Chemguide: Support for CIE A level Chemistry

Learning outcome 16: Hydroxy compounds

16.1: Alcohols

Statement 16.1.2

This statement covers a whole lot of reactions of alcohols. You will need to keep referring to your syllabus as you read down this page.


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 16.1.3. However, you will find these terms used throughout the Chemguide pages you will read, and you need 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 16.1.2(a): 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 16.1.2(b): 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.

Notice that the syllabus talks about "PCl3 and heat". I haven't mentioned the need for heat on the Chemguide page, and actually haven't been able to find another source which mentions it either. In fact the PCl3 reaction is more complicated than this anyway. Just learn what CIE want.

You will find an exact copy of this statement in statement 15.1.1(c) as a means of preparing halogenoalkanes.

Statement 16.1.2(c): 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 16.1.2(d): Oxidation to carbonyl compounds and carboxylic acids

There is no logic in the syllabus at this point! This can't be done until you know about primary, secondary and tertiary alcohols - which is why I stressed at the beginning of this page that you should sort this out first.

This statement includes the facts about the oxidation of the various sorts of alcohols.

You will find this covered on the page about oxidation of alcohols.

Before you start on this page, you will need to follow the second link in the red box at the top of the page so that you understand what aldehydes and ketones are. Read the whole of the first section of that page explaining what aldehydes and ketones are. In the Bonding and Reactivity section, just read the short bit about where aldehydes and ketones differ. Ignore the rest of the page for now.

In the equations on the main page about the oxidation of alcohols, you can ignore the full versions or the electron-half-equations. All CIE will expect are the versions involving oxygen in square brackets.

You can ignore the use of these reactions in distinguishing between primary, secondary and tertiary alcohols until statement 16.1.3.

The syllabus statement also talks about using acidified potassium manganate(VII) as the oxidising agent and that isn't included on the page you have read.

You can use the same equations (the ones with oxygen in square brackets) for potassium manganate(VII) as with potassium dichromate(VI). For CIE purposes the products are the same. The only essential difference is the colour change.

When acidified potassium manganate(VII) oxidises things it turns from deep purple to colourless.

Statement 16.1.2(e): 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 sulfuric 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 16.1.2(f): Ester formation from carboxylic acids

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 18.2, but you need to understand these names and structures now.

You can ignore the formation of esters using acyl chlorides or acid anhydrides at the bottom of the page for now. You will meet the acyl chloride reaction in statements in the second half of your course. The acid anhydride reaction isn't on the syllabus anywhere.

Go to the Section 16 Menu . . .

To return to the list of learning outcomes in Section 16

Go to the CIE Main Menu . . .

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Go to Chemguide Main Menu . . .

This will take you to the main part of Chemguide.

© Jim Clark 2020