Chemguide: Support for CIE A level Chemistry

This statement covers a number of important reactions of the alcohols.

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

Important

Despite the fact that this is all included in one statement, there is a lot of chemistry to learn here. Take your time over it.

This statement does not require any knowledge of the mechanisms for the reactions covered.

Background work

Start by reading the page introducing alcohols.

This covers how the alcohols are named, and their physical properties. Make sure you understand the importance of hydrogen bonding in the physical properties of the alcohols.

Technically, you don't need to know about the differences between primary, secondary and tertiary alcohols until statement 10.4(b). However, you will find these terms used throughout the Chemguide pages you will read, and it would be a good idea to get this sorted out now. In fact, the use of these terms is exactly the same as with halogenoalkanes, so there is nothing very difficult involved.

Statement 10.4(a)(i): Combustion

You will find the combustion of alcohols discussed briefly on the page about uses of alcohols.

Make sure that you can write the equation for the complete combustion of methanol and ethanol, or any other given alcohol. Do not try to learn these equations - just make sure that you can work them out if you need to.

Statement 10.4(a)(ii): Substitution to give halogenoalkanes

You will find this described at some length on the page about replacing the -OH in alcohols by a halogen.

This page covers several different ways of doing this, and you need to be familiar with all of them. Every single one of these methods has cropped up either in an exam question or as a possible answer in a mark scheme.

Don't forget to notice that the addition of PCl₅ can be used to test for an -OH group.

Statement 10.4(a)(iii): Reaction with sodium

This is covered on the page about the reactions of alcohols and sodium.

You can ignore the section about the reactions of alkoxide ions.

Statement 10.4(a)(iv): Oxidation to carbonyl compounds and carboxylic acids

There is no logic in the syllabus at this point! This can't be done until you have covered statement 10.4(b), and I will leave it until then to discuss it.

Statement 10.4(a)(v): Dehydration to alkenes

You will find this on the page about the dehydration of alcohols.

You need to know about both the catalytic dehydration, and also dehydration using concentrated sulphuric or phosphoric(V) acids.

There was one question about using an apparatus similar to the catalytic dehydration which asked why you why you had to remove the delivery tube from the water before you stopped heating the horizontal tube.

If you have done any practical work at all, this will be obvious to you. If you haven't, the answer is that as the very hot horizontal tube cools, the pressure inside it falls, and cold water is sucked back into it, cracking it. (Actually, if you want to be pedantic, water is forced into the hot tube by the higher air pressure acting on the water in the beaker.)

Don't forget to read about the dehydration of more complicated alcohols at the bottom of the page. You may well be asked a question about this.

What you need to remember is that the -OH group gets removed together with a hydrogen atom from the next-door carbon atom. Obviously, if the -OH group is in the middle of the chain, it has two next-door carbon atoms, and a hydrogen could get removed from either of them. Therefore there is more than one possible alkene formed.

Make sure that you understand how you can get 3 different alkenes from the dehydration of butan-2-ol. This has appeared in a past question.

Statement 10.4(a)(vi): Ester formation

Read the page about esterification.

It is essential to take your time over the names and structures of esters. You will eventually meet these again in section 10.6, but you need to understand these names and structures now.

You may have noticed that making esters from acyl chlorides is in bold type in the syllabus. That means that it will only be examined at the end of a two year course.

So, for now, you can ignore the formation of esters using acyl chlorides or acid anhydrides at the bottom of the page. You will eventually meet the acyl chloride reaction in statement 10.6(e) in the second half of your course. The acid anhydride reaction isn't on the syllabus.

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