Acetylation of an Unknown Alcohol An Introductory 'H-NMR Experiment Stephen E. Branz San Jose State University. San Jose, CA 95192 NMR is of such pivotal importance to the organic chemist that its a1,sence from an organic laboratory cuurse u,ould he a daring omisiion. Severtheless, it has laen our experienre that students are slow t o appreciate the multifaceted power of NMR. We attribute their difficulties to two principal factors. First, the focus on the carbon framework runs counter to the usual concern with functional group chemistry. (In this respect, IR, with its focus on functional groups, is conceptually much easier for students to assimilate). Second, the way in which NMR is typically appended to the laboratory experiment does not encourage students to discover the utilitv of NMR for themselves. NMR is most commonly used as an analytical tool for "after-the-fact" characterizatimofa known nmpound. Our ohjertive u,as todesign a lahoratory experiment that would overcome these two main barriers to NMR understanding. The concept of an integrated NMR experiment is not new1, but it is still quite rare? We chose to implement our plan with an otherwise routine acetylation reaction. Fischer (acid-catalyzed) esterification-undeniably a key reaction of organic chemistry-is almost uniformly included in any organic lahoratory syllabus. The "synthesis of banana oil" (isoamyl acetate)3 is the particular reaction that we adapted for our purposes, hut the principles embodied in our approach might he fruitfully applied in a nnmher of other reactions. Students are asked to read the lab text describing the preparation of hanann oil by aretylntion of isoamyl alcohol (;3-methyl-l-b~ttnnol~. The important exception to the standard procedure3 is that an unknown five- or six-carbon fully saturated acyclic alcohol is used instead of isoamyl alcohol. Ignoring stereoisomers, there are 25 alcohols defined hy the formulas CsHlzO and C ~ H M OOf . these, all eight of the fivecarbon alcohols and 13 of the six-carbon alcohols are commercially availahle.Wnknown to the students, we have further restricted the possibilities to eliminate tertiary alcohols and alcohols beyond our budgetary means. The table lists the six alcohols that we have used su~cessfully.~ T h ~ set ~ ~ d e nwho t s rcrrive a six-carbun starting alcohol must a d p i t quantities to maintain the same molar ratios as ei\.pn rn the texthcmk for isoamvl alcohol. Thus, they are required to run the proton spectraof their unknowns pGor to heeinninr exoerimental work. Introductory lectures on chemicaishifi and integration allow assignment of the OH resonance to the peak furthest downfield. Relative integrals allow each studeht to readily determine whether (s)he has been given a five- or six-carbon alcohol.6 I t is wise to warn students that a small amount of water carelessly introduced into the sample will artificially enhance the OH peak. This provides the instructor an opportunity to discuss exchongeable hydroaens in a relevant situation. Further ramifications . of slow proton exchange may be demonstrated. We have found that many of our commercial starting alcohols are pure enough to show coupling between the OH and the aCH's. The demonstration that a single crystal of para-toluenesulfonic acid will collapse the OH multiplet t o a singlet7 is graphic proof of a phenomenon that is conceptually difficult for students to grasp. The acetylation procddure is followed by a simple distillation in which the students record the boiling points of their
Unknown Alcohols Number Compound
ol Carbons
Number of a-H's
Number of 831'9
acetates. An NMR of the product shows several distinct changes from that of the starting material. The OH resonance hasdisappeared, a new methyl singlet is observed, and the n-CH proton(s) are shifted downfield. Noting these changes illustrates the utility of NMR in following the promess of a reaction. Finally, each student must attempt to identify hisher unknown.8 Integration has already determined whether the unknown is a fi;e- or six-carbon alcohol. The Lucas Test is the traditional means of classifying an alcohol as primary, secondary, or tertiary, hut the integration of the a-CH multiplet (relative to the other H's) carries this same informations Classical chemical tests will not readily yield any information about the 8-carbonfs). The multiplicity of the n-proton's (or n-protons') resonance clearly allows the number of p-hydrogens to he ascertained. Most students initially find
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For other examples of integrated NMR experiments, see: Markow. P. G.. and Cramer, J. A,, J. Chem. Educ., 60, 1078 (1983),and reference* cited therein. A random SUNey of eight recent organic laboratory texts found only one such experiment designed for introducing NMR: Helmkamp, G. K.. and Johnson. H. W., "Selected Experiments in OrganicChemistry," 3rd ed.. W. H. Freeman and Co.. New York. 1983, Expt. 7-D. This experiment may be found in many commonly used lab texts, e.g., (a) Lehman, J. W.. "Operational Organic Chemistry." Allyn and Bacon, Boston, 1981, Expt. 5: (b) Pavia, D. L., Lampman, G. M., and Kriz. G. S.. "introduction to Organic Laboratory Techniques," 2nd ed., Saunders. -~~ ~. Philadelohia. . . 1982. Exot. 10: ic) . . Miller. J. A,. and Neuzil. E. F 'Modern Exper mental Organlc Chemistry." O.C. Heath and Co.. Lexington. MA, 1982. Expt 8-3; and (dl Ault. A,, "Techniques and Exper ments for Organic Chemistry." 4th ed., Allyn and Bacon. Boston, 1983, Expt. 57. Aldrich Chemical Company, 1984-1985 catalog. lsoamyl alcohol was eliminated from our list because the odor of the esterified product is an unmistakeable clue. The e-CH's provide the most reliable base integral in determining the carbon number of the unknown. A referee has suqqested the use of an internal standard lo aid in the assignment of c a r h number. 'The a-CH's wlli, of course, also coilapsetoasimpler multiplet on addition of tosic acid. Boiling point and IR evidence may also be integrated into the solution of the unknown structure. In principle, the CO stretching vibrations in lhe IR may also be used to classify an alcohol as primary, secondary, or tertiary, but the assignment is not 100% reliable: Bellamy. J. L., "The Infrared Spectra of Complex Molecules." 3rd ed., John Wiley and Sons, New York, 1975, Voi. I. Chap. 6.
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Volume 62
Number 10 October 1965
899
this piece of evidence elusive. Some thought is required in that most students will individually study and learn these order to appreciate that information about the 8-hydrogens concepts during this experiment." is available even though the peaks caused by these same Our experience with this experiment has been very favorhydrogens are buried in a complex upfield multiplet. able. We invite others to explore the possibilities of using The NMR evidence accumulated thus far-Cs versus Cs, "unknown starting material adaptations" to introduce other and the numbers of a- and 8-hydrogens-allows an unamhispectroscopic methods. guous structure to he assigned for only one of the alcohols we Acknowledgment have actually used: 2-methyl-1-butanol.lQFor the other nnknowns, we expect the students to provide either a partial I would like to thank Harland Embree, Gerald Selter, and structure or a list of possible structures consistent with their Lanny Replogle for their helpful suggestions and comments. obsewations. In summary, we have successfully adapted an existing experiment in a way that adds little time to the laboratory schedule, yet adds immensely to the students' understand'O 1-Pentanol is unambiguously identified if olfactory evidence is ing of 'H-NMR. The concepts of chemical shift, integration, also consideredl The only other alcohol consistent with these three exchangeable hydrogens, and multiplicity have been intr0simple pieces of NMR evidence is isoamyl alcohol; the odor of its duced in the context of solvine the (nartial) struct,ura an nrntetc ~m ~ ~ ~~ ~ ~ ~of . --~ ..,. ~ - . .i." ~ m r n i a t n l r a h". unknown. As students are led'to believe that unknowns are " We have obtained satisfactoryresults on a 30-MHz (Varian EM300) NMR. A copy of the handout we use to accom~anv chosenfrom the full list of 25 constitutional isomers. we- have . the textbook - - - ~experiment canbe obtained by writing to the author. found the actual list of six alcohols to be sufficient to ensure
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Journal of Chemical Education
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