Chemguide: Support for CIE A level Chemistry

Learning outcome 15: Halogen compounds

15.1: Halogenoalkanes

Statements 15.1.5 and 15.1.6

These statements cover the mechanisms for nucleophilic substitution in the halogenoalkanes.

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

There is a lot of detail in this. In principle, you could be asked for the mechanism for any of the substitution reactions that you have come across in statement 15.1.3. There are also two different mechanisms depending on what sort of halogenoalkane you have got - primary, secondary or tertiary.

That's a lot of possible mechanisms you could be asked on this one statement.

Trying to learn all of this reaction-by-reaction would be hopeless! You have to understand what you are doing. As long as you can understand it, you can work out what happens in all of these cases (and in any similar cases) if you have to.

Important background work

First, if you are at all uncertain about primary, secondary or tertiary halogenoalkanes, start by reading the page halogenoalkanes in the mechanisms section of Chemguide. Unless you feel completely comfortable with these terms, you won't understand important bits of the rest of this topic.

Now read the most important page of all, about nucleophilic substitution. This is a general introduction.

CIE expect you to know about both SN1 and SN2 reactions - so you will need everything on this page.

The page mentions the term "carbocations" in several places, and in particular the relative stabilities of primary, secondary, and tertiary carbocations. You will find this discussed in more detail on this page about carbocations.

CIE may also ask about the energy profiles for SN1 and SN2 reactions.

These are different, because one goes through an intermediate, and the other through a transition state. You will find this discussed on the page energy profiles for simple reactions in the Rates of Reaction section of Chemguide. You will find the examples used on that page involve halogenoalkanes.

Leaving groups

When a bromoalkane undergoes nucleophilic substitution, whichever mechanism is used, a bromide ion is pushed out of the organic compound. The bromide ion is described as the "leaving group".

Reactions involving iodoalkanes are faster. In these, iodide ions are pushed out, and the iodide ion is a leaving group. Iodide ions are described as better leaving groups than bromide ions.

How good a particular halogen is as a leaving group depends on the strength of the carbon-halogen bond, and also on the stability of the ion formed. Factors which affect the stability of an ion are beyond what you need to know for CIE purposes.

Of the four halogens, iodide ions are the best leaving group, and fluoride ions the worst. You can explain that perfectly well in terms of the very strong carbon-fluorine bond, and the much weaker carbon-iodine one.

Exploring the individual reactions

Reactions involving ions

There are two of these mentioned by the syllabus - the reactions with hydroxide ions and with cyanide ions.

First read the page about the reactions between halogenoalkanes and hydroxide ions. Don't forget that you will need both SN1 and SN2 reactions. Follow the Help! link at the bottom of the page for a detailed explanation.

You should find that most of this is familiar to you from what you have read previously. CIE will accept either way of writing an SN2 reaction (either showing the transition state or not). Learn whichever you feel happier with.

Now do the same thing for the reactions between halogenoalkanes and cyanide ions.

By the time you have read all the way through this, you will probably be getting bored by it all, because it is almost a word-for-word repeat of everything you have done before. But that's not surprising - the hydroxide and cyanide reactions work in exactly the same way.

Reactions involving water and ammonia

Now repeat the whole thing with:

Make sure that you understand the minor difference between the reaction between the halogenoalkane with water and with hydroxide ions - where the odd hydrogen ion is removed at the end.

With ammonia, ignore any link which takes you to further substitution. This isn't needed for the CIE exam.

In each case, follow the Help! link at the bottom of the page.

A final comment

I have asked you to do a lot of work for this statement, and it will probably have taken you quite a long time. If you have come out of it feeling "Well, that was tedious, but at least it all makes sense", then you are over a major hurdle in organic chemistry, and shouldn't have any real problems from now on, because you now have the confidence that you can understand it.

On the other hand, if the only way you are going to be able to cope with this is to try to learn it without understanding it, you are heading for problems in the future. Learning without understanding isn't a sensible option.

You really need to talk to someone who can help you to work out exactly what is causing your difficulties. Sometimes this can be as simple as misunderstanding one word early on in the topic. Don't just give up! Work out what the problem is and get it sorted out. That's what successful students do.

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© Jim Clark 2020