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Why are carbonyl bonds stronger than those of alkenes? I've read several times that carbonyl bonds are shorter and that you need to apply a higher amount of energy to break them. Why is that so?

Is it due to the electronegativity of oxygen, which pulls the bonding electrons near it, thus attracting the carbon as well?

Buck Thorn
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Αντώνιος Κελεσίδης
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    Yes you are correct the electronegativity difference between C and O imparts partial ionic character to the bond. Consider that there is a partial negative charge on O and a partial positive charge on C and these charges attract each other. – Tan Yong Boon Oct 09 '18 at 23:03
  • I see. So the same can be said regarding the triple bond between carbon and nitrogen when compared to the alkyne bond? – Αντώνιος Κελεσίδης Oct 10 '18 at 07:54
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    Yes you are right. These charge separations may be slight in some cases but they do have significant consequences on bond strength. – Tan Yong Boon Oct 10 '18 at 12:01

3 Answers3

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Why are carbonyl bonds stronger than those of alkenes?

I will use homolytic bond dissociation energies here to quantify bond strength. Other ways to quantify it exist.

I've read several times that carbonyl bonds are shorter and that you need to apply a higher amount of energy to break them. Why is that so? Is it due to the electronegativity of oxygen, which pulls the bonding electrons near it, thus attracting the carbon as well?

There is a pattern of bonds between different atoms being stronger than bonds between identical atoms. If you take a table of bond dissociation energies to calculate the differences $$\ce{2 BDE(X-Y) - BDE(X-X) - BDE(Y-Y)},$$

where BDE stands for bond dissociation energy in kJ/mol, you get the following picture for single bonds:

Single H C N O
C 44
N 183 101
O 344 224 93
F 539 469 248 79

So for example, two C-H bonds are approximately 44 kJ/mol stronger than a C-C bond and a H-H bond.

For double bonds, you get (taking the bond in dioxygen as a double bond):

Double C N
N 210
O 498 298

Bond dissociation energies are from http://ch301.cm.utexas.edu/thermo/#thermochemistry/enthalpy-bonds.html.

There is a trend as you turn two homo-atomic bonds into two "mixed" heteroatomic bonds: The higher the electronegativity difference, the higher the gain in energy. This is a rough calculation because it is based on average bond dissociation energies. To make a more rigorous case, you would have to go to specific examples, e.g. comparing a given amount of ethane and hydrazine with twice the amount amino methane: $$\ce{H3C-CH3 + H2N-NH2 -> 2 H3C-NH2}$$

This only shows the correlation, not the cause. In the most direct way, the reason why two C=O double bond is more stable than a C=C bond and a "O=O" bond is because that's what we get when we calculate an approximation of the solution of the Schrödinger equation. And we can test it experimentally.

Trying to explain it with partial ionic interactions, partial charges, or electronegativity is a slippery slope because these are all constructs we have come up, rather than measurable quantities.

Karsten
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It's stronger because of a dipole moment. oxygen is more electronegative than carbon, so there is a stronger attraction between C and O than a C=C bond.

TheScienceGuy
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    A more polar bond is not necessarily stronger than a less polar one. – Martin - マーチン Nov 12 '18 at 17:48
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    @Martin I agree with your point. None of the answers here seem to establish the necessary causal relationship. Would you care to answer the question, or point me towards a helpful resource? – Ray Bradbury Feb 09 '21 at 04:31
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    @Ray I don't think I could answer this in depth here. Get yourself a book on quantum chemistry and explore for yourself. It'll be more rewarding. I liked Jensen's book quite well: https://chemistry.stackexchange.com/questions/37303/resources-for-learning-chemistry/37304#37304 – Martin - マーチン Feb 09 '21 at 17:55
  • A very simplistic approach is that electrons in MOs are attracted to both nuclei and the added protons in the O atom with similar shielding lowers the energy of the bond a bit. This can be interpreted as increased bond polarity or additional resonance structures, but they are an invention also. – jimchmst Jun 04 '22 at 23:17
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Basically there can be two reasons •electronegativity of oxygen is more than carbon which creates partial negative charge on oxygen and partial positive on carbon and they attract each other resulting in a shorter bond length • i consider the second reason more dominant- as oxygen is in same period as that of carbon and has greater atomic number too that is it is already having shorter size than carbon this means we will have C-C double bond longer than C-O double bond which will increse the bond strengh as bond length is shorter.....similarly the case of C-C triple bond and C-N triple bond

KSHITIJ TRIPATHI
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