IR spectroscopy 1

In A-level chemistry, questions requiring you to interpret IR spectra have a list of wavenumbers and the bonds they represent. Many undergraduate exams will not provide this, and will expect you to recognize the main absorptions from memory.

The most important absorption to memorise is the carbonyl stretch around 1715 cm^-1. You should also recognize the C-H stretch, which is just below 3000 cm^-1.

Unfortunately the carbonyl stretch can dip into the 1600s, and the 1600s is where the C=C stretch is located. But there is a difference - the carbonyl stretch is much more intense:

Another important stretch is the broad O-H peak at 3400:

The broadness is caused by hydrogen bonds. So the O-H stretch becomes a sharp peak if the spectrum is done on gaseous alcohol.

The N-H stretch can also be found in this region, at similar intensity. However, the N-H stretch is shaper, and for primary amines it is split into two peaks: