noun
or benzene nucleus
: the structural arrangement of atoms believed to exist in benzene and other aromatic compounds showing six carbon atoms in planar symmetrical hexagonal fashion numbered 1 to 6 for convenience in designating the location of substituting groups and linked by alternate single and double bonds with each carbon attached to hydrogen in benzene itself or to other atoms or groups in substituted benzenes of which there is but one variety for each monosubstituted product (as toluene, phenol) but three for each disubstituted product — see meta- 4b, orth- 3b, para- 2b(1); compare cyclohexane , pyridine , structural formula