SolutionsBond Properties
1. Strength and length
Order the C–F, C–Cl, C–Br and C–I bonds by strength and relate it to their length.
C–F (110 kcal/mol, 1.39 Å) > C–Cl (85, 1.78 Å) > C–Br (71, 1.93 Å) > C–I (57, 2.14 Å). The longer the bond, the poorer the orbital overlap and the weaker the bond.
2. Dipole moments
Fluorine is much more electronegative than chlorine, yet the dipole moment of CH3F (1.81 D) is almost the same as that of CH3Cl (1.86 D). Why?
The dipole moment is the product of charge and distance. The larger charge separation of the C–F bond is offset by its shorter length.
3. The electrophilic centre
In CH3CH2Br, which atom is the electrophilic centre? Which part of the nucleophile attacks it?
The carbon bonded to bromine, which bears a partial positive charge (δ+). It is attacked by the lone pair (or the negative charge) of the nucleophile.
4. Which reacts faster?
Which will react faster with a nucleophile, CH3F or CH3I?
CH3I. Although the C–F bond is more polar, the C–I bond is much weaker and iodide is an excellent leaving group. Reactivity follows the order R–I > R–Br > R–Cl >> R–F.