Chemguide: Support for CIE A level Chemistry

Learning outcome 14: Hydrocarbons

14.2: Alkenes

Statements 14.2.4 and 14.2.5

These statements are about the electrophilic addition mechanism in the reactions of ethene.

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

This will probably be the second organic reaction mechanism that you have met. You have probably already done the free radical substitution reaction with alkanes like methane.

This electrophilic addition mechanism is more typical of the sorts of mechanism you will meet from now on, and it is very, very important that you should really understand it. This mechanism is relatively simple to understand, and will give you confidence in your ability to do organic chemistry.

To help you understand it, I am going to ask you to look at quite a lot of Chemguide pages. Don't try to short-cut this! You will come to regret it if you do. A sensible amount of time spent on this now will make your life much easier later on in the course.

First, read the introductory page about electrophilic addition.

Towards the bottom of that page, you will be given a link to a short page explaining the use of curly arrows in reaction mechanisms. You must follow that link, and you must understand exactly how these are used. You can ignore the bottom half of the page about using arrows to show the movement of single electrons. CIE don't seem to use these.

The reactions between alkenes and HBr

Symmetrical alkenes like ethene

Read the page about the mechanism for the reactions between symmetrical alkenes and hydrogen halides. It is important that you should follow the "Help!" link at the bottom of the page.

Symmetrical alkenes are ones which have exactly the same groups at both ends of a carbon-carbon double bond. You will find two examples of this on these pages - ethene and cyclohexene. Although the syllabus talks about ethene, you are just as likely to get asked a question about something more complicated like cyclohexene.

Unsymmetrical alkenes like propene

This is where things are going to get a bit more difficult. Don't start this until you are reasonably confident about what you have read so far.

You first need to read the page about carbocations.

Don't leave this until you are sure that you understand it. The most important thing for you to remember is that . . .

primary carbocations are less stable than secondary ones which in turn are less stable than tertiary ones.

. . . but you must also understand the reason for that.

Now read the page about electrophilic addition to unsymmetrical alkenes.

And, at last, you can read about the reaction that is actually mentioned in the syllabus - between unsymmetrical alkenes and hydrogen halides. Don't forget to read the "Help" page via the link at the bottom.

You do not need to worry about the alternative reaction between propene and HBr. That isn't an electrophilic addition, and so its mechanism isn't required by the syllabus.

The reaction between alkenes and bromine

Finally, read the pages about the reactions between symmetrical alkenes and bromine. It is again important that you should follow the "Help!" link at the bottom of the page, although by this time, it should all be getting very familiar to you - perhaps even boringly familiar. If you have got to the "boringly familiar" stage, then you have won this particular battle!

You will find two different versions of the mechanism for the alkene-bromine reaction. CIE have said in a mark scheme that they will accept either version. In that case, go for the easy one! Look only at the parts headed "The simplified version of the mechanism". Completely ignore the alternative version - it will only confuse you.

There is one final important point that I haven't covered on any of these pages, mainly because it confuses everything to include it at this stage. However, CIE sometimes asks questions about it, and so you need to be able to cope with it.

In everything you will have read, we have looked at the reaction from the point of view of the alkene. We have said that this is attacked by an electrophile, which is a species with a slight positive charge on it somewhere.

But that is just a convention. We could equally well look at it from the point of view of the other molecule. After all, all that is happening is that the two molecules collide and react. You haven't got one molecule chasing the other one around to try to react with it.

If you look at it from the point of view of the other molecule (such as HBr), it is being attacked by something which has an electron-rich region (the pi bond of the ethene). That electron-rich region can be thought of as attacking the slightly positive part of the hydrogen bromide molecule, or whatever.

So you could describe the ethene molecule as something which seeks after positive parts of other molecules. We call such a species a nucleophile. You will come across this term again when you look at reactions of halogenoalkanes later.

I would suggest that you put off worrying about this until later on, after you have done some more mechanisms - perhaps even until you start doing past papers to practise for your exam. You need to keep everything straightforward and simple until you gain confidence.

And a last thought . . .

If you have got all the way through all these pages I have asked you to read about this, and have understood most (or even better, all) of it, congratulations.

I am not suggesting that you will necessarily have remembered it all at this stage, but if you have understood it, well done! You have just got over one of the major hurdles in organic chemistry.

All of the other mechanisms that you will meet in the future are based on the same sort of principles. You may find that some are more difficult than others, but actually nothing is more difficult than when you meet this sort of stuff for the first time.

If you aren't happy about this, you must have another go at it. Try to work out what is bothering you and then talk to your teacher or to friends who have managed to get through it. But don't whatever you do just move on to the next topic and hope that it will all become clear eventually. It won't - it will just get worse!

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© Jim Clark 2020