Lesson 5 of 6
Bond length, energy and reaction
Distinguish bond strength from polarity and avoid single-factor reactivity rules.
A-Level 9476 (2026-2027)
I can define covalent bond energy and length.Syllabus
Syllabus 9476, 2(j). Define covalent bond energy and length.
I can compare bond reactivity using structure.Syllabus
Syllabus 9476, 2(k). Compare bond reactivity using structure.
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 exampleWhy polarity alone fails for C-halogen reactivity
C-F is very polar. Does that make a fluoroalkane the easiest halogenoalkane to hydrolyse?
Try it first. Then show the worked stepsHide the worked steps
- 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.
Check your understanding
C=C has a larger total bond energy than C-C. Why is ethene still more reactive than ethane?
Show the answer
The pi bond is weaker than the sigma bond and its electrons are exposed, so it reacts first.
Total bond energy is the sigma and pi together. An electrophile attacks the exposed pi electrons, and only the pi bond breaks; the sigma framework stays.