0
THE INTEGRATION OF INFRARED SPECTROSCOPY INTO ORGANIC QUALITATIVE ANALYSIS ROBERT T. CONLEY Canisius College, Buffalo;New ~ o ; k
INRECENT years the analytical chemistry of organic substances has become an important problem which the teacher of organic chemistry and qualitative organic analysis must consider in order to prepare the student for either an industrial career or advanced study. In particular, infrared spectroscopy, the indispensable tool for organic research, has caused many problems in the teaching of organic qualitative analysis. Authors of recent textbooks have tried t o include infrared spectroscopy in the course in qualitative organic analysis on the undergraduate and even in the first year course in organic chemi~try.~ 1 SAHINER, R. L., R. C. FUSON,AND D. Y. CURTIN,"The Syetematic Identification of Organic Compounds," 4th ed., John Wley & Sons,-Inc., New Yark, 1956, pp. 167-79. CHEEONI~, N. D., AND J. B. ENTRIKIN,r'Semimic~~ Qualitative Organic Analysis," 2nd ed., Intersoience Publishers, Inc., New Yark, 1957,pp. 275-86. FIESEE,L. F., liExperiments in Organic Chemistry," 3rd ed., D. C. Heath & Co., Boston, 1955, pp. 170-84. J
VOLUME 35, NO. 9, SEPTEMBER, 1958
After considerable experimentation, a method has been developed which successfully integrates infrared analysis into the qualitative organic chemistry courses. It gives each student an opportunityto gain an excellent understanding of the important principles and methods involved in infrared analysis without any serious diversion of his attention from the chemical behavior of the unknowns. Four lecture periods of one hour each were devoted to infrared spectroscopy. These lectures were given sequentially early in the semester to provide the student with a sufficient background for laboratory assignments involving the use of infrared techniques. The general topics discussed in these lectures were designed to acquaint the student with the position of the infrared region in the general electromagnetic spectrum and to correlate this method of analysis with other more familiar methods such as visible and ultraviolet spectrophotometry.
CLASSROOM WORK
Following a brief introduction designed to stimulate the student's interest in the large number of applications of the method of analysis, the types of vibrational excitations found in this region were presented. It was found useful t o use slides and models for clarifying the lecture material. By issuing and referring to a comprehensive bibliography sufficient impetus was given to the student to undertake the necessary library study for the adequate understanding of the subject matter. A simplified schematic diagram of the instrument was developed for the class, followed by a discussion of the optical features of a single and a double beam spectrophotometer. In these discussions, no emphasis was given to the electronic portions of the instrument since it was felt this topic was not of sufficient importance to warrant detailed coverage in this type of course. The optical path of the double beam instrument was demonstrated to the student by removing the cover of the machine and tracing the optical path from the source of the detector, indicating the function of each of the instrument components. Diagrams of the optical systems of several of the instruments in common use were distributed t o the student for inclusion in the lecture notes.4 The various source materials, instrument components, and types of detectors in commercial usage were briefly discussed during this demonstration. The methods used to obtain an infrared spectrogram were treated, emphasizing the applications and limitations of each method. LABORATORY WORK
With this background, the student was allowed to choose for identification two of the assigned six single component unknowns; one, a solid, the other, a liquid (see Table 1). The students were given individual instruction in the operation and care of the instrument and in sample preparation, including the preparation of mull samples and potassium bromide pellets. Each student was then allowed to run two spectra, using a different method of sample preparation, for each of the unknowns chosen. Most of the common methods of sample preparation were exemplified, and some indications were given which showed the effect of the method of sample preparation on the spectrum of organic materials (see Figures 1 4 ) . When all the students had completed the laboratory work, a simple Colthup chart5 was developed by calculating the approximate position of absorption of several functional groups by the methods outlined by Gordy6 and Badger,? followed by placing the more common group assignments for stretching and bending vibrations directly on a blank spectrogram sheet. Spectra-structure correlation sheets8 (Colthup charts) were distributed to each student, and a series of simple spectra, exemplifying the common aliphatic and aromatic functional groups, were interpreted in two recitation periods. The members of the class were then allowed two weeks to develop their interpretations of the unknowns (see 'Available a n request from the instrument. manufacturers. The diagrams used in this course were obtained from BairdAtomic, Inc., and Beckman Instruments, Inc. WOLTHUP, N. B., J. Opt. Sw. Am., 40,397 (1950). ' G o ~ o r W., , J . Chem. Phys., 14, 305 (1946). BA~GER R., M., J . Chem. Phys., 2, 128 (1934). 8 Available on request from the Stamford Research Lahoratories, American Cyanamid Company, Stamford, Connecticut.
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454
Table 2) and to submit a complete written report including the spectra. The student was advised before running the spectrum to obtain the boiling point, refractive index, and/or the melting point of the unknown: After the report was filed with the instructor, the remaining portion of the unknown was returned to the student for the preparation of two derivatives based on the student's interpretation of the spectrum of the unknown. After the completion of this assignment a final report was submitted. The student was graded on his comprehension of the subject and on the accuracy and completeness of the report. The reports were returned t o the student and the spectra compared with a known spectrum of the material. Known spectra were obtained from three sources: the instructor's personal file built up from samples run during the course of the experiment, the Sadtler Standard Spectra File (Midget Series), and the American Petroleum Institute Infrared TABLE 1
List of T m i c a l Unknowns
-
I. Acids and Ueil- derivatives Acids: acetic, sdipic,, anthranilie, benzilic, henzoic, n-hutyric, isobutyrie, cmnamlc, oxalie, phenylaeetir, salicylic, tartaric Amides: acetamide, acetanilide, benaamide, henzanilide, butyramide, eapralactam, N,N-dimethyl formamide, salicylamide, suceinamide Anhydrides: acetic, heneoic, phthdic, succinic Esters: Iliethyl malonate, ethyl acetate, ethyl scetoacetate, ethyl benzoate, phenyl acetate, phenyl benzoate Lactones: hutyrolactone, valerolactone 11, Aldehydes, ketones, and pinones Aldehgdes: benzaldehyde, .isobutyraldehyde, cinnamaldehyde, salicdaldehyde Ketones: acetone, acetophenone, henzalacetone, benzil, benzophenone, cainphor, cycloheptanone, cyclohexanone, cyclopentanone, indanone, tetralone Quinones: anthrequinone, naphthaquinone, quinone 111. Alcohob, phenols, and diols Alcohols: dlyl, hensyl, isobutyl, n-hutyl, tert-butyl, cetyl, cyclohexxnol, cyclopentanol, ethyl Phenols: catechol. m-cresol. o-cresol., n-cresol., naohthol-1, phenol, resorcho~ Dials and polyals: benzopinacol, butandiol-2,3, eyclohenane-1,2-did, glueosr, gl,veerol, hexanediol-2,5, lactose oro~anediol-1.3
.
.
IV. knlines
v.
Aniline, henzidine, decylamine, diethylamine, ethylamine, N,Ndimethyl aniline, N-methyl aniline, piperidine, propyl, triethylsmine , Ftl"-"
"b,oc,"
Dibutyl, diethyl, diphenyl, phenyl methyl VI . Halogen eompounds Chloro compounds: chlorobenzene, chloroform, o-dichlorobenzene, p-dichloroheneene, dichloroethane-l,l, dichloroethane-l,2, hexyl chloride, isopropyl chloride, trichlorobenzene-l,2,3, trichlorohenzcne-l,2,4 Bromo eompounds: hromobenaenc, p-dibromobenrene, n-propyl bromide, isopropvl hromide YII. Hydrocarbons Aliphatic and alieyclic: cyclohexane, cyclohexene, cyclopentane, diisobutylene, hexane, hexene-1, 3-methylpentsne, neopentane, Aromatie: anthracene, hename, diphenylethane, ethyl benzene, fluorene, isopropyl benzene, naphthalene, terthutyl hemene, toluene, m-xylene, o-xylene, p-xylene VIII. Nitriles Acetanitrile, benzonitrile, hutyronitrile, vdwonitrile IX. Nitro eompounds o-Chloronitrohensene, o-nitromisole, p-nitroanisole, nitrobenzene, p-nitrobenaoio acid, nitrocyclohexane, onitroethylbenzene, nitropropane-I, o-nitrotoluene, mnitrotoluene X. Salts Anilinc hydrochloride, meth,vlamine hydrochloride, sodium acetate, sodium benzoate XI. Miscellaneous 2,4-dinitrophenylhydraaone, azoheneene, Acetone cyelohexanone oxime, cyclohexznone semicarbazone, inrlnln nvridine. nuinnline
JOURNAL OF CHEMICAL EDUCATION
TABLE 2 Typical Student Interpretation of the Infrared Spectrum of Benxoic Acid Figure 1 2-3fi = light scattering due to particle,size Bsnds a t 3.5, 6.9, and 7 . 3 ~are due mainly to C 2
3
Bensoio
2
3
4
acid.
4
5
6
7
8
9
10
11
12
13
Pi1 Student curve obtained using the Nujol Mull
5
6
7
8
9
lo
11
12
13
14
IS
method.
14
H stretching, CH2 bending, and C-CHa groupings, respectively, of the paraffin oil dispersing medium Band8 a t 3.8 and 3 . 9 ~are due to strongly associated hydroxyl Band s t 5 . 9 5 ~is indicative of a conjugated carbonyl of an acid Bsnds a t 6.25 and 6 . 3 ~are due to C=C stretching of the benzene ring ~ indicative of the carbox I group Band a t 7 . 8 is Band a t 1 0 . 8 ~ is due to hvdroeen bondeidimer of the carboxvl " group Band st 1 4 . 2 ~is due to C-H wagging and indicative of a. manoaubstituted benzene
15
Fig"= 2 Bmaoic
aoid. student curve obtained in carbon tetrachloride solution.
100 80
60
6 . 7 ~is due t o the C 4 stretching of the benzene ring, and the band a t 7 . 1 is~ due to the earbaxylgroup. (This band was interfered with in Figure 1 because of th? absorption of the CH, groups in this region.)
The student then comvared his svectrum with the reference
a 20 0
2
3
Brnsoic
2
3
4
acid.
4
5
6
7
8 9 Fig"r. 3
10
11121314
15
Reference curve obtained using the Nujol Mull method.
5
6
7
8
9
10
11
12
13
14
15
Fivr. 4 Benaoio acid. ader.
Reference ourve obtained from a potsasium bromide
Spectrum Index. The student was then asked to prepare a short report accounting for any differences observed between the standard spectrum and that obtained in his identification of the unknown. Since two or more spectra were available for comparison, this port,ion of the assignment proved to be worth while in giving the student experience in observing the effect of varying purity and of the differences in the spectra ohtained by different methods of sample preparation. As a final assignment, the student was given twenty spectra, reproduced photographically (two t o a page), for practice in interpretation. The first ten spectra were "knowns" identified by having the structure directly on the spectra sheets. The second ten were complete "unknowns" with reference made only t o the physical properties of the material and the methods used t o obtain the spectra. I n the former case, the student was t o identify and assign all possible bands and, in the latter case, to propose a structure for the material and substantiate it by his interpretation. In this assignment, which is rather difficult for a beginning student in spectroscopy, a logical interpretation was emphasized rather than a correct one. The students were encouraged to cooperate and try to establish the gross structural features of the molecule whose spectrum was given. The correct structures were then discussed with the class and t,he important bands in each of the latter ten spectra were assigned. VOLUME 35, NO. 9, SEPTEMBER,1958
The total number of class hours in the above program waslimited to four lecture hours, three recitation hours, and one four hour laboratory period. Throughout the course other topics were discussed pertaining to the use of infrared analysis as a tool for the organic chemist. Probably the most important of these was the desire of most students to discuss the quantitative applications of the infrared method of analysis. Student interest in the course was remarkably improved since the manner in which the material was presented encouraged independent study. This interest was also manifested in greater enthusiasm in the chemical methods of identification of organic compounds and in independent student research. The recitation sections were more active and stimulating as a result of increased student library work and study. The written reports were more complete and the students showed increased imagination in choice of a mode of attack on the identification of the unhown samples. Another advantage was that the amount of material covered in the course was appreciably increased, much of it by independent study. The major disadvantage t o the program is that it seems unlikely that the course could function smoothly with classes in excess of 30 stndents. I n this experiment the maximum class size was 19 stndents, but it was felt that the class could he increased without additional burden on the instructor. ACKNOWLEDGMENTS
The author wishes to express his gratitude t o Mr. Norbert Helmer of the National Aniline Company for his many suggestions regarding the subject matter and t o Dr. Herman A. Szymanski, Chairman of the Department of Chemistry, for allowing the necessary time on the infrared recording spectrophotometer as well as for his cooperation in developing the necessary theoretical aspects of the subject in his companion course in physical chemistry.
