Haloalkanes and Haloarenes - Physical and Chemical Properties

The $C-X$ bond is polar due to the electronegativity difference between Carbon and Halogen ($X$). The dipole moment follows the order: $CH_3Cl > CH_3F > CH_3Br > CH_3I$.

Boiling Points: For the same alkyl group, the boiling point increases with the atomic mass of the halogen: $R-I > R-Br > R-Cl > R-F$. For isomeric haloalkanes, boiling point decreases with branching because the surface area decreases, weakening Van der Waals forces.

Solubility: Although polar, haloalkanes are only slightly soluble in $H_2O$ because they cannot form hydrogen bonds with water molecules nor break the existing $H$-bonds between water molecules.

$S_N2$ Mechanism (Bimolecular Nucleophilic Substitution): Occurs in a single step via a pentacoordinate transition state. It involves the inversion of configuration (Walden Inversion). Reactivity order: $CH_3X > 1^\circ > 2^\circ > 3^\circ$.

$S_N1$ Mechanism (Unimolecular Nucleophilic Substitution): Occurs in two steps involving a carbocation intermediate. It usually results in racemization. Reactivity order (based on carbocation stability): $3^\circ > 2^\circ > 1^\circ > CH_3X$.

eta-Elimination (Dehydrohalogenation): When heated with alcoholic $KOH$, haloalkanes form alkenes. According to Saytzeff's Rule, the highly substituted alkene (more alkyl groups attached to the double-bonded carbons) is the major product.

Reactivity of Haloarenes: They are extremely less reactive towards nucleophilic substitution due to resonance (partial double bond character of $C-X$), $sp^2$ hybridization of carbon, and instability of the phenyl cation.

Electrophilic Substitution: In haloarenes, halogens are deactivating but $ortho$- and $para$-directing due to the $+R$ effect (resonance) which increases electron density at $o$ and $p$ positions.