Structural determination of monosubstituted alkylbenzenes by proton

from azobenzene studies. Phenol red is an ideal system because of the behavior cited above where reliable measurements could be made in the absence of gelatin, as well as on samples which provided useful polarograms only in the presence of gelatin. Moreover, for this system the correct rate constant (in the absence of gelatin) was known. Measurements were performed on both purified and unpurified samples of phenol red with various concentrations of gelatin. In every case the systems behaved as would be expected for the disproportionation reaction, with no other complications (presumably the value of k , would be altered, but for scan rates employed in this study, electron transfer for the first wave remained Nernstian). In addition, rate constants measured from cyclic voltammograms in the manner described above were within experimental error of the value that was obtained by independent spectrophotometric measurements (3.4 X 102M-1 sec-l), regardless of gelatin concentration. Typical results from such experiments on phenol red and cresol red are summarized in Table IV. As these data show, use of gelatin (at least for these systems) is an expedient and acceptable approach, provided, of course, that the interest is not in the adsorption phenomena per se, or in details of the heterogenous electron transfer reaction. The effect of Triton X-100 as a maximum suppressor for phenol red was also investigated since Senne and Marple employed Triton X-100. Qualitatively, the effects of Triton X-100 and gelatin are the same. However, Triton X-100 is unacceptable for quantitative measurements. For example, apparent rate constants measured with cyclic voltammetry in the presence of Triton X-100depend on the concentration ~~

of Triton X-100as well as on scan rate and phenol red concentration. Typical results are shown in Figure 10 where apparent rate constants are plotted us. scan rate for several bulk concentrations of phenol red. These data are reminiscent of data reported by Wopschall and Shain (20) for azobenzene. In fact, these authors suggested using plots like Figure 10 to obtain homogenous rate constants for systems showing weak adsorption; the value of k extrapolated to zero scan rate is ostensibly the correct k . Interestingly, the data of Figure 10 all extrapolate to a common rate constant close to the correct value for phenol red. This fact suggests that Triton X-100 suppresses adsorption enough to eliminate obvious anomalies, but that some residual adsorption still remains. Unfortunately, this simple explanation is not correct, since apparent rate constants measured in the presence of both gelatin and Triton X-100 simultaneously also are a function of Triton X-100 concentration. Obviously, one has to be careful in using empirical approaches, such as “gelatin” electrodes, in electrochemical kinetics. ACKNOWLEDGMENT

We wish to thank Stan Crouch and Paul Beckwith for permitting us to use their stopped-flow apparatus.

RECEIVED for review March 1, 1972. Accepted May 2, 1972. Financial support for this research was provided by the National Science Foundation through Foundation Grant GP-

10671.

~~~

Structural Determination of Monosubstituted Alkylbenzenes by Proton Magnetic Resonance Osamu Yamamoto, Kikuko Hayamizu, Kiyoharu Sekine, and Shuji Funahira Gocernment Chemical Industrial Research Institute, Shibuya-ku, Tokyo, Japan In order to obtain the basic data for the structural determination of monosubstituted alkylbenzenes by PMR, the PMR parameters are determined for 66 such compounds, and the additivity rules for the ring proton chemical shifts and the coupling constants are examined. General features of the ring proton signals in the monosubstituted alkylbenzenes are discussed in detail. A simple method is proposed for presuming the substituent and discriminating the isomers by use of the center of gravity and the pattern features of the ring proton signals. The effect of the alkyl chain on the PMR parameters is also studied. HIGHRESOLUTION PROTON MAGNETIC RESONANCE spectrometry (PMR) has been widely used for identification and structural determination of organic compounds. One of the significant features of PMR spectra is the simplicity of the spectral pattern and the ease of its interpretation. In many cases, a visual inspection of the spectrum based on very simple rules is sufficient to assign each spectral peak and to deduce the proposed structure of the molecule from the information contained therein. Thus, many organic chemists now employ PMR spectra as a powerful tool for identification of the organic compounds obtained in the course of or in the final stage 1794

of their studies. For these purposes, extensive efforts have been made to obtain graphic representation of proton chemical shifts, which have been used in many laboratories (1-8). In some cases, however, PMR spectra show complicated patterns, where three or more spins are strongly coupled with each other. In such cases, PMR signals no longer correspond to the usual patterns governed by the first order spin-multiplicity rules, and the complete analysis by a computer is usually necessary to extract PMR parameters that are correlated with the molecular information. A typical and important example is the ring protons of benzene derivatives with one or two substituents. Generally ring protons resonate in a rela(1) L. H. Meyer, A. Saika, and H. S. Gutowsky, J. Amer. Chem. SOC., 75,4567 (1953).

( 2 ) N. F. Chamberlain, ANAL.CHEM., 31,56 (1959). (3) K. Nukada, 0. Yamamoto, T. Suzuki, M. Takeuchi, and M.

Ohnishi, ibid., 35, 1892(1963). (4) F. C. Stehling, ibid., p 773. ( 5 ) K. W. Bartz and N. F. Chamberlain, ibid.,36,2151 (1964). ( 6 ) F. C. Stehling and K . W. Bartz, ibid., 38,1467 (1966). (7) 0. Yamamoto, T. Suzuki, M. Yanagisawa, K. Hayamizu, and M. Ohnishi, ibid., 40,568 (1968). ( 8 ) N. F. Chamberlain, ibid., p 1317.

ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972

tively narrow range, i.e., 8.5-6.0 ppm downfield from TMS, while the protons are coupled strongly with each other on the order of 7-9 Hz between ortho positions and of 1-2 Hz between meta positions. The chemical shift difference between two protons in the compounds is often so small that resulting patterns are very complicated. Thus organic chemists are usually satisfied with knowing only that benzene protons are present, and it is difficult to know which substituent exists or at which position the substituent is placed, except for paradisubstituted benzenes, which show a fairly simple pattern of an AA’BB’ spectrum. Nevertheless, the chemical shifts of ring protons have been studied from early days, relating to .rr-electron density on the ring structure or other interesting topics, and many important correlations with the proton chemical shifts have been found, among which the relation to Hammett’s u constants is the most familiar (9). .From a more practical point of view, the additivity rule of the ring proton chemical shifts depending upon the substituent was proposed by many workers (10-14), and has been considered as an aid for the identification of the organic molecules with a benzene ring showing a relatively simple PMR pattern. Later the additivity rule for the ring proton coupling constants was also found (15-17). The characteristic PMR spectra of the ring protons are mainly revealed by these two additivity rules. Although the thorough computer analysis is not practical for the determination of the molecular structure by the PMR spectrum, some trial simulations of the spectrum are convenient for the identification of the spectral pattern. For this purpose it is sufficient to make a few trial calculations without the iteration technique, by slightly changing the chemical shifts corresponding to the changes in solvent and concentration. The PMR parameters estimated, for example, by the two additivity rules may be used for these trial calculations. Even a mini-computer, which has been rapidly developed recently, can make such calculations if the suitable soft-ware is available, and such an approach seems quite feasible for practical purposes. In this respect, it is desirable that PMR parameters be accumulated for the basic compounds of this type. Furthermore, if the characteristics resulting from the two additivity rules are reflected in the ring proton pattern, it may be expected that there would be some regularity in the PMR pattern of the ring protons, however complex it may be. And if it is possible to extract by close inspection of the pattern some information other than that obtained by the computer analysis, it will be very useful, even if insufficient, for the practical inference of the structure of the molecules. The importance and usefulness of the signal patterns for the structural determination have already been suggested for paraffinic hydrocarbons by Bartz and Chamberlain (5). Monosubstituted alkylbenzenes are an important class of benzene derivatives, but their PMR spectra have not been (9) J. W. Ernsley, J. Feeney, and L. H. Sutcliffe, “High Resolution Nuclear Magnetic Resonance Spectroscopy,” Vol. 2, pp 749-82, Pergarnon Press, Oxford, 1966. (10) P. Diehl, Helc. Chim. Acta, 44,829 (1961). (1 1) J. S. Martin and B. P. Dailey, J. Cl7em.Pliys., 39,1722 (1963). (12) G. W. Srnith,J. Mol. Spectrosc., 12,146(1964). ~~, (13) J. J. R. Reed,ANAL. C H E M . ,1586(1967). (14) B. Richardson and T. Schaefer, Cuti. J. Cliem.,46,2195 (1968). (15) S. Castellano and K. Kostelnik, Tefruhedrotz Lett., 1967, 5211. (16) K. Hayarnizu and 0. Yarnamoto, J. Mol. Spectrosc., 25, 422 (1968). (17) J. M. Read, Jr., R. W. Crecely, R. S. Butler, J. E. Loemker, and J. H. Goldstein, Tetrul7edronLett., 1968,1215.

studied extensively, except for some of para-derivatives ( l e ) , particularly from the standpoint of analytical chemistry, because of the complexity of the spectra. Furthermore, the ring protons are coupled, though weakly, with the protons in the alkyl chain, which leads to broadening the spectral lines in the ring protons. In the present work, we determine the PMR parameters for a total of 66 kinds of 0-,m-, andp-monosubstituted alkylbenzenes for the purpose described above, and the additivity rules for the ring proton chemical shifts and the coupling constants are tested. The chemical shifts for each proton are graphically presented. Then a simple method for estimating the substituent and discriminating the o- and m-isomers is proposed. Furthermore the effect of the alkyl chain on the ring proton chemical shifts is examined. These data and methods are very useful for the structural determination of the monosubstituted alkylbenzenes. EXPERIMENTAL

Apparatus. Proton magnetic resonance spectra of the m-, and p-substituted alkylbenzenes were obtained by a Varian HA-100D spectrometer operating at 100 MHz for about 5 mol % solutions in CC14, in which a small amount of TMS was included as an internal lock signal. The sweep rate was 0.1 Hz/sec. All the spectra were obtained by decoupling the alkyl protons at the cy-position to the benzene ring. For the protons of the methylene or methine group in the alkyl chain other than toluene derivatives, a Wavetek voltage-controlled oscillator Model 114 was used in the sweep mode to decouple the proton signals spreading over a rather wide range. The initial and the final frequencies in the sweep range of the voltagecontrolled oscillator were set slightly within the frequency range of the proton signals to be decoupled. The sweep rate and the intensity of the decoupling frequency were adjusted to obtain an optimum result. If the sweep rate is too slow, much power (HJ is required, while if it is too fast, the resulting decoupled signals are undesirably modulated. An optimum sweep time over the desired frequency range seems to be from 0.5 to 1 sec. In this technique, critical adjustment of decoupling frequency is unnecessary, and it is very easy to decouple, for example, the methine proton in an isopropyl group, for which signals are spread over about 40 Hz. When the substituent contains protons coupled with the ring protons, e.g., -OCH3 and -N(CH&, the triple resonance technique was used to obtain the completely decoupled spectrum. Materials. Since the ring proton shifts are sensitive to the concentration (19), the samples were prepared to have a concentration of 5.0 + 0.5 mol % in CC14 TMS, if permitted by solubility. Otherwise a saturated solution was used. The samples were not degassed with a few exceptions. Most of the materials were obtained from commercial sources. The following substances were prepared according to the method described in literature : o-bromoethylbenzene,m-bromoethylbenzene, p-bromoethylbenzene (20), and m-ethylaniline (21). m-Nitroethylbenzene was prepared from 4-ethyl-2-nitroacetanilide, which was prepared by Case’s method (22), by treating with KOH followed by deamination. The purity of all the materials was checked by means of PMR spectra and gas-chromatography, and, if necessary, the materials were purified by preparative gas-chromatography. 0-,

+

(18) J. S. Martin and B. P. Dailey, J. Chem. Phys., 37,2594 (1962). (19) K. Hayarnizu and 0. Yamamoto, J. Mol. Spectrosc., 28, 89 (1968). (20) Org. Syiz., 1, 133 (1956). Methods of preparation for bromotoluenes were applied to the ethylbenzene derivatives. (21) M. Ohki and T. Sato, Bull. Clzem. SOC.Jup., 30,508 (1957). (22) F. H. Case, Z. B. Jacobs, R. S. Cook, and J. Dickstein, J. Org. Chem., 22,390(1957).


1795

0000000000000000

&i

Lq

1796


Spectral Analysis. The spectra obtained were analyzed as an ABCD (0-and m-derivatives) or AA'BB' (p-derivatives) spin system with the iteration mode by use of LAOCOON MBYH program (23) by a FACOM 270/30 computer. RESULTS AND DISCUSSION

The results of the spectral analysis for the toluene derivatives are shown in Table I. The labeling of the protons is indicated in Table I. The rms errors between the observed and the calculated lines are well below 0.1 Hz, and typically 0.04-0.07 Hz. Fitting of the calculated pattern with the observed spectrum is generally good. Figure 1 is a graphic presentation of the obtained chemical shifts of the ring protons in the toluene derivatives. This figure is compared with the corresponding chart for monosubstituted benzenes (19). The similarity of the trend in the ring proton chemical shifts between the two cases seems to be natural because the substituent effect of the methyl group on the ring proton chemical shifts is not so different from that of the hydrogen atom. Examination of the Additivity Rules. Since the excellent work of Diehl (IO) on the additivity rule of the ring proton chemical shifts, many refinements of the rule have been made (11-14, 24-28). For the correction of the apparent discrepancies, some parameters have been introduced on a reasonable physical basis. Although these efforts are useful for understanding the nature of the ring proton chemical shifts, many parameters introduced to reflect many physical factors are too troublesome to be used for practical purposes: Apart from the physical interpretation, it seems much more convenient and possibly sufficient to assume two or three correction parameters. Among others, Reed (13) has introduced two correction parameters for crowding of the substituents, one of which is called the direct effect, allowing for the interaction of two substituents ortho to one other. The other is the sandwich effect, which is characteristic of the substituent pair at the meta position. In Table I, the differences from the predicted values, 6,, based on the simple additivity rule in the following way are shown in parentheses

*o

n N

n-

LhN

6,

=

6,

- CA: i

(11

where 6, is the chemical shift of benzene, and Af" = 6, - :6 in which Sf is the chemical shift of the proton at position i of the monosubstituted benzene having the substituent R. The values for A: are taken from references 16,19, and 29. Table I shows that in m-, and p-derivatives, the additivity rule predicts the ring proton chemical shifts well within 0.03 ppm (3 Hz), with only a few exceptions. This means that the sandwich effect is not significant for the toluene derivatives. (23) A program compiled by the authors by modifying LAOCOON I1 of A. A. Bothner-By and S . Castellano [J. Chem. Phys., 41, 3863 (1964)l. (24) T. K. Wu and B. P. Dailey, J . Chem. Phys., 41,2796(1964). (25) F. Hruska, H. M. Hutton, andT. Schaefer, C ~ H J. .Chem., 43, 2392 (1965). (26) W. B. Smith and J. L. Roark, J. Amer. Chem. Soc., 89, 5018 (1967). (27) Y. Nomura and Y. Takeuchi, Org. Magrz. Reson., 1, 213 (1969), and references cited therein. (28) W. B. Smith, A. N. Ihrig, and J. L. Roark, J. Phys. Chem., 74,812(1970). (29) K. Hayamizu and 0. Yamamoto, J. Mol. Spectrosc., 29, 183 (1969). ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972

1797

Figure 1. Schematic diagram of ring proton chemical shifts of monosubstituted toluenes Numbers in the figure indicate the position of the proton, and dotted lines show the center of gravity of the signals. The chemical shifts are referred to internal TMS

In the ortho derivatives, large discrepancies are observed between the experimental and the calculated values. The large deviations for 61 and aq (ortho to the substituent) are ascribed to the so-called ortho effect, and may be corrected, for example, by introducing the crowding parameters of Reed (13). In other words, these discrepancies as such are considered as the crowding parameters for the toluene derivatives. It is noted, however, that in the o-derivatives, and c?~,in which each proton is not positioned ortho to any substituents, deviations are often larger than 0.05 ppm. These are observed in the compounds with strong donor or acceptor substituents, and the improvement cannot be attained without further introduction of some other parameters. These remote effects probably come from the change in the mesomeric or inductive effect of the substituent due to the presence of the methyl group, or some specific interaction between the molecules. This discussion seems very interesting from the viewpoint of the electronic structure of the benzene ring, which is closely related to the proton chemical shift. However, the main purpose of the present paper is the determination of the molecular structure of monosubstituted alkylbenzenes by PMR from the analytical standpoint, and not to engage in a detailed discussion of the anomalies of the additive nature of the ring proton chemical shifts. Thus, such a sophisticated discussion will not be made here. The data obtained herein will be used as a starting point for the identification of the alkylbenzenes by PMR. They are 1798

also usable, for example, as the basic data for computer simulation as described above, or as the data for formulating the prediction scheme involving more elaborate correction parameters in the additivity rule. An additivity scheme for the ring proton coupling constants proposed by the authors (16) is ji,n-X,m-Y

=

J

13

n-X

+ J a1m-Y

- Jobenzene

(2)

where the subscripts i and j refer to protons i and j , and the superscripts n-X and m- Y refer to substituent X in position n and substituent Y in position m. The deviations between the observed and the calculated values are also given in parentheses in Table I. The agreement is good for all the compounds, with maximum deviations of less than 0.2 Hz except for two coupling constants. These are J23 in N-methyl-otoluidine (difference 0.23 Hz) and J3ain N,N-dimethyl-otoluidine (0.22 Hz). It has been pointed that the additivity rule for the ring proton coupling constants fails in some o-disubstituted benzenes with two strongly interacting substituents (15). But it is not immediately concluded that the deviations from the additivity rule in the above two molecules definitely reflect the effect of such an interaction, because the deviations are not so large. Generally it can be said that the additivity rule for the ring proton coupling constants holds well for the monosubstituted alkylbenzenes. The Center of Gravity of the Signals. Figure 1 shows that the entire signal of the ring protons moves from low to high


Type A

Table 11. Comparison between Calculated and Observed Centers of Gravity of the Ring Proton Signals for Some Compounds Center of gravity (Hz) Obsd by Differ- Type Compound Calcd integration ence pattern 0-Methylaceto730.2 730.2 0.0 A phenone m-Methylaceto746.0 -0.3 B 745.7 phenone 0-Bromotoluene 716.1 717.7 -1.6 A m-Bromotoluene 712.5 712.9 -0.4 B 711.9 709.4 2.5 A 0-Chlorotoluene m-Chlorotoluene 705.4 706.2 -0.8 C 0-Nitroethyl746.2 745.6 0.6 A benzene m-Nitroethyl771.2 771.7 -0.5 B benzene 0-Ethylaniline 671.3 670.2 0.9 B m-Ethylaniline 654.5 657.2 -2.7 A

field as the substituent changes from an electron acceptor to an electron donor. Since the center of gravity of the signals is an average of their chemical shifts, however complicated the signals are, it provides a primary measure of the substituent in the alkylbenzene derivatives. The center of gravity of the signals is obtained by the integration technique or the equivalents on the usual spectrometer on one hand, and by averaging of four chemical shifts obtained from the exact analysis on the other hand. The comparison for some of the compounds between the two methods is given in Table 11. In the p-derivatives, which show a symmetrical AA'BB' pattern, the center of gravity of the signal is easily obtained by simply averaging the line positions of the four main signals. For the other derivatives, however, some techniques should be employed to determine the center of gravity; the integration method seems the most convenient. Strictly speaking, the integration technique cannot determine the center of gravity of the signals unless the signal pattern is symmetrical. But when the entire signal is concentrated into a relatively narrow range, they can be regarded as approximately symmetrical, because many split lines are nearly symmetrically distributed over this narrow range because of strong coupling. In this instance, the approximate value of the center of gravity may be obtained by integrating the entire signal and determining the frequency at the half-height point of the integration curve. But when the signals are divided into two or more groups, this is not the case; here the center of gravity should be a weighted average of the centers of gravity obtained by integrating each subgroup. For the integration, the decoupling of the alkyl protons as has been done in the exact analysis is unnecessary, and hence it is a very simple procedure. A more refined but expensive technique for obtaining the center of gravity is that of on-line data acquisition by mini-computer followed by some calculations. Table I1 shows that the center of gravity of the signals obtained by the integration technique agrees with the average of the chemical shifts obtained from the exact analysis within 3 Hz,and for most cases within 2 Hz. The difference arises from the experimental errors in the integration and from the unsymmetrical signal pattern. In spite of this relatively large allowance, the center of gravity of the signals can still be used as a measure of the identification of the monosubstituted alkylbenzenes. In Figure 1 , there are also shown the centers of gravity of the signals obtained from the spectral analysis, which give the characteristic information on the series of the

A

ryL

d L Figure 2. Typical ring proton patterns in monosubstituted toluenes

o-methylacetophenone (type A), (b) rn-methylanisole (type A), m-methylacetophenone (type B), (4 o-methylanisole (type B), and ( e ) m-tolunitrile (type C). Both undecoupled (upper) and decoupled (bottom) spectra are shown (a) (c)

substituents for all isomers. Thus, in combination with the procedure of use of the signal pattern discussed in detail in the next section, it is useful for estimating the group substituted in the benzene ring. The chart for the center of gravity of the ring proton signals is a counterpart of the graphic representation of the chemical shifts for methyl, methylene, and methine protons (2-8). General Features of the Ring Proton Patterns for the Monosubstituted Alkylbenzenes. Roughly speaking, all of the coupling constants between the ring protons remain nearly constant while the chemical shift differences between them change from about 0 to 50 Hz or more approximately continuously according to the nature of the substituents. Thus the PMR patterns of the ring protons in monosubstituted alkylbenzenes should vary in some regular manner as the substituent changes. The PMR patterns of the 0- or m-monosubstituted alkylbenzenes may be divided into three categories when the finer structures are disregarded. (The symmetrical pattern in p derivatives does not need to be discussed further.) Type A consists of two groups of signals with relative intensities of 1 :3, the group with intensity of 1 always appearing at lower field. Type B consists of two groups of signals with equal intensities, and type C of only one group of signals concentrated in a relatively narrow range. The examples are shown in Figure 2 . In Figure 2 , the pattern features will appear more clearly in the undecoupled spectra than in the decoupled ones, since in the former the fine structures are somewhat collapsed due to the long range couplings. Figures 2(a) and 2(6) show the type A pattern, where the group of the signals at the lower field side has the relative intensity of 1 , and the higher field side group has the intensity of 3. The type B pattern is shown in Figures 2(c) and 2(d). It will be observed that the type B pattern consists of two groups with equal relative intensities.


1799

Table III. Pattern Features of Ring Proton Signals for Monosubstituted Toluenes Approximate range of the center of gravity of the Overall frequency Substituent signals, HZ spread Characteristics of pattern

Type A

B

~

Electron acceptor o-NO2 0-1 0-COCl o-Br 0-COOR 0-C1a 0-COOH 0-CN 0-COR Electron donor m-OCOR m-NH2 m-OR m-NHMe m-OH m-NMe2 Electron acceptor m-NO2 m-COR m-COC1 m-I m-COOH m-Br m-COOR Electron donor 0-OR

Two groups with relative intensity of 1 :3. The group with intensity of 1 is at the lower field

The lower field group is split into two

755-710

Larger than that of the corresponding m-isomer

690-645

Larger than that of the corresponding o-isomer

770-710

Smaller than that of the corresponding o-isomer

690-685

Smaller than that of the corresponding m-isomer

735-705

Smaller than that of the corresponding o-isomer

705-680

Smaller than that of the corresponding m-isomer

subgroups

The lower field group is split into 3 subgroups Two groups with relative intensity of 1 :1

0-"2

o-"Me Electron acceptor One group concentrated in the narrow range m-CN m-C1 Electron donor 0-OCOR 0-OH o-NMez The lower field group seemingly split into three subgroups.

C

5

Figure 2(e) shows the type C pattern. In this example, the entire signal is concentrated within only about 20 Hz. In the patterns of types A and B, an interesting feature is observed. The molecules with an electron acceptor substituent (X,) show type A of the pattern for the o-derivatives and type B for the m-derivatives, while the compounds with an electron donor substituent (X,) show type B for the oderivatives and type A for the m-derivatives. This characteristic of the ring proton patterns in the alkylbenzenes results for the following reason : An electron-accepting substituent X , moves the ring proton chemical shifts downfield more significantly at the ortho position to the substituent than at the other positions. Thus the alkylbenzenes with substituent X, a t the ortho position show the type A pattern because they have only one proton ortho to the substituent, while those having the substituent at the meta position show the type B pattern because there are two protons ortho to the substituent. On the other hand, since an electron-donating substituent X D moves the ring proton chemical shifts to higher field at the ortho and para positions, and barely affects the meta-proton chemical shift, the reverse trend is obtained. Further close inspection of the spectra gives more detailed information. In the type A pattern, the lower field group of signals with intensity 1 is due to the 4-proton (ortho to the substituent, see Table I) in the o-derivatives having a substituent X,, or to the 2-proton (meta to the substituent) in mderivatives having a substituent X D . The 4-proton in the o-derivatives has only one adjacent proton and its signal roughly splits into two subgroups from vicinal coupling on the order of 7-8 Hz, while the 2-proton in the m-derivatives has two such protons and exhibits roughly three subgroups due to the two adjacent protons (with a splitting of 7.5-8 Hz). These rules can be conveniently employed for discerning the 1800

0- and m-isomers of the monosubstituted alkylbenzenes, which are summarized in Table 111. For the compounds showing the type C pattern, it is difficult t o discriminate the isomers by applying such rules. But even in the type C pattern and hence in all the types of patterns, a n additional feature is observed. For the compounds with an X A substituent the overall frequency range over which the entire signal spreads is always larger in a n oisomer than in the corresponding m-derivative, while for the compounds with a n X, substituent the overall frequency spread in a meta compound is always larger than in the corresponding o-derivative. This feature of the pattern arises from the fact that the methyl group in toluene makes its ortho and para protons shift t o a position about 20 Hz higher upfield compared to benzene, while the meta proton is shifted to a higher field by only about 10 Hz by the methyl group, Le., the 0- and p-proton shifts due to the methyl group are twice that of the m-proton. For the compounds with a substituent X A , all of the proton shifts are moved to the lower field side by XA, the extent being largest for the proton ortho to the X , substituent and smallest for the meta-proton. Thus the overall frequency spread of the ring proton signals in the toluene derivatives with a n X , substituent is the largest when the proton ortho to the substituent X A is at the meta position to the methyl group and at the same time the proton meta to the X A substituent is ortho or para to the methyl group. This is the case in the o-derivatives with a n X A substituent. On the other hand, when the proton ortho to the X , substituent is ortho or para t o the methyl group and at the same time the proton meta to the X, substituent is meta to the methyl group, as in the case of m-derivatives with an X A substituent, the frequency spread will be smallest. The net effect is a larger overall frequency spread of the signals in an o-derivative than


. . . .

--e-

n---

0000

0

3

0000

mmwm

%

C;&& I I I I


1801

720

710

700

690

680

670

660

6$0

6 1 Hz

ortho Me

Et n -Pr

i-Pr sec -Bu’ t,-Bu

mefa Me

Et t-Bu

para Me Et

n-Pr sec-Bu t -Bu‘

Figure 3. Change of the ring proton chemical shifts in monoalkylphenols. Chemical shifts are referred to internal TMS

in the corresponding m-compound. In the molecules with a substituent X,, the reverse is true because the protons ortho and para to the X , substituent move to higher field to a larger extent than the meta proton does. The net effect derived from a similar discussion is the larger overall frequency spread of the signals in a m-derivative than in the corresponding oisomer. The features of the pattern just described may also be applied to the discrimination of the o- and m-isomers. But caution must be exercised that the high field shift effect of the alkyl group on the proton ortho to it tends to decrease as the alkyl group becomes bulky, as will be discussed in the next section, and eventually the reverse trend prevails, i.e., a downfield shift is observed for a tert-butyl group. Thus the features in the overall frequency spread are not revealed for compounds with a bulky alkyl group such as tert-butyl. The Effect of the Alkyl Chain on the PMR Parameters. Generally the PMR parameters of the monosubstituted alkylbenzenes are determined mainly by the substituent other than the alkyl group and not so much affected by the change of the latter. The PMR parameters for monosubstituted ethylbenzenes and other higher alkylbenzenes are listed in Table IV. In ethylbenzene derivatives, four substituents, -Nos, -Br, -OH, and -NH2, are selected as typical examples according to their electronic properties, and in the higher alkylbenzenes the phenolic compounds are studied in detail as an example. In Table IV, the figures in the parentheses, unlike in Table I, show the differences from the corresponding value of the toluene derivatives to make clear the effect of the alkyl groups. The small deviations for the coupling constants indicate that the alkyl chain does not substantially affect the ring proton coupling constants unless a “bulky” substituent is present at the a-position to the benzene ring, where the term “bulky” refers to the extent of branching at the a-carbon atom. Indeed the o-derivatives show relatively larger deviations in Ji2and J3*in compounds having iso-propyl, sec-butyl, 1802

and terr-butyl groups. This may be a steric effect on the coupling constants (15). Generally the chemical shifts are nearly the same in both methyl and ethyl derivatives. Some of the chemical shifts in ethyl derivatives, however, deviate to a large extent from the corresponding toluene derivatives. The large deviations are found in the 4-proton of o-nitroethylbenzene (the difference is 9.3 Hz) and in the 1-, 2-, and 4-protons of m-ethylaniline (about -6 -9 Hz), each of which has a substituent of strong acceptor or donor properties, but m-nitroethylbenzene and o-ethylaniline d o not show such peculiarities. Thus an unambiguous conclusion for this will require more reliable experimental evidence for many related compounds. Close inspection of the data for the phenolic compounds shows a trend in the ring proton shift in relation to the alkyl group. As seen from Figure 3, also in this case, the predominant factor is the bulkiness of the alkyl group at the a-position to the benzene ring rather than the length of the alkyl chain. It is natural that the proton para to the alkyl group is not substantially affected. The ortho and meta protons move to a lower field as the bulkiness of the alkyl group increases, the extent being larger for the ortho proton. This makes the entire signal spread. The low field shift of the ortho proton may be interpreted as the steric compression effect of the alkyl group. In conclusion, although the trends described above are observed in the PMR parameters of the monosubstituted alkylbenzenes with a higher alkyl group, the effects are generally small except for the bulky groups, and the alkyl group does not substantially affect the general features of the ring proton signals. Thus the procedures described in the preceding sections may generally be applied to the structural determination of these compounds. The Methyl Proton Shift in the Monosubstituted Toluenes. Since the methyl proton shift changes, reflecting the nature of the substituent and its substituting position in the monosubstituted toluene derivatives, it can be used for the identi-


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ACKNOWLEDGMENT

fication of the toluene derivatives, which may complement the estimation from the ring proton region. The methyl proton shifts are listed in Table I. Roughly speaking, the behavior of the methyl proton shift in the 0-,m-, and p-monosubstituted toluenes is similar to that of the 0-,m-,and p protons in the monosubstituted benzenes, respectively (19). However, some studies of the methyl proton shift in the toluene derivatives have already been made (4, 30), and further discussion is not intended in this work.

The authors express their gratitude to Tokyo Kasei Kogyo Co., Ltd., by whom the majority of the samples were provided. They are also indebted to Hiroshi Tomita for the analysis and preparation of the samples by gas chromatography, to Masaru Yanagisawa, Tomoko Tanahashi, and Yukio Mochizuki for their help in the experimental work, and to Fumiko Taka for her data compilation.

(30) H. Yamada, Y. Tsuno, and Y. Yukawa, Bull. Chem. SOC.Jap., 43,1459 (1970).

RECEIVED for review February 18, 1972. Accepted April 11, 1972.

Rapid, Phase-Sensitive, Three-Electrode Alternating Current Polarography A. M. Bond Department of Inorganic Chemistry, University of Melbourne, Parkville, 3052, Victoria, Australia

D. R. Canterford Department of Physical Chemistry, University of Melbourne, Parkville, 3052, Victoria, Australia Previously, short controlled drop times have been employed with considerable advantage i n ac polarography to permit faster scan rates of potential and short recording times. I n this work, the possibility of using 3electrode phase-sensitive detection with this so-called “rapid” ac polarographic technique has been investigated. Results show that theoretical relationships derived for natural drop time ac polarography can be extended to the rapid ac method. Thus, with 3-electrode phase-sensitive instrumentation, excellent discrimination against the charging current is still obtained at short controlled drop times. I n fact, the degree of discrimination under rapid conditions was better than with natural drop time phase-sensitive ac polarography. This was particularly evident at high frequencies. With the rapid phase-sensifive ac technique, copper(l1) and cadmium(l1) could be detected down to the 5 x lO-’M to 1 x 10-6M level. The introduction of phase-sensitive readout to the rapid ac technique, therefore, provides a considerable improvement to results reported previously with non phase-sensitive instrumentation. Indeed, the technique appears to be highly attractive i n many aspects, having the advantage of permitting fast scan rates, and thus short analysis times, while maintaining excellent discrimination against the charging current. This i s i n contrast t o other polarographic techniques where the use of short drop times necessarily results i n a lowering of ability to discriminate against the charging current, and therefore a loss in sensitivity. IN A RECENT ARTICLE (I), Bond pointed out the considerable advantages of using rapid alternating current (ac) and direct current (dc) polarographic techniques, with short controlled drop times, compared with the usual polarographic approach of using natural drop times of between about two and eight seconds. It was concluded in this article that the rapid techniques could be given much wider usage than presently accorded. In rapid polarography, the use of short controlled drop times permits fast scan rates of potential to be used, without (1) A. M. Bond,J. Electrochem. SOC.,118,1588(1971).

loss in precision of measurement ( I ) . Thus, considerable time saving, compared with conventional polarography, may be achieved in recording a polarogram, which makes the technique most attractive for routine analysis. Ac polarography has many advantages over dc polarography, particularly for fast electrode processes (2-4). These advantages are still maintained at short controlled drop times, and endeavors to improve the rapid ac polarographic technique even further would seem most desirable. In practice, the detection limit for a particular polarographic technique often occurs at a concentration of electroactive species, at which the charging or capacitive current masks or “swamps out” the faradaic current. Therefore, it is not only the absolute magnitude of the faradaic current but also the ratio of the measured faradaic current to the measured charging current which often determines the sensitivity of a technique. Many of the extensions to polarographic techniques developed in the past 20 years have aimed at providing discrimination against the charging current. For instance, in ac polarography, the employment of phase-sensitive detection provides considerable improvement in the detection limit and the precision of measurement at low concentrations ( 4 , 5). With dc polarography, techniques such as Tast (currentsampled) and pulse polarography, also provide discrimination against the charging current. In a recent comparison of a wide variety of polarographic techniques, (6), it was observed that with the dc techniques (2) B. Breyer and H. H. Bauer, “Chemical Analysis, Vol. XIII,

Alternating Current Polarography and Tensammetry,” Interscience, New York/London, 1963. (3) H. Schmidt and H. von Stackelberg, “Modern Polarographic Methods,” Academic Press, New York/London, 1963. (4) A. M. Bond, ANAL.CHEW,44,315(1972). (5) D. E. Smith in “Electroanalytical Chemistry,” A. J. Bard, Ed., Marcel Dekker, New York, N.Y., 1966, Chap. 1, Vol. 1. (6) A. M. Bond and D. R. Canterford, ANAL.CHEM.,44, 721 (1972).


1803

Structural determination of monosubstituted alkylbenzenes by proton

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