Carbonyl shifting in IR spec

[super112]

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Why would a carbonyl IR value be shifted to the right (from 1710 to 1685) and not the left if there is conjugation or a benzene ring? I was thinking it would shift left.

 

[DAT Destroyer]

Why would a carbonyl IR value be shifted to the right (from 1710 to 1685) and not the left if there is conjugation or a benzene ring? I was thinking it would shift left.

A great question......If we have a true carbonyl with much double bond character we see a signal around 1720 such as that exhibited in a ketone. Conjugation with a group such as a benzene ring allows for resonance. This means the carbonyl group has LESS double bond character since the electrons will now be shared. Less double bond character translates into a weaker bond, hence a lower IR wave number. ...perhaps around 1685.

Hope this helps

Dr. Jim Romano

 

[super112]

Makes sense now, thanks!