Nature of Carbon-Halogen Bond
Halogens are more electronegative than carbon. Therefore, in the carbon-halogen covalent bond, the halogen atom pulls the bonded electrons towards itself. Due to this unequal sharing of bonded electrons, the C–X bond becomes polar in nature. The carbon atom bears a partial positive charge (δ+) (electron deficient) and the halogen atoms bears a partial negative charge (δ–) (electron suficient). Depending upon the type of halogen attached to the haloalkanes, the partial charge on C–X bond and the polarity may differ.
Fluorine is the most electronegative element, attracts the bonded electron pair more strongly than the carbon atom, resulting in a highly polarized bond with a significant dipole moment. On the other hand, as we move down the halogen group, the electronegativity decreases, leading to a decrease in the polarity of the C-X bond. Based on electronegativity difference, C-I bond should be considered as non-polar since both carbon and iodine atoms have same electronegativity i.e. 2.50. This affects the reactivity of these compounds, with C-F bonds being the most stable and least reactive, while C-I bonds being the most reactive and least stable.
The nature of the C-X bond is different for Haloalkanes and Haloarenes.
Haloalkanes: Halogen atoms are more electronegative than carbon. Due to this, they share the pair of electrons that lies closer to the halogen atom. As a result, the halogen carries a small negative charge (i.e. δ-) while the carbon carries a small positive charge (i.e. δ+). Such polarized bond is called a polar covalent bond.
Haloarenes: In the case of haloarenes carbon of the benzene ring attached to a halogen is sp2 hybridized which have a short length and can hold more electron pair and the C-X bond is polar in nature. so, the lone pair of electrons undergo the resonance with the benzene ring due to this delocalization of the lone pair of electrons leading to the formation of double bond character. Due to this double bond character of C- X bond in haloarenes, the C-X bond is shorter in length and stronger than in haloalkanes.
Salient Features of the nature of C-X bond in Haloarenes and Haloalkanes
✍︎ The C-X bond of haloalkanes is more polar than the C-X bond of haloarenes. This is due to the higher electronegativity of halogen over carbon.
✍︎ There is a delocalized lone pair of electrons on the X atom present over the benzene ring which results in the double bond character in the C-X bond of haloarenes.
✍︎ As we move in the periodic table the atomic size down the group increases hence C-X bond length in haloarenes also increases.
✍︎ Also moving down the group the dipole moment decreases as the dipole moment depends on the electronegativity difference. As we know, Cl has less electronegativity than F, but the dipole moment of the C–Cl bond is more than C–F which is an exceptional case.
