Resonance and pi bonds

Moderators: Chem_Mod, Chem_Admin

Rob Tsai 2F
Posts: 99
Joined: Wed Sep 30, 2020 10:09 pm

Resonance and pi bonds

Postby Rob Tsai 2F » Mon Nov 23, 2020 4:42 pm

Because the actual bonds in a molecule with resonance would have a bond length somewhere in between, for example, a single bond and double bond, does this effect formations of pi bonds? I am a little unclear as to how even if a molecule has resonance (like benzene in today's lecture) it would still have pi bonds if the bond isn't exactly a double bond? Does only the side-by-side overlapping of the two-orbitals matter in this case, and not necessarily the fact that a double bond exists?

Posts: 136
Joined: Wed Sep 30, 2020 9:37 pm
Been upvoted: 2 times

Re: Resonance and pi bonds

Postby Claire_Latendresse_1E » Mon Nov 23, 2020 6:48 pm

I think it would probably be fair to say that, in a resonance structure with alternating bonds order like benzene, each of the bonds has a mixture of sigma and pi bond character. As the bond length would be a combination of the shorter double bonds and the longer single bonds, the "actual" bonds would be a combination of a sigma bond and a pi bond. This is a pretty tricky topic, though, so my reasoning may not be a very precise model for resonance.

Megan Singer 3D
Posts: 141
Joined: Wed Sep 30, 2020 9:33 pm
Been upvoted: 1 time

Re: Resonance and pi bonds

Postby Megan Singer 3D » Mon Nov 23, 2020 7:28 pm

Dr. Lavelle talked about molecular orbitals today during the lecture when he was talking about benzene, but he said that's beyond the scope of our course and not to worry too much about it. From the picture in the lecture though, it seems like the 2p orbitals for all of the carbons in benzene were involved in a pi bond that is spread out over the entire molecule. Here's a picture of what that looks like

pi bonds benzene.png

As far as the actual bonds go, it's my understanding that when there is a sigma bond and a pi bond (in other words, a double bond) both of those bonds are present which results in the atoms being pulled closer together and a shorter overall bond length. There is no combination of the sigma and pi bond into some sort of hybridized sigma-pi bond.

Samudrala_Vaishnavi 3A
Posts: 98
Joined: Wed Sep 30, 2020 9:34 pm

Re: Resonance and pi bonds

Postby Samudrala_Vaishnavi 3A » Tue Nov 24, 2020 9:03 am

These are just my thoughts. Resonance structures, by definition, have a partial double bond character so all of the bond lengths are the same overall since they are a mixture or average of all the bond lengths of the lower energy Lewis structures. In this way, sigma and pi bonds wouldn't apply to these models since we can't tell which bond is definitively a double bond or a single bond since all of the bond lengths and character are the same. I would think that this would only apply to the experimental structures where the bond length/types are more definitive. From what we learned in the lecture, the pi bond character is applied to the whole model due to the partial double bonds so, it contributes to the idea that a molecule is restricted to one plane. Just understanding the character of the bonds matters to an extent since helps us get an idea of the shape of the molecule. Beyond that, I don't think understanding the specifics of these bonds in resonance structures will be covered in class.

Return to “Sigma & Pi Bonds”

Who is online

Users browsing this forum: No registered users and 1 guest