someone please help me 10) The stretching frequency range for a carbonyl group,

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someone please help me

10) The stretching frequency range for a carbonyl group, CoO, is greater than that for C-o. For the former the range is 1670-1820cm1 depending on whether the carbonyl group is a part of an aldehyde, ketone, ester, amide, carboxylic acid, or anhydride. For the latter the range is 1100- 1300cm. Note that in each carbon and oxygen are the two atoms participating in the two different bonds. Yet, the IR stretching frequency is different. Hmmm. .how can that be? Offer an explanation to the "parodox described obove. Explanation

Explanation / Answer

Ester has a stretch of 1000-1300 cm-1 with an intensity of two bands or more but in carbonyl we have a strong intensity stretch of 1670-1820 cm-1.

Some bonds absorb infrared light more strongly than others, and some bonds do not absorb at all. In order for a vibrational mode to absorb infrared light, it must result in a periodic change in the dipole moment of the molecule.  In general, the greater the polarity of the bond, the stronger its IR absorption. The carbonyl bond is very polar, and absorbs very strongly. The carbon-oxygen single bond in ether in contrast, is much less polar, and thus a stretching vibration does not result in a large change in the overall dipole moment of the molecule.

Ester groups absorb rather weakly compared to carbonyls.So we have different IR stretching frequency .

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