Some General Principles for Writing Reaction Mechanisms
(how to avoid the most common student errors)
1) conservation of matter
write balanced equations
show electron movements (arrows)
2) simplicity is preferred
Occam's Razor
Principle of Least Motion
3) reactions follow the lowest energy pathway
balance bond breaking with bond-making
4) most commonly, the types of intermediates encountered are
in acid, cations
in base, anions
in the gas phase, radicals
Reaction Energy Diagrams
simple 2D diagrams: reaction coordinate, energy, Ea, delta H
multiple reaction coordinates - 3D surfaces
examples - vitamin D3 conformations
intermediates and transition states
the Hammond Postulate
Radicals
EPR detection of unpaired spins
analogies to NMR - hyperfine coupling constants
stabilization: hyperconjugation of SOMO
structure: CH3 is planar, others pyramidal
generation: initiators
bond dissociation energies
reactions: chain reactions
initiation, propagation, termination
spin trapping, ring closure effects, radical "clock"
radical anions, radical cations
Carbenes
singlet and triplet states
generation: diazo compounds, alpha-eliminations, photosensitization, carbenoids
reactions: cycloadditions, insertions, rearrangements
Carbocations
(more coverage in Chapter 8)
nomenclature: carbenium ions (classical, trivalent), carbonium ions (nonclassical, hypervalent)
structure: planar, strong hyperconjugation and resonance effects
generation : superacids
rearrangements: H and C shifts
the 2-norbornyl cation
Carbanions
structure: pyramidal
importance of counterion (metal), covalent bonding, and solvation effects
generation: acid-base reactions, metal-halogen interchange
reactions: protonation, reactions as strong nucleophiles
stereochemistry: examples of retention of configuration