Nucleophile (electron-rich species) attacks the electron-deficient carbon atom bonded to halogen.
Halogen atom acts as a leaving group (halide ion).
$S_N2$ (bimolecular nucleophilic substitution):
- Concerted, one-step reaction. No intermediate formed.
- Rate depends on concentration of both haloalkane and nucleophile.
- Inversion of configuration (Walden inversion).
- Steric hindrance hinders reaction: Methyl halides > Primary halides > Secondary halides > Tertiary halides.
$S_N1$ (unimolecular nucleophilic substitution):
- Two-step reaction. Carbocation intermediate formed in the slow step.
- Rate depends only on concentration of haloalkane.
- Racemization occurs due to planar carbocation (attack from either side).
- Stability of carbocation determines reactivity: Tertiary halides > Secondary halides > Primary halides.
- Allylic and benzylic halides show high reactivity due to resonance stabilization of carbocations.
Ambident Nucleophiles: Possess two nucleophilic centers (e.g., $CN^-, NO_2^-$).
- $KCN$ (ionic) forms alkyl cyanides (C-C bond).
- $AgCN$ (covalent) forms alkyl isocyanides (C-N bond).
- $KNO_2$ forms alkyl nitrites (O-N bond).
- $AgNO_2$ forms nitroalkanes (N-O bond).

(2) Elimination Reactions ($\beta$-Elimination / Dehydrohalogenation):

When a haloalkane with $\beta$-hydrogen is heated with alcoholic $KOH$.
Hydrogen atom from $\beta$-carbon and halogen from $\alpha$-carbon are removed, forming an alkene.
Saytzeff's Rule: The major product is the alkene with the greater number of alkyl groups attached to the doubly bonded carbon atoms.
Example: 2-Bromopentane $\xrightarrow{alc.KOH}$ Pent-2-ene (major) + Pent-1-ene (minor)
Elimination vs. Substitution:
- Depends on nature of alkyl halide, strength/size of base/nucleophile, and reaction conditions.
- Bulky nucleophiles prefer elimination.
- Primary alkyl halides prefer $S_N2$.
- Tertiary alkyl halides prefer $S_N1$ or elimination.

(3) Reaction with Metals:

Grignard Reagents (Alkyl magnesium halides, $RMgX$):
$R-X + Mg \xrightarrow{dry \ ether} R-MgX$
- Highly reactive, react with any source of proton (e.g., water, alcohols) to form hydrocarbons ($R-MgX + H_2O \longrightarrow R-H + Mg(OH)X$).
- Must be prepared in dry ether.
Wurtz Reaction:
$2R-X + 2Na \xrightarrow{dry \ ether} R-R + 2NaX$
- Forms hydrocarbons with double the number of carbon atoms.

(1) Nucleophilic Substitution Reactions:
- Extremely less reactive than haloalkanes due to:
  1. Resonance effect: C-X bond acquires partial double bond character, making cleavage difficult.
  2. Difference in hybridization: Halogen attached to $sp^2$ carbon (more electronegative, shorter