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Representing Organic Molecules

Representing organic molecules

There are a number of ways to represent organic compounds. It is useful to know all of these so that you can recognise a molecule regardless of how it is shown. There are four main ways of representing a compound in two dimensions (on your page). We will use the examples of two molecules called 2-methylpropane and butane to help explain the difference between each.

Structural formula

The structural formula of an organic compound shows every bond between every atom in the molecule. Each bond is represented by a line. The structural formulae of 2-methylpropane and butane are shown in the figure below.

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The structural formula of (a) 2-methylpropane and (b) butane.

Different ways of representing a carbon atom bonding to four hydrogen atoms.

Semi-structural formula

It is possible to understand the structure of an organic molecule without writing out all the carbon-hydrogen bonds. This way of writing a structure is called a semi-structural formula and is shown in the figure below.

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The semi-structural formulae of (a) 2-methylpropane and (b) butane.

Compare these semi-structural representations with the structural representations shown above.

Tip:

A substituent is an atom or group of atoms that replaces a hydrogen atom on the main chain of an organic molecule. Therefore a brached group is a substituent. A halogen atom can also be a substituent.

Condensed structural formula

It is also possible to represent a molecule without showing any bonds between atoms at all. This is called a condensed structural formula (see figure below). As for a semi-structural representation, the carbon atoms are grouped with the hydrogen atoms bonded directly to it. The bonds between these groups are not shown. Branched or substituent groups are shown in brackets after the carbon atom to which they are bonded.

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The condensed structural formulae of (a) 2-methylpropane and (b) butane.

Note that in the (b) part of the figure above, the two \(\text{CH}_{2}\) groups can be abbreviated to \((\text{CH}_{2})_{2}\). Compare these condensed structural representations with the structural and the semi-structural representations above.

Molecular formula

The molecular formula of a compound shows how many atoms of each type are in a molecule. The number of each atom is written as a subscript after the atomic symbol. The molecular formula of 2-methylpropane is:

\(\text{C}_{4}\text{H}_{10}\)

This means that each molecule of 2-methylpropane consists of four carbon atoms and ten hydrogen atoms. The molecular formula of butane is also \(\text{C}_{4}\text{H}_{10}\). Molecular formula gives no structural information about the compound.

Of course molecules are not two-dimensional so shown below are a few examples of different ways to represent methane (\(\text{CH}_{4}\), see figure below) and ethane (\(\text{C}_{2}\text{H}_{6}\), see second part of figure below).

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Different ways of representing methane.

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Different ways of representing ethane.

This means that butane can be represented in two dimensions as shown in figure (a) below but it actually looks more like the three-dimensional representation given in figure (b) below.

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(a) Two-dimensional and (b) three-dimensional representations of butane.

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