[rleung]

Hi,

I am a little confused about this.  Any help would be much appreciated  It asks me to list the following in order of decreasing frequency of the carbon-oxygen double bond stretch:

CH3COOCH3
CH3COCl
CH3COH
CH3CONH2

The answer is CH3COCl>CH3COOCH3>CH3COH>CH3CONH2.  I do not seem to understand why CH3COH would come in third on the list.  Since there is absolutely no electron delocalization in order to decrease the double-bond character of the C=O, shouldn't it be hardest to stretch (harder than CH3COCl and CH3COOCH3, both of which can delocalize the electrons of the C=O, decreasing its double-bonded character).  Any ideas?  Thanks so much.

Ryan

[movies]

Shouldn't it be H₃CCO₂H?

[plu]

I believe he's referring to CH₃CHO which would have a C=O stretch at a lower frequency than both CH₃COOCH₃ and CH₃COCl.  As to the reason this is this case, I am at a loss as well  

[Winga]

CH3COCl>CH3COOCH3>CH3COH>CH3CONH2

More electronegative the atom attached to carbonyl carbon, more partial positive charge the carbon carried. Therefore, this increases the charge-charge interaction between C & O and makes the C=O stronger.
Beside the inductive effective, there is also a resonance effect (mesomeric effect), which weaken the C=O bond by delocalization of electron density.

According to your answer, I got this explanation.

1. CH3COH, you can use it as reference.

2. CH3COCl, the inductive effect is greater than resonance effect (resonance is not appreciated, Cl is Period 3, poor overlapping of p-orbitals), so, inductive effect is predominant.

3. CH3COOCH3, the inductive effect is also greater than resonance effect (but resonance is more available, O is Period 2), therefore, overall the inductive effect is still predominant, but less effective than CH3COCl (although O is more electronegative than Cl).

4. CH3CONH2, resonance effect > inductive effective.