Organic Chemistry
Organic Chemistry tests functional-group recognition, stereochemistry, reaction mechanisms, aromaticity, and the electronic effects that control reactivity. NBE Sciences items commonly ask you to rank acidity, assign configuration, or predict a major product.
Core rules
- CIP priorities follow atomic number; ties are broken by the next atoms along the branch.
- For R/S, orient the lowest priority away from you. Clockwise 1-2-3 is R; counterclockwise is S. If the lowest priority points toward you, reverse the result.
- SN2 is concerted, backside attack, second order, and favored by methyl or primary substrates, strong nucleophile, and polar aprotic solvent. SN1 proceeds through a carbocation and is favored by tertiary substrates and polar protic solvents.
- E2 needs an anti-periplanar beta hydrogen and a strong base; E1 shares the carbocation intermediate with SN1.
- Huckel aromaticity: cyclic, planar, fully conjugated, with 4n + 2 pi electrons.
- Electron-withdrawing groups increase acidity; resonance stabilization of the conjugate base is powerful.
Worked example
A chiral carbon bears OH (priority 1) pointing toward you, ethyl (2), methyl (3), and H (4) pointing away. The sequence 1-2-3 is clockwise, so the configuration is R. If the same drawing had H pointing toward you, the observed clockwise order would be reversed to S.
Traps
- Do not rank by chain length alone; apply CIP atom-by-atom.
- Aromatic rings need a continuous p-orbital system and the correct electron count.
- SN1 and E1 share intermediates, so heat and weak nucleophile favor elimination.