Organic chemistry stops being descriptive and becomes mechanistic

Alkanes, alkenes, alcohols, carboxylic acids named for the first four members. Cracking, combustion, addition polymerisation, the bromine water test, fermentation. Reactions are given as facts. No mechanisms. No isomerism beyond, at most, a mention.

Full IUPAC nomenclature (Structure 3.2.5) including branched chains, multiple functional groups and priority. Multiple representations — empirical, molecular, condensed, full structural, skeletal, stereochemical (3.2.1) — and you must convert fluently between them. Structural isomerism (3.2.6) and stereoisomerism: cis/trans and E/Z, and optical isomerism/enantiomers at HL (3.2.7). Then reactions become mechanisms with curly arrows:

• Radical substitution of alkanes: initiation, propagation, termination, homolytic fission (Reactivity 3.3).
• Nucleophilic substitution of halogenoalkanes, SN1 vs SN2, leaving-group ability, carbocation stability (3.4.1–3.4.3, 3.4.9–3.4.10).
• Electrophilic addition to alkenes, Markovnikov selectivity explained by carbocation stability (3.4.5, 3.4.11–3.4.12).
• Electrophilic substitution of benzene (3.4.13). (HL)

Plus spectroscopic identification (3.2.8–3.2.12): MS fragmentation, IR functional groups, ¹H NMR chemical shift / integration / splitting, and combining all three.

Organic goes from "a list of reactions to remember" to "a small set of electron-movement principles from which reactions are predicted". Curly arrows always start at an electron pair (lone pair or bond) and point where the pair goes. This is the single largest volume of genuinely new content.

Mechanisms require the polarity/electron-density ideas of R-01.

Trying to memorise organic reactions individually — this works at GCSE and fails at HL because the volume is 10× larger; drawing curly arrows from atoms rather than from electron pairs; missing that a chiral carbon has four different groups; getting Markovnikov backwards because carbocation stability isn't understood.