Organic Chemistry Mechanisms - SN1/SN2 & E1/E2 Reactions

An electrophile is an electron-deficient species (Lewis acid) that accepts a pair of electrons from a nucleophile to form a new bond. In SN/E reactions, the electrophile is typically the carbon atom bonded to the leaving group.

A reaction where a nucleophile (electron-rich species) replaces a leaving group (electron-withdrawing group) on an electrophilic carbon atom. The general form is $R-X+N{u}^{-}ightarrowR-Nu+X^{-}$. The carbon atom bonded to the leaving group is the electrophilic center.

A reaction where a molecule loses two atoms or groups, typically from adjacent carbons ($\alpha$ and $\beta$), to form a new $pi$ bond (usually a double bond). The general form is $R_{2}CH-C{R}_{2}Xightarrow{R}_{2}C=C{R}_2+HX$.

A nucleophile is an electron-rich species (Lewis base) that donates a pair of electrons to form a new bond with an electrophile. Examples include $O{H}^{-}$, $R{O}^{-}$, $C{N}^{-}$, $N{H}_3$, $H_{2}O$, $R_{3}N$.

A leaving group is an atom or group that departs with the pair of electrons that originally bonded it to the substrate. Good leaving groups are weak bases (e.g., halides $C{l}^{-}$, $B{r}^{-}$, $I^{-}$, tosylates $OT{s}^{-}$, water $H_{2}O$). They are stable once they leave.

The SN2 (Substitution, Nucleophilic, Bimolecular) reaction is a concerted, one-step process. The nucleophile attacks the electrophilic carbon from the backside, simultaneously displacing the leaving group.
Mechanism: Nu:^- + R_3C-X \\xrightarrow{} [Nu \\cdots C \\cdots X]^\\ddagger \\xrightarrow{} Nu-CR_3 + X^- (The transition state shows partial bonds forming and breaking).