Question

Assign the peak in the IR spectrum of dibenzalacetone that is at 1639 cm^-1. Is this...

Assign the peak in the IR spectrum of dibenzalacetone that is at 1639 cm^-1. Is this the normal absorption for this functional group? In benzaldehyde there is an absorption at 2738 cm^-1 but it is absent in the IR of dibenzalacetone. Why?

Homework Answers

Answer #1

Benzylideneacetone is Ph-CH=CH-CO-CH=CH-Ph. The peak at 1639 cm-1 is the ketone C=O stretch of this compound is instead of the normal 1700 cm-1 is because of conjugation. The ketone of this molecule is highly conjugated through the double bond and into the aromatic ring. Hence the electrons of the ketone carbonyl are delocalized throughout the molecule and into the aromatic ring thus, lowering its observed IR wavenumber to 1639 cm-1.

The peak at 2738 cm-1 in benzaldehyde is due to aldehydic –C-H stretching. It is absent in Benzylideneacetone since it does not contain any aldehyde functional group.

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