Organic Chemistry - Reaction Mechanisms & Stereochemistry

An $S_{N}1$ (Substitution, Nucleophilic, Unimolecular) reaction is a two-step process involving a carbocation intermediate. The rate-determining step is the formation of the carbocation. It favors tertiary ($3^{\circ }$) over secondary ($2^{\circ }$) substrates, is promoted by polar protic solvents, and often leads to racemization (a mixture of enantiomers) if the carbocation is chiral.

An $S_{N}1$ energy diagram typically shows two transition states and one intermediate (the carbocation). The first transition state (leading to the carbocation) is higher in energy, representing the rate-determining step. The second transition state (leading from the carbocation to the product) is lower in energy.

An $S_{N}2$ (Substitution, Nucleophilic, Bimolecular) reaction is a concerted, one-step process where the nucleophile attacks the electrophilic carbon from the backside, simultaneously displacing the leaving group. This leads to an inversion of configuration at the reaction center (Walden inversion). It favors primary ($1^{\circ }$) over secondary ($2^{\circ }$) substrates due to steric hindrance, is promoted by polar aprotic solvents, and is bimolecular (rate depends on both nucleophile and substrate).

An $S_{N}2$ energy diagram shows a single transition state, representing the simultaneous bond breaking and bond forming. There are no intermediates.

An $E1$ (Elimination, Unimolecular) reaction is a two-step process, similar to $S_{N}1$, involving a carbocation intermediate. The rate-determining step is the formation of the carbocation. It favors tertiary ($3^{\circ }$) over secondary ($2^{\circ }$) substrates, is promoted by polar protic solvents, and typically follows Zaitsev's rule, forming the more substituted (more stable) alkene product. It often competes with $S_{N}1$ reactions.

An $E2$ (Elimination, Bimolecular) reaction is a concerted, one-step process where a strong base removes a proton and the leaving group departs simultaneously. It requires an anti-periplanar geometry between the proton and the leaving group for optimal orbital overlap. It favors tertiary ($3^{\circ }$) over secondary ($2^{\circ }$) substrates (though primary can occur), is promoted by strong bases and polar aprotic solvents, and typically follows Zaitsev's rule. It often competes with $S_{N}2$ reactions.

EAS involves two main steps: 1. Attack of an electrophile ($E^{+}$) on the aromatic ring, forming a resonance-stabilized carbocation intermediate called a sigma complex or arenium ion, which temporarily breaks aromaticity. 2. Deprotonation of the sigma complex by a base (often the conjugate base of the electrophile or the solvent) to restore aromaticity and yield the substituted product.

Activating groups are electron-donating (e.g., $-OH$, $-N{H}_2$, alkyl groups) and increase the electron density of the ring, making it more reactive towards electrophiles. They are typically ortho/para directors. Deactivating groups are electron-withdrawing (e.g., $-N{O}_2$, $-COOH$, halogens) and decrease the electron density, making the ring less reactive. Most deactivating groups are meta directors, except for halogens, which are deactivating but ortho/para directors.