Terpenoid pathway

This is the largest pathway in aromatics. The plant starts from a five-carbon block called isoprene and joins blocks together: two blocks make a ten-carbon monoterpene, three make a fifteen-carbon sesquiterpene.

That way of assembling explains why every monoterpene has ten carbons and every sesquiterpene fifteen. Look at the formula column in the library and you will see it at once: C10 and C15 again and again.

Once the skeleton is assembled, the plant adds functional groups — hydroxyl to make an alcohol, carbonyl to make an aldehyde or ketone. That is why linalool, citral, camphor and menthol are all terpenoids even though they belong to four different chemical families. Chemical family is about the functional group; pathway is about the skeleton.

This library classifies compounds along three axes, and each answers a different question.

Boiling point tells you when you smell a compound — early or late on the smelling strip. Chemical family tells you how it behaves — stable or fragile, and how to recognise it on an analysis. Biosynthetic pathway tells you where it comes from.

The third axis is the explanatory one. The other two describe how compounds cluster into groups; this one says why they cluster that way.