivity.

Carbon-1 that forms 1-chloropropane is a primary carbon, so we multiply its probability 3 by reactivity 1 to give final proportion 3.

Carbon-2 that forms 2-chloropropane is a secondary carbon, so we multiply its probability 1 by reactivity 4 to give final proportion 4.

Finally the ratio of 1-chloropropane to 2-chloropropane, after taking into consideration of reactivity, will be 3:4.

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

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