[Orgo] Swern Oxidation - Why O reforms double bond?

[R35]

Hello. I noticed the title may be confusing, but here is what I am asking:

In the Swern Oxidation, DMSO reacts with (COCl)2. After the DMSO attacks the carbonyl carbon, the oxygen on the (COCl)2 becomes anionic.


Now, why does the O reform the double bond to kick off Cl? Why does this step happen? I've made a screen shot:

http://imgur.com/63vIeiy

Why does THAT happen the way it does? Thanks!

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[polyatomic]

Chlorine is a VERY good leaving group because of its high electronegativity. At the same time, oxygen has a negative charge that would be like to be neutralized, which is done by forming a double bond.

These two favorable forces cause the oxygen to come down, reform the double bond and kick off the chlorine.

 

[R35]

Thanks a lot!

(Sorry for the late reply!)