Chemguide: Support for CIE A level Chemistry

Learning outcome 21: Organic synthesis

21.1: Organic synthesis

Learning outcomes 21.1.2 and 21.1.3

These statements are about using all the organic chemistry you know so far. Read the statements before you go on.

As I said in statement 21.1.1, you shouldn't even think of doing this section until right at the end of your AS level course when your knowledge of organic chemistry is at a very high level. Trying to do this section if you aren't confident about organic chemistry is a waste of time.

You should be able to find a good number of past questions,and that is actually the only real way of dealing with this section. Make sure that you only look at questions from papers 1 and 2 at this stage.

As you do more of these questions, you may well find that patterns appear - some reactions or reaction types will occur more often than others. Those will then start to become familiar.

A couple of specific points . . .

In 21.1.2, it talks about multi-stage synthetic routes. In the past, as far as I can see, this has never gone beyond a two stage process:

reactant molecule - intermediate - product molecule

You are likely to be given the structures of the reactant and product molecules, and asked for a suitable intermediate and the reactions needed to produce it from the reactant and convert it into the product.

Questions like this are usually fairly obvious if you have reasonable knowledge of organic chemistry.

In 21.1.3, I am not absolutely sure what they are thinking of in talking about "possible by-products".

One example might be the electrophilic addition of, say, HCl to a carbon-carbon double bond with different groups at both ends. The main product will come from an addition according to Markovnikov's Rule, but a small amount of anti-Markovnikov addition will happen as well. That would be a by-product.

Or a reaction between, say, a secondary halogenoalkane and sodium hydroxide solution can produce both substitution by an OH group to form an alcohol, or elimination to produce an alkene. If you wanted just one of these, the other would be a by-product.

As I said at the beginning of this, your organic chemistry has to be very good to be able to recognise these and similar cases.

And finally . . .

If you read the Examiner's Reports, you will quite often come across a statement which says something like "There were few good answers to this", or "Few candidates could do this". That's because the question is too difficult, or badly phrased. Don't worry about that - the grade boundaries are adjusted to take account of it.

If you practise doing this sort of question enough so that you have got the answers to one of these questions right whereas most people didn't, that should be a major boost to your confidence. It is worth spending quite a lot of time on this!

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© Jim Clark 2020