IR spectroscopy: answer to quiz

Dear Students,

The recent quiz question requires you to choose the correct compound between two compounds, C and D, to which a given IR spectrum belongs to:

Let us examine and compare them. Both are aldehydes (having CHO groups) and have aromatic rings. The difference between them is compound C has a C=C while D does not. And this one difference makes for a lot of differences between the IR spectra given off by C and D. Let us look at the spectrum.

IR spectrum given in the quiz

Having this double bond means C is a fully unsaturated compound: all the carbons in the molecules are sp² carbons. Looking at the 3000 cm^-1 region, we see sharp, moderately intense bands to the left of 3000 cm^-1 (3062, 3083), C-H stretching vibrations of sp² or unsaturated carbons. No bands are seen to the right of 3000 cm^-1  except for a pair of bands (2826, 2724) which are stretching vibrations of C-H of the CHO group. This clearly shows that this is a spectrum of an unsaturated compound. So this spectrum belongs to C.

Other IR features that set the two compounds apart is the position of the sharp and strong C=O stretch band. This C=O stretching vibration is at 1678 cm^-1, rather low (<1700). This lowered band frequency could be due to conjugation of the carbonyl to a double bond and the aromatic system, and this is indeed seen in compound C. Additionally, there is the presence of the C=C stretch band at 1615 cm^-1.

Now, shown below is an actual spectrum of compound D. You can see many differences if you compare it with that of C.

IR spectrum of compound D