Directive Influence of Substituents in Monosubstituted Benzene

NOTE ON THE SOURCE TEXT: the book's own page prints this heading as '13.5.1 directive influence...' (reusing the number 13.5.1, already used above for 'Nomenclature and Isomerism') -- a verified duplicate-numbering slip, filed here as 13.5.7 in sequence. When a monosubstituted benzene undergoes electrophilic substitution, the group already on the ring controls both the RATE of the reaction and the SITE (ortho/meta/para) the incoming electrophile prefers. Groups that speed up the reaction relative to benzene are ACTIVATING groups; those that slow it down are DEACTIVATING groups. Independently, groups are classed by where they direct the incoming electrophile: those that raise electron density at the ortho and para positions are ORTHO-PARA DIRECTORS, and those that raise it at the meta position are META DIRECTORS. ORTHO-PARA DIRECTING GROUPS: every ACTIVATING group is an ortho-para director, e.g. -OH, -NH2, -NHR, -NHCOCH3, -OCH3, -CH3, -C2H5. For the phenolic (-OH) group specifically: phenol is a resonance hybrid of structures in which the oxygen's lone pair is pushed into the ring, placing extra negative charge specifically at the ortho and para ring positions -- so -OH activates the ring (makes it MORE electron-rich than benzene overall) and directs incoming electrophiles to ortho/para. Aryl halides are a special, partly-exceptional case: the halogen's strong inductive (-I) electron-withdrawal LOWERS the ring's overall electron density (making halogens DEACTIVATING overall), yet a halogen's lone pair can still donate into the ring by resonance, which selectively raises electron density at ortho/para relative to meta -- so a halogen substituent is simultaneously a DEACTIVATOR (rate) and an ORTHO-PARA DIRECTOR (site), an important combination not shared by any other common substituent. META DIRECTING GROUPS: essentially every DEACTIVATING group is a meta director, e.g. -NO2, -CN, -CHO, -COR, -COOH, -COOR, -SO3H. For the aldehyde (-CHO) group specifically: benzaldehyde is a resonance hybrid of structures that place extra POSITIVE charge …