Topic 5 of 6
Bond length, energy and reaction
Distinguish bond strength from polarity and avoid single-factor reactivity rules.
A-Level 9476 (2026-2027)
A strong bond is not automatically a non-polar bond
Energy, length and polarity answer different questions about a reaction.
Bond length is the equilibrium distance between two bonded nuclei. Bond energy is the energy required to break one mole of specified covalent bonds in gaseous species. A mean bond energy averages the same bond type across compounds; its use in a calculation is approximate, not an exact measurement for every molecule.
For bonds between the same elements, higher bond order generally means a shorter, stronger bond: C≡C is shorter and has greater total bond energy than C=C, which is shorter and stronger than C-C. But the pi part of a multiple bond can react while the sigma framework remains. A large total double-bond energy does not make an alkene generally less reactive than an alkane.
Worked example
Why polarity alone fails for C-halogen reactivity
C-F is very polar. Does that make a fluoroalkane the easiest halogenoalkane to hydrolyse?
- Polarity creates an electron-deficient carbon that can attract a nucleophile.
- However, the C-F bond is particularly short and strong, so breaking it is difficult.
- Down the common Cl, Br, I series, C-halogen bonds generally get longer and weaker; comparable iodoalkanes often hydrolyse faster.
No. Bond polarity identifies a possible attack site; bond strength and the reaction pathway also control the rate.