Anhydride IR

[justadream]

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"Anhydride: the double C=O doesn't show up as a single band. Instead, 2 bands shows up between 1700 cm-1 and 1800 cm-1."

That is from mcat-review.

But here, the C=O group looks identical...

 

[The Brown Knight]

On the mcat-review, were there substituents other than methyl on the carbonyl?

 

[justadream]

@The Brown Knight

Nah, it was just listed as a general statement.

 

[Shirafune]

Did a quick google search and found this:
http://books.google.com/books?id=Ywzf4GyoUaoC&pg=PA106&lpg=PA106&dq#v=onepage&q&f=false

Discussion of anhydride IR spectra begins at the bottom of pg 105, but the appearance of two bands is specifically explained on pg 106.

Basically, the two carbonyl bonds can stretch in the same direction (symmetrically) or in different directions (asymmetrically), giving rise to two different bands.