Please help with this Ochem problem regarding IR frequency! The stretching frequ
ID: 895831 • Letter: P
Question
Please help with this Ochem problem regarding IR frequency!
The stretching frequency range for a carbonyl group, C=0, is greater than that for C-O. For the former the range is 1670-1820Cm^-1 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^-1. 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 "paradox" described above. Explanation:Explanation / Answer
Solution-
The infra-red (IR) covers the range of the electromagnetic spectrum between 0.78 and 1000 mm.
The most useful I.R. region lies between 4000 - 670cm-1
IR spectroscopy, wavelength is measured in wavenumbers which have the units cm-1.
Wave number = 1 / wavelength in cm
Absorption of IR is restricted to compounds with small energy differences in the possible vibrational and rotational states. Because of vibration & rotation within a molecule must cause a net change in the dipole moment of the molecule. The alternating electrical field of the radiation interacts with fluctuations in the dipole moment of the molecule. If the frequency of the radiation matches the vibrational frequency of the molecule then radiation will be absorbed, causing a change in the amplitude of molecular vibration.
Absorption bands associated with C=O bond stretching are usually very strong because a large change in the dipole takes place in that mode.
Some General Trends:
i) Stretching frequencies are higher than corresponding bending frequencies. (It is easier to bend a bond than to stretch or compress it.)
ii) Bonds to hydrogen have higher stretching frequencies than those to heavier atoms.
iii) Triple bonds have higher stretching frequencies than corresponding double bonds, which in turn have higher frequencies than single bonds.
iv)Double bond have higher frequency than single bond.
Carbonyl (C=O) higher frequency than (C-O) ether,