Year 1: A Short Introduction to Infrared Spectroscopy
This is an introduction to infrared spectroscopy.
Infrared or IR spectroscopy is an important analytical technique available to chemists.
Virtually any sample can be studied.
Organic chemists most often prepare their IR sample by first grinding a solid with potassium
bromide and then compressing the mixture into a clear transparent disk— or, if the compound
is a liquid, by placing a drop of the sample between two flat plates of sodium chloride. The
procedure is neat, easy and fast.
In IR spectroscopy we pass infrared radiation through the sample and monitor the frequencies
absorbed. Energy from the absorbed radiation causes bonded atoms to vibrate.
All molecules vibrate. The vibrations may be quite simple when a molecule consists of only
two atoms. When three atoms are involved, however, several vibrations are possible. And in
larger molecules, all bonds vibrate at the same time and the combinations become more
complex still. The good news is that vibrations of key functional groups occur at characteristic
frequencies. Infrared spectroscopy is particularly good at detecting different types of C-H
bonds, alkenes and benzenes, carbonyl compounds, alcohols, amines and many other
functional groups that might be present in a compound.
An infrared spectrometer exposes a sample to infrared light and detects the frequencies at
which bonds vibrate and infrared light is absorbed. Infrared spectra are simple absorption
spectra with peaks plotted upside down along a frequency scale based on reciprocal
centimeters or wavenumbers. This may seem an odd frequency unit at first, but it gives
convenient numbers that every chemist understands. All vibrations of interest are found in the
range between 4000 at the high frequency end and 600 cm-1 at the low frequency end. Note
the change in scale at 2000 cm-1 so that the right-hand half of the spectrum is more detailed
than the left-hand half.
Stronger bonds vibrate faster at higher frequencies, and so do bonds involving lighter atoms.
We find therefore vibrations of bonds to hydrogen at the highest frequencies, above 2500
cm-1, because hydrogen is the lightest atom. Vibrations of single bonds between two "heavyweight" atoms come to the right in the IR spectrum. The strongest bonds are triple bonds,
such as the CC triple bond of alkynes and CN triple bond of nitriles; these show up in the IR
spectrum towards the left at frequencies between 2500 and 2000 cm-1. Notice however that
triple bond absorptions occur at lower frequencies then absorptions for single bonds involving
the light hydrogen atom. Double bonds are weaker than triple and so their IR absorptions
occur at lower frequencies from about 2000 to 1500 cm-1. And these are followed, below 1500
cm-1, by absorptions from single bonds involving atoms other than hydrogen.
The region below 1500 cm-1 is called the fingerprint region. This region is not normally
interpreted and, because it is often more complex, we only occasionally look at it more
closely. We'll ignore the fingerprint region at this level.
Most organic compounds contain C-H bonds. Now, let's have a look at the C-H vibration, also
called the C-H stretch. The IR spectrum of an alkane is seen here. It shows a typical alkane CH stretch at slightly below 3000 cm-1. Since most organic compounds contain C-H bonds, this
is not very informative at first. However, the C-H bond of alkenes and benzenes is observed
instead slightly above 3000 cm-1. So, just by looking at an IR spectrum, we can distinguish
between saturated and unsaturated compounds.
An alkene such as 1-hexene will, of course, display C-H stretches of both the alkane and the
alkene part of the molecule. In addition, 1-hexene will also have a C=C double bond stretch in
the double bond region of the IR spectrum. We have therefore two pieces of evidence for an
alkene: its C-H stretch a little above 3000 and the C=C double bond stretch at around 1640
cm-1. Unfortunately, the C=C double bond stretch is not always strong. At times, it can be
quite small and hard to detect, particularly when the alkene double bond has identical
substituents on both sides. We'll just have to live with that.
Aromatic compounds, too, have C=C double bonds. The C=C double bond stretches of
benzene are observed between 1600 and 1450 cm-1. Sometimes you will find 3 IR peaks,
sometimes 4 for the benzene ring vibrations. Intensities may vary from one aromatic
compound to another, but they are always in the same region.
The carbonyl group is probably the most important functional group in Organic Chemistry.
The C=O double bond stretch comes right here, practically in the centre of the IR spectrum
and well away from most other functional group stretches. Depending on the type of carbonyl
group, the C=O double bond stretch may be slightly below or a bit above 1700 cm-1. Luckily,
the C=O stretch is always strong and hard to miss. It is so important that the first thing you do
when you see an IR spectrum is to check whether it has a C=O double bond stretch or not.
Back to single bonds involving hydrogen. The O-H stretch of alcohols is found above 3200
cm-1. It is quite broad because alcohols love to form hydrogen bonds. The peak is strong and
easily spotted. When an O-H is located next to a carbonyl group, we have a carboxylic acid,
which gives rise to an even broader O-H stretch in the IR spectrum extending from almost
3500 to 2500 cm-1.
The N-H stretch of an amine tends to be smaller and shows up in the same area occupied by
O-H vibrations. Unlike O-H stretches which are broad, N-H stretches are much narrower. You
can even distinguish a primary amine, which gives rise to two stretches, and a secondary
amine, which possesses only a single IR peak.
IR spectroscopy is the first technique you get to know for identifying organic compounds. It
allows you to prove that a reaction has been successful, that you made an alcohol, an alkene, a
substituted benzene, an ester.
You can even follow reactions where a characteristic IR peak of your starting material
vanishes and a new IR peak for your product appears. You will get some experience with IR
spectra in the lab.
Analysing IR spectra requires practice. It is not something that you can learn in 5 minutes.
You will find an IR trainer on Vision that will help you to familiarise yourself and test your
skills in detecting key functional groups. Try it out next time you are on Vision, or download
the training program and install it on your own PC. Have fun with learning how to deduce
chemical structures.