introduced Ch 01; returns Ch 04 (nomenclature), Ch 14 (first synthesis), Ch 21 (Friedel-Crafts in ibuprofen), Ch 26 (acetylation for aspirin), Ch 35 (drug design capstone) - **SN1/SN2/E2/E1 decision framework** — built Ch 10-13; returns in every chapter thereafter that features substitution or elimi