Nucleophilic substitution and elimination reactions are among the most fundamental transformations in organic chemistry. Understanding the four competing mechanisms—SN1, SN2, E1, and E2—is essential for predicting reaction products and designing synthetic routes.
SN2 Mechanism (Substitution, Nucleophilic, Bimolecular)
The SN2 mechanism is a one-step, concerted process where the nucleophile attacks the electrophilic carbon from the back side while the leaving group departs from the front side. The reaction rate depends on the concentration of both the substrate and the nucleophile: rate = k[substrate][nucleophile]. Walden inversion occurs, meaning the stereochemistry at the reaction center is completely inverted (R to S or S to R). SN2 is favored by primary alkyl halides, strong nucleophiles, and polar aprotic solvents (acetone, DMF, DMSO).
SN1 Mechanism (Substitution, Nucleophilic, Unimolecular)
The SN1 mechanism is a two-step process: first, the leaving group departs to form a planar carbocation intermediate; second, the nucleophile attacks from either face. The reaction rate depends only on the substrate concentration: rate = k[substrate]. Racemization occurs because attack from either face of the planar carbocation gives a 1:1 mixture of enantiomers. SN1 is favored by tertiary alkyl halides, weak nucleophiles, and polar protic solvents (water, alcohols, acetic acid). Carbocation stability follows the order: tertiary > secondary > primary.
E2 Mechanism (Elimination, Bimolecular)
The E2 mechanism is a one-step, concerted elimination where a base abstracts a proton from the β-carbon while the leaving group departs from the α-carbon, forming a double bond. The reaction rate depends on both the substrate and the base: rate = k[substrate][base]. E2 requires anti-periplanar geometry — the β-hydrogen and the leaving group must be in the same plane and opposite to each other for optimal orbital overlap. E2 is favored by strong, sterically hindered bases such as potassium tert-butoxide (t-BuOK), sodium ethoxide (NaOEt), and sodium hydroxide.
E1 Mechanism (Elimination, Unimolecular)
The E1 mechanism is a two-step process: first, the leaving group departs to form a carbocation; second, a base abstracts a β-proton to form the alkene. The reaction rate depends only on the substrate concentration: rate = k[substrate]. E1 often competes with SN1 because both proceed through the same carbocation intermediate, and higher temperatures favor elimination over substitution. E1 is favored by tertiary alkyl halides, weak bases, and polar protic solvents. The Zaitsev product (more substituted alkene) is the major product due to greater carbocation stability.
Predicting the Dominant Mechanism
The dominant mechanism depends on several factors. Substrate structure: methyl and primary substrates favor SN2; tertiary substrates favor SN1/E1; secondary substrates can follow any pathway depending on conditions. Nucleophile/base strength: strong nucleophiles favor SN2; strong bases favor E2; weak nucleophiles/bases favor SN1/E1. Leaving group: good leaving groups (I-, Br-, OTs-, OTf-) favor all mechanisms while poor leaving groups (OH-, OR-, NH2-) do not participate. Solvent: polar aprotic solvents accelerate SN2 while polar protic solvents accelerate SN1/E1. Temperature: higher temperatures favor elimination over substitution (E2 and E1).
Examples
CH3CH2Br + NaOCH3 undergoes SN2 (primary substrate, strong nucleophile/base) to produce CH3CH2OCH3 + NaBr. (CH3)3CBr + H2O undergoes SN1 (tertiary substrate, weak nucleophile) to produce (CH3)3COH + HBr. CH3CH2Br + t-BuOK follows E2 (strong, hindered base) giving CH2=CH2 + KBr + t-BuOH. (CH3)3CBr + EtOH with heat follows E1 (tertiary, weak base, heat) producing (CH3)2C=CH2 + HBr.
