Understanding Peptide Group Planarity and Resonance Stabilization

[yungwun22]

Homework Statement

I read that the peptide group that links two amino acids is planar because of the partial double bond character arising from resonance. I don't understand how the planarity arises though. Also, how is resonance stabilizing?

Homework Equations

The Attempt at a Solution

 

[Bacat]

Resonance is stabilizing because it gives multiple configurations for the electrons to arrange themselves in. By Hund's Multiplicity Rule, the more possible states the electrons occupy the more stable they are. Think of it this way:

If you think of an electron on a three-dimensional coordinate grid and then let it orbit around the x-axis, it has a certain probability of leaving the orbit. If you let the same electron orbit around the y-axis, it can also leave the orbit. But if it can now orbit around both axes this increases the overall stability of the system because if it leaves one axis it can orbit around the other one. It can still leave orbit, but it has a lower probability of doing so.

Of course, a bond is not exactly a three-dimensional grid, but you can think of the nuclei of each of the atoms as being planets that the electron orbits around (instead of the x-axis or y-axis). Resonant bonding allows the electron to move to more places without leaving the system (breaking the bond).

Double bonds are planar because of the way that they are formed. It is very difficult to describe without a model, but I will try. Take your index fingers on both hands and put them together by pointing them at each other. You see that you can twist your hands around without breaking the contact of the fingers at the tip. This is like a sigma (single) bond. The atoms are free to rotate around the bond. Now take your index fingers and touch them side by side so the whole length of one finger matches the length of the other. Now you notice that if you try to move your hands you have to move both hands together. You cannot rotate your hands around the bond without breaking it. This is the pi (double) bond. Because the atoms cannot rotate around the bond they are forced into a planar configuration. But it isn't so much a force as a lower energy configuration. If you give the molecule enough energy the atoms will break the double bond and rotate freely. But with a small amount of energy they are likely to maintain the double-bond (planar) configuration because it is stable.

I hope this helps.