Deduce Propagation Step for Free Radical Substitution of Ethane

Let's take a look at our discussion question this week!

221031 question

We want to deduce which is a propagation step in the free radical substitution between bromine and ethane.

Check out the following video to learn more about FRS mechanism of alkane.

Let's recap the features of propagation step.

221031 propagation properties

1. The number of reactive radicals stay the same hence the reaction continues or propagates.
2. Only involves breaking 1 bond and forming 1 bond homolytically.

Here are the standard propagation steps for monosubstitution of ethane.

221031 ethane propagation

1. Bromine radical attacks ethane and takes H to form HBr and ethyl radical
2. Ethyl radical attacks bromine molecule and takes Br to form bromoethane and bromine radical

The bromine radical is regenerated to attack another ethane for another monosubstitution, or bromoethane for disubstitution.

Let's run through the options and see which is a possible propagation step.

A. CH3• + Br2 → CH3Br + Br•

221031 option A

Since we are using ethane, there is no methyl radical formed during the reaction.

Hence option A is wrong.

B. Br• + CH3CH3 → CH3Br + CH3

221031 option B

This involves breaking C-C bond which is not observed during FRS.

Hence option B is wrong.

C. CH3CHBr• + Br2 → CH3CHBr2 + Br•

221031 option C

This step is possible during disubstitution of ethane from CH3CH2Br to CH3CHBr2.

Hence option C is correct.

D. Br• + CH3CH2Br → CH3CHBr2 + H•

221031 option D

When bromine radical attacks alkane, it should take the hydrogen since hydrogen is more exposed.

Hence we should form HBr and alkyl radical as products instead.

In FRS, H radicals are not formed hence option D is wrong.

Therefore the answer to this question is option C.

221031 answer

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

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