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Empirical Equations for 13C Nmr Chemical Shifts of Alkanesulfonic Acids

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Empirical Equations for 13C Nmr Chemical Shifts of Alkanesulfonic Acids BUNSEKI KAGAKU Vol. 33, pp.E47-E53, 1984 C The Japan Society for Analytical Chemistry, 1984 EMPIRICAL EQUATIONS FOR 13C NMR CHEMICAL SHIFTS OF ALKANESULFONIC ACIDS Yoshio KOSUGI R* and Hideyuki KONISHI ** *D epartment of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Chikusa-ku, Nagoya 464 ** Ch emistry Section, Aichi Educational College, Kariya, Aichi 448 13C NMR data of eleven sodium alkanesulfonates in deuterium oxide have been obtained. Simple equations for prediction of chemical shifts have been derived. Maximum deviation between observed and predicted values is •} 0.8 ppm. Substituent effect of sulfonate group on ƒ¿-carbon is much larger than the degree predicted by linear relationship with electro- negativity of 1-substituted alkanes. On the other hand, ƒÀ-effect is not much variable with kinds of substituents, but comparatively smaller effect is found for sulfonate group. Thus the difference between the effect on ƒ¿- and ƒÀ-carbon caused by sulfonate group is larger (about 36 ppm), which is contrasted with the effect of, for example, methyl group (•`0 ppm). Numerous organic compounds containing sulfonyl group are encountered in natural products, medical drugs, biochemical compounds, artificial sweetenings, synthetic detergents etc. Analysis or determination of sulfonic acids are less convenient comparing with amines, alcohols, ketones and carboxylic acids. In fact, spectral data of sulfonic acids are relatively few. Although we have demonstrated mass spect- rometric analysis1), carbon-13 nuclear magnetic resonance (13C NMR) spectroscopy still seems suitable for sulfonic acid which is very acidic and poorly extractable from aqueous solutions. It is also interesting to see substituent effect of sulfonic acid on 13C NMR chemical shifts. Generally, substituent effect has still been an important and topical subject for understanding the essential nature of chemical shifts2). On the other hand, empirical equations are practical use for full analysis of spectra. Based on 13C NMR data, chemical shifts of alkanes3)4), alcohols5), and amines6) are formulated. In our previous paper, 13C NMR data of several simple alkanesulfonates have been reported7). Herein added are eleven alkanesulfonates and empirical equations are derived. EXPERIMENTAL Chemicals. Sodium salts of linear alkanesulfonates from n-C5H11 up to n-C11H23 were purchased from Tokyo Kasei Kogyo Co. Ltd., and used without purification. Four kinds Ad sodium salts of branched alkanesulfonates, such as 2-propane-, 2-butane-, iso- butane-, and 2-ethylhexanesulfonic acids were prepared by the Strecker reaction reported8). Alkyl bromides and sodium sulfite were of chemical pure grade. Better E48 BUNSEKI KAGAKU Vol. 33(1984) yields were achieved by employing a mixture of dimethyl sulfoxide and water to homogenize the reaction mixture. Crystallization of products was obtained from 75% aq. alcohol. Experimental conditions and results are summarized in Tablel. Table 1 Preparation of branched alkanesulfonates by Strecker reaction. NMR measurements. A 200 mg of sodium alkanesulfonates was dissolved in 3 ml of deuterium oxide containing ca. 5 mg of sodium 3-trimethylsilylpropionate-d4 (TSP-d4). The solution in a NMR sample tube of 12 mm was set on a Varian XL-100-15 NMR Spectrometer operating at 25.16 MHz. Proton-decoupled spectra were recorded with 2 KHz band width at 8W. Off-resonance spectra were also recorded when necessary. Normally 1000 transients were accumulated using pulse angle of ca. 45° in repetitive interval of 3 sec. A Varian DATA 620i computer of 8K was used for Fourier transformed spectra of 5000 Hz width. RESULTS AND DISCUSSION Chemical shift data of a number of aromatic and aliphatic sulfonic acids includ- ing methane-,ethane--, n-propane-, and n-butanesulfonic acids have been reported previously7). There, it has been found the chemical shift (6c) is least influenced by pH values of aqueous solution. These alkanesulfonic acids are strongly acidic enough to completely dissociate into sulfonate ion even at lower pH values. In the present studies, for the sake of convenient handling, sodium salts were used and NMR measurements were carried out in deuterium oxide but not in concentrated sulfuric acid. Assignments of spectral peaks were made based on data of the corresponding E4 9 alkanes9) and based on consideration of gradual attenuation of influence by sulfonate group or terminal methyl group with increase of distance on alkyl chain. The results of alkanesulfonates of straight chain were summarized with the previous data. Table 2 13C NMR data of sodium alkanesulfonates(RSO3Na) in deuterium oxide.*1 Electron density is one of the important causative factors governing 13C NMR chemical shift. For the present 13C NMR data, however, electron density calculated by CNDO/2 method was not useful. It is interesting to note the increase of chemical shift values of terminal methyl at the highest field with the length of alkyl chain. Similar observations have been reported in other series of straight chain, such as alkanes9 alcohols 5), and alkyl sulfates 10) In the last case, terminalmethyls of dodecyl-,tetradecyl- and cetyl- sulfates give slight, but evident differences in chemicalshift ( (Ile) enoughto quantitize amounts of these three mixturps. In order to derive empirical equations for the alkanesulfonates, corrections for the chemical shift of each carbon are made by addingm e = 16.4 -Me(RSONa) or multiples of the value and, if necessary, by adding effects of terminal methyl on a- and (3-carbons. Thus, chemical shifts of each carbon1610)in Table 2, with the exception of methane- and ethanesulfonates and C2 and C3 of propanesulfonate, are fairly well expressed by simple equations. E50 BUNSEKI KAGAKU Vol. 33(1984) (1 ) Table 3 Constant values in Eqs.1-3. ( 2 ) (3 ) where TMeand TMe13area- anda- effects of terminal methyl, respectively. Values of constants in Eqns.1-3 are given in Table 3. /1614e for C5H11S03Na' for example, is 16.4-15.7=0.7. An empirical equation for C9 or higher carbons is simple: that is (4 ) where Rn denotes alkanesulfonate with carbon number of n. For example, 69 of decane sulfonate (R10) is Calculated chemical shifts are collected in Table 4. Maximum errors are also given in the last column. Table 4 Calculated chemical shifts (ppm) of alkanesulfonates(RSO3Na) using Eqs.1-4. E51 Thus predictions are successful in spite of complication of the effects by terminal methyls. Substituent effect of sulfonate group is derived from the comparison with the corresponding alkanes 9). Similar data of various substituents are available in literatures (Table 5). Taft's substituent parameters ( oi) are useless for unifica- tion of these data. A linearity between chemical shifts and electronegativity has been first reported on 1H and 13C NMR of CH3X, where X = H, C, N, 0, F, or X = 1, S, Br, Cl, F 12). Some of the 13C NMR data in Table 5 are correlated with the electro- negativity of directly bonded atom. A fairly good linear relation is observed for the chemical shift of a-carbons ( 6 ) with the electronegativity, Table 5 Substituent effect*1 on a, $, and 1-carbons of 1-substituted alkanes (R-X). E52 BUNSEKI KAGAKU Vol. 33(1984) However substituents bearing double-bonded oxygen(s), such as NO2, COX, CO2R,and S03, afford greater a-effect on carbons. Extremely large deviation of NO2and SO; from the linearity may be attributable to their resonant characters. Our failure of MO calculations for the present alkanesulfonates may also be related to the difficulty in determination of molecular structures. The only negative a-effect by iodo group in Table 5 is due to its large shielding effect, which ig manifested in tetraiodo- methane of 0=-293.3 ppm 13). No remarkable difference in (3-effect is observed among various substituents. Only carboxyl and sulfo groups give smaller (3-effect. Therefore magnitude of difference between a- and 13-effects is very large (36 ppm) in sulfonate and very small (-0 ppm) in methyl or cyano group. The y-effect is small shielding effect resulting in negative sign or up-field shift. Primary mechanism of the effect is deemed to be through space interaction rather than through bonds. However, the present results in Table 5 are better correlated with Q values14-15) than electro- negativity. The semiempirical parameter Q is composed of ionization potential and polarity of the substituent. Chemical shifts of ortho-substituted benzenes and fluoro- benzenes are well correlated with Q values. Nitro group disturbs the linear relation again. Abnormality of sulfo group has been found previously7)in smaller shift change upon dissociation comparing with carboxylic acids, amines or phenol. Therein it has been assumed that the generated charge remains within the large and symmetric sulfo group. Several kinds of non-linear alkanesulfonates were also examined. The results are summarized in Table 6. Table 6 13C NMR data of non-linear alkanesulfonates. Chemical shifts are expressed in ppm downfield from TSP -d 4. E53 Contrary to the linear alkanesulfonates, more parameters are required for empirical equations. This is in the same situation as alkanes where more than twenty parameters are used in Lindeman-Adams equation 3). The The present small body of data does not suffice for many parameters, but a tentative equation for ƒ¿-carbon of non-linear alkanesulfonate can be derived. (5 ) Table 7 Constant values in Eq. 5 where m and n are the number of ƒ¿- and β-carbons, B and Γ are constants given in Table 7. REFERENCES 1) Y.Kosugi, K.Matsumoto: Fresenius' Z.Anal.Chem., 312,317 (1982). 2) Y.Kosugi, Y.Furuya: Tetrahedron, 36, 2741 (1980). 3) L.P.Lindeman, J.Q.Adams: Anal.Chem., 43, 1245 (1971). 4) D.M.Grant, E.G.Paul: J.Am.Chem.Soc., 86, 2984 (1964). 5) J.D.Roberts, F.J.Weigert, J.I.Kroschwitz, H.J.Reich:J.Am.Chem.Soc.,92,1338(1970). 6) J.E.Sarneski, H.L.Surprenant, F.K.Molen, C.N.Reilley:Anal.Chem., 47, 2116 (1975).
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