Reactivity 3. What are the mechanisms of chemical change? - Electron sharing reactions

Bond Fission: Chemical reactions involve the breaking of covalent bonds. This can occur via homolytic fission, where each atom retains one electron from the shared pair, forming radicals ($X\cdot$), or heterolytic fission, where one atom retains both electrons, forming ions ($X^+$ and $:Y^-$).

Lewis Acids and Bases: A Lewis acid is an electron pair acceptor (e.g., $BF_3$, $AlCl_3$, or $H^+$), while a Lewis base is an electron pair donor (e.g., $:NH_3$, $H_2O:$, or $OH^-$).

Nucleophiles and Electrophiles: Nucleophiles ($Nu^-$) are electron-rich species with a lone pair that seek electron-deficient centers. Electrophiles ($E^+$) are electron-deficient species that seek electron-rich centers like double bonds.

Curly Arrow Notation: Double-headed curly arrows represent the movement of an electron pair ($\curvearrowright$). Single-headed 'fishhook' arrows represent the movement of a single electron ($\rightharpoonup$).

Radical Substitution: Occurs in alkanes (e.g., with $Cl_2$ or $Br_2$ in UV light). It involves three stages: Initiation (homolytic fission), Propagation (regeneration of radicals), and Termination (combination of radicals).

Nucleophilic Substitution ($S_N1$ and $S_N2$): Mechanisms for the reaction of haloalkanes. $S_N2$ is a concerted one-step process favored by primary haloalkanes, while $S_N1$ is a two-step process involving a carbocation intermediate ($C^+$), favored by tertiary haloalkanes.

Electrophilic Addition: The characteristic reaction of alkenes. The high electron density of the $\pi$ bond attracts electrophiles (e.g., $HBr$ or $Br_2$), leading to the addition of atoms across the double bond.