Introductory Reaction Mechanism and Kinetics

I. Nature of Covalent Bonds

• Non-Polar Covalent Bonds: Formed when atoms involved in bonding are the same (e.g., H–H). The electron cloud is symmetrically disposed between the two atoms.
• Polar (Partial Ionic) Covalent Bonds: Formed between dissimilar atoms with different electronegativities (e.g., H–Cl). The electron cloud is displaced toward the more electronegative atom, creating a non-symmetrical bond with partial charges ($\delta+$ and $\delta-$).

II. Fundamental Electronic Effects

Electronic effects describe how electron density shifts within a molecule, which ultimately determines its reactivity.

1. Inductive Effect:

   • Definition: A permanent effect where a non-polar bond (like C–C) becomes polarized due to the polarity of an adjacent bond (like C–Cl).
   • Mechanism: It occurs exclusively through sigma (single) bonds and its influence diminishes as the distance from the polar group increases.
   • Negative Inductive Effect (-I): Caused by electron-withdrawing groups such as halogens (-Cl, -F, -I, -Br), $-NO_2$, $-CN$, and $-COOR$.
   • Positive Inductive Effect (+I): Caused by electron-donating groups, primarily alkyl groups (e.g., $-CH_3$, $-C_2H_5$). The magnitude of the +I effect follows the order: tertiary alkyl > secondary alkyl > primary alkyl.

2. Electromeric Effect:

   • Definition: A temporary effect involving the complete transfer of a shared pair of electrons in a multiple bond (double or triple) to one of the atoms.
   • Trigger: It only occurs at the approach of an attacking reagent and is restored once the reagent is removed. It never occurs in single (sigma) bonds.

3. Resonance (Mesomeric) Effect:

   • Definition: The ability of pi electrons in a conjugated system to move between atoms, resulting in a simultaneous decrease in electron density at one position and an increase elsewhere.
   • Conjugated vs. Isolated Systems: Resonance occurs in conjugated systems (where double bonds are separated by exactly one single bond). It does not occur in isolated systems (double bonds separated by more than one single bond).
   • Rules for Resonance: Nuclei positions remain fixed; only pi electrons or unshared valence electrons are delocalized.
   • Types:
     • +R or +M: Electron-donating groups that displace electrons away from themselves (e.g., $-NH_2$, $-OH$).
     • -R or -M: Electron-withdrawing groups that pull electron displacement toward themselves (e.g., $-NO_2$).

4. Steric Effect:

   • Definition: The hindrance caused by bulky groups that can modify or nullify electron availability.
   • Impact: If bulky groups prevent p-orbitals from becoming parallel, delocalization (resonance) can be inhibited.

III. Bond Cleavage and Reactive Intermediates

Chemical reactions proceed through the breaking of bonds to form intermediates.

• Homolytic Cleavage: The bond breaks such that each atom receives one electron from the shared pair, forming free radicals ($R^\bullet$).
• Heterolytic Cleavage: The bond breaks unequally. If the carbon retains the electron pair, it becomes a carbanion ($C^-$); if it loses the pair, it becomes a carbocations/carbonium ion ($C^+$).

Stability Orders of Intermediates: