On an analysis you will meet names that differ only by a Greek letter or a number. Alpha and beta. Cis and trans. 2- and 5-.
These are not superfluous technical details. They are different compounds.
Same formula, different structure
Isomers are compounds with the same molecular formula but a different arrangement of atoms.
Alpha-pinene and beta-pinene are both C10H16. They differ only in the position of one double bond. Their boiling points are about ten degrees apart, enough for a chromatography column to separate them into two peaks, and the nose can tell them apart: alpha is brighter and drier, beta darker and less sweet.
A note to avoid confusion: furfural and 5-methylfurfural are not isomers. They differ by exactly one methyl group, so their formulas differ too — C₅H₄O₂ and C₆H₆O₂. That relationship is called homology, and it illustrates a different principle: adding a methyl group raises the boiling point by about twenty-five degrees. Isomers must have the same formula.
Optical isomers
There is a subtler kind of isomer: two molecules that are mirror images of each other, like a left and a right hand. All their ordinary physical properties are identical — same mass, same boiling point, same mass spectrum.
But the smell differs.
Carvone in the R form smells of spearmint. Carvone in the S form smells of caraway seed. One formula, every number the same, two unrelated smells.
We deliberately do not write “that is, the smell of garden mint (húng lủi)” here, even though it would be the most convenient way to say it. The reason is in the garden mint entry: in Vietnam that name does not correspond to a single species, so attaching a particular molecule to it would be attaching it to the wrong address.
This is direct evidence that olfactory receptors recognise three-dimensional shape, not just atomic composition. A receptor has a fixed shape, and a left hand does not fit a right glove.
Why GC/MS usually misses it
An ordinary chromatography column separates by volatility and polarity. Two optical isomers are identical in both respects, so they leave the column at the same moment and give a single peak.
Mass spectrometry cannot tell them apart either, because they break apart in exactly the same way.
Separating them requires a chiral column, a column with an asymmetric stationary phase. That is a topic for Part Four, and one of the two rare ways of telling natural origin from synthetic.
What to do when reading
When you see two near-identical names on an analysis, don’t add them together. They are two separate lines because they are two separate compounds.
And when an analysis merges them into one line — for example writing citral instead of listing geranial and neral separately — know that you are losing information. The separate version says more about origin and about processing.