Why can the single bond in an amide or ester not rotate?

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Offline mpt-matthew

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Why can the single bond in an amide or ester not rotate?
Biological Sciences - University of Leicester, England.

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Offline mpt-matthew

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Why can the single bond in an amide or ester not rotate?
« Reply #1 on: 22/01/2011 20:24:07 »
This answer was from someone on Yahoo who i asked:

UMMMMMM........they DO rotate. Who told you it didn't? If you take an NMR spectrum of DMF at different temperatures, you will see that the splitting of the 2 methyls varies which is caused by the changing of the roatational rates along the C-N bond as a function of the temperature. Esters totally do it which is one reason they are more reactive than amides. Draw the resonance structures for them:

O................................O-
II..................................I..
C--NR2.......and........C=NR2

O................................O-
II..................................I..
C-OR........and..........C=OR

These are both present in the molecules, the amide's C-N bond rotation is MUCH slower than the C-OR of the ester but they still rotate. There is TONS of experimental data to support this too.
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Thanks to them:
My university professor told me this (im doing biology, but we still have chemistry lectures for stuff).
I presume he told us just to make it simple for us to understand the resonance and different forms.
Unfortunately for him i ask WHY? allot.
Biological Sciences - University of Leicester, England.