Lab 2 - Infrared Spectroscopy (IR)
1To learn various functional groups encountered in Organic Chemistry
2To learn of the important role of infrared spectroscopy in the study of structure of organic compounds
3To develop skill in the recognition of characteristic absorption bands
4To identify a compound by an investigation of its infrared spectrum
IntroductionDuring the course of this year we will study several different classes of compounds including alcohols, alkenes, and ketones. Each of these classes is distinguished by the presence of a "Functional Group" in the molecule. For example, all alcohols contain an 'O-H' group attached to an sp3 hybridized carbon atom. Alkenes contain a carbon-carbon double bond (C=C), and ketones contain a carbon-oxygen double bond (C=O). As you will learn, IR may actually be thought of as a Functional Group detector. The quickest and easiest way to determine the presence of one of these "Functional Groups" is to take the IR spectrum of the compound. The technique is simple and can often provide a definitive answer in less than ten minutes. Evidence provided by IR is widely respected. It is commonly used in judicial proceedings as much as fingerprints are used. In fact, the IR of a pure compound bears the same relationship to that compound as fingerprints do to an individual. To save space, we will not discuss the theory of IR Spectroscopy. Consult the index of your lecture text. Every modern Organic text discusses IR theory in some detail. Be sure to read that section of your text before doing your first IR so that you will understand what you are doing.
Figure 1: IR Instrument
Figure 2: Close-up of the salt plates in the IR instrument, ready to take a sample
SamplingIR spectra can be determined for solids, liquids, or gases. IR gas analysis is a common analytical tool for those involved in studies of atmospheric pollution. The only draw-back is that it is very expensive and delicate cells are needed. IR spectra of solids and liquids are usually obtained by dissolving the sample in a relatively IR transparent solvent such as CCl4 and using simple liquid cells. A solid could also be ground to a fine paste with NUJOLTM (a mixture of highly purified hydrocarbons) and the resulting 'mull' studied directly. The NUJOL exhibits only a very few well defined peaks that can be ignored when examining the spectrum of the mull. Solid spectra may also be obtained by mixing the solid with dry KBr, grinding to a fine, well mixed powder, and then forming a disk of the mixture by applying high pressure in a specially designed device. The resulting 'KBr Disk' will produce a spectrum free of almost all extraneous peaks. Liquids are the easiest to study by IR. A pure sample of the liquid (1-2 drops) may be placed between two disks of pure NaCl or KBr and the resulting 'sandwich' placed directly in the sample holder of the spectrometer. Excellent spectra can be obtained in a matter of a few minutes with minimum expense. This technique is called running a "neat" spectrum, meaning the spectrum is of the pure liquid only, without solvent.
Reference SpectraThe Infrared spectra of thousands of compounds have been determined and compiled by several different companies. Two of the most popular collections are the Sadtler Index of IR Spectra and the Aldrich Library of Infra-red Spectra Both collections are easily accessible in 'hard copy' form in most major university libraries. They are also available in computer readable format for rapid searching and spectrum matching. All modern FT-IR spectrometers are controlled by computers. The operating system often has the capability of searching one or more databases of spectra and finding the spectra that most closely match the spectrum that was just run. It is always a good idea to compare your IR spectrum with an authentic spectrum of the material you think you have. ALL IR Spectra in this manual are reproduced with permission from: The Aldrich Library of FT-IR Spectra, Edition 1, Charles J. Pouchert, Volume 1, 1985.
Figure 3: Methyl formate - An ester
Figure 4: Isopentyl acetate - An ester
Figure 5: 3-Pentanone - A ketone
Figure 6: 2-Octanone - A ketone
Figure 7: 1-Hexanol - An alcohol
Figure 8: 2-methyl-2-propanol
Figure 9: 1,2,4,5-Tetramethylbenzene - An aromatic
Figure 10: Benzene - An aromatic
Figure 11: Cyclooctane An alkane
Figure 12: Decane - An alkane
Functional GroupsWe have seen that IR is really a "Functional Group Detector." As such, it will be a valuable tool throughout this year. You should become accustomed to examining an IR spectrum and looking for characteristic bands that indicate the presence of a Functional Group. This lab manual contains copies of spectra of various compounds. These will serve as the reference spectra for the infrared experiment. Study these spectra and become familiar with the regions that indicate the presence of -OH, C=O, C-H, etc. Where is the "Fingerprint Region" and what is its significance? Consult your lecture text for help with these questions. Make your own Table of Functional Groups and IR Frequency Ranges. (Consult your lecture text for these important numbers) Some IR stretching frequencies are located here.
Pre-LabAnswer all assigned WebAssign questions.
- (a) C=O _______________
- (b) O-H _______________
- (c) C=C _______________
- (d) C-C _______________
- From my TA
- In the lab
- Down the nearest stairs to the first floor, meet the rest of the class at the front of the building
- Take the elevator to the basement and leave on my own
- Take the elevator to the first floor, meet the rest of the class at the front of the building
- (a) 3200-3600 cm–1
- (b) 1600 cm–1
- (c) 1680-1750 cm–1
- (d) 2500-3300 cm–1
- (I) C-H
- (II) N-H
- (III) O-H
- (IV) F-H
In-Lab QuestionsThe laboratory work involves identification of an unknown by recording its infrared spectrum, investigating the major absorption bands, and comparing the spectrum with spectra of a group of known compounds. Please print the worksheet for this lab. You will need this sheet to record your data.
- benzyl alcohol