Organic LAB: Infrared Spectroscopy
- To learn various functional groups encountered in Organic Chemistry
- To learn of the important role of infrared spectroscopy in the study of structure of organic compounds
- To develop skill in the recognition of characteristic absorption bands
- To identify a compound by an investigation of its infrared spectrum
A systematic procedure can help you identify your unknown. All of the unknowns contain a carbonyl (C=O) group. Begin your analysis of the IR spectrum by determining the wavenumber (cm-1) of the absorbance of your carbonyl group(s). This can give you valuable information as to which carbonyl-containing functional group is present. Look at Table 1 to determine which possible functional groups you have. I recommend looking at the table for the following example. If you observe a C=O stretch at 1725 cm-1, you could determine that you have either an aliphatic ketone, an aliphatic aldehyde, an -unsaturated ester, an aryl ester, or a formate ester. Therefore, we have eliminated the other functional groups as possibilities for the structure of the unknown. We can look for additional frequencies to help us narrow the functional groups even more. (Note: the error in the wavenumbers for our IR specrometers are ± 4 cm-1.)
Take a closer look at your IR spectrum. What stands out? Do you have a carboxylic acid (a large, broad peak that centers at ~3000 cm-1 due to the O-H stretch)? Do you have an ester (prominent peak due to C-O stretch in the 1150-1050 cm-1 range)? Do you have an amide (N-H stretches in the 3400-3250 cm-1 range)? Do you have two peaks in the carbonyl frequency? That would indicate the symmetric and asymmetric stretches of the carbonyls of an acid anhydride. (Be careful not to confuse a C=C stretch with a C=O stretch.) How can you tell if you have an aldehyde? Look for the characteristic aldehyde C-H stretch in the 2830-2695 cm-1 range. Sometimes there is more than 1 band in this range due to the aldehyde C-H. They are typically small peaks, but they are quite distinct. If you don’t have any of these peaks in addition to your carbonyl stretch, then you must have a ketone or an acid chloride.
Now there’s one more piece of information that can be very helpful to you. You’ll need to determine whether your compound is aliphatic (sp3 carbons near the carbonyl group) or aryl (aromatic ring directly attached to the carbonyl.) For all of our aryl-containing molecules, the carbonyl group is CONJUGATED with the ring. Conjugation shifts the IR absorbance to the right by ~30 cm-1. That’s why Table 1 lists aliphatic and aryl absorbances separately. There are a couple of places to look on the IR spectra to determine whether or not you have an aromatic ring. All aromatic compounds give very weak bands in the 2000-1600 cm-1 range. The peaks are so weak that sometimes it is difficult to determine if they are really peaks or just noise in the baseline. Fortunately, you can look elsewhere in the IR spectra to help confirm the aromatic ring. Out-of-plane bending frequencies for aromatic rings can typically be observed below 900 cm-1. Monosubstituted rings typically have out-of-plane bending peaks, one in the 770-730 cm-1 range and one in the 710-690 cm-1 range. These are usually strong, sharp peaks, so they should be easy to spot. You can also look for the C=C stretch of an aromatic ring in the 1600-1585 cm-1 range.
Procedure interpret your IR spectra:
- Determine which functional groups you may have present by comparing your C=O stretching frequency with the values listed in Table 1 and 2. Eliminate the compounds that don’t match these functional groups.
- Look for additional functional group information by looking specifically for peaks you would expect to find if that functional group were present (i.e. if an aldehyde is one possibility, look to see if you have an aldehyde C-H stretch, etc.)
- Determine whether your structure is aromatic or aliphatic.
- Write the name, structure and number of your unknown in your lab notebook. Discuss in detail how you deduced the structure of your unknown.
Table 1: IR frequencies.
O-H stretch of carboxylic acid
3300-2800 (strong, broad, centers at ~3000)
3400-3250 (medium intensity, often broad)
sp2 C-H stretch
sp3 C-H stretch
aldehyde C-H stretch
See Table 1
aromatic C=C stretch
alkene C=C stretch
aromatic ring bends
770-730 and 710-690 (for mono-sub’d ring)
Table 2: Characteristic Infrared Carbonyl Stretching Frequencies
R C C OH
Aliphatic acid chlorides
Aryl acid chlorides
R C C H
O O R O R
Acyclic anhydrides (2 peaks)
R C C OR’
R C C R’
You may not have come across some of the terms mentioned in Table 1. Here is an explanation a few of them. You should already know the structure of each of the functional groups mentioned (acid chloride, ester, amide, aldehyde, etc.)
That simply means that the structure includes an aromatic ring, typically bonded directly to the C=O.
Alkane-like would be a crude definition. Basically, the carbons bonded to the C=O are sp3 hybridized.
A double bond is located between the carbons that are and to a functional group, typically a carbonyl
This is an example of an -unsaturated ketone. The common name for this particular compound is methyl vinyl ketone (MVK).
formate ester This is the common name of esters that are similar in structure to the aldehyde, formaldehyde, which is H2C=O.
H O a formate ester
You will need to watch a demonstration of how to use the FTIR spectrometers. You will be expected to know how to use the FTIRs later in chm122L and in chm223L. Please remember to include a discussion in your lab notebook of your logic in determining your unknowns. Also, include a table in your notebook that identifies the most important stretches and bends in each of your unknowns. The table should include a column for the wavenumber (in cm-1) and the assignment of that absorbance to a particular stretch or bend (i.e. sp3 C-H stretch.)
The table above is not comprehensive, but it provides a nicely condensed list of important functional groups in IR.
Cyclohexanone C=O sharp and very strong.
Cyclopentanamine N–‐H (two absorbance for the two N–‐H bonds in a 1° amine).
Group Problem Solving Activities
There are two groups of problems. One group from the University of Colorado and another from Notre Dame University.
http://www.orgchemboulder.com/Spectroscopy/irtutor/tutorial.shtml University of Colorado link:
Notre Dame link:
Q. Draw Possible Spectrum for the following structures
ethyl benzoate ethyl acetate
OH OH H
trans-2-hexenoic acid butanoic acid
acetophenone cyclohexanone 3-hepten-2-one