Structural Isomer Identification via NMR A Nuclear Magnetic Resonance Experiment for Organic, Analytical, or Physical Chemistry Zvi Szafran Merrimack College, North Andover, MA 01845
Over the past 25 years, nuclear magnetic resonance spectroscopy (NMR) has proven to he a most valuable tool in the elucidation of structure and bonding in molecules. There have been several useful experiments published showing the utility of NMR in studying biochemical kinetics ( I ), group electronegativity (2). solution concentrations (3),and multicompone& systems (4). among others. Advanced chemistry students are well acquainted with the technique, but laboratory experiments are necessary to acquaint sophomore and junior students to its utility. This exoeriment examines the ahilitv of NMR to distinguish between structural isomers via resonance multiplicities and chemical shifts. The exoeriment is useful for a number of reasons:
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It serves as a eood introduction to the use of NMR. 2) Students pain experience in assigning N M R spectra. 3) It graphically illustrates rhedifl'rrencr between orgnnicsrructurd isomers. 4) The topic of spin-spin coupling is introduced in a concrete fashion. Furthermore, the experiment is easily completed in one lahoratory period. 1)
Background and Theory The transmission of magnetic information hetween two nuclei takes place through ;he electrons hetween the nuclei. In the liquid state or in solution this nuclear interaction is readily observable as a splitting of the nuclear resonance. The value of this splitting is J,the coupling constant, which is independent of the magnetic field strength (5). The multiplet splitting can most easily he calculated using the relationship where M = maximum multiplicity n = number of equivalent coupled nuclei I = spin of the coupled nuclei For protons, I = 112 and eqn. (1) reduces to the familiar It should be recognized that in many cases, coupling constants for nonequivalent nuclei may be similar (as may the chemical shifts), and overla~pingof spectral lines can occur, rendering the observed muliiplicjty smaller than the calculated value of M. Coupling information, together with chemical shift data (which &in simple cases prop&ional to the electronegativity of the attached suhstituent ( 2 ) )make NMR a very powerful analvtical tool. Comoounds of identical molecular formula can easiiy he differentiated by their NMR spectra, due to their different structures. Integration can also be used to differentiate between structural isomers, as the number of methyl and methylene groups differ from isomer to isomer. A methyl group would have a relative area of 3 as compared to an area of 2 for a 260
Journal of Chemical Education
methylene group. These areas are, of course, for the entire multiplet. Analysis of Data The compounds used in this experiment are 1-chlorobutane (I),2-chlorobutane (11). 1-chloro-2-methylpropane (1111, and 2-chloro-2-methylpropane (IV). All have the molecular formula CdH&l. These compounds are commercially available, the most expensive being compound 111 (100 g/$20.10) (6). The experiment is thus inexpensive to run. The structures of the compounds are shown in I-IV below. d d
c
b
a
CH&HX-CH~-CH~-CI I-ehlorobutane (I1
c
a
h
CHg-CHZ-CH-CH3 CI 2-chlorobutane (111
The spectrum of compound N is the simplest to analyze. All nine protons are chemically equivalent ( 5 ) ,and give rise to a single unsplit resonance. The protons are two carbons removed from the chlorine, so the chemical shift is small (1.61 ppm (7)). Integration is of no help for this compound, since there is only one resonance. Compound I11 has three different types of protons, designated a, h, and c in Figure 1. Methylene group a (relative integrated area of 2) is split into a doublet by proton h (3.32 pprn). Proton b (integrated area of 1) is split into a multiplet bv orotons a and c (1.91 oom). Methvl erouos c (inteerated are; of 6) are split into a hbud~etby proton d (1.0'3 ppL). AS ex~ected.the resonances shift to lower field as the mouo gets closer to the chlorine. Compound I exhibits three signals. Methylene group a (integrated area of 2) is split into a triplet (3.49 ppm) by methylene group b. The resonances for methylene groups b and c (total relative integrated area of 4) occur as overlapping multiplets (1.1-2 pprn). The multiplicities are caused by coupling to groups a and c in the first case, and b and d in the second. Methyl group d (relative integrated areaof 3) is split hv methvlene .. erouo oom). . c into a triolet . (0.95 . .. . Compound 11 exhibits four resonances. I'rotn a (integrated oornr. areaof I .) is solit . intoa multiolet bv mouos hand c (3.88 . .& . Methyl group h (integratedareaof3) i's split into a doublet by proton a (1.49 pprn). Methylene group c (integrated area of 2) is split into a multiplet by groups a and d (1.72 pprn). Methyl group d (integrated area of 2) is split into a triplet by methylene group c (1.01 pprn). Experimental Using a dispcsable pipet, the instructor should place about 0.5 mL of the liquid to be examined in an NMR tube and add a few drops of
TMS as a standard. The four sample tubes are then labeled A, B, C, and D, and given to the student to identify via their NMR spectra. The student will analyze the spectra in terms of their chemical shifts..solittines. . - . and inteerations and oredict which isomer is mesent. To confirm their prediction, the student may consult standard spectra, such as found in reference (7).
C O ~ C ~ S ~ O ~ The experiment is easily performed a t minimal expense, showing the usefulness of NMR as an analytical tool in
structural elucidation. If so desired, aromatic isomers may also be used. Literature Cited Kasperek, G , J., and Pratt,R. F., J. CHBM.EDUC.,54,515 11977). ~ r e o w rJ.c., , J.CHEM. E~uc.,55,538(1978). Silueira, A. JI., Brethorick, H. D. Jr., and Negiahi, E., J. CHRM. EDUC.56,560 119791. Ciaasen, R.. Woleott, R.. and Reinbold, P. E.. J. CHEMEDUC55.542 (19781. Peulder, W. W., "Nuclear Magnetic Rp\nanee." Allyn and Bacon, lac. Boatoo. MA, 1971. (6) Aldrich Chemical Company. 1982.1988 Price. (71 Simons. W. W., "The Sadtbr Handbmk of Pmton NMR Spectra," Sadtler Rsseareh Laboratories. Philadelphia. PA. 1978. ~ ichemical i shifu arereported in ppm, derhidded from TMS.
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Volume 62
Number 3
March 1985
261
