Detailed step-by-step reaction mechanisms: SN1, E1, E2 (alongside SN2 already in organic-chemistry), radical-chain, and pericyclic (Diels-Alder) mechanisms. Each characterised by molecularity, stereochemistry, and rate-determining step.
reaction-mechanisms
SN1 mechanism
Unimolecular nucleophilic substitution: rate-determining ionisation to a carbocation intermediate followed by fast nucleophilic attack.…
E1 mechanism
Unimolecular elimination: rate-determining ionisation to a carbocation (same first step as SN1) followed by β-proton loss. Zaitsev's rule:…
E2 mechanism
Bimolecular elimination: concerted anti-periplanar loss of a proton and leaving group. Rate = k[RX][base]; stereospecific with…
Radical chain mechanism
Initiation (homolysis) → propagation (radical regeneration) → termination (radical-radical coupling). Governs halogenation of alkanes,…
Diels-Alder reaction
Canonical pericyclic [4+2] cycloaddition between a conjugated diene and a dienophile, yielding a cyclohexene with predictable regio- and…
Woodward–Hoffmann rules
Orbital-symmetry conservation rules that determine which concerted pericyclic reactions (electrocyclic, cycloaddition, sigmatropic) are…
Frontier molecular orbital (FMO) theory
Reactivity and selectivity in many reactions are dominated by the interaction between the highest occupied MO (HOMO) of one reactant and…
Marcus theory of electron transfer
ΔG‡=(λ+ΔG°)²/(4λ) with reorganisation energy λ. Inverted region: faster rate as driving force exceeds λ. Nobel 1992. Explains…
Eyring equation (transition state theory)
k = (kBT/h) e^(-ΔG‡/RT). Thermodynamic formulation of TST; ΔH‡ from slope, ΔS‡ from intercept of Eyring plot. Relates kinetics to…
Hammond postulate
Transition state structurally resembles the nearer-energy species. Late TS for endothermic steps, early TS for exothermic. Rationalizes…
Kinetic isotope effects
Primary KIE kH/kD 2-7 for C-H cleavage via zero-point energy difference. Inverse KIE when bond strengthens. Tunneling contributions (>10…
Concerted vs stepwise mechanism tests
KIE, stereochemistry, Hammett ρ, linear Eyring detect concerted; stepwise via intermediate trapping, crossover.
Radical clocks
Cyclopropylmethyl → butenyl (2×10⁸ s⁻¹) radical rearrangement; distinguishes radical vs polar; Griller–Ingold tables.
Mechanism of asymmetric induction
Stereoselection via diastereomeric TS; ΔΔG‡ from ee: 1 kcal/mol ≈ 70% ee at RT; Curtin-Hammett control.
Carbanion stability & orbital analysis
s-character in lone pair increases stability (sp>sp²>sp³); α-heteroatoms stabilize (nitro, cyano, sulfonyl); cation-π.
Carbocation stability & rearrangements
3°>2°>1°>methyl; benzyl, allyl resonance; hydride/alkyl shifts to more stable cation; non-classical ions.
Carbenes: singlet vs triplet
Singlet concerted stereospecific insertion; triplet diradical stepwise; CH₂ ΔE_ST ≈ 9 kcal/mol (triplet GS).
Nitrene chemistry
Analogous to carbene with N; metal-nitrenoids in C-H amination; Rh(II), Cu catalysts.
Radical-polar crossover mechanisms
Radical step → cation/anion via SET; enables combined photoredox/2-electron chemistry; Davis, MacMillan examples.
Bema–Hapothle extension of Hammond
Parallel/perpendicular effects of substituents on 2D energy surface; More O'Ferrall-Jencks diagrams map mechanism change.
Neighbouring-group participation (NGP)
Intramolecular assistance in SN/E via bridged intermediate; acetate, bromine, aryl NGP; Winstein ionization/return.