Functional groups

Organic chemistry could in principle mean memorising the reactions of every one of the millions of known carbon compounds. It does not, because reactivity in an organic molecule is almost entirely local: it lives in small clusters of atoms called functional groups — the –OH of an alcohol, the C=O of a carbonyl, the –COOH of a carboxylic acid — and the rest of the molecule, the carbon skeleton the group is attached to, is largely a spectator.

This is why ethanol and a fatty alcohol with twenty carbons both fizz with sodium metal, releasing hydrogen: both carry an –OH group, and that group is what reacts. The chain length changes physical properties — boiling point, solubility, viscosity — substantially, but it does not change which reactions occur. Learning "alcohols react with sodium" is learning a fact about every alcohol at once, not about ethanol specifically.

This locality is what makes organic chemistry learnable rather than an impossible memorisation task. A first course is organised around perhaps a dozen functional groups precisely because that dozen accounts for the overwhelming majority of reaction types a molecule can undergo. Recognising which functional groups are present in an unfamiliar molecule, then, is the single most useful skill in the subject: it converts "I have never seen this exact compound" into "I know how each of its pieces behaves."