Chapter 53: Organic Chemistry and Aromatic Systems

Carbon occupies a unique position in chemistry: four valence electrons, intermediate electronegativity, three hybridisation states, and the ability to form stable chains of arbitrary length. These properties generate the vast landscape of organic chemistry—over ten million known compounds. This chapter surveys the major classes of organic molecules (hydrocarbons, functional groups, isomers), develops the concept of chirality and its biological significance, and then turns to aromaticity: the remarkable stability of benzene and related cyclic systems, governed by H"uckel’s 4n+2 rule. Throughout, organic structure is interpreted in Category τ as τ² mode decoration on molecular graphs, with aromaticity as optimal τ² delocalisation in cyclic topologies.