Answer to Question #155765 in General Chemistry for PARSA SHAFAIT

Question

The liquid film IR spectrum of acetylacetone shows absorption bands at 3400-2500centi per meter ( a very broad band which remains unchanged on dilution), 1710centi per meter and 1600 centi per meter. Assign the group vibrations to these bands.

Answer 1

The easiest thing to do is look at some example spectra. All of them are taken from the Spectral Database for Organic Compounds.

This is 1-propanol, first as a liquid film and then in CCl₄ .

In the liquid, the broad peak at 2933 cm^-1

 is due to intermolecular hydrogen bonding, just as you would see in an IR spectrum of bulk pure water. The CCl₄

spectrum shows that as you reduce the probability of 1-propanol (or any alcohol) molecules contacting each other, the free stretch appears at 

3137 cm^-1. This is similar to water in a gas-phase spectrum.

water is explicitly made from O−H bonds whereas the O−H bond in ethanol only represents a fraction of the bonds in the molecule.

Close. It isn't about the fraction of total bonds, in this case it's simply that water has two hydroxyl stretches and ethanol only has one. As water is added, the intensity of the hydrogen bonding O−H band should increase over the pure alcohol hydrogen bonding band, but we cannot know quantitatively what the increase will be without performing an experiment.

Yes. In the gas phase, the water symmetric and asymmetric O−H stretches are at 3000 cm ^1-and 3156 cm^-which then red-shifts to 3004 cm^-1 in the bulk due to hydrogen bonding. Compare this to 1-propanol, where the "free" and hydrogen bonding values are 3137 cm^-1 and 3040 cm^-1

3040 cm^-1, respectively. The water stretches are blue-shifted (higher in energy) compared to the alcohol.