[M249]

Hi all,

I'm trying to do my gchem lab write up, and I'm stuck on a few questions. In class I'm pretty sure I remember talking about how resonance effects will weaken a C=O (carbonyl) bond, and inductive effects will make it stronger (we were looking at stretching frequencies). Is this right?

I also have to explain why resonance and inductive effects make the bond stronger or weaker, but I am stuck on that and cannot seem to find a definitive answer anywhere. Thanks for any help you can give!

[nj_bartel]

I think your problem lies in that both resonance and inductive effects and either strengthen the double bond of a carbonyl or weaken it.  What happens depends on whether the effect is electron donating or electron withdrawing.

If the carbonyl carbon is made more electron deficient, do you think that the double bond form of the carbonyl would be strengthened or weakened?

If the carbonyl carbon is made more electron rich, do you think that the double bond form of the carbonyl would be strengthened or weakened?

[M249]

I would think that making the carbonyl carbon more electron deficient would weaken the bond and making it more electron rich would strengthen it. Is this on the right track?

[nj_bartel]

Edit: I take that back.  I'm just going to explain, because I fear me trying to lead you in the right direction isn't going well (my fault).

A carbonyl carbon is electron deficient, due to having a double bond to a more electronegative atom (oxygen).  The carbonyl bond has two resonance structures - one with the double bond between the carbon and the oxygen, and one with a single bond between the carbon and oxygen with a formal negative charge on the oxygen and a formal positive charge on the carbon.  Adding electron donating substituents to the carbonyl carbon makes the overall stability of the carbonyl group increase, but it weakens the importance of the resonance structure where the carbon and oxygen have a double bond.  This is because the carbon is now more electron rich and more willing to give up the pi bond electrons to oxygen.  Vice versa applies too.  When you add electron withdrawing substituents to the carbonyl carbon, it becomes even more electron deficient and is less willing to donate the pi bond electrons to the oxygen - the importance of the c=o double bond resonance structure increases.

With IR spectroscopy, a more importance c=o resonance structure means a higher stretching frequency because double bonds are stronger than single bonds.

[M249]

Thank you for your reply. I apologize if I am frustrating you, I'm just trying to apply what you said to my lab worksheet. I think I understand what you said in your last post, but I'm still a little confused as to what I would say to answer my question. The question is as follows on the lab:

"Consider the carbonyl bond. If a molecule has resonance structures that involve this carbonyl bond, how does that impact the strength of the carbonyl bond? Also, explain why the bond strength is impacted in this manner."

Then there is a second question that reads exactly the same, except instead of resonance it addresses inductive effects.

[nj_bartel]

You're not frustrating me, no worries.  I was just having a hard time leading you to the explanation I gave rather than just typing it out.

I'd address what a resonance EDG/EWG and inductive EDG/EWG would do in each case.

So you'd end up with

1) If a resonance EDG was attached to the carbonyl carbon....
    If a resonance EWG was attached to the carbonyl carbon....

2) If an inductive EDG was attached to the carbonyl carbon....
    If an inductive EWG was attached to the carbonyl carbon....