1898
R. B. DUNLAP, G. A. GHANIM,AR'D E. H. CORDES
Secondary Valence Force Catalysis. IX. Catalysis of Hydrolysis of para-Substituted Benzaldehyde Diethyl Acetals by Sodium Dodecyl Sulfate1 by R. Bruce Dunlap, Ghanim A. Ghanim, and E. H. Cordes2 Contribution No. 167R from the Department of Chemistry, Indiana University, Bloomington, Indiana (Received September 16, 1968)
$7401
The acidic hydrolysis of substitut,edbenzaldehyde diethyl acetals is subject to catalysis by dilute aqueous solutions of sodium dodecyl sulfate. Second-order rate constants for these reactions are independent of surfactant concentration below the critical micelle concentration, then increase rapidly with increasing surfactant concentration, level off, and finally decrease slightly with further increases. Second-order rate constants for the reactions in aqueous solution and in the presence of optimal concentrations of the surfactant are well correlated by Hammett u constants. Values of p of -3.3 and -4.1 were obtained for the purely aqueous medium and the surfactant -solutions, respectively. Thus, catalysis of hydrolysis of benzaldehyde diethyl acetals by sodium dodecyl sulfate is more important for those substrates possessing electron-donating substituents than for those possessing electron-withdrawingsubstituents. The maximum rate increase observed with the p-nitro substrate is only 17-fold while that observed with the p-methoxy substrate is 146-fold.
Introduction Previous investigations from this laboratory have established that the hydrolysis of certain ortho esters is subject to marked catalysis by sodium dodecyl sulfate and by other anionic surfactant^.^-^ One of the surprising conclusions derived from this work is that surfactant catalysis of hydrolysis of a series of methyl orthobenzoates becomes more marked as the reactivity of the substrate is increasedV6 In an effort to generalize this conclusion and to broaden the scope of our investigations of surfactant catalysis for organic reactions, we have carried out a study of the kinetics of hydrolysis of a series of six p-substituted benzaldehyde diethyl acetals in the presence of aqueous solutions of sodium dodecyl sulfate. The results of this investigation are presented herein.
Experimental Section Materials. p-Fluorobenzaldehyde diethyl acetal was synthesized from the corresponding benzaldehyde by the general method of Fife and Jao.? Anal. Calcd: C, 66.64; H, 7.63; F, 9.58. Found: C, 66.77; H, 7.87; F, 9.49. Other benzaldehyde diethyl acetals were previously synthesized by Fife and Jao and were obtained as described by these workers.' Sodium dodecyl sulfate was the best grade available from Distillation Product Industries and was used without further purification. Other chemicals were of a reagent grade. Glass-distilled water was employed throughout. Kinetic Measurements. These were carried out spectrophotometrically with a Zeiss PMQ I1 spectrophotometer equipped with a heatable cell holder through which water from a bath thermostated a t 25" was continuously circulated. Hydrolysis of p-NOz-, p-C1-, p-F-, p-H-, p-CH3-, and p-0CH3-substituted The Journal of Physical Chemistrv
benzaldehyde diethyl acetals in aqueous solution and in the presence of sodium dodecyl sulfate was followed by observing the appearance of the corresponding benzaldehyde a t 267, 280, 257, 243, 243, and 256 mp, respectively. First-order rate constants were obtained from plots of (OD, - ODt) against time in the usual fashion. Second-order rate constants were obtained by dividing the first-order rate constants by the activity of the hydrated proton. Values of pH were determined with a Radiometer PHM 4c pH meter equipped with an internal glass electrode. Values of pH were maintained constant throughout the kinetic runs by the use of cyanoacetate, acetate, or phosphate buffers in appropriate ranges of pH.
Results I n Table I, second-order rate constants for hydrolysis of six para-substituted benzaldehyde diethyl acetals a t 25" are collected as a function of the concentration of sodium dodecyl sulfate. Each of the substrates behaves in a qualitatively similar fashion. Below the critical micelle concentration for sodium dodecyl sulfate in 0.01 M acetate buffer, near 0.004 M, secondorder rate constants are substantially the same as those obtained in purely aqueous solution. Above this value, (1) Supported by Grants AM08232-04 and -06 from the National Institutes of health. (2) Career Development Awardee of the National Institutes of Health, Grant K3 GM 10-248-02. Fellow of the Alfred P. SlOan Research Foundation. (3) M . T. A . Behme, J. G. Fullington, R. Noel, and E. H. Cordes, J . Amer. Chem. Soc., 87, 266 (1965). (4) L. R . Romsted. R. B. Dunlap, and E. H. Cordes, J . Phys. Chem., 7 1 , 4681 (1967). (5) R . B. Dunlap and E. H. Cordes, J. Amer. Chem. Soc., 90, 4395 (196s). (6) R. B. Dunlap and E. H. Cordes, J . Phys. Chem., 73, 361 (1969). (7) T. H. Fife and L. K. Jao, J . Org. Chem., 30, 1492 (1965).
SECONDARY VALENCE FORCECATALYSIS
1899
Table I: The Effect of Concentration of Sodium Dodecyl Sulfate on Second-Order Rate Constants for Hydrolysis of a Series of para-Substituted Benzaldehyde Diethyl Acetals a t 25' Substituent P-NOzO
p-CP
[Sodium dodecyl sulfate], A I
0.000 0.008 0.012 0.018 0.024 0.036 0.048 0.060 0.072 0.084 0.000 0.004 0.008 0.012 0.018 0.024 0.036 0 048 0.060 0.000 0.004 0.008 0.012 0.018 0.024 0.036 0.048 0.060 I
P-F"
a
kz,
&I-1 min-1
Substituent p-Hb
20.3 158 291 354 343 314 346 287 279 255 2,505 3,250 39,350 80,900 85 ,400 70 ,800 92 ,500 92,100 79 ,500 7,122 6,425 104,200 236,000 252,000 272,000 333 ,500 334,500 318,000
p-CHad
p-OCHad
[Sodium dodecyl sulfate], A 4
kz, M-1 mn-1
7,046 8,140 120,470 221,300 312,100 339 ,000 305 ,000 317,200 27 ,485 37,600 1,480,000 2,158,000 2,348,000 2 ,250,000 2,510,000 1,945,000 2,220,000 77,170 172,000 5,175,000 9 ,630,000 9,495,000 11,230,000 10,790,000 10,560,000 10,310,000
0.000 0.004 0.008 0.012 0.018 0.024 0.036 0.048 0.000 0.004 0.008 0.012 0.018 0.024 0.036 0.048 0.060 0.000 0.004 0.008 0.012 0.018 0.024 0.036 0.048 0.060
Carried out in 0.01 M cyanoacetate buffer, 40y0 base, at an initial concentration of p-nitrobensaldehyde diethyl acetal near 6.67 X 10-6
M. Carried out in 0.01 M acetate buffer, 80% base, at an initial concentration of p-chlorobensaldehyde diethyl acetal near 6.67 X
10-6
M or of benzaldehyde diethyl acetal near 5 X 10-6 M .
Carried out in 0.01 M acetate buffer, 50% base, at an initial concentration of p fluorobensaldehyde diethyl acetal near 6.67 X 10-6 M . Carried out in 0.01 M sodium phosphate buffer, pH 6.4, at an initial concentration M. of either p-methyl- or p-methoxybensaldehyde diethyl acetal near 6.67 X
rate constants increase rapidly with increasing surfactant concentration and reach a maximum in the neighborhood of 0.036 M surfactant. Further increases in surfactant concentration have little effect on the second-order rate constants although a significant decrease in these values is usually observed a t higher concentrations. Closer inspection of the data in Table I will reveal that those substrates possessing the more electron-donating substituents in the p ura position are more susceptible to catalysis by sodium dodecyl sulfate than those possessing electron-withdrawing substituents. This point is brought home more clearly in Table I1 in which second-order rate constants for the uncatalyzed reaction, second-order rate constants for the catalyzed reaction a t the optima1 concentration of sodium dodecyl sulfate, and the over-all rate increase are collected. Clearly the hydrolysis of the p-nitro substrate is least susceptible ta catalysis and that of the p-methoxy substrate is most susceptible to catalysis. Second-order rate constants obtained in purely aqueous solution and those obtained a t optimal concentrations of sodium dodecyl sulfate are well correlated
by the Hammett u constants as shown in Figure 1. Least-squares analysis of the data in Tables I and I1 yields a value of p = -3.3 for the uncatalyzed and a value of -4.1 for the surfactant-catalyzed reaction. Table 11: Second-Order Rate Constants for Hydrolysis of a Series of para-Substituted Benzaldehyde Diethyl Acetals a t 25" in Aqueous Solution and in the Presence of Optimal Concentrations of Sodium Dodecyl Sulfate
Substituent
P-NOz p-c1 P-F P-H P-CH~ p-OCHa
[Sodium dodecyl ko? sulfatela, M M-1 min-1
0.018 0.036 0.048 0.024 0.036 0.024
20.3 2,505 7,122 7,046 27,485 77,170
kz,C M-1
min-1
354 92,500 334,000 339,000 2,510,000 11,230,000
Rate increase 17.4 36.9 46.9 49.1 91.3 146
a The indicated concentrations are those at which maximum oatalysis occurs (Table I). Second-order rate constants for the reactions in aqueous solution. Second-order rate constants at the indicated concentration of sodium dodecyl sulfate.
Volume 75, Number 6 June 1969
R. B. DUNLAP, G. A. GHANIM,AKD E. H. CORDES
1900
Discussion
that hydrolysis of acetals is subject to such catalysis is not surprising since the mechanisms of these reactions are closely related. For each type of substrate, the transition state involves the cleavage of a carbonoxygen bond unaided by nucleophilic assistance from the solvent; hydrolysis of ortho esters probably involves proton transfer from the hydrated proton to the substrate in the transition state as well, while proton transfer seems to be a preequilibrium process in the hydrolysis of acetals and k e t a l ~ . A ~ t any event, the catalysis can be understood in terms of electrostatic stabilization of the developing carbonium ion in the transition state. The kinetics of hydrolysis of meta- and para-substituted benzaldehyde diethyl acetals at 30' has previously been studied by Fife and Jao, who employed 50% dioxane-water as solvent.' Our results obtained employing purely aqueous media are consistent with those of the previous workers although our rate constants are 3-100 times greater than those in 50% aqueous dioxane. Inhibition of hydrolysis of benzaldehyde diethyl acetals by organic solvents is similar to that previously observed for the hydrolysis of methyl orthoben~oate.~The effect of polar substituents on the second-order rate constants for these reactions is similar in the two solvent systems although, as noted above, the rate constants in purely aqueous media are correlated well by the u constants while those in 50y0 aqueous dioxane are correlated by substituent constants intermediate in value between those for u and u+. The effect of sodium dodecyl sulfate concentration on second-order rate constants for the hydrolysis of substituted benzaldehyde diethyl acetals is closely related to that previously observed for the hydrolysis of substituted methyl orthobenzoate~.~-~ The triphasic dependence of rate constants upon surfactant concentration is also observed for the hydrolysis of ortho esters and the explanation provided in that case would seem to be applicable to this as ell.^-^ That is, below the critical micelle concentration no catalysis is observed because no micelles exist. Above this concentration the increasing catalysis with increasing surfactant concentration reflects the absorption of the substrates onto the surface of the micelles which becomes complete a t sufficiently high concentrations of surfactant. The small rate decreases observed at relatively high surfactant concentrations are reasonably interpreted as inhibition by the counterion of the surfactant i t ~ e l f . ~ , ~ Furthermore, the magnitudes of the rate increases for the hydrolysis of the substituted benzaldehyde diethyl acetals and substituted methyl orthobenzoates in the presence of sodium dodecyl sulfate are similar. Direct comparisons are precluded since the values of p for the two series of reactions are substantially different.
Previous studies have established that ortho ester hydrolysis is subject to marked catalysis by anionic s~rfactants.~-6 The observation made in this study
(8) Y. Yukawa and Y. Tsuno, Bull. Chem. SOC.Japan, 32, 971 (1959). (9) E . H.Cordes, Progr. Phys. Org. Chem., 4, 1 (1967).
%
-a2
ox)
to.2
SOA
+os
to.8
0Figure 1. Logarithms of second-order rate constants (in units of MP rnin-') for the hydrolysis of a series of para-substituted benzaldehyde diethyl acetals in aqueous solution (lower line) and in the presence of sodium dodecyl sulfate (upper line) plotted againRt the Hammett substituent constants.
Although these tables contain single entries for the uncatalyzed rate constants, these were, in most cases, measured at three values of pH and the values recorded in the tables are the mean of these. This value of p is in excellent agreement with that previously observed for meta-substituted benzaldehyde diethyl acetal hydrolysis by Fife and Jao in 50% aqueous dioxane a t 30°, -3.35.7 Under these conditions, second-order rate constants for the para-substituted acetals depend upon values of the substituent constants intermediate between those of u and u+. Analysis of the data of Fife and Jao for the para-substituted substrates according to the method of Yukawa and Tsuno8 yields values of p = -3.35 and r = 0.5,9 again in agreement with our value. It is not clear why the reaction in water follows u and that in 50% aqueous dioxane 0.5(a+ - u)]. The difference in values follows [u of p for the reaction in aqueous solution and for that in the presence of sodium dodecyl sulfate is, of course, required by the observation above that the most reactive substrates are the most susceptible to catalysis.
+
The Journal of Physical Chemistry
THESTRUCTURE OF DIIMIDE
1901
One of the more interesting observations recorded in this study is that the rate of hydrolysis of benzaldehyde diethyl acetals in the presence of sodium dodecyl sulfate is more sensitive to the nature of the polar substituent than is the same reaction in purely aqueous solution. Values of p of -4.1 and -3.3 were obtained for these two reactions, respectively. This finding is consistent with our earlier observation that values of p for the hydrolysis of substituted methyl orthobenxoates in the presence of sodium dodecyl sulfate and in purely aqueous solution are -2.5 and -2.0, respectively.6 This finding suggests that acetal hydrolysis, as well as ortho ester hydrolysis, occurs with somewhat more carbon-oxygen bond cleavage in the transition state when it occurs in the micellar phase as compared to the aqueous phase. As developed earlier, this behavior can be rationalized on the basis that electrostatic
stabilization is decreasingly important as the positive charge is dispersed.'j Thus stabilization would be near a maximum for the protonated substrate and then decrease with carbon-oxygen bond cleavage and subsequent delocalization of charge. Lending decreasing stabilization t o the system as it proceeds along the reaction coordinate will result in reaching the transition state farther along that coordinate. It is certainly true that the variation in p values between the reactions in surfactant and aqueous systems may be the consequence of factors other than that just developed. For example, as the nature of the substituent changes, the positioning of the substrate on the micellar surface may change as well and such changes may have kinetic consequences. At the moment, there seem to be no direct methods for probing these and related considerations.
The Structure of Diimidel by L. J. Schaad and H. B. Kinser2 Department of Chemistry, Vunderbilt Universdly, Nashville, Tennessee
(Received September 1 7 , 1 9 6 8 )
Calculations using a restricted basis of Gaussian orbitals have been carried out on diimide (NtH2) with particular attention to the mechanism of double bond reduction by N2H2. The trans structure (NNH angle = 104', N N length = 1.28 A, N H length = 1.13 A) is computed to be 7.5 kcal/mol lower in energy than the cis (NNH angle = 112', N N length = 1.27 A, N H length = 1.13 A). Linear, twisted, and bent forms of NzH2 were also considered, and the bent form was found to be the transition state in cis-trans isomerization. CzH2, Nz, Cz, and Hz were studied by the same methods, in part to check the reliability of the NzHz results, and in part in connection with the NZHZisomerization calculations.
Introduction Since the work of van Tamelena and Corey4 in 1961, diimide ( = diazine = NzHz) has been used successfully as a reducing agent for symmetrical double bonds. Diimide is not stable, but is generated in the reaction vesseI by decomposition of such precursors as anthracene-9,lO-biimine or salts of azodicarboxylic acid. Details of the chemistry of these reactions may be found in the fairly complete list of references given by Baird, Franzus, and Surridge,6and in a 1965 review by Miller.6 The point of particular interest here is that diimide gives cis addition of hydrogen to a double bond; there appears to be no exception. This stereospecificity is usually interpreted in terms of a cyclic transition state,
\
/
b
/
C
\
+
N&2
\ -+ -C-C--.
/ --+ -C-C\ / / \ I I H M H H \WN / .
+ Nz
which seems to imply a cis ( = s y n ) structure for diimide, or perhaps an easy interconversion between cis and trans ( = a n t i ) forms. We report here a nonempirical theoretical investigation of this question. It is widely appreciated that the results of semiempirical methods such as the Hiickel and extended Hiickel must be accepted cautiously; if they happen to be accurate for a given molecule, perhaps they will also be for other similar molecules. Unfortunately the same warning also applies t o most nonempirical calculations. For very small systems such as HZand H3+,it is possible (1) Presented in part a t the Ohio State Symposium on Molecular Structure and Spectroscopy, Sept 1967. (2) Present address: Chemistry Department, Iowa State University, Ames, Iowa. (3) E. E. van Tamelen, R. 9. Dewey, and R . J. Timmons, J. Amer. Chem. Soc., 8 3 , 3725 (1961). (4) E. J. Corey, W. L. Mock, and D. J. Pasto, Tetrahedron Lett.. 347 (1961). ( 5 ) W. C. Baird, Jr., B. Franzus, and J. H. Surridge, J. Amer. Chem. Soc., 89, 410 (1967). (6) 0.E. Miller, J. Chem. Educ., 4 2 , 254 (1965). Volume YS,Number 6 June 1969
