Analysis of Functional Groups by solubility and Infrared ~nalysis William N. Turek St. Bonaventure University. St. Bonaventure. NY 14778
In recent vears there has develoued a trend to introduce the elements of infrared and nuclea; magnetic resonance spectroscopy early in the basic organic laboraton, course. We have developed an experiment that introduces the students to infrared spectroscopy while at the same time familiarizing them with the solubility behavior of various organic compounds. Since the solublity behavior of organic compounds is related t o their functional groups and infrared spectroscopy is an excellent method of ascertaining that functional groups are present in an organic structure, this experiment serves-to integrate some of the basic chemical reactions of functional erouDs with their snectral orooerties. T i e experiment>onsisg ofihe following parts. The solubilitv' of a series of known comnounds was determined in the well:known classification solvents consisting of water, 5% sodium hvdroxide. 5% sodium bicarbonate. 5% hvdrochloric acid, andeoncentrated sulfuric acid (la). ?;he inFrared spectrum of each of the knowns was interpreted in terms of the absorption bands of their functional groups. Finally the identification of the functional =our, in an unknown was made n ginfrared by determining its s o ~ u b i l i t ~ ~ n d i n t e r p r e t iits spectrum. An intelligent choice of know1 compounds helps tocorrelate the relationship between the chemical r roper ties of functional groups and their infrared spectrum. A choice which includes a water-soluhlr alcohol, a water-insoluble csrhoxvlic acid. a water-insoluble primary or secondary amine, a water-insoluble ketone, an alkene, and an aromatic hydrocarbon would provide the opportunity to discuss the factors necessary for water solubility, solubility in 5% sodium bicarbonate, in 5% hydrochloric acid, and in concentrated sulfuric acid ( I b ) .The student consequently is taught early in the course to relate chemical ~ r o ~ e r t ito e sstructure. The inirared spectra of the known liquids should be determined and made available to the student if he is to learn some of the basic nuances ot' infrared spectral interpretation hefore heing required to analyze the infrared spectrum of an
unknown. T o facilitate the student's learning the correlation of structure with spectrum, the infrared rang& divided into three reeions: I. 4000-2500 cm-': . 11.1850-1500 cm-': and 111. . 1500-1600 cm". A summarv of the reeions and absomtion bands is eiven in the table. I t is well ton& that the v OH bands of alcojlols can be distinguished from those of carboxylic acids by shape and frequency and from those of amines by shape, intensity, and sometimes number, while the v=CH bands of alkenes and aromatics are discerned from the v CH of alkyl groups by using 3000 cm-I as a line of demarcation. In region I1 the v C=O bands of aldehydes, ketones, and acids are readily distinguished from the v C=C bands of alkenes by intensity if not by frequency. The C-0 bands in region 111 are somewhat broader than other bands in this region. The final part of the experiment involved a determination of the solubility of an unknown and its functional moup. Most of the students, twenty-six out of thirty-two, correclty determined the solubility of their unknowns. Those that had received water-soluble amines or carboxvlic acids as unknowns were able to get a clue about the functional group present by checkiie the aciditv of the aoueous solution with litmus DaDer. The percentage ofihe students that properly interpretkd'the infrared mectrum of their unknowns was lower (twentv out of thirty-two). Most of the students that misinterpreted the infrared s ~ e c t r u mdid so bv confusing the spectrum of a ketone for that of an aldehyde. They failed tiobserve the absence of the C-H absorption band associated with the CHO group a t approximately 2720 cm-I (2d). Another common mistake was to confuse an m i n e with an alcohol, however, this error could be avoided in the case of a primary amine by instructing the student to give close attention to the 1600 region where a somewhat broad band due to the 6,NHz mode appears. In a unique case when cyclopentanone was the unknown, several students confused a rather strong band at 3500 due to the C=O overtone with the v OH band in alcohols. The above mistakes can be minimized by providing representative spectra of known samples and carefully instructing the student about the absorption bands associated with the various functional groups, noting the position, intensity, number, and shape of these bands.
' Specit:cally. 2 drops of the liquid solute and 30 drops of the solvern are added to a 13 X lOOmm test lube and thorouahlv - . mixed bv stinina with a glass rod over a period of two minutes.
-
Correlation of Compound (Group), Frequency, and Vlbratlonal Mode Region (cm- ') I
4000-2500
I1 1850-1500
Ill
1500-1000
Compound (Group) alcohols (OH) carboxylic acids (OH) amines (NH. or NH) alkenes, aromatics (=CH) aldehydes (=CH) alkyl group(C-H) aldehydes, ketones, carboxylic acids. (-0) alkenes ('24) aromatics (ring) amines (NH.) alcohols (C-H) carboxylic acids (CO)
Frequency, intensity.
Vibrational
shape
Mode
Reference
3550-3200 s. broad 3300-2500 s, very broad 3500-3300 wm. lo-2 bands. 2'-1 band 3070-3010 w 2830-2695 w
v OH Y OH v NH A H A H
I2a. 3 3 )
ZS~O-Z~~OS
v CH
1760-1665s 1670-1640 w-m
vm v C=C v ring 6 NH2
.
.
1600, 1500 1650-1580 m,broad
1205-1050
6,
somewhat broad
v CO
1320-1210s
(2b. 36) (2C. 331 (3d. 30 (2@ I2e. 33)
(M (231 (2h, 3h) (21) (2i.31) 1% 33,)
Key: v = stretching mode, d =deformation, o = mong, m = msdium, w = weak.
Volume 61 Number 6
August 1984
709
Literature Cited
710
Journal of Chemical Education
(nPP. 117.120:
(e) PP. ~ 0 6 1 0 7 ;
"
,q,
(g) pp. 1 ~ 1 0 9 (h) : P. 111: (i) P. 1 2 7 ; ~ P. ) 115: (k)
