Organic chemistry
SN1/SN2 substitution, E1/E2 elimination
Compare concerted and stepwise substitution and elimination, and predict their rates, stereochemistry, and product distributions.
IntuitionIntuition: competing paths for a leaving group
A nucleophile can replace a leaving group at carbon, while a base can remove a neighboring proton as the leaving group departs. The substrate, reagent, solvent, and temperature together determine which pathway wins.
SchoolSchool level: identify what changes
Definition: Nucleophilic substitution
A nucleophile replaces a leaving group bonded to an electrophilic carbon.
In an elimination, atoms are removed from adjacent carbons and a carbon–carbon π bond forms. Substitution changes the group attached to carbon; elimination usually increases unsaturation.
UndergraduateUniversity: four mechanistic archetypes
| Path | Rate law | Key stereochemical outcome | — | — |
|---|---|---|---|---|
| SN2 | k[RX][Nu⁻] | backside attack; inversion | ||
| SN1 | k[RX] | planar carbocation; often partial racemization | ||
| E2 | k[RX][base] | anti-periplanar geometry favored | ||
| E1 | k[RX] | carbocation intermediate; rearrangement possible |
Example
A secondary alkyl bromide reacts with a strong, bulky base in ethanol on heating. Which pathway/product tendency is favored?
Solution
E2 is favored because the reagent is a strong base and the bulky base is a poor nucleophile; an alkene forms, often with less-substituted (Hofmann) product favored by steric effects. Exact selectivity depends on substrate and conditions.
AdvancedAdvanced: structure, solvent, and selectivity
SN2 requires accessible backside approach to σ* C–leaving-group and is slowed by substitution at the reacting carbon. SN1/E1 require ionization; polar protic media often stabilize ions, but nucleophile activity and ion pairing matter. E2 requires an aligned C–H bond and leaving group; cyclic conformations can constrain which β-hydrogen is removed.