Nature of the C--X Bond in Haloarenes

In a haloarene, the halogen is bonded to a ring carbon that is $sp^2$-hybridised and part of the ring's continuous, conjugated $\pi$ system. This has two linked structural consequences that together explain why aryl halides are so much less reactive towards nucleophilic substitution than haloalkanes.

Resonance gives the $\text{C--X}$ bond partial double-bond character. One of the halogen's lone pairs, sitting in a $p$-type orbital, can overlap sideways with the ring's $\pi$ system, delocalising electron density from the halogen into the ring (this is drawn as resonance structures placing a formal negative charge on the ring at the ortho and para positions and a positive charge on the halogen). This partial delocalisation means the $\text{C--X}$ bond is not a pure single bond but has some double-bond character mixed in, which:

• shortens the bond, from roughly $1.77$--$1.78\ \text{\AA}$ typical of a saturated $\text{C--Cl}$ bond in a haloalkane down to about $1.69\ \text{\AA}$ in chlorobenzene, and
• strengthens the bond, making it harder for the halogen to depart as $\text{X}^-$ than the corresponding haloalkane bond.

The $sp^2$ ring carbon itself also matters. An $sp^2$ carbon has more $s$ character in its bonding orbitals than an $sp^3$ carbon, and orbitals with more $s$ character form shorter, stronger sigma bonds -- reinforcing the same trend independent of resonance. …