A level chemistry student here, fairly basic question. We always learnt (from GCSE bond enthalpies) that "Bigger bonds are stronger", so to speak; my teacher often quoted $\ce{N#N}$ as very unreactive as the triple bond is very strong. However, now we are covering organic chemistry in a bit more depth at A-level, they are saying that the alkanes are generally unreactive as the $\sigma$-bond (which I read as: single bond) is fairly strong, but the $\pi$-bond (read: double bond) has more exposed electrons and so breaks more easily.
This is quite hard to accept, considering it contravenes everything I've ever learnt about bonds (more shared electrons will surely exert a stronger electrostatic attraction and have a higher bond enthalpy, no?) but then again we are not taught about bond energies in any great detail. I am hoping someone could explain or motivate this a little (I imagine a fully correct answer would be way over my head ;)): why are double/triple bonds sometimes weaker, but most of the time stronger, than single bonds? And what properties of the different elements (e.g. this holds for carbon but not for nitrogen) cause the discrepancy?
chem.libretexts.org Bond_Energies:
Bond | Bond energy [kJ/mol] | Incremental bond energy [kJ/mol] |
---|---|---|
$\ce{C-C}$ | 347 | - |
$\ce{C=C}$ | 614 | 264 |
$\ce{C#C}$ | 839 | 225 |
$\ce{N-N}$ | 160 | - |
$\ce{N=N}$ | 418 | 258 |
$\ce{N#N}$ | 941 | 523 |