Chemguide: Support for CIE A level Chemistry
Learning outcome 36: Organic synthesis
36.1: Organic synthesis
Learning outcomes 36.1.1, 36.1.2 and 36.1.3
These statements (the whole of Section 36) are about using the organic chemistry you know in certain types of exam question.
Before you go on, you should find and read the statements in your copy of the syllabus.
You shouldn't even think of doing this section until right at the end of your 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.
If you aren't confident, then the first thing you have to do is to revise the organic chemistry so that your understanding is good. It doesn't matter at the moment if you can't always remember the detail, but you must understand the main processes.
This section represents the sort of questions that CIE have asked for quite a lot of years, so it is easy to find examples to practise on. And that is actually the only real way of dealing with this section.
These questions will often start with a scarily complicated molecule, often drawn as a skeletal formula. It is important that you can identify the functional groups in the molecule (ester, amide, carbonyl group, carbon-carbon double bond and so on), so that you know how that part of the molecule will react with various things.
You can often recognise these questions (apart from the scary molecule!) because the page may have a whole lot of boxes for you to draw structures in. Find as many questions like this as you can, and practise doing them, referring to the mark scheme and Examiner's Report as you go.
Don't worry if they seem difficult to start with. In most cases, they refer to straightforward chemistry, but set in difficult looking surroundings. Don't think you have to start with the beginning of a sequence. Quite often, you will find that it is better to start somewhere towards the end, and work back.
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 36.1.2 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:
You may 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 36.1.3 I am not entirely sure what they are thinking of in talking about "possible by-products".
One obvious one would be if they wanted you to substitute, say, a nitro group into the 4-position on an aromatic ring. A group that directs into the 4-position also directs into the 2-position. The 2-position substitution could be counted as a by-product.
Another 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 this, because it will be reflected in lower grade boundaries.
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 (and your final grade). It is worth spending quite a lot of time on this!
© Jim Clark 2020