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# 4 The molecules involved

## 4.1 Salicylic acid

The structural formula of salicylic acid, 2.1, looks quite complicated. However, it becomes less daunting if you unpack it a bit. One of the first things to do when confronted with an unfamiliar structure is to check that all the valencies are correct (four for carbon, two for oxygen and one for hydrogen). If any atoms have the wrong valency, it follows that there is a mistake somewhere and the molecule does not exist as drawn. It looks OK for the structure of salicylic acid. You probably noticed that some of the carbon atoms have two bonds joining them to another atom. These are called double bonds and they contain four electrons, two per bond. They are quite common in chemical structures. There are two types of double bond in salicylic acid, carbon-carbon double bonds (C=C) and a carbon–oxygen double bond (C=O). Two of the thinner, more flexible bonds in the model kit are used when making models requiring a double bond in the structure. Make a model of each type of double bond in 2.1 and keep them for using later.

Let's have a closer look at the salicylic acid molecule. For a start, focus on the ring part of the structure. If the two groups attached to the ring (the side chains) are removed and replaced with hydrogen atoms, we are left with the hydrocarbon, benzene. This is a liquid, present in coal-tar, which used to be widely used as a solvent by chemists until it was discovered just how poisonous it is. Nowadays benzene is a product of the petrochemical industry.

### Activity 1

• 1. Make a model of a benzene molecule, 2.2, with your model kit.

What do you notice about the model that makes it different to all the previous molecular models that you have made?

All the atoms lie in one plane; the ring structure is flat (planar). The presence of the double bonds is the cause of this.

• 2. Now turn your attention to the rest of the salicylic acid molecule. Remove the hydrogen from one of the benzene's carbons (it does not matter which one) and replace it with an oxygen joined to a hydrogen (an OH group). When this type of reaction is carried out for real, chemists refer to it as a substitution reaction. The structure that you have made is 2.3. You may need to rotate your model, or the —﻿OH group on it, to match 2.3.

Note that the C—﻿O—﻿H sequence in 2.3 is not linear. Also the —﻿O—﻿H group in 2.3 can rotate freely around the C—﻿O bond, giving many possible overall shapes for the molecule only one of which is planar.

The presence of the —﻿OH group gives the molecule 2.3 particular properties that are not possessed by benzene. Such a group is called a functional group. The —﻿OH functional group is called a phenol (pronounced fee-nol) group if it is joined to a benzene ring. The same word, phenol, is also used as the name of the compound you made (2.3) consisting of a benzene ring carrying an —﻿OH group and no other substituent groups.

### Activity 2

Now look at the other group on the benzene ring in salicylic acid, 2.1. Make a model of this group and substitute it for another hydrogen atom on your model of phenol. This time you will have to be a bit more particular about which hydrogen atom you substitute. It must be on a carbon atom that is adjacent to the phenol group. There is still some choice, though, as there are two of them.

Does it matter which of the two carbon atoms you change?