A tutorial on IR Spectroscopy

Dear students,

This is a tutorial on reading IR spectra to make your preparation for the exam complete. I take the opportunity to discuss the IR question from the previous quiz.

Here is the spectrum given in the quiz:

You were asked to choose between two compounds, C and D as to which the spectrum belongs to. C is an aldehyde (1-pentanal) and D is a carboxylic acid (pentanoic acid).

Let’s look closely at the spectrum. The most obvious peak is the intense and sharp carbonyl (C=O) stretch at 1727 cm^-1 signalling the presence of a carbonyl which both compounds have in common, both are carbonyl compounds. Next, look closer at a moderately intense peak at 2719 cm^-1. There are actually two peaks at ~ 2700 and ~2800 cm^-1, one of the peaks being half hidden behind the bigger saturated C-H stretch bands. This two peaks show the presence of C-H stretch of an aldehyde. So, this is clear indication that this spectrum belongs to compound C.

Some of you may have been misled by a rather medium size peak at around 3500 cm^-1. This is NOT an O-H stretch. It is probably an overtone of our intense carbonyl band at  ~1700 cm^-1. If you remember the IR spectrum of myristic acid from our Nugmeg myristicin practical, you will see that the O-H stretch band of myristic acid is distinctively different: it is broader, smooth, and spreads over a larger region (overlapping the saturated C-H bands). Have a look at it again.

An actual IR spectrum of compound D, pentanoic acid (it’s also called valeric acid) is shown below:

Hope this little discussion clears the haze regarding IR spectrum.

I include here a video on reading IR spectra (a substitute tutorial teacher ) that I find useful for your preparation. Pleasant viewing and good luck for the exams.

Source: Youtube channel Sarutahiko1 (www.youtube.com/watch?v=XIWc9eT476c)