Alan P. Marchand and Danny Jackson Unwersrty of Oklahoma Norman, 73069
Structural Assignment of a Cl0Hl2O3 Ester by Mass S p e ~ t r 0 ~ ~ 0 p y An undergraduate organic problem
With the current emphasis on integrating spectroscopic methods (ir, uv, nmr, and mass spkctrometry) into basic undereraduate oreanic chemistrv lecture courses, there is a corresponding demand for chBllenging and illustrative nroblems. Tvaicallv. is placed upon in.. .. vrimarv . . emuhasis . terpretation of nmr, ir, and uv spectra, as these three spectrosconic techniaues are eenerallv extremelv useful aids to determining structures of organic compounds. The increasnmr, ir, and uv specing availahilitv of moderatelv . vriced . trimeters hasealso contributed to the preeminence of these spectroscopic techniques in undergraduate organic instruction. Students learn how to derive useful structural information from nmr, ir, and uv spectra relatively quickly and easily. To derive additional information from mass spectra often requires considerably more effort on the part of beginning students for all hut the simplest organic systems. In fact, the other three spectroscopic methods appear to he so powerful that beginning students might be led to question the necessity (or, indeed, the usefulness) of going to the trouble and expense to obtain and analyze mass spectra. In considering ways to impress students with the potential usefulness of mass spectroscopy to organic chemistry, we were led to a problem presented in Morrison and Boyd (Third Edition) in which students were asked to suggest a structure for a compound having the molecular formula C I ~ H 1 2 0on? the basis of its proton nmr spectrum alone ( I ) . The nmr spectrum displayed an AA'BB' pattern centered a t 6 7.40 (area 4H), a quartet centered a t 6 4.33 (area 2H), a singlet a t 6 3.80 (area 3H), and a triplet centered a t 6 1.35 (area 3H). The answer given (2) was "ethyl anisate" (i.e., ethyl p-methoxyhenzoate (I)). However, it occurred to us that a second possible structure, methyl p-ethoxyhenzoate (11) was also consistent with this nmr spectrum, and that it would, in fact, be very difficult to choose a priori between (I) and (11) on the basis of the one nmr spectrum given in the problem without recourse to additional nmr literature on model compounds (such as anisole, methyl benzoate, phenetole, and ethyl benzoate). It was anticipated that the pedagogical value of this problem would he increased should both the nmr and mass spectra of compounds (I) and (11) be made available for direct comparison. Although one would expect the nmr spectra (or, for that matter, the ir and uv spectra) of these compounds to he closely similar, their mass spectral fragmentation patterns should be quite different, thereby readily affording a basis for differentiation. Synthesis of Esters (I) and (11) Sophomore-level organic chemistry students were afforded samples of p-ethoxybenzoic acid (111)and p-methoxybenzoicacid (IV). The ethyl ester (I) was simply prepared by refluxing (IV) (5.0 g) overnight in excess absolute ethanol (30 ml) containing concentrated sulfuric acid (1 ml) as catalyst (3).After work-up (which included washing the organic product with dilute aqueous sodium bicarbonate solution to remove unreaeted (IV), the product (I) was purified uio reduced pressure distillation. A colorless ail was thereby obtained (1.7 g), nn2' 1.5238, bp 81-84°C/0.3 mm; (lit. ( 4 ) nu14:' 1.5274, bp 136-13'1°C/13 mm). Unlike (IV), acid (111) was not sufficiently soluble in the hailing 390 / Journal of Chemical Educafion
I
1
8
Figure
7 1. 60 MHz
6
proton
* nrnr
PPM
3
2
spectra of esters (I) and
1
(11) (CCI,
0
solvent,
M&S~internal standard)
Figure 2. 70 eV Mass spectrum of ester(1).(Peak intensities are expressed as percents of the base peak (m/e 28 = 1001. alcohol (here, methanol) to permit esterification over a reasonable reaction time in good yield. Accordingly, ester (11) was synthesized hv refluvine (111) (5.0 e ) in excess thionvl chloride (15 ml) overnight. The yeaction miiture was then concentrated in uocua, and the residue was triturated with methanol (30 ml). The resulting solution was refluxed an additional 24 hr. After work-up, ester (11) was purified uio reduced pressure distillation. A colorless oil was thereby obtained (1.4 g), bp 80-85°C/0.3 mm. The distillate salidified upon standing to afford a colorless solid, mp 36-3'1°C(uncarr.) (lit. ( 5 ) mp 37.5-38'C). Nmr spectra of esters (I) and (11) were ohtained on a Varian T-60 nmr spectrometer (CCla solvent, MeaSi internal standard); these spectra are shown in Figure 1. The corresponding mass spectra were ohtained on a Hitachi-Perkin Elmer RMU-7E mass spectrometer operating a t 70 eV. The individual peaks (mle) were calibrated by the addition
Figure 3. 70 eV Mass spectrum of ester(l1).(Peak intensities are expressed as percents of the base peak (m/e 28 = 100).
of a small amount of perfluorokerosene to the sample being run. The mass spectra of (I) and (11) (intensity (base peak = 100) versus mle) are shown in Figures 2 and 3, respectively.
m/e 92
den
Spectral Analysis
Comparison of the nmr spectra of (I) and (11) (Fig. 1) reveals the anticinated small deshieldine effect of a carhoxvl ether oxygen atom relative to that of an ether oxygen atom on adjacent methyl and methylene protons; (compare the chemical shifts of ArC02CH3 with ArOCH3 and the corresponding shifts of ArC02CH2CHs with ArOCH2CHz in Fig. 1). On the other hand, the mass spectra of (I) and (11) (Figs. 2 and 3, respectively) hear little mutual resemblance other than the fact that they display the same base peak (mle 28) and parent ion (mle 180). I t is instructive to first examine the mass spectrum of (I). Major fragment ions,occur a t rnle 180,152,135,107,92,77, 50, 40, 32, and 28. Mechanistic rationalization of some of the major fragments is shown in Figure 4. The postulation of the fragmentation pathway mle 135-mle 107 is supported by the appearance of a metastable ion a t mle 84.8. The pathway m/e 107-mle 77 is suggested by analogy to the known ability of anisole to expel formaldehyde, affording C6H6t (mle 78) which can lose an additional hydrogen atom to afford a phenyl cation (CsHs+, mle 77) ( 6 ) .Fragment ions a t rnle 152 and 28 can be accounted for in terms of a McLafferty rearrangement on the molecular ion (7). The mass spectrum of (11) displays major fragment ions a t mle 180, 149, 121, 93, 65, 40, 32, and 28. Figure 5 suggests mechanistic pathways to account for the production of some of the major fragment ions. Unfortunately, no metastable ions could he observed in the mass spectrum of (11). The fragmentation pathways shown in Figure 5 were suggested by analogy to the fragmentation behavior of (I) and also by analogy to the known fragmentation behavior of aromatic ethers and esters of aromatic carhoxylic acids (8). Conclusions
Esters (I) and (11) display closely similar nmr spectra. However, their 70 eV mass spectra are quite different, thereby providing a spectral method through which the two compounds may readily he differentiated. Students are thus provided with a concrete example which should impress them with the utility of mass spectroscopy as an aid toward assigning structures of organic compounds.
Figure4. Mechanistic pathways to account far some of the major fragments shown in the mass spechum of (11.
mle 65 Figure 5. Mechanistic pathways to account for some of the main fragments shown in the mass spectrum of (11).
v&A.
Practbal O~ganicChemistry:' 3rd Ed., Lonemens. I., .'A Text-Book Green. New Yerk, 1956.~.781. 16) usn Auwern, K.. Jmiur L i e b i ~ 8 A n n .Chem.. 408.254 119151. 151 West. R., Omstein. S.. McKpo, D..and Lqner, R., J. A m e r Chem. Soe.. 14, 39M)
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Literature Cited
16) M w r . F..snd Harriron.A. G..Con. J Chem.. 42.2MR (19641. 17) Siiverrtein. R. M.,Bassler, G. C.. and Morrill. T. C.. "Spectromatrie identification or Orpanic Compounds." 3rd Ed.. John Wiley and Sona, Inr.. New York. 19'74, pp.
i l l Morrison, R. T.. and Boyd. R. N.. "Organic Chemirfry.ll 3rd Ed., AIlyn and Bacon, Ine.. Bnrton. 1973. Chap. 20. problem 26.pp.694.699. (21 Hof. ll1.p. 1199.
161 Rudzikiewin, H.. Djeraasi. C.. and Williams. D. H.. "Mars Spectrometry a1 Ownie Cumpuunds," Holden-Day. Ine.. San Wancisru, 1967, pp. 197-205.237-241.
..
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Volume 53.Number 6, June 1976 / 391
