Reaction Mechanisms (SN1, SN2, E1, E2) Cheat Sheet

Key Facts

• $S_N1$ is a two-step substitution mechanism with carbocation formation, and its rate law is $\text{rate} = k[\text{RX}]$.
• $S_N2$ is a one-step substitution mechanism with backside attack, and its rate law is $\text{rate} = k[\text{RX}][\text{Nu}^-]$.
• $E1$ is a two-step elimination mechanism with carbocation formation, and its rate law is $\text{rate} = k[\text{RX}]$.
• $E2$ is a one-step elimination mechanism, and its rate law is $\text{rate} = k[\text{RX}][\text{Base}]$.
• The usual substrate reactivity order for $S_N1$ and $E1$ is $3^\circ > 2^\circ \gg 1^\circ$ because more substituted carbocations are more stable.
• The usual substrate reactivity order for $S_N2$ is $\text{methyl} > 1^\circ > 2^\circ \gg 3^\circ$ because steric hindrance blocks backside attack.
• $S_N2$ at a chiral center causes inversion of configuration, while $S_N1$ often gives racemization because the carbocation is planar.
• For many $E2$ reactions, the major alkene follows Zaitsev's rule, so the more substituted alkene is favored unless a bulky base favors the Hofmann product.

Vocabulary

Nucleophile

A nucleophile is an electron-rich species that donates an electron pair to form a new bond.

Leaving group

A leaving group is an atom or group that departs with an electron pair during substitution or elimination.

Carbocation

A carbocation is a positively charged carbon intermediate, usually written as $\text{C}^+$, that is stabilized by alkyl substitution and resonance.

Rate law

A rate law shows how reaction rate depends on reactant concentrations, such as $\text{rate} = k[\text{RX}][\text{Nu}^-]$ for $S_N2$.

Anti-periplanar

Anti-periplanar describes two bonds in the same plane but pointing in opposite directions, a geometry often required for $E2$ elimination.

Zaitsev product

The Zaitsev product is the more substituted alkene that often forms as the major product in elimination reactions.