Organic Chemistry - Reaction Mechanisms & Functional Group Transformations

Carbocation stability increases with substitution: $3^{\circ }>2^{\circ }>1^{\circ }>\text{methyl}$.

Driven by:
1. Hyperconjugation: Delocalization of electrons from adjacent $\sigma$-bonds (like $C-H$) into the empty $p$-orbital.
2. Inductive Effect: Electron-donating alkyl groups push electron density through $\sigma$-bonds toward the positively charged carbon.

A single-step, concerted mechanism where the nucleophile attacks the electrophilic carbon from the backside while the leaving group departs.

A stepwise mechanism involving rate-determining leaving group departure to form a carbocation intermediate, followed by nucleophilic attack.

A concerted, single-step elimination where a strong base abstracts a $\beta$-proton while the leaving group departs, forming a $\pi$-bond.

A stepwise elimination reaction where the rate-determining step is the loss of a leaving group to form a carbocation intermediate, followed by deprotonation by a weak base.

Key Distinction: Steric hindrance drastically decreases nucleophilicity without significantly affecting basicity (e.g., tert-butoxide, $(C{H}_3{)}_{3}C{O}^{-}$, is a strong base but a poor nucleophile).

The total inversion of stereochemical configuration ($R\to S$ or $S\to R$, assuming priority stays constant) at a chiral carbon center during an $S_{N}2$ reaction.

Occurs because the nucleophile must approach from the side $180^{\circ }$ opposite to the leaving group to minimize electrostatic repulsion and overlap with the C-X ${\sigma }^{\ast }$ antibonding orbital.