Basic Concepts of Organic Chemistry

Naming organic compounds

To avoid confusion, there are certain rules we follow when naming organic compounds. The IUPAC system is used to give compounds a systematic name using the rules described below.

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. 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.

Make sure you are counting the number of carbon atoms on the longest chain. Anything else needs to be named as a side-chain at the beginning of the name.

For example, if an alkane’s longest chain consists of five carbons, with a side chain containing one carbon branching from the third carbon, we would name it 3-methylpentane.

If you have multiple side chains with different numbers of carbon atoms, these need to be written in alphabetical order i.e. ethyl comes before methyl. If you have multiple side chains which all contain the same number of carbon atoms, you need to use the prefix di- or tri- or tetra- depending on whether you have two, three or four side chains respectively.

For example, an alkane with five carbons making up the longest chain and with two side-chains (consisting of just one carbon) branching off the third carbon atom, would be called 3,3-dimethylpentane.

The functional group will give you the ending of the name. Here’s a list of the different functional groups and their corresponding names:

• Alcohols will have the ending –ol e.g. pentanol

• Alkanes will have the ending –ane e.g. pentane

• Alkenes will have the ending –ene e.g. pentene

• Carboxylic acids will have the ending –oic acid e.g. pentanoic acid

• Aldehydes will have the ending –al e.g. pentanal

• Ketones will have the ending –one e.g. pentanone

Representing organic compounds

Organic molecules can be represented using different types of formula. The table below shows different formulas for the hydrocarbon propene.

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, we would use a displayed formula.

• General formula: a formula which can be applied to a whole homologous series of compounds e.g. alkenes

• Empirical formula: the simplest whole number ratio of atoms of each element in a compound

• Molecular formula: the actual number of atoms of each element in a compound

• Structural formula: shows the arrangement of atoms in a molecule by writing groups of atoms in a list

• Displayed formula: shows the arrangement of atoms in a molecule and the bonds between them

• Skeletal formula: shows the bonds of the carbon skeletal only with functional groups attached. The carbon atoms and hydrogen atoms in the chain are not shown.

Functional groups

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

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.

At A-Level, we also need to know the following terminology:

• Aliphatic – a straight-chain, branched or non-aromatic cyclic compound.

• Alicyclic – a compound arranged in non-aromatic rings

• Aromatic – a compound containing a benzene ring

• Unsaturated – a compound that contains at least one double or triple carbon bond, or an aromatic functional group

• Saturated – a compound which only contains single carbon bonds

Isomerism

Structural 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:

1.      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.

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.

3.      Functional group isomerism: This is where two molecules have the same atoms arranged into different functional groups.

Reaction mechanisms

Covalent bonds can break in two ways:

1. Heterolytic fission – the covalent bond breaks so that both bonding electrons go to the same atom. One of the atoms gets both electrons and the other gets none, forming positive and negative ions.

2. Homolytic fission – the covalent bond breaks so that one of the bonding electrons goes to each atom, forming two radicals.

A radical is a species with an unpaired electron and is represented with a dot to show the unpaired electron.