Why does a carbon double bond "break readily" but this is not so for other elements (nitrogen double and triple bonds are stronger)? [duplicate]








**This question already has answers here**:
 
 




[Is the transformation of pi bonds to sigma bonds always energetically favorable?](/questions/16767/is-the-transformation-of-pi-bonds-to-sigma-bonds-always-energetically-favorable)

 (3 answers)
 

Closed last year.



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:](https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Chemical_Bonding/Fundamentals_of_Chemical_Bonding/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 |


 

The contexts are different here. Yes, nitrogen features in many explosive compounds because the explosion forms nitrogen gas. In this case, the N–N triple bond does not even exist in the starting material, so 'all three' of the bonds have to be formed at once, and that releases that huge amount of energy (941 kJ/mol according to your table).


Conversely, when one talks about alkenes or alkynes being relatively reactive, the issue is not about how strong the triple bond itself is. These reactions usually involve a double bond being converted to a single bond, or a triple being converted to a double. Thus, the energetic term under consideration is only the **difference** between the double and a single bond, which is much smaller.


So, it's not so much that nitrogen is special and carbon isn't. It's true that the nitrogen bonds are slightly stronger than the carbon bonds, but that is not really a factor here. The main issue is that you're comparing two different things (formation of a full triple bond vs the breaking of 'half' a double bond, or perhaps even less than half).


 3