Effect of the acyl substituent on the equilibrium constant for hydration of esters

1980 ◽  
Vol 58 (13) ◽  
pp. 1281-1294 ◽  
Author(s):  
J. Peter Guthrie ◽  
Patricia A. Cullimore
Keyword(s):  
Aqueous Solution ◽  
Equilibrium Constant ◽  
Rate Constant ◽  
Rate Constants ◽  
Acid Catalysis ◽  
Methyl Esters ◽  
Equilibrium Constants ◽  
Free Energies ◽  
General Acid ◽  
Aldehydes And Ketones

Heats of hydrolysis have been measured for the trimethyl orthoesters of isobutyric, propionic, benzoic, methoxyacetic, chloroacetic, and cyanoacetic acids using aqueous acid with an organic cosolvent where necessary, and of the corresponding esters in alkaline solution. Solubilities or free energies of transfer from gas to aqueous solution have been measured, permitting calculation of the free energies of formation of the aqueous orthoesters, and by methods which we have published previously, calculation of the free energies of formation of the covalent hydrates of the esters, and the free energy changes for hydration of these esters.Using estimated pKa values equilibrium constants were calculated for the addition of hydroxide to the esters. The data are in good agreement with the appropriate Marcus equation relating rate and equilibrium constants with a value for b of 8.99 ± 0.17. This line was used to estimate the equilibrium constant for addition of hydroxide, and thence of water, to some additional esters where only the rate constant was available. Rate constants for hydrolysis of methyl esters in aqueous solution at 25 °C were calculated from literature data, correcting for the effect of other conditions as necessary. From the equilibrium constants for addition of water we could estimate the rate constants for uncatalyzed hydrolysis; for the cases where this rate constant has been measured, the agreement was satisfactory. For acid catalyzed hydrolysis the data permit a test of the two alternative mechanisms considered previously, namely specific acid catalysis and general acid catalysis with hydronium ion acting as a general acid. For esters the mechanism is clearly specific acid catalysis, but for aldehydes and ketones it appears very likely that the mechanism is general acid catalysis.

The Strecker reaction — an examination in terms of no-barrier theory

2008 ◽  
Vol 86 (4) ◽  
pp. 285-289 ◽  
Author(s):  
J Peter Guthrie ◽  
Goonisetty Bhaskar
Keyword(s):  
Aqueous Solution ◽  
Equilibrium Constant ◽  
Rate Constants ◽  
Reaction Rate ◽  
Equilibrium Constants ◽  
Reaction Rate Constant ◽  
Cyanide Ion ◽  
Strecker Reaction ◽  
Rate Determining Step ◽  
Iminium Ion

For those examples of the Strecker reaction where information about both rate and equilibrium is available, we have been able to calculate rate constants for the addition of cyanide ion to the iminium ion by the no-barrier theory (NBT) approach. Both experimental and calculated values are for reaction in aqueous solution. Only for the reactions of benzaldehyde with benzyl or allyl amines and HCN are the equilibrium constants and rate constants for the final, rate-determining, step directly available from the literature. For the reactions of acetone with ammonia, methylamine, or dimethylamine and HCN rate constants for the retro-Strecker cleavage and the equilibrium constants for the overall Strecker reaction have been reported. These equilibrium constants, combined with equilibrium constants for iminium ion formation, which can be extracted from information in the literature, allow calculation of the equilibrium constants for the final step of these Strecker reactions. No-barrier theory has already been applied to carbonyl additions, including cyanohydrin formation; this report provides further evidence for the generality of this approach for calculating rate constants without using any kinetic information.Key words: Strecker reaction, rate constant, equilibrium constant, no-barrier theory, computation.

scholarly journals Carbonyl Addition Reactions: Factors Affecting the Hydrate–Hemiacetal and Hemiacetal–Acetal Equilibrium Constants

1975 ◽  
Vol 53 (6) ◽  
pp. 898-906 ◽  
Author(s):  
J. Peter Guthrie
Keyword(s):  
Aqueous Solution ◽  
Carbonyl Compounds ◽  
Equilibrium Constants ◽  
Substituent Effects ◽  
Steric Effects ◽  
Addition Reactions ◽  
Free Energies ◽  
Electronic Effects ◽  
Factors Affecting ◽  
Analogous Formula

Equilibrium constants for hydrate–hemiacetal interconversion in aqueous solution at 25° have been measured for four fluorinated carbonyl compounds: compound, alcohol, K4 (M−1): CF3CHO, C2H5OH, 2.3; CF3COCH3, CH3OH, 1.0; CF3COPh, CH3OH, 3.5; CF3COCF3, CH3OH, 0.14. These values, combined with values from the literature, permit an examination of substituent effects upon the equilibrium constant for[Formula: see text]The free energy change for this process, corrected for symmetry and steric effects, follows the equation[Formula: see text]Thus electronic effects upon this equilibrium are generally small and in fact are often smaller than steric effects.This analysis permits and justifies the calculation of free energies of formation of [Formula: see text] compounds from the (more generally measurable) free energies of formation of the analogous [Formula: see text] compounds.

The retroaldol reaction of 3-methyl-2-butenal

1983 ◽  
Vol 61 (1) ◽  
pp. 171-178 ◽  
Author(s):  
J. Peter Guthrie ◽  
Brian A. Dawson
Keyword(s):  
Sodium Hydroxide ◽  
Equilibrium Constant ◽  
Rate Constants ◽  
Equilibrium Constants ◽  
Aqueous Sodium Hydroxide ◽  
Initial Increase ◽  
Aqueous Sodium ◽  
Rate Determining Step ◽  
Kinetics Of

In aqueous sodium hydroxide solutions at 25 °C, 3-methyl-2-butenal, 1c, undergoes retroaldol cleavage to acetone and acetaldehyde. The kinetics of the retroaldol reaction were followed spectrophotometrically at 242 nm and showed simple first order behavior. When 3-methyl-3-hydroxybutanal, 2c, was added to aqueous sodium hydroxide solutions at 25 °C, there was an initial increase in absorbance at 242 nm, attributed to formation of 1c, followed by a 20-fold slower decrease; the rate of the slow decrease matches the rate of disappearance of 1c under the same conditions. Analysis of the kinetics allows determination of the three rate constants needed to describe the system: khyd = 0.00342; kdehyd = 0.00832; kretro = 0.0564; all M−1 s−1. The equilibrium constant for enone hydration is 0.41. Rate constants for the analogous reactions for acrolein and crotonaldehyde could be obtained from the literature. There is a reasonable rate–equilibrium correlation for the retroaldol step. For the enone hydration step, rate and equilibrium constants respond differently to replacement of hydrogen by methyl. It is proposed that this results from release of strain after the rate-determining step by rotation about a single bond; this decrease in strain is reflected in the equilibrium constant but not in the rate constant.

Acid Ionizations of Nitroalkanes in Aqueous Solution

1973 ◽  
Vol 51 (12) ◽  
pp. 1941-1944 ◽  
Author(s):  
Takeki Matsui ◽  
Loren G. Hepler
Keyword(s):  
Aqueous Solution ◽  
Carboxylic Acids ◽  
Equilibrium Constants ◽  
Free Energies ◽  
Methyl Substitution ◽  
Calorimetric Measurements ◽  
Acid Ionizations

Calorimetric measurements have led to ΔH0 values for ionization of nitromethane, nitroethane, 1-nitropropane, and 2-nitropropane in aqueous solution at 298°K. Combinations of these enthalpies with free energies from equilibrium constants for ionization have led to ΔS0 values for the ionization reactions. It is noted that the trend toward decreasing pK with methyl substitution in nitroalkanes is unusual compared to phenols and carboxylic acids. Similarly, correlations of ΔS0 with ΔG0 and ΔH0 are different for nitroalkanes than for other acids.

The nature of the transition state in diarylmethyl cation – nucleophile combination reactions as probed by secondary α-deuterium isotope effects

2001 ◽  
Vol 79 (12) ◽  
pp. 1887-1897
Author(s):  
Thuy Van Pham ◽  
Robert A McClelland
Keyword(s):  
Transition State ◽  
Isotope Effect ◽  
Rate Constant ◽  
Rate Constants ◽  
Flash Photolysis ◽  
Isotope Effects ◽  
Equilibrium Constants ◽  
Bond Formation ◽  
Kinetic Isotope ◽  
Rate Limiting

Transition-state structures for the carbocation–nucleophile combination reactions of (4-substituted-4'- methoxydiphenyl)methyl cations with water, chloride, and bromide ions in acetonitrile–water mixtures have been investigated by measuring the secondary α-deuterium kinetic and equilibrium isotope effects. Rate constants in the combination direction were measured with laser flash photolysis. Equilibrium constants were measured for the water reaction by a comparison method in moderately concentrated sulfuric acid solutions, for the bromide reaction via the observation of reversible combination, and for the chloride reaction from the ratio of the combination rate constant and the rate constant for the ionization of the diarylmethyl chloride product. The fraction of bond making in the transition state has been calculated as the ratio log (kinetic isotope effect):log (equilibrium isotope effect). For the water reaction, there is 50–65% bond making in the transition state; this is also true for cations that are many orders of magnitude less reactive. The same conclusions, 50–65% bond formation in the transition state independent of reactivity, have previously been made in correlations of log kw vs. log KR. Thus, two quite different measures of transition structure provide the same result. The kH:kD values for the halide combinations in 100% acetonitrile are within experimental error of unity. This is consistent with suggestions that these reactions are occurring with diffusional encounter as the rate-limiting step. Addition of water has a dramatic retarding effect on the halide reactions, with rate constants decreasing steadily with increased water content. Small inverse kinetic isotope effects are observed (in 20% acetonitrile:80% water) indicating that carbon—halogen bond formation is rate-limiting. Comparison of the kinetic and equilibrium isotope effects shows ~25 and ~40% bond formation in the transition states for the reactions with bromide and chloride, respectively.Key words: carbocation, isotope effect, transition state, halide.

Kinetics and mechanism of gold(III) incorporation into tetrakis(1-methylpyridium-4-yl)porphyrin in aqueous solution

2004 ◽  
Vol 08 (11) ◽  
pp. 1269-1275 ◽  
Author(s):  
Ahsan Habib ◽  
Masaaki Tabata ◽  
Ying Guang Wu
Keyword(s):  
Aqueous Solution ◽  
Rate Constants ◽  
Kinetic Data ◽  
Ph Dependence ◽  
Equilibrium Constants ◽  
Hydroxyl Group ◽  
Net Charge ◽  
First Order ◽  
Kinetics Of ◽  
Hydrolysis Of

The kinetics of the reaction of the tetrakis(1-methylpyridium-4-yl)porphyrin tetracation, [ H 2( TMPyP )]4+, with gold(III) ions were studied along with equilibria of gold(III) species in aqueous medium at 25°C, I = 0.10 M ( NaNO 3). The equilibrium constants for the formation of [ AuCl 4-n( OH ) n ]- ( n = 0,…,4), defined as β n = [ AuCl 4- n ( OH ) n ]- [ Cl -] n / [ AuCl 4-][ OH -] n were found to be that log β1 = 7.94 ± 0.03, log β2 = 15.14 ± 0.03, log β3 = 21.30 ± 0.05 and log β4 = 26.88 ± 0.05. The overall reaction was first order with respect to each of the total [ Au (III)] and [ H 2 TMPyP 4+]. On the basis of pH dependence on rate constants and the hydrolysis of gold(III), the rate expression can be written as d [ Au ( TMPyP )5+]/ dt = ( k 1[ AuCl 4-] + k2[ AuCl 3( OH )-] + k3[ AuCl 2( OH )2-] + k4[ AuCl ( OH )3-])[ H 2 TMPyP 4+], where k1, k2, k3 and k4 were found to be (2.16 ± 0.31) × 10-1, (6.56 ± 0.19) × 10-1, (1.07 ± 0.24) × 10-1, and (0.29 ± 0.21) × 10-1 M -1. s -1, respectively. The kinetic data revealed that the trichloromonohydroxogold(III) species, [ AuCl 3( OH )]-, is the most reactive. The higher reactivity of [ AuCl 3( OH )]- is explained by hydrogen bonding formation between the hydroxyl group of [ AuCl 3( OH )]- and the pyrrole hydrogen atom of [ H 2( TMPyP )]4+. Furthermore, applying the Fuoss equation to the observed rate constants at different ionic strengths, the apparent net charge of [ H 2( TMPyP )]4+ was calculated to be +3.5.

scholarly journals Mechanism of Allosteric Modulation of Rod Cyclic Nucleotide–gated Channels

1999 ◽  
Vol 113 (5) ◽  
pp. 601-620 ◽  
Author(s):  
Elizabeth R. Sunderman ◽  
William N. Zagotta
Keyword(s):  
Transition State ◽  
Rate Constant ◽  
Rate Constants ◽  
Cyclic Nucleotide ◽  
Equilibrium Constants ◽  
Rod Photoreceptor ◽  
Open Probability ◽  
Allosteric Transition ◽  
Cng Channel ◽  
Single Channel Currents

The cyclic nucleotide–gated (CNG) channel of retinal rod photoreceptor cells is an allosteric protein whose activation is coupled to a conformational change in the ligand-binding site. The bovine rod CNG channel can be activated by a number of different agonists, including cGMP, cIMP, and cAMP. These agonists span three orders of magnitude in their equilibrium constants for the allosteric transition. We recorded single-channel currents at saturating cyclic nucleotide concentrations from the bovine rod CNG channel expressed in Xenopus oocytes as homomultimers of α subunits. The median open probability was 0.93 for cGMP, 0.47 for cIMP, and 0.01 for cAMP. The channels opened to a single conductance level of 26–30 pS at +80 mV. Using signal processing methods based on hidden Markov models, we determined that two closed and one open states are required to explain the gating at saturating ligand concentrations. We determined the maximum likelihood rate constants for two gating schemes containing two closed (denoted C) and one open (denoted O) states. For the C ↔ C ↔ O scheme, all rate constants were dependent on cyclic nucleotide. For the C ↔ O ↔ C scheme, the rate constants for only one of the transitions were cyclic nucleotide dependent. The opening rate constant was fastest for cGMP, intermediate for cIMP, and slowest for cAMP, while the closing rate constant was fastest for cAMP, intermediate for cIMP, and slowest for cGMP. We propose that interactions between the purine ring of the cyclic nucleotide and the binding domain are partially formed at the time of the transition state for the allosteric transition and serve to reduce the transition state energy and stabilize the activated conformation of the channel. When 1 μM Ni2+ was applied in addition to cyclic nucleotide, the open time increased markedly, and the closed time decreased slightly. The interactions between H420 and Ni2+ occur primarily after the transition state for the allosteric transition.

Equilibrium constants and heats of formation of methyl esters and N,N-dimethyl amides of substituted benzoic acids

1992 ◽  
Vol 70 (6) ◽  
pp. 1671-1683 ◽  
Author(s):  
J. Peter Guthrie ◽  
David C. Pike ◽  
Yiu-Chung Lee
Keyword(s):  
Methyl Esters ◽  
Equilibrium Constants ◽  
Heats Of Formation ◽  
Free Energies ◽  
Ester Formation ◽  
Free Energy Of Transfer ◽  
Partition Constants ◽  
Substituted Benzoic Acids ◽  
Energy Of Transfer ◽  
Benzoyl Chlorides

Heats of methanolysis and dimethylaminolysis of substituted benzoyl chlorides (4-X-C6H4-COCl, X = H, CH3, OCH3, Cl, NO2) have been measured, as have the heats of hydrolysis of the esters, permitting the calculation of the heats of formation of the benzoyl chlorides (4-X-C6H4-COCl, X, DHf: CH3O, −80.29 ± 0.70; CH3, −48.10 ± 1.46; NO2, −47.70 ± 0.87), methyl benzoate esters (4-X-C6H4-COOCH3, X, DHf: CH3O, −124.50 ± 0.39; CH3, −93.99 ± 0.58; Cl, −92.09 ± 0.53; NO2, −92.55 ± 0.31), and N,N-dimethylbenzamides (4-X-C6H4-CON(CH3)2, X, DHf: CH3O, −75.87 ± 1.42; CH3, −46.62 ± 1.99; H, −40.96 ± 1.41; Cl, −49.33 ± 1.09; NO2, −48.05 ± 1.53). Free energies of transfer from methanol to water and from gas to water were determined for the esters and amides. Free energies of formation in aqueous solution were calculated for the acids, esters and amides, making use of thermodynamic estimation procedures where necessary. Equilibrium constants were measured for ester formation in water (X, K (M−1): CH3O, 0.14; CH3, 0.14; H, 0.12; Cl, 0.15; NO2, 0.13) and N,N-dimethylaminolysis in methanol (X, K (M−1): CH3O, 8.16; CH3, 17.5; H, 26.5; Cl, 22.6; NO2, 41.0). Partition constants for esters and amides were measured for methanol/dodecane and dodecane/water, permitting calculation of the free energy of transfer from methanol to water (4-X-C6H4-COOCH3, X, DGmw: CH3O, 3.12; CH3, 3.17; H, 3.01; Cl, 3.43; NO2, 2.89; 4-X-C6H4-CON(CH3)2, X, DGmw: CH3O, 0.96; CH3, 1.48; H, 0.92; Cl, 1.77; NO2, 0.99). These data permit calculation of the equilibrium constants for dimethylaminolysis of substituted methyl benzoates in water, and for amide formation in water (4-X-C6H4-CON(CH3)2, X, K(M−1, reactants and products as neutral molecules): CH3O, 767; CH3, 752; H, 2050; Cl, 1020; NO2, 2350). In the course of our calorimetric measurements we derived an improved value for the heat of solution of HCl in methanol.

scholarly journals Thermodynamics of ester and orthoester formation from trifluoroacetic acid

1976 ◽  
Vol 54 (2) ◽  
pp. 202-209 ◽  
Author(s):  
J. Peter Guthrie
Keyword(s):  
Aqueous Solution ◽  
Free Energy ◽  
Equilibrium Constant ◽  
Trifluoroacetic Acid ◽  
Sodium Methoxide ◽  
Standard Free Energy ◽  
Steric Effects ◽  
Addition Reactions ◽  
Free Energies ◽  
Acetic Acids

The equilibrium constant for the addition of sodium methoxide to methyl trifluoroacetate, in methanol as solvent, has been measured by 19F nmr, and is 7 M−1. From this was calculated an equilibrium constant, 2 × 10−4 M−1, for addition of methanol to the ester. The equilibrium constant for formation of methyl trifluoroacetate in aqueous solutions is 0.06 M−1. These results, with literature data, permit calculation of the free energies of formation in aqueous solution of orthotrifluoroacetic acid and its mono-, di-, and trimethyl esters. These in turn permit calculation of the standard free energy changes for addition of water and methanol to trifluoroacetic acid and its methyl ester. These combined with the analogous values for formic and acetic acids permit evaluation of ρ* values for these addition reactions. Linear plots are obtained if correction is made for steric effects, and the ρ* values are somewhat larger, 2.1–2.9, than was observed for the analogous carbonyl addition reactions.

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