Phenylpropanoid pathway

The plant’s second pathway, starting from the amino acid phenylalanine. The products always have a six-carbon benzene ring, usually with a three-carbon chain — hence the name.

This is the source of most rich spice smells: eugenol in clove, cinnamaldehyde in cinnamon, anethole in star anise. It is also the source of vanillin and of the rose note of 2-phenylethanol.

One easy mistake: indole and methyl anthranilate are also aromatic rings made by the plant, but they follow the anthranilate branch rather than passing through phenylalanine. The telltale sign is the nitrogen atom — no phenylpropanoid has one.

The same pathway produces lignin — the substance that makes wood hard. That is why, when wood burns, the breakdown products carry the unmistakable mark of phenylpropanoids.

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.