Heterocyclic chemistry — IUPAC-III organic chemistry of ring systems containing one or more non-carbon atoms (N, O, S, P, Se) in the ring. Foundations: Hückel's 4n+2 rule (Hückel 1931): a planar, fully-conjugated, monocyclic system with…
heterocyclic-chemistry
Hückel (4n+2) rule: planar conjugated monocycle aromatic iff π-count mod 4 = 2
Hückel's 4n+2 rule (Hückel 1931, degenerate-perturbation theory on the cyclic MO ladder): a planar, fully-conjugated, monocyclic system…
Pyridine pKaH ≈ 5.2 vs pyrrole pKaH ≈ −3.4 (8.6-unit lone-pair topology split)
Canonical pKaH split driven by lone-pair topology. Pyridine's ring nitrogen contributes two electrons to a σ lone pair *in the ring plane*…
[4+2] Diels-Alder: aromatic 6π transition state (thermally allowed, Woodward-Hoffmann)
Diels-Alder reaction (Diels & Alder 1928) — concerted [4+2] cycloaddition between a conjugated diene (4π) and a dienophile (2π) — proceeds…
Hückel π-counts: benzene 6 (mod 4 = 2, aromatic); cyclobutadiene 4 (mod 4 = 0, anti-aromatic)
Sympy-exact witness of Hückel classification for three canonical heterocyclic and carbocyclic systems. Benzene: π=6, 6 mod 4 = 2, aromatic…
pKaH pyridine = 26/5, pyrrole = -17/5, Δ = 43/5 (exact rational textbook pins)
Sympy-exact witness of the pyridine-vs-pyrrole basicity split as exact rational textbook values. pKaH(pyridine) = 26/5 = 5.2;…
[4+2] TS π = 6 (4n+2, n=1): thermally allowed, photochemically forbidden (flag split)
Sympy-exact witness of the Woodward-Hoffmann signature for the canonical [4+2] Diels-Alder. Diene contributes 4 π-electrons, dienophile 2,…