3852 Fendler and Fendler
`The Journal of Organic Chemistry
`Hydrolysis of Nitrophenyl and Dinitrophenyl Sulfate Esters1
`E. J. Fendler and J. H. Fendler
`Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15318, and
`Radiation Research Laboratories, Mellon Institute, Camegie-Mellon University, Pittsburgh, Pennsylvania 15818
`Received May 6,1968
`
`The pH-rate profile for the hydrolyses of o-, m-, p-nitrophenyl and 2,4- and 2,5-dinitrophenyl sulfates is char-
`acterized by a plateau in the pH 4-10 region preceded by a more rapid acid-catalyzed reaction and followed by
`feeble base catalysis. A linear free-energy plot for log k¡, (see ref 11) against the p   of the corresponding phenol
`with a slope of —1.2 was obtained. Sodium chloride, sulfate, and fluoride and copper(II) sulfate and silver per-
`chlorate only slightly affect ka for 2,4-dinitrophenyl sulfate, but sodium perchlorate significantly decreases it.
`Acetonitrile decreases k<¡ for the same ester; dioxane in concentrations up to 80% has the same effect, but, when it
`reaches 90%, the rate is increased by a factor of 70; and dimethyl sulfoxide and  , -dimethylformamide increase
`it exponentially. The effects of electrolytes, acetonitrile, and dimethyl sulfoxide on the molar activity coeffici-
`ents of 2,4-dinitrophenyl sulfate and its hydrolysis transition state have been separated by distribution experi-
`ments. Sodium chloride slightly “salts in,” and sodium sulfate and perchlorate, and the dipolar aprotic solvents
`“salt out” 2,4-dinitrophényl sulfate, but all these salts and acetonitrile destabilize the transition state while di-
`methyl sulfoxide has little effect on it.
`Linear free-energy correlations have been obtained between the acid-
`catalyzed rate constants, hp, and the dissociation constants of the leaving groups for all three acids at all con-
`centrations and temperatures investigated. The slopes of these plots are between 0.22 and 0.26 indicating that
`the acid-catalyzed hydrolysis of these sulfate esters is relatively insensitive to the electron-withdrawing power of
`the leaving group. The catalytic effectiveness of the acids is HjSCh > HClCh > HC1. Plots of log     + Ho
`against Ho + Ch + are linear, but their slopes are different for the different acids. This behavior is rationalized by
`assuming that the acids, in addition to their proton-donating power, exert specific electrolyte effects on both the
`initial and the transition states of these hydrolyses.
`Recently, considerable interest has been shown in
`mechanistic studies concerning the hydrolysis of sulfate
`esters2-6 owing to their biochemical importance.6 The
`spontaneous2 perchloric acid,3 base,2 and amine2 cat-
`alyzed hydrolyses of p-nitrophenyl sulfate and car-
`boxyl group participation in salicyl sulfate hydrolysis4
`have been reported. Lack of sufficient data has frus-
`trated attempts to compare the mechanisms of aryl
`sulfate hydrolyses with those of the much more widely
`It has been sug-
`investigated monoaryl phosphates.7
`gested that a study of the hydrolysis of sulfate esters
`containing good leaving groups might substantiate the
`proposed unimolecular mechanism.2 Although 2,4-
`and 2,5-dinitrophenyl sulfates have such good leaving
`groups, their preparation and consequently kinetic in-
`vestigations of their hydrolysis have so far eluded the
`attention of chemists and biochemists alike. We,
`therefore, have undertaken a systematic investigation
`of the mechanisms of hydrolysis of both mono- and di-
`nitrophenyl sulfates and report the results of a study of
`their neutral and acid-catalyzed hydrolyses.
`
`hr at 35-40° and for approximately 14 hr at room temperature.
`After the mixture was cooled to 0°, it was poured rapidly with
`stirring into cold 4   KOH (50 ml). The precipitate was
`twice with cold 95%
`filtered immediately, washed at
`least
`ethanol, and dried in vacuo.
`The potassium salts of o- and ro-
`recrystallized from 90% ethanol.
`nitrophenyl sulfates were
`The labile dinitrophenyl sulfates were purified by the addition
`of dry acetonitrile to the solid, centrifugation, and rotary evapora-
`tion of the solution followed by the same procedure but using
`a minumum amount of cold aqueous ethanol. Rapid rotary
`evaporation of the solvent at 10-15° and drying in vacuo gave
`shiny white crystals. The purity of all the sulfates was estab-
`lished to be not less than 99% by complete hydrolysis of known
`quantities in aqueous buffer solutions at pH 10.00, assaying the
`amount of phenoxide ion liberated, and by their ir and nmr spec-
`tra.
`Deionized distilled water was used to make up the buffer
`solution and the standard acid and alkali solutions. The pH of
`the buffer solutions was adjusted by addition of hydrochloric
`acid or sodium hydroxide at 25.00° using an Orion Model 801 pH
`meter. The concentrations of the acid solutions were determined
`by titration with standard 0.10 or 1.00 M NaOH (BDH) using
`Lacmoid as the indicator. The alkali solutions were standardized
`by titration with standard 1.00 M HC1 using Lacmoid as the
`indicator.
`1,4-Dioxane was purified by the method of Fieser*
`and stored in polyethylene bottles in a refrigerator. Spectro-
`grade dimethyl sulfoxide,  , -dimethylformamide, and aceto-
`nitrile were
`used without further purification and stored over
`molecular sieve. The aqueous organic solutions were made up
`by using the appropriate volumes measured at room temperature.
`Kinetics.—The hydrolysis was followed spectrophotometrically
`by determining either the phenoxide ion or
`the phenol concentra-
`tion. The phenoxide ion formation was
`followed in the higher
`pH region and in alkaline solutions. At lower pH values and in
`taken of the fact that the extinc-
`the acid region advantage was
`the nitro- and dinitro-substituted phenol
`tion coefficients of
`sulfates are very much smaller than those of the corresponding
`in strong acid solutions, the formation
`phenols. For a few runs
`followed by adjusting the pH of the in-
`of phenoxide ion was
`dividual samples to 9.0 by the addition of buffer solutions. The
`agreed within 3% with those
`rate constants for
`these runs
`formation.
`The
`obtained by direct measurement of phenol
`following wavelengths (  µ) were used-for following the phenoxide
`ion and phenol formation, respectively: o-nitrophenyl sulfate,
`410, 290; ro-nitrophenyl sulfate, 380, 350; p-nitrophenyl sulfate,
`403, 320; 2,4-dinitrophenyl sulfate, 360, 320, and 2,5-dinitro-
`phenyl sulfate, 440, 270. The Beer-Lambert law was obeyed over
`
`(9) L. F. Fieser, “Experiments in Organic Chemistry,” 3rd ed., D. C.
`Heath and Co., Boston, Mass., 1957, p 284.
`
`Experimental Section
`Materials.—p-Nitrophenyl sulfate potassium salt (Eastman)
`was recrystallized from 90% ethanol.
`o-Nitro-, m-nitro-, 2,4-dinitro-, and 2,5-dinitrophenyl sulfates
`were prepared by a modified procedure of Burkhardt and Wood.8
`Chlorosulfonic acid (35 mmol, 2.32 ml) was added dropwise with
`stirring to a solution of dimethylaniline (12.5 mmol, 11.73 ml)
`in carbon disulfide (12.5 ml) at —16 to —21°. The mixture was
`then warmed to 35-40°;
`the appropriate dry phenol (25 mmol)
`was added with stirring; and the mixture was stirred for 1-1.5
`(a) For a preliminary report, see E. J. Fendler and J. H. Fendler,
`(1)
`(b) Supported in part by the U. S. Atomic
`Chem. Commun., 1261 (1967).
`Energy Commission.
`(2) S. J. Benkovic and P. A. Benkovic, J. Amer. Chem. Soc., 88, 5504
`(1966).
`(3) J. L. Kice and J. M. Anderson, ibid,, 88, 5242 (1966).
`(4) S. J. Benkovic, ibid,, 88, 5511 (1966).
`(5) R. W. Hay and J. A. G. Edmonds, Chem. Commun., 967 (1967).
`(6) K. S. Dodgson, B. Spencer, and K. Williams, Biochem. J., 64, 216
` . Bostrón, K. Bernstein, and M. W. Whitehouse, Biochem. Pharma-
`(1956);
`col., IS, 413 (1964).
`(7) J. R. Cox, Jr., and O. B. Ramsay, Chem. Rev., 64, 317 (1964).
`(8) G.  . Burkhardt and H. Wood, J. Chem. Soc,, 141 (1929).
`
`MAIA Exhibit 1034
`MAIA V. BRACCO
`IPR PETITION
`
`

`

`Vol. 33, No. 10, October 1968
`
`Hydrolysis of Nitrophenyl Sulfate Esters 3853
`
`the range of concentrations employed for all the sulfates. Ki-
`netic runs at 14.90, 25.00, and 45.00° were
`followed directly
`in the thermostated cell compartment of a Beckman DU-2
`carried
`spectrophotometer. Those at 75.00 and 100.00° were
`out in sealed tubes which were placed in a thermostat, individually
`withdrawn at the required times, quickly cooled, and analyzed
`For
`the appropriate wavelength.
`spectrophotometrically at
`the hydrolyses in sodium hydroxide solutions, alkali resistant
`glass (Corning No. 7280) was used. The temperature in the
`thermostats and inside the cell compartment was measured by an
`NBS thermometer and maintained to be ±0.02°. Examples of
`are shown in Figure 1.
`typical kinetic runs
`Partitioning Experiments.—The measurement of the distribu-
`tion of 2,4-dinitrophenyl sulfate between water and an immiscible
`temperature and in the presence
`organic solvent, at a constant
`and absence of salts or of other media, gives the effect of the
`electrolytes or of the media on the molar activity coefficient of
`the sulfate ester.10 Water and cyclohexane, water and hexane,
`cyclohexane and dimethyl sulfoxide, and hexane and acetonitrile
`the salts are not extracted into
`are almost completely immiscible;
`the organic layer; and the density differences of these solvents
`readily allows them to be separated quickly and cleanly after
`shaking. The hydrolysis of 2,4-dinitrophenyl sulfate at 25.00°
`was negligible compared with the distribution time. Even in 75%
`dimethyl sulfoxide, representing the fastest hydrolysis rate, the
`half-life for the hydrolysis of 2,4-dinitrophenyl sulfate is ap-
`proximately 30 min and generally the distribution was carried
`in less than 2 min. Typically a known concentration of 2,4-
`out
`dinitrophenyl sulfate was made up by weight in a buffer or elec-
`trolyte solution (25.00 ml), and the concentration of this solution
`redetermined by measuring the absorption at 320   µ of a
`was
`suitable dilution using the appropriate blank solution. These
`two independently determined concentrations usually agreed
`within ±3%. A 10.00-ml portion of
`the aqueous 2,4-dinitro-
`phenyl sulfate was vigorously mixed with 10.00 ml of cyclo-
`hexane or the same volume of hexane in a thermostated separatory
`funnel at 25.00°. After the two layers had separated, the con-
`centration of 2,4-dinitrophenyl sulfate was measured spectro-
`photometrically in each layer at 320   µ using the appropriate
`blank solution. The distribution coefficient,
`is defined as
`r,
`in hexane/[2,4-
`in cyclohexane or
`[2,4-dinitrophenyl sulfate]
`in aqueous media. The sub-
`in water or
`dinitrophenyl sulfate]
`script o refers to water and s to salt solutions or other media.
`The ratio re/r„ is equal to the ratio /,//„, where /s is the molar
`activity coefficient of 2,4-dinitrophenyl sulfate in the presence
`of electrolytes or other media, and /„ is that
`in water.
`Six
`independent values for r0 were determined, and they agreed
`within ±5%.
`Furthermore, 2,4-dinitrophenyl sulfate was
`distributed between 3.75 Af NaCKX and cyclohexane, and in a
`separate experiment hexane was used instead of cyclohexane.
`Excellent agreement was obtained between the values found in
`these two experiments (Table I).
`
`Table I
`Activity Coefficients of 2,4-Dinitrophenyl Sulfate
`Its Hydrolysis Transition State in the Presence of
`and
`Electrolytes, Acetonitrile, and Dimethyl Sulfoxide0
`Medium
`/a//o
`1.25 Af NaCKX
`1.38
`1.92s
`2.50 Af NaCKX
`1.19
`2.75s
`3.75 Af NaCKX
`1.37
`4.25s
`3.75 Af NaCKXc
`1.38
`4.24s
`0.75 Af NaCl
`0.94
`1.05s
`1.50 Af NaCl
`0.91
`1.00s
`2.25 Af NaCl
`0.82
`0.97s
`0.75 Af NaeSO,
`1.73
`1.70s
`23.40
`22.40
`25.00% CH3CN
`214.00
`147.00
`50.00% CHaCN
`200.00
`372.00
`75.00% CHaCN
`25.00% DMSO
`0.80
`1.85
`50.00% DMSO
`6.23
`1.30
`70.00% DMSO
`0.87
`8.67
`° Using cyclohexane unless otherwise stated; correlated with
`rate constants at 25.00° unless otherwise stated.
`6 Rate con-
`stants measured at 45.00°.
`0 Using hexane.
`(10) F. A. Long and W. F. McDevit, Chem. Rev.. 61, 119 (1952).
`
`Figure 1.—Plot of 1 + log (OD« — ODt) against time for the
`neutral hydrolysis of sulfate esters. A =
`2,4-dinitrophenyl
`sulfate; pH 5.90, 75.00°. B = o-nitrophenyl sulfate; pH 8.00,
`time scale in 102. C = 2,5-dinitrophenyl sulfate; pH
`100.00°;
`5.90, 100.00°.
`
`Figure 2.—Variation of fa with pH for o-nitrophenyl sulfate at
`100.00°.
`
`Figure 3.—Plot of 8 + log fa at 100.00° against pK„ of the cor-
`responding phenol.
`Results
`for the neutral11
`The first-order rate constants, k0,
`hydrolysis of nitrophenyl and dinitrophenyl sulfates
`The pH-rate profile for o-nitro-
`are given in Table II.
`phenyl sulfate at 100.00° is in Figure 2. Figure 3
`the neutral hydrolysis refers to the hydrolysis of the
`In this paper,
`(11)
`sulfate eeters, in the plateau region of the pH-rate profile, i.e., from pH 4 to
`12, and is symbolized by fco.
`
`

`

`3854 Fendler and Fendler
`
`The Journal of Organic Chemistry
`
`Table II
`Hydrolysis of Nitrophenyl and Dinitrophenyl Sulfates in the pH Region
`10*ti, sec-1"-----
`
`,
`
`----------
`
`—
`
`...
`
`..
`
`—
`
`o-Nitrophenyl sulfate
`100.00°
`75.00°
`1307.0
`306.0
`178.0
`120.7
`40.2
`6.78
`
`pH
`Buffer®
`0.10 M HC1
`1.00
`0.01    ,  ,
`2.50
`0.01 M KH2P04
`2.70
`0.01 M KH2P04
`3.00
`0.01 mkh2po4
`3.50
`0.01 M KHC8H404
`4.00
`0.01 M KHCsHA
`4.32
`5.90
`6.80
`0.01 MKHC,H404
`0.01 Af KH2P04
`7.10
`6.00
`0.01 Af KH2P04
`6.85
`7.00
`8.00
`0.01 Af Na2B407
`7.17
`0.01 Af Na2B407
`9.00
`6.80
`0.01 Af Na2B407
`7.45
`10.00
`0.01 Af K2HP04
`7.20
`11.00
`0.10 Af NaOH
`8.40
`13.00
`1.0 Af NaOH
`68.10
`14.00
`“ The pH of buffers were adjusted at 25.00°.
`
`0.659
`
`0.642
`
`0.644
`
`pKa of the phenol.
`Figure 4.—Plot of 6 + log k$ at 1.00 Af acid against pXa of
`the corresponding phenol at 25.00°: O, HC10«; B, HC1;
` ,
`H2S04.
`
`shows the linear free-energy relationship between the
`dissociation constant of
`the leaving group (nitro or
`dinitrophenol) and the rate constants for the neutral
`hydrolysis. The Arrhenius parameters for the neutral
`hydrolysis are collected in Table III.
`The effects of
`the neutral hydrolysis of 2,4-dinitrophenyl
`salts on
`sulfate at 45.00° are summarized in Table IV, and
`Table V gives the solvent effects on the hydrolysis of
`the same compound. The effects of electrolytes, ace-
`tonitrile, and dimethyl sulfoxide on the molar activity
`coefficient of 2,4-dinitrophenyl sulfate and on that of
`its hydrolysis transition state are collected in Table I.
`The first-order rate constants, k+, for the acid-cat-
`alyzed hydrolysis of nitrophenyl and dinitrophenyl
`sulfates are given in Table VI. Good linear free-energy
`correlations have been obtained between k+ and the
`dissociation constants of the leaving groups for all three
`the concentrations and temperatures in-
`acids at all
`vestigated. A typical plot for a 1.00 M acid at 25.00°
`is given in Figure 4. The slopes of these plots are
`0.22-0.26. Plots of log fc* against Ho12 generally gave
`taken from M. A. Paul and F. A. Long, Chem.
`(12) Values for Ho were
`Rev., 67, 1 (1957), and when available from more
`recently determined values
`such as those which have been reported by M. J. Jorgesen and D. R. Hartter,
`J. Amer. Chem. Soc., 86, 878 (1963), and K. Yates and H. Wai, ibid., 86, 5408
`(1964).
`
`m-Nitrophenyl
`sulfate
`100.00°
`948.0
`
`2,4-Dinitrophenyl sulfate
`45.00°
`75.00°
`326.0
`269.0
`
`2,5-Dinitrophenyl sulfate
`100.00°
`75.00’
`3200.0
`1680.0
`
`72.5
`
`0.31
`0.32
`0.32
`0.31
`0.32
`
`3700.0
`3680.0
`3720.0
`
`295.0
`
`279.0
`278.0
`272.0
`273.0
`
`150.0
`148.0
`149.00
`
`1220.0
`1250.0
`
`1180.0
`1180.0
`
`1160.0
`
`1510.0
`
`Table III
`Arrhenius Parameters for
`the Neutral Hydrolysis
`of Nitrophenyl and Dinitrophenyl Sulfates
`AS*, eu“
`Sulfate
`E, kcal/mol
`-17.4
`24.7
`o-Nitrophenyl
`18.5*
`p-Nitrophenyl
`—18.  8
`-18.0
`18.8
`2,4-Dinitrophenyl
`-17.4
`19.4
`2,5-Dinitrophenyl
`Calculated at 75.00°.
`6 See ref 2.
`
`NasSO,
`CuS04
`AgC104
`NaF
`
`NaCl
`
`NaC104
`
`Table IV
`Salt Effects on the Neutral Hydrolysis of
`2,4-Dinitrophenyl Sulfate at 45.00°
`Concn of salt, M
`106  ·, sec-1
`0.00
`27.9
`1.00
`25.7
`2.00
`25.0
`24.8
`3.00
`20.3
`1.00
`2.00
`15.2
`3.00
`11.00
`4.00
`8.19
`5.00
`5.78
`1.00
`28.0
`0.1
`33.0
`0.1
`35.0
`0.5
`29.5
`1.0
`32.2
`lines whose slopes are given in Table
`good straight
`VII.
`The plot for o-nitrophenyl sulfate is illustrated
`in Figure 5. The scatter between the different acids
`might be due to the different electrolyte effects of the
`acids on the sulfate esters or on their hydrolysis trán-
`sition states.
`'¡To test this point we replotted oür ex-
`perimental datá as suggested by Bunnett18 and later
`modified by Bunnett and Olsen.14 Plots of log k^ + Ho
`against —log oH,o are curved and are different for the
`different acids; some of them show a minimum. Fig-
`ure 6 shows these plots for o-nitrophenyl sulfate.
`In
`their modified treatment Bunnett and Olsen suggest
`that the slope,  , of plots of log
`+ Ho against H0 +
`
`(13) J. F. Bunnett, ibid., 88, 4956 (1961), and accompanying papers.
`(14) J. F. Bunnett and F. P. Olsen, Can. J. Chem., 44, 1917 (1966).
`
`

`

`Vol. S3, No. 10, October 1968
`
`Hydrolysis of Nitrophenyl Sulfate Esters 3855
`
`Table V
`Neutral Hydrolysis or 2,4-Dinitrophbnyl Sulfate in Aqueous Organic Solvents
`Volume %
`E,
`organic solvent
`kcal/mol
`18.8
`0
`18.6
`25
`19.2
`50
`18.4
`75
`20.0
`40
`22.3
`60
`24.9
`80
`25.9“
`90
`18.0
`25
`20.0
`50
`19.4
`75
`19.5
`85
`22.2
`25
`22.1
`50
`= 76 sec-1 at 14.90°.
`
`10*  , sec-1, at 45.00°
`27.9
`27.4
`21.0
`13.1
`50.0
`49.0
`42.0
`
`62.1
`142.0
`302.0
`1130.0
`90.8
`183.0
`
`10*i^, sec-1, at 25.00°
`4.00
`3.85
`2.74
`1.86
`5.94
`4.60
`3.50
`260.00
`9.20
`18.00
`38.7
`143.7
`8.62
`17.6
`
`Medium
`CHjCN-HjO
`
`Dioxane-HsO
`
`DMSO-HjO
`
`DMF-HjO
`
`1 Calculated from 1
`
`AS*, eu, at 25.00°
`-18.0
`-18.5
`-16.0
`-15.5
`-10.0
`-5.2
`0.1
`14.01
`-18.0
`-12.0
`-10.5
`-6.5
`-3.7
`-3.2
`
`Table VI
`Acid-Catalyzed Hydrolysis of Nitrophenyl and Dinitrophenyl Sulfates1
`
`HC1
`
`HCIO,
`
`HiSO,
`
`Concn of acid, M
`1.03
`1.03»
`2.06
`3.06
`4.12
`5.15
`5.15»
`6.18
`7.12
`1.00
`1.00»
`2.00
`3.00
`4.00
`5.00
`5.00»
`6.00
`7.08
`1.00
`1.00»
`2.00
`3.00
`4.00
`5.00
`5.00»
`6.00
`1 At 25.00°, unless specified otherwise.
`
`&copy;-Nitrophenyl
`sulfate
`1.58
`17.8
`4.44
`11.2
`24.5
`48.6
`
`126.5
`258.0
`1.70
`22.2
`4.16
`9.92
`24.0
`60.1
`
`191.0
`759.0
`1.76
`20.1
`4.33
`11.2
`33.6
`92.4
`
`252.0
`» At 45.00°.
`
`m-Nitrophenyl
`sulfate
`0.91
`12.0
`3.26
`6.47
`15.0
`34.3
`387.0
`79.5
`201.0
`0.958
`13.0
`3.06
`5.67
`12.8
`28.0
`297.0
`78.7
`
`0.98
`11.6
`2.94
`7.13
`20.3
`54.1
`456.0
`151.0
`
`p-Nitrophenyl
`sulfate
`2.35
`43.5
`6.30
`18.0
`39.4
`85.5
`
`2,4-Dinitrophenyl
`sulfate
`13.3
`135.0
`29.7
`64.7
`109.0
`276.0
`
`2,5-Dinitrophenyl
`sulfate
`5.09
`45.0
`9.08
`23.3
`52.9
`98.7
`
`184.0
`
`2.35
`45.6
`7.25
`15.2
`35.7
`68.2
`
`228.0
`
`2.36
`44.2
`9.58
`18.5
`51.7
`138.0
`
`651.0
`
`12.7
`126.0
`27.7
`63.6
`142.0
`300.0
`
`862.0
`
`13.2
`151.0
`44.0
`
`225.0
`
`1226.0
`
`293.0
`
`5.17
`53.6
`11.5
`24.5
`52.1
`105.0
`
`307.0
`
`5.27
`49.3
`10.4
`23.4
`69.0
`182.0
`
`743.0
`
`Table VII
`the Acid-Catalyzed Hydrolysis of Nitrophenyl
`  Values1 and Slopes of Log k^ vs. —Ho Plots for
`Dinitrophenyl Sulfates
`p-Nitrophenyl
`m-Nitrophenyl
`o-Nitrophenyl
`sulphate
`sulphate
`sulphate
`-0,05
`-0.07
`-0.03
`HC1
`-0.39
`-0.20
`-0.39
`HCIO,
`-0.20
`-0.15
`-0.20
`,H,SO,
`0.95
`0.94
`0.96
`HC1
`Log k^ vs. —Ho slope
`0.86
`0.85
`0.86
`HCIO,
`at 25.00°
`0.83
`0.85
`0.86
`H,SO,
`log Ch+ + Ho at 25.00° in the 1.00-6.00 M acid range.
`slope of plots of log k+ + Ho vs.
`
` 
`
`1  
`
`=
`
`and
`
`2,5-Dinitro-
`pbenyl sulphate
`-0.10
`-0.38
`-0.14
`1.10
`0.73
`0.73
`
`2,4-Dinitro-
`phenyl sulphate
`-0.10
`-0.46
`-0.32
`0.98
`0.85
`0.86
`
`

`

`3856 Fendler and Fendler
`
`The Journal of Organic Chemistry
`
`-Ha.
`Figure 5.—Plot of 5 + log fa against —H<¡ for o-nitrophenyl
`sulfate at 25.00°:
` , HCIO,; 0, HC1;
` , H2SO,.
`
`Figure 7.—Plot of 6 + log fa + Ho against —
`for o-nitrophenyl sulfate at 25.00°: O, HCIO,;
`H2SO«.
`
`(log Ch+ + Ho)
` ,
`0, HC1;
`
`coefficients of 2,4-dinitrophenyl sulfate to be accurate
`within ± 10%, although the errors
`are smaller for the
`electrolytes. Errors for the transition-state activity
`coefficient ratios are inevitably compounded, but even
`for acetonitrile, representing the most complex system,
`the results are correct within ±25%.
`
`Discussion
`Neutral Hydrolysis.—The pH-rate profile for the
`hydrolysis of aryl sulfate esters involves a plateau, pH
`4-12, preceded by a more rapid acid-catalyzed reaction
`and followed by an exponential curve
`(see Figure 2)
`due to the incursion of base catalysis.15 Superficially
`this hydrolytic behavior is very similar to that which
`has been observed for the hydrolysis of alkyl,7 aryl,15·17
`and acyl18·19 phosphate dianions.
`The present results on the hydrolysis of nitrophenyl
`and dinitrophenyl sulfate anions will be discussed with
`respect to the hypotheses that (a) the rate-determining
`step involves unimolecular sulfur-oxygen bond fission
`with the elimination of S03,
`the hydrolysis pro-
`(b)
`ceeds by bimolecular nucleophilic attack of water on
`sulfur, and (e) water molecules participate in the tran-
`sition state by ion-dipole or dipole-dipole interactions
`or by hydrogen bonding to such an extent that the
`molecularity of the reaction is a matter of its academic
`definition.
`The sensitivity of the rate of hydrolysis to changes
`in substituents in the leaving group (p) has been found
`to be a useful mechanistic probe in studies of phosphate
`ester hydrolysis.16·17·19 We have found that a plot of
`log fa for the sulfate anion hydrolysis against the p  
`is linear with a slope of
`of the corresponding phenol
`— 1.2 (Figure 3). Such a relatively large substituent
`is consistent with a unimolecular mechanism,
`effect
`since in this case the only requirement for hydrolysis is
`sulfur-oxygen bond breaking. However, for the hy-
`drolysis of phosphate monoanions, which exhibits
`many characteristics of a unimolecular reaction but
`(15) The powerful base catalysis at high pH values might involve a change
`from a unimolecular to a bimolecular mechanism involving attack by hy-
`droxide ion on both carbon and sulfur.2 In addition, these processes could
`be complicated by specific electrolyte effects of the alkali metal hydroxide.
`Clearly our data on the base-catalyzed hydrolysis are insufficient to elucidate
`the base-catalyzed mechanism in greater detail.
`(16) C. A. Bunton, E. J. Fendler, and J. H. Fendler, J. Amer. Chem. Soc.,
`89, 1221 (1967).
`(17) A. J. Kirby and A. G. Varvoglis, ibid., 89, 415 (1967).
`(18) G. DiSabato and W. P. Jencks, ibid., 88, 4400 (1961).
`(19) A. J. Kirby and W. P. Jencks, ibid., 87, 3209 (1965).
`
`Figure 6.—Plot of 6 + log fa + Ho against log omo for o-nitro-
`phenyl sulfate at 25.00°:
` , HCIO,; 0, HC1;
` , H2SO<.
`
`log Ch+ indicates the effect of activity coefficient
`ratios on
`the reaction rate.14 Figure 7 shows such
`plots for o-nitrophenyl sulfate. Corresponding plots
`for the other sulfate esters are similar. The slopes of
`these plots are given in Table VII.
`The energies and
`entropies of activation are collected in Table VIII.
`
`Table VIII
`Energies and Entropies of Activation for
`Acid-Catalyzed Hydrolysis of Nitrophenyl
`Dinitrophenyl Sulfates
`
`Sulfate
`o-Nitrophenyl
`
`m-Nitrophenyl
`
`p-Nitrophenyl
`
`2,4-Dinitrophenyl
`
`2,5-Dinitrophenyl
`
`Solvent
`1.03 Af HC1
`1.00 M HCIO,
`1.00 M H2SO,
`1.03 MHC1
`5.15 Af HC1
`1.00M HCIO,
`5.00 M HCIO,
`I.OOMHüSO,
`5.00 M H2SO,
`1.03 MHC1
`LOOM HCIO,
`1.00 M H2SO,
`1.03 MHC1
`LOOM HCIO,
`LOOMHüSO,
`1.03 MHC1
`LOOM HCIO,
`1.00 M H2SO,
`
`B,
`koal/mol
`22.9
`23.9
`21.6
`27.8
`22.4
`25.2
`22.3
`23.4
`20.1
`22.6
`22.2
`23.1
`21.8
`21.6
`22.9
`21.0
`21.2
`21.9
`
`the
`and
`
`AS  , eu
`at 25.00°
`-5.7
`-5.5
`-9.0
`1.0
`-0.4
`1.6
`-1.4
`-1.0
`-7.0
`-2.0
`-6.0
`-1.5
`-1.5
`-5.0
`-1.3
`-8.0
`-6.0
`-7.0
`
`Errors, standard deviations, in individual rate con-
`those in E are
`stants are not greater than ±4%;
`approximately ±0.7 kcal mol-1; and those in AS*
`are ±2 eu. We consider values for the molar activity
`
`

`

`Vol. 33, No. 10, October 1968
`
`the
`involves a proton transfer to the leaving group,
`In this case,
`slope of a corresponding plot is —0.32.20
`a small substituent effect is reasonable since leaving
`groups which weaken the phosphorus-oxygen bond de-
`the basicity of the ester oxygen atom and vice
`crease
`the slope for
`In addition,
`the magnitude of
`versa.
`sulfate anions is considerably greater than seems prob-
`able for a mechanism involving bimolecular displace-
`the bimolecular
`ment by a water molecule.
`Indeed,
`reaction between hydroxide ion and a series of fully
`substrated phosphates with different leaving groups dis-
`plays little sensitivity to the nature of
`the leaving
`group; i.e., a plot of log k against the pK& of the leaving
`group has a slope of —0.43.21 However, the effect of
`substituents in the leaving group on the rate of hy-
`dolysis of nitro and dinitrophenyl sulfates is quite
`similar to that found for the hydrolysis of the dianions
`of substituted acetyl, benzoyl, and aryl phosphates for
`which the corresponding slope is —1.2.16-19 Con-
`siderable evidence supports the conclusion that
`the
`hydrolysis of these phosphates proceeds by unimolecu-
`lar phosphorus-oxygen bond fission with elimination of
`the metaphosphate ion.7 Thus, the available evidence
`for substituent effects suggests that the hydrolysis of
`nitro and dinitrophenyl sulfates proceeds by a uni-
`molecular mechanism, but it does not exclude the al-
`ternative formulation mentioned in hypothesis c.
`The presence of sodium chloride (up to 3.00 M),
`sodium sulfate (1.00 M), and sodium fluoride (1.00 M)
`only slightly affects the rate constant for the neutral
`hydrolysis of 2,4-dinitrophenyl sulfate, but the addition
`of sodium perchlorate significantly decreases it (Table
`I\r). The addition of sodium perchlorate, chloride,
`and sulfate was found to increase the rate of hydrolysis
`of the dianion of 2,4-dinitrophenyl phosphate, but this
`the cation
`effect was more
`specifically dependent on
`than on
`the anion.16 The available data concerning
`the effects of 0.8 M potassium iodide, 0.8 M sodium
`fluoride, and 0.25 M sodium sulfite on the hydrolysis of
`p-nitrophenyl sulfate at 75.00°2 has been interpreted as
`evidence supporting a unimolecular mechanism.22
`However, an intercomparison of these salt effects between
`the various sulfate and phosphate esters is somewhat mean-
`ingless since they can only be described in terms of free-energy
`changes of the initial state relative to the transition state.
`Such changes are best described by the Br0nsted-Bjer-
`rum rate equation which relates the rate constant in the
`presence of electrolytes, k, to the rate constant in their
`absence, k0, and to the ratio of activity coefficients of
`the initial and transition states, fjf*, which follows.
`k = kof./f*
`(1)
`A meaningful interpretation of electrolyte and medium
`reaction rate must necessarily involve a
`effects on
`terms in eq 1
`separation of
`the activity coefficient
`rather than a gross comparison of the observed rate
`constants for seemingly similar reactions. We have
`attempted to do this by partitioning 2,4-dinitrophenyl
`
`(20) C. A. Bunton, E. J. Fendler, E. Humeres, and K-U Yang, J. Org.
`Chem., 32, 2806 (1967).
`(21) D. F. Heath,
`‘Organophosphorus Poisons,” Pergamon Press, New
`York, N. Y., 1961, p 79.
`(22) These salt-effect studies were
`carried out, however, at a constant
`ionic strength of one using potassium chloride which seriously complicates any
`interpretation of
`the effect of 0.8 M added electrolyte on
`the rate of hy-
`drolysis.
`
`Hydrolysis of Nitrophenyl Sulfate Esters
`
`3857
`
`sulfate between cyclohexane or hexane and the aqueous
`solution (see Experimental Section). Combining rjr0
`fjfa with eq 1 one obtains
`
`=
`
`=
`
`/,*//„*
`(2)
`k0rB/kBr„
`which allows separation of the activity coefficient ratios
`for both the initial and transition states. Sodium
`chloride slightly “salts in” and sodium perchlorate and
`sulfate “salt out” 2,4-dinitrophenyl sulfate, but all
`these salts destabilize the transition state (Table I).
`The significant
`rate deceleration caused by sodium
`perchlorate (Table IV) is explicable in terms of a very
`pronounced transition-state destabilization which is
`not canceled by initial state interactions.
`It
`is not
`surprising that sodium chloride and sulfate affect the
`hydrolysis rate insignificantly since the former salt
`only slightly alters the molar activity coefficient of the
`initial and transition state and interactions of the latter
`with the initial state and transition state cancel.
`Copper (II) and silver(I)
`ions have no significant
`the hydrolysis of 2,4-dinitrophenyl sulfate
`effect on
`a considerable copper(II)
`(Table IV).23 Recently,
`reported for 8-hydroxyquinoline
`ion catalysis was
`sulfate.5 Although the exact nature of metal
`ion
`rate enhance-
`catalysis is not perfectly understood,
`ment is generally discussed in terms of chelation with
`the substrate7 or charge neutralization in the transition
`state.25 Not unexpectedly, metal ions do not seem to
`chelate with 2,4-dinitrophenyl sulfate nor do they en-
`hance its hydrolysis rate by any other mechanism.
`In their recent study Benkovic and Benkovic2 have
`reported that the rate constant for p-nitrophenyl sulfate
`hydrolysis was a factor of two slower in 50% aqueous
`acetonitrile than in water.
`these authors
`Indeed,
`used this fact as one of their mechanistic probes to
`substantiate the proposed unimolecular mechanism2
`since similar effects have been reported for hydrolysis
`of acetyl phosphate dianion.18 On the other hand, the
`hydrolysis rate of 2,4-dinitrophenyl phosphate was
`found to increase by a factor of four in aqueous 50%
`the
`acetonitrile,17 and considerable enhancement of
`hydrolysis rate of steroid sulfates28 by moist dioxane
`has been noted for
`time.
`These conflicting
`some
`effects of dipolar aprotic solvents on the hydrolysis of
`sulfate and phosphate esters question the validity of
`the use of solvent effects as mechanistic criteria, at
`least for these compounds. We,
`therefore, have ex-
`amined the effect of acetonitrile, dimethyl sulfoxide,
`dioxane, and  , -dimethylformamide on hydrolysis of
`2,4-dinitrophenyl sulfate (Table V). Changing the
`aqueous medium to aqueous acetonitrile steadily de-
`the hydrolysis rate. Dioxane has the same
`creases
`effect up to 80% dioxane concentration, but, when the
`water content of the medium is 10%, the neutral rate
`is increased by a factor of 70. Dimethyl sulfoxide and
` , -dimethylformamide increase the rate of the neu-
`tral hydrolysis exponentially.
`
`(23) However, the presence of 0.01 M cetyltrimethylammonium bromide,
`a cationic micelle, increases the rate of both the neutral and the base-cata-
`lyzed hydrolysis by a factor of two but very significantly decreases the acid-
`catalyzed rate of pH 2.0, whereas 0.01 M sodium lauryl sulfate, an anionic
`micelle, slightly decreases the rate of the neutral hydrolysis.54
`(24) E. J. Fendler and J. H. Fendler, to be published.
`(25) C. H. Oestreich and  . M. Jones, Biochem., 2926, 3151 (1966); 1515
`(1967).
`(26) J. McKenna and J. K. Norymberski, J. Chem. Soc., 3889 (1957).
`
`

`

`3858 Fendler and Fendler
`
`The effects of these dipolar aprotic solvents on the
`hydrolysis of 2,4-dinitrophenyl sulfate is revealed
`strikingly by their entropies of activation. Supported
`by a large body of experimentally determined entropies
`of activation for reactions of known mechanism, posi-
`tive AS* values have been assigned to unimolecular
`to bimolecular mecha-
`mechanisms and negative ones
`nisms.27 The entropies of activation for the neutral
`hydrolysis of nitrophenyl and dinitrophenyl sulfates
`are more negative than those generally associated with
`a unimolecular mechanism (Table III). The inter-
`pretation of the magnitude of entropy effects for uni-
`molecular reactions is, however,
`less straightforward
`than that
`reactions. The ordering
`for bimolecular
`effect in the latter case arises from the covalently bound
`in the transition state, while the former situa-
`water
`tion is a composite of two opposing effects. The one-
`step process of forming sulfur trioxide and phenoxide
`ion results in a decrease in the order of the system,
`hence an increase in the entropy. On the other hand,
`the internal mobility of the solvent
`is reduced by a
`greater solvation of the entities in the transition state
`resulting in a decrease in entropy. Apparently for
`aryl sulfate esters, the net effect on the entropy differ-
`ence between the initial and the transition state due to
`these opposing factors results in a moderatively nega-
`tive entropy of activation. Several reactions whose
`mechanisms have been independently established to be
`unimolecular have fairly considerable negative entro-
`pies of activation.28·29 The hydrolysis of 2,4-dinitro-
`phenyl sulfate in aqueous solvent mixtures containing
`dipolar aprotic solvents exhibits significantly higher
`entropies of activation (Table V). The effectiveness
`of the solvents in increasing the entropies of activation
`is dioxane >  , -dimethylformamide > dimethyl
`sulfoxide > acetonitrile. Tentatively one may ascribe
`the entropy increase to a decrease in transition-state
`solvation.28
`best described in
`Medium effects, however,
`are
`terms of eq 1 which allows a separation of the activity
`coefficient effects in the initial and transition states.
`We have attempted to do this for the acetonitrile and
`It
`is evident
`dimethyl sulfoxide systems (Table I).
`that, although acetonitrile considerably increases the
`molar activity coefficient of 2,4-dinitrophenyl sul-
`fate, the corresponding transiti