what's the stronger bond?

[ladymiresa]

Hi guys, I'm getting all turned around in my logic and could use some insight.

My MCAT prep book says that a C-N bond is stronger than a C-O bond, and uses this as a reason why when an amide is under nucleophilic attack, the O is the leaving group.

It makes sense to me that N would bond to C better than O, because O is more electronegative and thus wouldn't want share it's electrons as well, weakening the bond.

But then why does C-O have a greater bond energy than C-N?

What am I misunderstandinggggg. Thanks.

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[Doctor or bust]

My understanding was that the greater the difference in electronegativity between atoms, the stronger the bond. The radius of the atoms also comes into play in bond strength but I believe that the electronegativity of each plays a larger role. Thus, since C-O has a greater electronegativity difference than C-N, it would have a larger bond energy.

The electronegativity argument is highlighted with C-F because they have pretty similar atomic radii and a large electronegative difference which leads to their high bond energy.

The atomic radius argument is shown with the bond energy of C-H because hydrogen is much smaller than carbon and thus puts up little resistance to being bound to another atom.

Does this help?

 

[BerkReviewTeach]

Hi guys, I'm getting all turned around in my logic and could use some insight.

My MCAT prep book says that a C-N bond is stronger than a C-O bond, and uses this as a reason why when an amide is under nucleophilic attack, the O is the leaving group.

It makes sense to me that N would bond to C better than O, because O is more electronegative and thus wouldn't want share it's electrons as well, weakening the bond.

But then why does C-O have a greater bond energy than C-N?
What am I misunderstandinggggg. Thanks.

In the specific case of an amide, you are comparing the pi-bond to O breaking to the sigma-bond to N breaking. This is an isolated case where the pi-bond of the carbonyl breaking supports the idea that the C=O pi-bond is weaker than the C-N sigma bond. But that is a specific case involving a pi-bond compared to a sigma bond. Is the question you are talking about making that comparison?

Overall, the chart you show indicates that a C-O sigma bind is stronger than a C-N sigma bond. C-O bonds are shorter (due to the smaller atomic radius of O compared to N), which makes them stronger. As mentioned above, the difference in electronegativity actually makes it harder for carbon to get an electron back from the other atom when breaking in a hemolytic fashion. This makes the C-O sigma bond stronger than the C-N sigma bond.