As well as being small molecules, odorants must be volatile molecules in order to be detected (or smelled, as is the case here). You will meet the concept of volatility again in Week 6 when you consider the laboratory process of distillation.
For something to be volatile, it means that a significant proportion of the molecules must be in the gaseous phase at room temperature. Indeed, one reason why molecules of larger mass cannot be detected is because they have insufficient volatility. It is possible to appreciate this from the boiling temperature at one atmosphere pressure of three simple odorants (Table 1):
- ethyl formate, which contains five ‘heavy’ atoms (three carbon, two oxygen)
- ethyl nonanoate, which contains 13 ‘heavy’ atoms
- ethyl palmitate, which contains 20 ‘heavy’ atoms.
Note that hydrogen is not classed as a ‘heavy’ atom.
Table 1 The boiling temperature (b.t.) at one atmosphere pressure of three odorants
|Ethyl formate||C3H6O2||74||strong, fruity, rum-like||54|
|Ethyl nonanoate||C11H22O2||186||cognac, nut-like||227|
|Ethyl palmitate||C18H36O2||284||faint, waxy-like||340|
At any given pressure, the more volatile substances are those with the lower boiling temperatures. This is because, for these substances, there are more molecules in the gaseous phase at lower temperatures – that is, they are easily vaporised.
So, at a given temperature and pressure, volatile compounds have more molecules in the gaseous phase than less volatile compounds and are therefore more easily detected. By detected, you can consider this as ‘smelled’ – the most volatile odorant, ethyl formate, has the strongest smell out of the three.
Note that for reference, the boiling temperature of water is 100 °C; water is relatively volatile (consider a day which feels very humid) but it has no odour.
From the boiling temperature data in Table 1, what can you say about the relative volatility of the three odorants?
Ethyl formate is much more volatile than the other two compounds; it boils at a temperature well below the boiling temperature of water (100 °C). Ethyl nonanoate boils at a temperature well above that of water and so is less volatile, and ethyl palmitate boils at a temperature that is even higher, so it is not very volatile at all and its smell is very faint.