Organic Chemistry and Reaction Mechanisms
Scope
The exam covers functional-group recognition, IUPAC naming, isomerism and stereochemistry, substitution and elimination competition, addition to alkenes, aromatic substitution, and carbonyl reactivity.
Core rules
- Hybridization tracks geometry: sp³ tetrahedral, sp² trigonal planar, sp linear.
- Stereochemistry: assign R/S with CIP priorities; SN2 gives inversion, SN1 gives racemization with possible carbocation rearrangement.
- SN1 is favored by tertiary substrates, polar protic solvents, and good leaving groups; SN2 by methyl/primary substrates, polar aprotic solvents, and strong nucleophiles. E2 needs an anti-periplanar β-hydrogen and a strong base.
- Markovnikov addition puts H on the carbon with more hydrogens; HBr with peroxides gives anti-Markovnikov addition.
- Aromaticity requires a planar, cyclic, fully conjugated system with 4n + 2 π electrons.
Worked example
Predict the pathway for 2-bromo-2-methylpropane with water versus 1-bromobutane with NaCN in DMSO. The first substrate is tertiary: the C–Br bond ionizes to a stable tertiary carbocation, so water captures it by SN1, giving a racemic or rearranged alcohol mixture. The second is primary and the nucleophile CN⁻ is strong in a polar aprotic solvent, so backside attack dominates: SN2 with clean inversion to pentanenitrile. Steric access and carbocation stability decide the mechanism.
Traps
- Forgetting that a 2° carbocation may rearrange (hydride or methyl shift) before capture.
- Assuming SN1 always gives 50:50 enantiomers; the leaving group can shield one face.
- Applying Hückel's rule without checking planarity: cyclooctatetraene is nonaromatic.
- Confusing pyrrole (lone pair in the π system) with pyridine (lone pair orthogonal, not counted).