Functional Groups, Formulae & Nomenclature (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about functional groups, formulae and nomenclature — the three things you need before you can do any organic chemistry at all.

Organic chemistry needs one shared language for describing molecules: the functional group says what a molecule is and how it behaves, the formula is how we draw it, and the name is how we say it.

Let's take that connecting idea in full. Every organic molecule gets described in three ways, and they are not independent. The functional group is the reactive part, and it decides how the molecule behaves. The formula is how you put it on paper — and there is more than one way to do that, depending on how much detail you need. The name is how you refer to it unambiguously.

The functional group is the thread running through all three: it defines a family of compounds, it is the part a skeletal formula must always show, and it fixes the ending of the name.

So we will take four things in turn. First some organic terminology. Then functional groups themselves. Then the different types of formula. And finally how compounds get named.

Organic chemistry is the study of carbon-containing compounds. Carbon dominates because it forms strong covalent bonds with other carbon atoms — a property called catenation — which lets it build long chains, branched chains and rings. That is why the number of possible carbon compounds is so vast.

A hydrocarbon is a compound made up of carbon and hydrogen atoms only. Ethanol is not one, because it also contains an oxygen atom.

Then the key organising idea: a homologous series. That is a group of compounds that share the same functional group and the same general formula, have similar chemical properties, and differ from each other by CH2. As you go up the series the molecules get bigger, so physical properties like boiling point and density show a trend.

A functional group is the part of a molecule that determines its physical and chemical properties. Change the functional group and you change how the molecule behaves, which is why it is the most useful thing to spot first.

You will see functional groups written with an R in front. R is any other atom or group of atoms, apart from hydrogen. So R-OH is any alcohol and R-COOH is any carboxylic acid.

Each family has its own group. Alkenes, halogenoalkanes where X is fluorine, chlorine, bromine or iodine, alcohols as R-OH, aldehydes R-CHO, ketones R-COR, carboxylic acids R-COOH, esters R-COOR, amines R-NH2 and nitriles R-C triple bond N.

That is the list worth learning, because everything downstream keys off it.

There are six ways of writing an organic compound, and they carry different amounts of information.

The general formula represents a whole homologous series. For the alkanes it is CnH2n plus 2.

Then three ways of drawing a single molecule. The structural formula shows how the atoms are bonded to each carbon. The displayed formula is a two-dimensional picture showing every atom by its symbol and every bond as a line. The skeletal formula strips out the carbon-hydrogen bonds, so the carbons sit at the angles and ends of the chain — but the functional groups still have to be shown.

Finally, two that just count atoms. The molecular formula gives the actual number of each type of atom: ethanoic acid is C2H4O2. The empirical formula gives the simplest whole number ratio, which for the same compound is CH2O.

Names are built systematically, and the alkanes are the basis. The stem tells you how many carbons are in the longest chain: meth- for one, eth- for two, then prop-, but-, pent- and hex- up to six.

If there are side chains or functional groups, number the carbons along that longest chain, starting from whichever end gives the lowest numbers. Side chains take the ending -yl, so a one-carbon branch is methyl. Groups like that are called alkyl groups.

If the same side chain appears more than once, use di-, tri- or tetra-. Commas separate numbers, hyphens separate numbers from words. And where there is more than one type of side chain, list them alphabetically.

Then the functional group sets the ending: -ol for an alcohol, -al for an aldehyde, -one for a ketone, -oic acid for a carboxylic acid, -oate for an ester.

Two things to remember, and two that cost marks.

To remember: you only need to name chains and rings with up to six carbon atoms each, which limits what you can be asked. And on amines, ethylamine can also be called aminoethane or ethanamine — the IUPAC name is ethanamine, but past papers favour ethylamine.

What costs marks: in a displayed formula, draw every bond and every atom. But do not use sticks for the carbon-hydrogen bonds in a skeletal formula. And watch the alcohol group — it is C-OH, never C-HO. Writing it backwards is a very common error.

Pulling it together. The functional group decides the chemistry, so spot it first: it tells you which family the compound belongs to and how it will behave.

There are six ways to write a formula, from the general formula for a whole series down to the empirical formula's simplest ratio, and you pick the one carrying the detail you need.

Names come from the group: the stem counts the longest chain, the numbers locate the branches, and the functional group fixes the ending.

That is the one shared language we started with — group, formula and name.

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