Haloalkanes and Haloarenes - Nomenclature and nature of C-X bond

Haloalkanes are compounds where one or more hydrogen atoms in an aliphatic hydrocarbon are replaced by halogen atoms ($X = F, Cl, Br, I$). Their general formula is $R-X$.

Haloarenes are compounds where one or more hydrogen atoms in an aromatic hydrocarbon are replaced by halogen atoms. The halogen is directly attached to an $sp^2$ hybridized carbon of the benzene ring.

Classification: Haloalkanes are classified as primary ($1^\circ$), secondary ($2^\circ$), or tertiary ($3^\circ$) depending on the nature of the carbon atom to which the halogen is attached.

IUPAC Nomenclature: Halogens are treated as substituent groups prefixed as fluoro-, chloro-, bromo-, or iodo-. The parent chain is the longest carbon chain containing the halogen, and numbering starts from the end nearest to the substituent.

Nature of C-X bond: The carbon-halogen bond is polar due to the higher electronegativity of halogens compared to carbon, resulting in a partial positive charge on carbon ($C^{\delta+}$) and a partial negative charge on the halogen ($X^{\delta-}$).

Bond Length Trend: Atomic size of halogens increases from $F$ to $I$, thus the $C-X$ bond length increases in the order: $C-F < C-Cl < C-Br < C-I$.

Bond Enthalpy Trend: As bond length increases, bond strength decreases. Therefore, bond enthalpy decreases in the order: $C-F > C-Cl > C-Br > C-I$.

Dipole Moment: Although $F$ is more electronegative than $Cl$, the dipole moment of $CH_3Cl$ ($1.860\,D$) is slightly higher than $CH_3F$ ($1.847\,D$) because of the significantly longer $C-Cl$ bond length.