Jerry W. Ellis
and David H. Buchanan Eastern Illinois University Charleston, lllinois 61920
The Preparation and Spectral Analysis of Toluene-a-d
Many modern organic textbooks introduce the common spectroscopic tools of the organic chemist early in their discussion of molecular structure. We describe here an experiment which can be performed successfully by first semester organic chemistly students which provides dramatic visual evidence of the changes in ir, nmr, and mass spectra upon substitution of deuterium for hydrogen in a simple molecule. Deuterium can be incorporated into simple hydrocarbons by deuterolysis of active organometallic compounds such as alkyllithium or a Grignard reagent with deuterium oxide or a deuterated acid. In this experiment, benzylmagnesium chloride is prepared from benzyl chloride and magnesium in diethyl ether. Benzyl Grignard reagent is then carefully quenched with deuterium oxide to form toluene-a-d. Toluene-a-d was chosen for this experiment because i t can be prepared in good yield and high isotopic purity by standard methods (I), has a convenient boiling point and molecular mass, is the analog of a compound already familiar to most students, and provides a simple example of deuterium-proton coupling in the nmr spectrum. Of the several published undergraduate experiments leading to deuterated compounds (2), none involve compounds in which geminal D-H coupling may be conveniently demonstrated. Although this experiment can he successfully completed by non-majors, the great number of experimental techniques and theoretical points it is capable of illustrating make it of special interest to chemistry majors. Among possible lecture subjects introduced by this procedure are: (1) Protonolysis of active organometallic compounds and the role of the metal in determining reactivity (3); (2) Alternate methods for the work-up of Grignard reactions; (3) Difficulties encountered in achieving anhydrous reaction media (4); (4) Infrared analysis; (5) Nmr analysis including D-H coupling and deuterium content by integration; ( 6 ) Isotopic analysis by mass spectroscopy; (7) Effect of ionizingvoltage on fragmentation in the mass spectrum. Deuterium content is a direct measure of student technique. Most students achieve 85-90% deuterium incorporation (mass spectroscopy). Nmr estimation of deuterium content (Varian T-60) was routinely 5-10% higher. Higher deuterium incorporation can be obtained by preparing a small amount of ethylmagnesium iodide in the reaction vessel before the addition of benzyl chloride ( I c , 2a). Freshly opened cans of anhydrous diethyl ether are adequate for these experiments. We recommend a light flame drying of the apparatus containing the magnesium metal to remove moisture, although the problem of surface oxide coating on the metal could be increased. It must be stressed that every student complete this step before the first can of ether is opened. We have found the reaction most conveniently done on a 0.1-mole scale in a 250-ml round bottom flask using the usual precautions for Grignard reactions. A total volume of 100 ml of ether should be used to minimize coupling reactions. After Grignard formation is judged complete, the mixture should he cooled to room temperature and 2.5 ml of DzO added dropwise. Care should be taken to minimize the exposure oFD20 to moist air (4).
Following the addition of D20, all volatile compounds are distilled from the reaction vessel using a flameless source of heat. When the head temperature reaches 11l0C, distillation is essentially complete. (Care should be taken, not to char the magnesium salts remaining as this complicates cleaning of the flask.) The distillate is dried and redistilled through a small fractionating column, collecting the fraction boiling between 105-1lO"C. Average yields in our labs are 3-5 g (35-50%). I R Spectrum
The C-D stretching band is seen in the ir spectrum of toluene-a-d at 2220 cm-1 when run as a thick film hetween specially prepared salt plates. (A small depression was made in one plate by scraping with a knife blade.) Spectra of toluene and toluene-a-d are run on the same chart paper using different colored pens. By assuming that the force constant for a C-H bond does not change upon substituting D for H, and using the value of 2960 cm-I for the stretching frequency of the C-H bond, the expected frequency of the C-D stretch can be calculated from
"=p +o
PCH
N M R Spectrum The number of possible spin states for a nucleus of spin quantum number I is 21 + 1. For a proton I = 5 ;for deuterium I = 1. A signal for a set of equivalent protons will be split by n neighboring nuclei of spin quantum number I into 2nl + 1 lines. Thus one neighbor proton will split a signal into 2 . 1 .1h + 1 = 2 lines. Equivalent protons do not give rise to splitting of their nmr signal. From this, students can predict the multiplicity of the signal of the henzylic protons of toluene-a-d. Prediction of relative intensities requ~resuse of a simple energy level diagram for the nuclear transition, a straightforward process. Because the gyromagnetic ratios of proton and deuterium differ, deuterium is not "seen" by the nmr spectrometer when operated at the magnetic field necessaly to observe protons. Since water is nearly ubiquitous in organic laboratories, some of the toluene produced may have arisen from quenching by water rather than DzO. From the integral of the nmr spectrum, it is possible to estimate the isotopic purity of the toluene-a-d. Comparison of peak areas for the nmr signals due to -CsH6 and -CH2D can, in principle, allow the calculation of an approximate isotopic purity; however, more accurate values of deuterium content can be obtained from mass spectroscopy. Mass Spectrum
Most molecules show a rich variety of peaks in their mass spectra due to fragmentation of the parent ion. The most abundant ion in the mass spectrum of toluene is the tropylium ion at mle 91 due to loss of a hydrogen atom from the parent ion (mle 92). A sample of pure toluenea-d (100%D) would show a parent peak at rn/e 93 and tropylium ions at m l e 92 (loss of H) and m/e 91 (loss of D). In a sample of partially deuterated toluene, the rnle 92 Volume 52. Number 4. April 1975
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peak will he due to undeuterated parent ions and deuteratd . P-l. ions. In .~ ~~~. -~~ addition. ~-~~~~ , naturallv occurrine 13C will give rise to a P 1 ion approximately 7.7% as'inrense as the narent ion. Thus. the s i m ~ l eratio of the intensities of mjeA93 to m / e 92 will not gi;e the isotopic purity. Since fragmentation results from excess energy in the parent ion, running the mass spectrum at an ionizing voltage close to the appearance potential of the parent ion rather than at the normal 7 0 eV can often eliminate fragmentation and thereby greatly simplify the analysis. In this experiment, the mass spectrum of toluene-a-d is run at about 9 eV such that any peak at m / e 92 is due to undeuterated toluene (5). After 7.7% of the m / e 92 intensity from the m / e 93 peak, the ratio of the corrected
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m / e 93 intensity (93*) to the sum of intensities of m / e 92 93%~ i v e the s fraction of deuterium in the sample.
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