Chemguide: Support for CIE A level Chemistry
Learning outcome 13
13.1: Formulae, functional groups and the naming of organic compounds
Learning outcomes 13.1.1, 13.1.2 and 13.1.3
These statements cover basic ideas about hydrocarbons and functional groups. Read the statements in your syllabus before you go on.
The first statement just defines what a hydrocarbon is - it is just a compound made up of carbon and hydrogen atoms only. You have probably known that for several years.
The other two statements introduce the importance of functional groups.
What are functional groups?
You will discover as you work through the course that organic chemistry is actually about the reactions of particular bonds in organic molecules. These bonds are a part of what are known as functional groups.
For example, the carbon-carbon double bond in alkenes is a functional group, because the properties of the alkenes are largely due to that bond.
The -OH group in alcohols is another functional group, because the properties of alcohols are dependent on the way it reacts.
It is possible to have more than one functional group in the same molecule. For example, consider the molecule:
The bromine attached to the left-hand carbon is a functional group, and that carbon-bromine bond will have a particular set of reactions. The -OH (alcohol) group at the other end is another functional group, and will add a second set of reactions to the molecule.
The chemistry of a big molecule may be due to the presence of a large number of functional groups scattered around the molecule.
This statement introduces alkanes as simple hydrocarbons. Alkanes include methane CH4, ethane C2H6, propane C3H8, and many more.
Apart from methane which only has one carbon atom, all the others have carbon-carbon single bonds and carbon-hydrogen bonds. These bonds are fairly unreactive, and so alkanes are also fairly unreactive and you will only come across a small number of reactions.
Alkanes are therefore considered not to contain a functional group.
You will find the names of the functional groups that you will be learning about later on in the sideways-printed table at the beginning of Section 13 in the syllabus.
You will find this starts on page 26 of the 2022 syllabus. My pdf reader actually finds it as page 28 - but what it says at the bottom of the page is 26.
This is an important table and you will need to come back to it later on. Don't, however, imagine that you have to learn this table. You will meet each of the compounds gradually throughout the course.
This is an important statement, although it isn't something that you necessarily need to learn.
What is important is to realise that a functional group on a carbon chain affects both the physical and chemical properties of the compound.
So, for example, an -OH group attached to a carbon chain affects its physical properties because -OH groups can form hydrogen bonds as well as permanent dipole-dipole attractions and dispersion forces.
That means that if you find a molecule which has an -OH group in it, you will know that it has a higher melting and boiling point than a similar size molecule without that group.
As far as chemical reactions are concerned, you will find that compounds containing an -OH group attached to a carbon chain can undergo reactions involving breaking the C-O bond or the O-H bond. All compounds with a functional group in common will share a set of similar chemical reactions.
You will come across heaps of examples of this as you work through the rest of the syllabus.
An important personal note
Organic chemistry has often been taught very badly in the past, leaving students confused and completely put off the subject. The problem with organic chemistry from a student's point of view is that you are suddenly landed with a whole lot of new words and ideas.
It is really important in the early stages not to try to do too much, and to take your time. Just concentrate on the demands of the syllabus statement you are working on, and don't worry about what comes later in the course until you are happy about what you have just done.
It is essential in organic chemistry not to go on to the next stage until you are confident about the material you have already met. If you try to rush it, you will fail.
© Jim Clark 2020