IR Spectroscopy
From MyMCAT
Contents |
Introduction
IR, or infared, spectroscopy is a vital tool to organic chemists wishing to identify what functional groups exist in unknown samples. The light our eyes see is but a small part of a broad spectrum of electromagnetic radiation. On the immediate high energy side of the visible spectrum lies the ultraviolet, and on the low energy side is the infrared. Having a wavelength range from 2,500 to 16,000 nm, the infared region is capable of revealing information not easily uncovered through basic means.
Photon energies associated with the infrared region are not large enough to excite electrons but they are still strong enough to induce vibrational excitation of covalently bonded atoms and groups. Covalent bonds are not rigid sticks or rods but are more like stiff springs that can be rotated (single bonds only), stretched, and bent. This wide variety of vibrational motions in turn is characteristic to a molecules component atoms. Consequently, virtually all organic compounds will absorb infrared radiation that corresponds in energy to these vibrations and infrared spectrometers permit chemists to obtain absorption spectra of compounds that are a unique reflection of their molecular structure.
An Example IR Spectroscopy
IR spectroscopy typically has a spectrum reading organized as seen below,
This spectrum is for formaldehyde and is typical of most infrared spectra. IR readings are inverted compared with UV-Visible spectra, thus a sample that did not absorb at all would record a horizontal line at 100% transmittance (top of the chart), while one that did absorb a significant amount would be towards the bottom. Each peak (dips on the graph) represent various bond characteristics and because most functional groups have specific bonds, they can be identified.
Functional Groups and their Peaks
While it is not necessary to remember all of the peaks for each functional group, learning a few key characteristics will be enough to answer 99% of all IR questions. In the reading of formaldehyde (example above), one can see that there are a few different possible ways that bonds can behave which leads to many different peaks. The complexity of infrared spectra in the 1450 to 600 cm-1 region makes it difficult to assign all the absorption bands, and because of the unique patterns found there, it is often called the fingerprint region. It is not necessary to remember any peaks in this region as they will not help you determine any significant functional groups. Absorption bands in the 4000 to 1450 cm-1 region are usually due to stretching vibrations of diatomic units, and this is sometimes called the group frequency region. It is this region which is most practical to identifying groups on the MCAT. Some notable frequencies that should be remembered include
| Functional Group | Assignment | Peak Type and Range |
|---|---|---|
| Alcohols, Acids | O-H | Broad Peak just above 3000cm-1 (usually 3200-3500) |
| Amines | N-H | Narrow Peak just above 3000cm-1 (usually 3300-3500) |
| Alkanes | C-H | Narrow Peak just below 3000cm-1 (usually 2800-3000) |
| Ketones, Acids | C=O | Narrow Peak at 1750cm-1 (usually 1700-1800) |
| Triple Bonds | CC or CN | Peak at 2200cm-1 (usually 2100-2300) |
Again, it should be emphasized that while this does not cover all the functional groups, it provides enough information for most MCAT questions.
Identifying a Molecule
One thing to keep in mind about IR spectra is that it can only tell you whether a group is present, or not, it will NOT tell you how many groups or how large the molecule is. Thus a sharp peak at 1750cm will tell us that there is some sort of a carbonyl group (ie ketone, ester, or part of an acid, etc) but it will NOT tell us if perhaps there are two or three ketone groups present.
With this in mind, and using only the select few important peaks we have to remember, we can easily guess what functional groups are present, and thus identify the molecule from a few (multiple choice) selections.
Consider the following IR spectra below. From the specific groups discussed above, we should focus our attention on the 3000 and 1750 areas. In doing so, one can most easily notice that there is a sharp peak at 1700cm-1, this corresponds well to a C=O group, thus it is most likely a ketone or aldehyde. Secondly, there is no broad group or sharp group above the 3000 mark, thus there should not be an O-H or N-H, respectively. There is however a sharp peak just below 3000, which most likely correlates to alkane CH, CH2, and/or CH3 bonds (but it could also be part of the aldehyde's CH bond).
If our choices were 2-methylcyclobutanol, 3-methyl-2-butanone, and 2-chloro-pentanoic acid, it should be clear to recognize that only 3-methyl-2-butanone has both the necessary C=O band and the lack of an O-H band.
Further Reading
IR Spectroscopy is just one of many tools used by chemists to identify molecules. On the MCAT you should be familiar with identifying the groups discussed above as well as with the other possible types of spectroscopy.
