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Topic: A question about the IR spectrum of benzyl chloride  (Read 9022 times)

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Offline sdfsfgfdgdfdf

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A question about the IR spectrum of benzyl chloride
« on: August 28, 2012, 01:11:44 PM »
I have a question about the infrared spectrum of benzyl chloride. The band from the C=O stretch vibration in the spectrum is a doublet of approximately equal strength. It is the result of Fermi resonance between the C=O band and an overtone of a strong band around 875cm-1. I have attached a photo of the band that I am interested in. My question is this:

What vibrational mode does that band at 875 cm-1 come from?

In some sources it says that it comes from an aryl-C stretch, in others that it comes from a C-Cl stretch. Is either of these true, or does it come from some other vibrational mode?   
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Offline Schrödinger

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Re: A question about the IR spectrum of benzyl chloride
« Reply #1 on: September 05, 2012, 06:30:45 AM »
My money would be on C-Cl bond stretching frequency. Maybe it has a partial double bond character due to resonance between Cl's lone pairs and C=O pi bond. This could be a plausible explanation for why the C-Cl band, that usually comes between 600-800 gets shifted to 875. Hence, the overtone of C-Cl stretch... Please correct me if I'm mistaken.
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Offline sdfsfgfdgdfdf

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Re: A question about the IR spectrum of benzyl chloride
« Reply #2 on: September 11, 2012, 09:10:55 PM »
I searched a little more on this after I posted the question, and I also think that C-Cl is the most likely vibration. I'm not sure about resonance though, I think resonance typically lowers absorption frequency. Maybe the rise in frequency is due to the electronegativity of the oxygen on C=O, which makes the C=O carbon more electronegative than a CH3- carbon. I haven't seen cases of Cl participating in resonance so far, but then again, I'm just a student...
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Offline Schrödinger

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Re: A question about the IR spectrum of benzyl chloride
« Reply #3 on: September 12, 2012, 07:09:29 AM »
The halogens do take part in resonance. Consider chlorobenzene. The reason the halo group is ortho/para directing is the resonance effect. The lone pairs on Cl atom are donated to the ring via resonance. (although there is a net reduction in electron density due to inductive effect, which predominates over resonance in the case of halogens thanks to their high electronegativity).

Similarly, resonance is very much plausible when it comes to the acid chloride system.
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Offline sdfsfgfdgdfdf

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Re: A question about the IR spectrum of benzyl chloride
« Reply #4 on: September 22, 2012, 11:29:05 PM »
You're right I forgot about resonance effects of chlorine, and I was just reading that recently. Sorry :)
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