Nature of C-X Bond

Why the C–X Bond Is Polarised

In every haloalkane, a halogen atom (F, Cl, Br or I) is joined to carbon by a single covalent bond. Halogens are considerably more electronegative than carbon, so the shared electron pair of this bond is pulled closer to the halogen. The bond is therefore not purely covalent but polar covalent: carbon develops a small deficiency of electron density and the halogen develops a corresponding excess.

This is written using the partial-charge notation:

Here $\delta^{+}$ marks the carbon atom carrying a fractional positive charge and $\delta^{-}$ marks the halogen carrying an equal and opposite fractional negative charge. This permanent charge separation is the single most important structural feature of the C–X bond, and everything about how haloalkanes behave chemically follows from it.

How Bond Length and Bond Strength Change Down the Group

Halogen size increases steadily on descending Group 17: fluorine has the smallest atomic radius and iodine the largest, with chlorine and bromine in between. Since bond length depends on the sizes of the atoms being joined, the carbon–halogen bond lengthens as the halogen gets bigger — the C–F bond is the shortest of the four, and the C–I bond is the longest.

A shorter bond generally overlaps atomic orbitals more effectively and is harder to stretch or break, so bond strength (bond enthalpy) runs opposite to bond length here: the C–F bond, being the shortest, is also the strongest, while the C–I bond, being the longest, is also the weakest. Going from C–F to C–I, bond length rises and bond enthalpy falls together, in step with increasing halogen size.

The dipole moment (a measure of how polarised the bond is, given the separated charge and the distance between the charge centres) does not follow this same simple size-based trend. It is highest for the C–Cl bond rather than for C–F, and then decreases through C–Br to C–I. This shows that polarity depends on both the electronegativity difference and the bond length together, not on atomic size alone — the exact figures for bond length, bond enthalpy and dipole moment across all four carbon–halogen bonds are collected in the accompanying data table.

Why This Polarity Drives Haloalkane Reactivity

Because the halogen end of the bond is electron-rich ($\delta^{-}$) and the carbon end is electron-poor ($\delta^{+}$), the carbon atom becomes an attractive site for any electron-rich species looking to donate a pair of electrons. Such an electron-rich attacking species is called a nucleophile, and it is drawn to the $\delta^{+}$ carbon precisely because of this polarisation.

• When a nucleophile attacks that carbon and displaces the halogen (which leaves as a stable halide ion, taking the bonding pair of electrons with it), the halogen is substituted by the incoming group — this is nucleophilic substitution. …