Chemguide: Support for CIE A level Chemistry
Learning outcome 15.4(a)
This statement is about the chemistry of the arenes - for example, benzene and methylbenzene.
Before you go on, you should find and read the statement in your copy of the syllabus.
This statement lists some important reactions of these compounds. In what follows, keep strictly to the facts, and ignore any reference to mechanisms for now. These appear in the next statement, 15.4(b).
What is important for this statement is that you learn the reagents, conditions, and products for all the reactions named in the statement.
You need to read some background pages before you start work on the details of this statement. Take your time over these - trying to rush through them would be a big mistake.
You should first read the page about naming aromatic compounds.
Read that page down to, and including, benzoic acid. Then jump down the page to the heading "Aromatic compounds with more than one group attached to the benzene ring". Read that section down as far as the green warning box.
Read the box so that you are aware of the problem, but it shouldn't affect you. CIE seem to take the same line with the names as I do.
Introduction and bonding
Read the page an introduction to arenes. It is important that you follow the links in the first green box to other pages about the bonding in these compounds.
You are unlikely to be asked anything about the Kekulé structure, but you might well come across it, and you should be aware of how it relates to the more modern structure. You don't need to spend too long on that page.
This is about substitution reactions involving chlorine or bromine. You will find these described on the page about the halogenation of benzene and methylbenzene.
You can ignore the addition reactions of both benzene and methylbenzene, but you must know about both sorts of substitution in methylbenzene - in the ring or in the side chain. It is essential that you know the conditions for all of these reactions, as well as what is formed.
Ignore any links to mechanisms for now.
This is about substituting a nitro group into a benzene ring. You will find everything you need on the page about the nitration of benzene and methylbenzene.
Once again, ignore any links to the mechanisms for these reactions for now. You can also ignore the final green box on the page.
This is about Friedel-Crafts reactions. You will find what you need on the page about the Friedel-Crafts reactions of benzene and methylbenzene.
This is new to the syllabus for 2016, so there is no indication at the moment of exactly how much you need. I think you could probably safely ignore the final section of the page about the manufacture of ethylbenzene.
This is about the oxidation of hydrocarbon side chains attached to a benzene ring. The methyl group in methylbenzene is a simple example of this.
Oxidation by alkaline potassium manganate(VII) solution
You will find what you need to know towards the bottom of the page about other reactions of benzene and methylbenzene.
Although it is mentioned in the final green box, you might miss it, and so I will repeat it here:
Because the potassium manganate(VII) solution is used under alkaline conditions, initially you get benzoate ions, C6H5COO-, produced.
The sulphuric acid (or hydrochloric acid would do just as well) is added to convert the benzoate ions into benzoic acid. The benzoate ions gain a hydrogen ion to give benzoic acid, C6H5COOH.
Oxidation by acidified potassium dichromate(VI) solution
In May/June 2013 Paper 43 Q5(c)(i), CIE asked a question which expected you to know that hot acidified potassium dichromate(VI) solution will also oxidise hydrocarbon side chains on a benzene ring. In this case, because you are doing it under acid conditions, you would go straight to the -COOH group.
Unfortunately, the example they chose was an exception to this.
In the question, the side chain had a carbon attached to the ring, and three CH3 groups attached to that. It turns out that oxidation by acidified potassium dichromate(VI) needs the carbon attached to the ring to have at least one hydrogen attached. This doesn't have any hydrogens attached, and so doesn't react.
Note: I have no idea how you were supposed to know that! It isn't mentioned in the syllabus, and although the official textbook mentions oxidation by acidified potassium dichromate(VI), it doesn't mention this problem. In over 50 years of teaching chemistry in one form or another, I had never come across this, and had to spend quite a long time tracking it down. Most sources ignore the reaction with acidified potassium dichromate(VI) entirely. Sometimes it is easy to get really irritated with CIE!
This is about the hydrogenation of the ring to give cyclohexane (from benzene) or cyclohexane with an alkyl group attached if you start from something like methylbenzene.
You will find what you need to know on the page you have just looked at about other reactions of benzene and methylbenzene. It is the second section.
© Jim Clark 2010 (last modified September 2018)