[ForumLoverGuy]

1. The problem statement, all variables and given/known data
1) Methyl vinyl ether (see sketch below) has been shown by spectroscopic methods to be most stable in the cis-conformation with a planar heavy atom skeleton. The bond angle ²C-³O-⁴C was found to be close to 120 degrees.



i) What C-O-C bond angle would you expect based on a simple VSEPR analysis?

ii) What hybridisation state should you use in valence bond theory to explain the observed C-O-C bond angle?

iii) Carry out an analysis on the pi-electron system in this molecule using the sigma-pi separation simplification and draw a simplified MO diagram for the pi-system. Based on your analysis explain what is the reason for the observed 120 degree bond angle?

iv) Based on your analysis in iii) write good contributing resonance structures and explain why the structures you have written are considered good structures.


2. The attempt at a solution

i) 2 bond pairs and 2 lone pairs on oxygen atom therefore bond angle of slightly less than 109.5 degrees, around 104 degrees.

ii) 120 degrees bond angle therefore sp2 hybridisation.

iii) From what I understand, the sigma-pi separation means that you can neglect interactions between the Pz and Px,y orbitals?

The CH2 carbon would form SP2 hybrid orbitals.


The CH3 carbon would form SP3 hydrid orbitals and would then form 3 sigma bonds with the hydrogen leaving only one orbital to interact with the oxygen. I assume that the oxygen S orbitals do not match the energy level of the carbon and will form non-bonding orbitals. Therefore the C-O sigma bond would be SP3-P.


I am unsure how to use this knowledge to actually draw this MO diagram? I have tried an attempt below. I can draw MO diagrams for simple diatomic molecules. And how would the MO diagram explain the observed 120 degree bond angle?


iv) I haven't used the MO diagram to draw these resonance structures but I have come up with two based on the octet rule:

[ForumLoverGuy]

1. The problem statement, all variables and given/known data
Cyanogen (NCCN) is a molecule that consists of two CN groups.
Using a Molecular Orbital approach, give an explanation for the unusual C-C bond length of 138 pm.

2. The attempt at a solution
I know that the MO diagram for CN- is this:

I am unsure how to draw the MO diagram for cyanogen. I know that the higher the bond order, the shorter the bond length.
I have tried to draw a MO diagram for cyanogen by combining two of the MO diagram for CN:


Not sure if this is necessary but I have drawn the following resonance structures:

I think that structure (I) contributes the most as structures (II) and (III) have a positive charge on the nitrogen.

I think the C-C bond in cyanogen is shorter due to delocalisation across it but I don't know how to explain this using Molecular Orbital theory.

[Arkcon]

I hope you don't mind my merging your two topics, they seem to covert much of the same ground, and it would be convenient to have all the answers together.

[Irlanur]

Forgive me for no looking at your solution in detail, but I think you're thinking way too far.

Regarding the methyl vinyl ester:
The question is "Carry out an analysis on the pi-electron system". So only look at the pi-system! which orbitals contribute to it?