Chemguide: Support for CIE A level Chemistry

Learning outcome 14: Hydrocarbons

14.1: Alkanes

Statement 14.1.3

This statement is about the mechanism of the free radical substitution reaction in alkanes and alkyl groups.

Before you go on, you should find and read the statement in your copy of the syllabus.

Organic reaction mechanisms

A reaction mechanism looks in detail at a reaction and explains exactly why and how it happens in terms of the bonds being broken and formed. I think reaction mechanisms are the most interesting and useful bits of organic chemistry.

But many students hate them! And the reason is that in the early stages of learning about organic chemistry, they (or more likely their teachers) try to do it all too quickly. Obviously, you have to learn the reaction mechanisms, but if you don't take the time to understand them as well, you will find the rest of the course really difficult.

Free radical substitution

First read the page What is free radical substitution?. This explains what substitution means and what free radicals are.

You should also make a note of the terms "homolytic fission" and "heterolytic fission". These aren't mentioned in this syllabus statement, but they are mentioned in section 13.2.

Now read the page about free radical substitution in the methane-chlorine reaction. This gives you the mechanism in a form which you can use in an exam or for revision purposes, but doesn't try to explain what is happening. So it is essential that you follow the link at the bottom of the page which says "Help! Talk me through this mechanism . . .".

Note:  This is what so often goes wrong when organic chemistry is being taught. You are given the mechanism and told to learn it. But it doesn't mean anything to you unless also have a detailed explanation of what is happening at every stage. If you really understand something, it makes learning it very easy.

It is very important that you don't leave this page until you are sure that you really understand it. It is also important to realise that there isn't anything very mysterious about the idea of a reaction mechanism. It is simply describing the sort of collisions and interactions that real particles are having in real reaction mixtures.

Now check whether you understand it by reading the page about multiple substitution in the methane-chlorine reaction. If that isn't clear to you, then you don't really understand what you have read about free radical substitution previously. Go back and read it again more slowly and carefully.

When you are happy with all this, quickly read through the corresponding set of pages about free radical substitution in the methane-bromine reaction. As you will see, it is exactly the same as the chlorine case, just replacing Cl by Br.

Free radical substitution in methyl groups

I am just including this to be on the safe side. The previous syllabus talked about substitution in methyl groups, and it has now been removed in the new syllabus. That doesn't necessarily mean that CIE won't ask a question involving a methyl group.

All the stuff you have looked at above applies as much to methyl groups as it does to methane itself.

So we will finish by looking at the mechanism for the reaction of chlorine gas in the presence of UV light with a compound CH3X.

That would include compounds like CH3CH3, CH3Cl, CH3COOH, and so on.

The mechanism for the substitution of the first hydrogen atom looks like this:

Chain initiation

Chain propagation steps

Chain termination steps

This is the clearest test of whether you really understand all this or not.

There are two possibly confusing points in the equations:

  • In the two chain propagation equations, I have had to draw the electron dot on the left of the formula in order to keep it attached to the carbon rather than the X atom or group.

  • In the final equation, I have flipped the left-hand formula over so that it is easier to see how the two radicals join together.

Apart from that you should be able to talk your way through this set of equations in such a way that you could explain them to someone else.

If you can't do that, you need to go back through all of this page again - concentrating especially on the "Help!" page in either the chlorine or bromine mechanism.

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© Jim Clark 2020