Question

A compound is known to be one of those shown here. What absorption bands in its IR spectrum allow you to identify the compound?

Short answer

A would have the band at 2700 cm^-1but not at 1380 cm^-1 .

B would have the band at 1380cm^-1 but not the one at 2700 cm^-1 .

C would have band at 2700cm^-1 and the one band at 1380 cm^-1 .

Step-by-step solution

IR absorptions in Aldehyde (CHO) and methyl group

In the IR spectra of aldehyde a peak usually appears around 2700 cm^-1. Simple ketone and aldehydes show a very strong band for the carbonyl group (C=O) that appears in the range of 1740-1710 cm^-1. This band is shifted to lower frequencies with conjugation to a C=C phenyl group. A very important doublet can be observed in the C-H stretch region for the aldehyde C-H near 2850 and 2750 cm^-1. The presence of this doublet allows aldehyde to distinguish from other carbonyl-containing compounds.

The methyl group usually shows a weak band at 1380 cm^-1 and a medium band at 1260 cm^-1.

Factors influencing C=O stretching vibrations

1. Conjugation effects: The introduction of C=C bond adjacent to carbonyl group result in delocalization of pi-electrons in the C=O and C=C bonds. The conjugation increases the single bond character of C=O and C=C bonds in resonance hybrid and hence lowers their force constant, resulting in lowering of the frequencies of carbonyl and double bond absorption.

2. Cis and Trans confirmations also shows variation in C=O frequencies. The s-cis confirmation absorbs at frequency higher than the s-trans confirmation.

3. Electron donating substituent on the carbonyl group stabilize the ionic resonance contributor and increase the single bond character of the C=O bond due to which frequency decreases.

4. Incorporation of the carbonyl group in small ring (5, 4 or 3-membered), raises the stretching frequency.

Identification of  the compound by its absorption band

The absorption band at 2700 cm^-1 for the aldehyde hydrogen and at 1380 cm^-1 for methyl group would help to distinguish the compounds.

A would have the band at 2700cm^-1 but not at 1380cm^-1 . Because compound A contain on aldehyde group (CHO).

B would have the band at 1380cm^-1 but not the one at 2700cm^-1 . Because compound B not containing aldehyde group (CHO) due to which they only show absorption at 1380cm^-1 due to methyl group.

C would have band at 2700cm^-1 and the one band at 1380cm^-1 . Because compound C contains both aldehyde (CHO) as well as methyl group as shown in the figure above.