Introduction to Organic Chemistry

An organic compound is essentially anything containing carbon. On this page you’ll learn about naming organic molecules and how they can be grouped into homologous series which share the same chemical properties.

Hydrocarbons

A hydrocarbon is a molecule made up of carbon and hydrogen atoms only. The carbon atoms can be joined together by only single bonds, which we refer to as a saturated hydrocarbon, or there may be some double carbon bonds which is known as an unsaturated hydrocarbon.

Representing organic molecules

Organic molecules can be represented using different types of formula, depending on what sort of information we want to know. The table below shows different formulas for the hydrocarbon propane. If we just want to know the number of atoms in the molecule, it’s best to use a molecular formula. However, if we want to see how those atoms are connected together then we would use a displayed formula.

Homologous series

A homologous series is a group of compounds that can be represented using the same general formula. They all react in the same way and they show trends in their physical properties - for instance, as the chain length increases, boiling point increases.

For example, the alkanes are a homologous series which all have the same general formula, C_nH_2n+2. The first member in the homologous series will have only one carbon with four hydrogen atoms attached and the second will have two carbons attached to six hydrogens and so on.

The image below shows the first six members in the homologous series on alkanes. You can see that each alkane is the same as the last, but has an extra CH₂ group attached to the chain.

Functional groups

Molecules which belong to the same homologous series will have the same functional group. A functional group is defined as a particular group of atoms which are responsible for how the molecule reacts. Functional groups include double carbon bonds (C=C) , alcohol groups (-OH), carboxylic acid groups (-COOH) and esters (-COO) which are drawn below.

Isomerism

Isomers are molecules with the same molecular formula but a different structural formula. This means that two isomers have the same number of atoms of a particular element but that they are arranged differently within the molecule. There are three different types of isomerism that you need to know about:

Chain isomerism: these are isomers where the carbon chain is arranged differently - for instance, instead of having one long straight chain, there may be side branches coming off a smaller chain instead. The example below shows pentane and dimethylpropane, which are chain isomers.

2. Position isomerism: These are isomers which contain the same functional group, but the functional group is located in a different part of the molecule. For example, in but-1-ene and but-2-ene, the double carbon bond functional group is attached to different carbon atoms.

3. Functional group isomerism: This is where two molecules have the same atoms arranged into different functional groups. In the example below, you can see how propanoic acid and methyl ethanoate have the same number of each type of atom, but they form a carboxylic acid in the first molecule and an ester in the second.

Naming compounds

The name given to hydrocarbons depends on the number of carbon atoms it has. The table below shows the names given to first six alkanes and alkenes (you won’t be asked to name a molecule that contains more than six carbon atoms). The best way to remember is to use a mnemonic such as monkeys eat peeled bananas to help you remember meth-, eth-, prop- and but-. Pent- and hex- for a five and six carbon chain are a little more obvious.