Order In Hydroxide Ion Research Articles

Unusual Stability and Carbon Acidity of a Dicationic Carbon Species

1,1'-Methylenebis(pyridinium) dication (MDP) is an unusual ion with two formal positively charged substituents attached to a central carbon, yet it is remarkably stable to hydrolysis at pH < 8. However, above this pH it undergoes a biphasic reaction liberating two equiv of pyridine sequentially. The rate of the first phase is second order in hydroxide ion, while that of the second is pH-independent. The first phase is also accompanied by the generation of a chromophore at 366 nm, which has been identified as a pyridine-ring-opened unsaturated iminoaldehyde formed by an ANRORC-type mechanism. This intermediate then undergoes ring closure to give the second equiv of pyridine and formaldehyde. Below pD 8 there is a very slow alternative pathway for degradation that is first order in hydroxide ion, liberates only one equiv of pyridine, and forms N-(hydroxymethyl)pyridinium ion. Deuterium exchange of the central methylene in D2O is faster than the breakdown of MDP and is predominantly OD(-)-catalyzed with a small amount of buffer catalysis. The estimated pKa of MDP dication in H2O, 21.2 at 25 °C and I = 1.0 M (KCl), is unexpectedly high but is about 9 units lower than that for the monocationic N-methylpyridinium ion. Deuterium exchange also occurs at the 2 and 6 positions of the pyridinium rings, but at a lower rate that is first order in deuteroxide ion and competitive with the breakdown of MDP only below pD 11.

Kinetics and mechanism of the reaction of N‐chlorosuccinimide with I− and OH−

AbstractIn this kinetic study of the reaction between the iodide ion and the N‐chlorosuccinimide, we have found that the reaction is first order in N‐chlorosuccinimide: namely, first order with respect to the iodide ion and first order with respect to the proton concentration. Based on the experimental results, we propose a mechanism consistent in Cl+ transfer from the N‐chlorosuccinimide to the iodide ion. We have found that the reaction presents general acid catalysis with a Brönsted exponent value of 0.10. In addition, given that the N‐chlorocompounds are not stable in aqueous solution, we have carried out a study on the influence of bases in the presence of N‐chlorosuccinimide. We have found that the reaction is first order in hydroxide ion, as well as in the 1,1,1,3,3,3‐hexafluoroisopropanoxide ion and the 2,2,2‐trifluorethoxide ion, which shows that the reaction is general‐base catalyzed. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 243–252, 2005

Hydrolytic Reactions of thecis-Methyl Ester of 3′-Deoxy-3′-thiothymidine 3′,5′-Cyclic(phosphorothiolate)

Hydrolysis of the cis-methyl ester of 3′-deoxy-3′-thiothymidine 3′-S,5′-O-cyclic(phosphorothiolate) (1a) has been followed by HPLC and MS. At pH 5 first order in hydroxide ion. The uncatalyzed and acid-catalyzed reactions yield two thiophosphate diesters, the 3′-S,5′-O-cyclic phosphorothiolate 2 and 3′-S-phosphorothiolate methyl ester 3, in a 9:1 and 1:3 molar ratio, respectively. The hydroxide ion catalyzed reaction gives the endocyclic P−O and P−S bond-cleavage products (3 and 4, respectively) in a 1:2 molar ratio. The pH-independent reaction is suggested to take place by attack of a water molecule on the carbon atom and concomitant C−O bond rupture, whereas the alkaline and acidic reactions involve attack of the nucleophile on the phosphorus atom and formation of a pseudorotating thiophosphorane intermediate. Under acidic conditions, cleavage of the N-glycosidic linkage competes with the phosphoester hydrolysis, corresponding to 20% of the hydrolysis products at pH < 1.

Kinetics of the Base-Catalyzed Permanganate Oxidation of Benzaldehyde

The kinetics of the base-catalyzed permanganate oxidation of benzaldehyde have been reexamined. The rate is proportional to the first power of the aldehyde and permanganate concentrations, and there are terms that are zero order, first order, and second order in hydroxide ion. The reaction has an isotope effect, and the effect of substituents gives rho = +1.58. The possible mechanisms for the reaction are discussed in the context of ab initio calculations at the B3P86/6-311+G and MP2/6-31G theoretical levels, and both one- and two-electron processes are possible. Benzaldehyde hydrate dianion is calculated to have a remarkably small C-H bond dissociation energy.

Kinetics and Mechanism of Reaction of 2,4-Dinitrobenzaldehyde with Hydroxide Ion

The reaction of 2,4-dinitrobenzaldehyde with hydroxide ion in water is first order in the substrate. The reaction order in hydroxide ion varies from one to two at lower and higher OH- concentrations, respectively. The reaction mechanism has been suggested, the pKa value of reaction product 2-nitro-4-nitrosophenol and the equilibrium constant of formation of the Meisenheimer adduct from 2-nitro-4-nitrosophenoxide and hydroxide ion have been determined.

Kinetics and mechanism of the formation of N-vinyl pyridinium cations in elimination reactions in aqueous base

The rates of the elimination reactions of N-(2-bromoethyl) pyridinium cations (1) and N,N′-ethylene bispyridinium dications (3) to give the corresponding N-vinyl pyridinium cations (2) have been measured spectrophotometrically in basic aqueous solutions (ionic strength 0.1, 25 °C) for a variety of substituents in the pyridine rings of each of these classes of pyridinium cation. The reaction kinetics are first order in 1 or 3 and first order in hydroxide ion. Brønsted-type plots of the second-order rate constants (kOH) as a function of the basicity (as pKBH) of the corresponding substituted pyridine are nonlinear for each of 1 and 3 and can be interpreted in terms of E1cb reaction mechanisms. For 1, the Brønsted-type plot displays two distinct "concave down" linear regions; rate-determining deprotonation for pKBH &lt; 5.16 (slope = −0.30), and a change in rate-determining step to bromide ion departure for pKBH &gt; 5.16 (slope −0.58). For 3, the Brønsted-type plot appears to be smoothly curved for symmetrically disubstituted bispyridinium dications, as a consequence of the multiple substituent effects upon each step of the E1cb reactions of these dications. However, log kOH for 3 is a smooth linear function of the previously reported log kOH for the E1cb reactions of N-(2-cyanoethyl) pyridinium cations over a range in which a change in rate-determining step has been directly demonstrated for these latter cations. Thus a change in rate-determining step as a function of pyridine basicity is also required within the E1cb mechanism for 3. The E1cb reactions of 1 are approximately 104-fold faster than the corresponding hydroxide ion catalyzed E2 eliminations from 2-phenylethyl bromides that are isoelectronic with 1.

Models of vitamin B6 enzymes. 3. Steric and electronic effects in carbon-hydrogen bond breaking reactions of bis(salicylideneglycinato)cobaltate(III) anions

Salicylaldehydes readily Schiff bases with glycine. These schiff bases form bis(salicylideneglycinato) cobaltate (III) complexes. Twelve cobalt (III) complexes have been synthesized with substituents variously on the 3-, 4-, 5-, and 6-positions of the aromatic ring of the salicylaldehyde moiety. The carbon-hydrogen bond breaking reactions of the two gem-methylenic protons of the glycine moiety have been studied by deuterium exchange. Rates are first order in complex and first order in hydroxide ion. Rates differ for the exchange of the two protons, and their ratio varies from 0.81 to 4.7. This demonstrates a reversal of stereoselectivity, depending on ring substituents. Rates vs substituents generally follow Hammett behavior. The structural and electronic features that lead to stereoselectivity are discussed

Structure–activity relations. Part 6. The alkaline hydrolysis of 3-methyl-5-methylidene- and 3,5-dimethylthiazolidine-2,4-diones. The addition of thiols to 3-methyl-5-methylidenethiazolidine-2,4-dione

The rate-coefficients for the alkaline hydrolysis of 3-methyl-5-methylidene- and 3,5-dimethyl-thiazolidine-2,4-diones in water at 25.0 °C have been measured. The reaction of the 5-methylidene substrate is first order in both substrate and hydroxide ion. The reaction of the 5-methyl substrate is two-phase. The first and more rapid reaction of this substrate is 1.4 times faster than the only observed reaction of the former substrate. The second reaction is the hydrolysis of an intermediate, which is first order in substrate and has two components, one first and the other second order in hydroxide ion. The rate coefficients for the addition of a series of thiols to the 5-methylidene substrate in water at pH 7.40 have been measured. A Brønsted coefficient of ca. 0.37 has been found. The detailed reaction pathway and relation to Marcus and related theories are discussed.

The hydrolysis of azetidinyl amidinium salts. Part 1. The unimportance of strain release in the four-membered ring

The alkaline hydrolysis of azetidin-2-ylideneammonium salts gives a mixture of β-lactams, by exocyclic C–N bond fission, and β-amino amides, by opening the four-membered ring. Despite the anticipated release of strain energy in opening the four-membered ring the β-lactam is usually the major product i.e. exocyclic C–N fission is favoured over endocyclic C–N fission. This occurs even when the basicities of the endo- and exo-cyclic nitrogens are similar. The apparent reluctance of the four-membered ring to open is also not the result of entropic or stereoelectronic factors. The kinetics of the reaction indicate the presence of a neutral tetrahedral intermediate because there are two changes in the rate dependence upon hydroxide ion with increasing base concentration. There is also a term in the rate law for the carbonate-catalysed reaction which is both first order in carbonate ion and first order in hydroxide ion. The neutral tetrahedral intermediate must be formed reversibly and undergo deprotonation of its hydroxy group at high pH. By assuming that this deprotonation by hydroxide ion is rate limiting and diffusion controlled, the equilibrium constants for the formation of the neutral tetrahedral intermediate can be calculated. These are reported together with the calculated microscopic rate constants for the formation and breakdown of the intermediate.

The electrochemistry of aqueous solutions of 4-methoxy-1,2-benzoquinone

Using cyclic voltammetry, the electrochemistry of aqueous solutions of 4-methoxy-1,2-benzoquinone was investigated over the pH range 0–8. From the variation of half-wave potential with pH, it was inferred that the quinone was reduced to the hydroquinone; no acid-base reactions involving either the quinone or hydroquinone were observed. At low pH ( < 3), the cyclic voltammogram of the benzoquinone changed rapidly to a second pair of peaks at lower potentials. These were shown to be due to 2-hydroxy-1,4-benzoquinone, formed by hydrolysis of the original compound. The other hydrolysis product, methanol, was detected chromatographically. The hydrolysis reaction was shown to be first order both with respect to the 1,2-benzoquinone and protons. At pH 6 and above, 4-methoxy-1,2-benzoquinone showed a second type of decomposition whose rate was approximately first order in hydroxide ion. Using electrochemical and spectroscopic measurements, it was also demonstrated that 4-methoxy-1,2-benzoquinone was formed during the tyrosinase-catalysed oxidation of 4-methoxyphenol. This confirms that this quinone could be involved in the melanocytoxic action of 4-methoxyphenol.

The electrochemistry of aqueous solutions of 4-methoxy-1,2-benzoquinone

Using cyclic voltammetry, the electrochemistry of aqueous solutions of 4-methoxy-1,2-benzoquinone was investigated over the pH range 0–8. From the variation of half-wave potential with pH, it was inferred that the quinone was reduced to the hydroquinone; no acid—base reactions involving either the quinone or hydroquinone were observed. At low pH (<3), the cyclic voltammogram of the benzoquinone changed rapidly to a second pair of peaks at lower potentials. These were shown to be due to 2-hydroxy-1,4-benzoquinone, formed by hydrolysis of the original compound. The other hydrolysis product, methanol, was detected chromatographically. The hydrolysis reaction was shown to be first order both with respect to the 1,2-benzoquinone and protons. At pH 6 and above, 4-methoxy-1,2-benzoquinone showed a second type of decomposition whose rate was approximately first order in hydroxide ion. Using electrochemical and spectroscopic measurements, it was also demonstrated that 4-methoxy-1,2-benzoquinone was formed during the tyrosinase-catalysed oxidation of 4-methoxyphenol. This confirms that this quinone could be involved in the melanocytoxic action of 4-methoxyphenol.

Kinetic and equilibrium study of the ring opening of 2-aryl-1-methyl-1-pyrrolinium ions in aqueous solution

Ultraviolet spectral and kinetic measurements are reported for the transformations in aqueous solution undergone by the cyclic iminium ion, the 2-aryl-1-methyl-1-pyrrolinium ion I+. In solutions with pH &lt; 9.5 this ion equilibrates with a ring-opened hydrated form, the protonated 4-methylaminobutyrophenone SH+. The concentration of the latter at equilibrium is small, but it can be observed using nuclear magnetic resonance spectroscopy. A kinetic analysis provides a measure of the [Formula: see text] equilibrium constant. Values range from 0.06 for 4-methoxyphenyl to 0.17 for 3-chlorophenyl. In acid solutions (pH &lt; 5) the equilibration is slow (half-life ≈ 20 min for phenyl) and pH independent. Above pH 5 this process becomes first order in hydroxide ion. In strongly basic solutions a neutral species is formed in equilibrium with these two cations, with pK(app) for the transformation cations [Formula: see text] neutral + H+ ranging from 11.8 to 11.3. Spectral arguments imply that the base species is the ring-opened 4-methylaminobutyrophenone S, with less than ten percent of the pseudobase of I+ present.

Kinetics and equilibrium in the ammonolysis of substituted phthalimides

Kinetic studies are reported for the base hydrolysis to phthalamic acid anions (H) and ammonolysis to phthalamides (A) for seven phthalimides (P): 1, unsubstituted; 2, 4-NO2; 3, 4-Cl; 4, 4-t Bu; 5, 3-NO2; 6, 3-Me; 7, 3-Me3Si. The hydrolysis kinetics require two mechanisms, one which is first order in neutral imide and first order in hydroxide ion, and a second, which is important only in quite concentrated NaOH, which is first order in neutral phthalimide and second order in hydroxide ion. Ammonolysis kinetics for 1–5 revealed the rate law: Rate = kN [Unionized phthalimide] [NH3][OH−]. A mechanism is proposed with rate-determining breakdown of the anionic form of the tetrahedral intermediate derived by addition of NH3 to the phthalimide. The ammonolysis is reversible. The phthalamide hydrolyzes to the phthalamic acid via cyclization to an intermediate phthalimide, which is detected in concentrated base where its formation from phthalamide is more rapid than its subsequent hydrolysis. Rate constants for the cyclization follow the rate law: Rate = kcyc [Phthalamide][OH−]. This reaction is the microscopic reverse of the ammonolysis, and the ratio kN/kcyc provides the equilibrium constant Keq for the reaction P + NH3 = A. Values for 1–5 lie in the range 2 × 102 – 4 × 103. With 3-methylphthalimide, kinetics in aqueous ammonia do not obey a first-order relationship, but they could be analyzed by a scheme whereby the phthalimide is converted reversibly to the phthalamide and simultaneously undergoes an irreversible hydrolysis. The value of Keq in the system is 1.8. With 3-trimethylsilylphthalimide the value of Keq is further reduced to 0.01. The ammonolysis reaction does occur more quickly than hydrolysis but the equilibrium is so unfavorable that even in concentrated ammonia only a small amount of the phthalamide is ever formed.

β-Ureido acids and dihydrouracils. Part 15. Effect of allylic strain on ring opening of 1,6-disubstituted dihydrouracils

The rate profiles for the alkaline hydrolysis of some dihydrouracil and dihydro-orotic acid derivatives have been measured in order to assess the effect of allylic strain on the ring opening of 1,6-disubstituted dihydrouracils. The introduction of a 1-nitrogen-substituent in the 6-substituted compounds brings about a substantial decrease (40–500 times) in the observed rate constant which is second order in hydroxide ion (k1k3/k–1). The rate decreases of the addition step, k1, are moderate and in the range expected from the observed shifts in conformational equilibria towards the axial conformation which gives rise to a hindered transition state. The major contributions to the rate decreases arise from the ring-opening step, k3/k–1, and have been attributed to strains of the type associated with the gem-dimethyl effect upon ring closure.

Chemistry of technetium in tripolyphosphate medium

Electroreduction of TcO 4 − yields only technetium(III) in acidic tripolyphosphate, while above pH7 it yields technetium(IV) and technetium(III), the oxidation state being controlled by the electrode potential. Above pH 9.5, the limiting current undergoes progressive suppression, but addition of ammonia reverses this effect. The technetium(III)/(IV) couple, which is reversible by pulse polarography, does not involve either free protons or free tripolyphosphate. The technetium(IV) tripolyphosphate complex undergoes irreversible alkaline hydrolysis, in a reaction that is first order in hydroxide ion.

Reactions of alkylnitrosoureas in aqueous solution

The acid- and base-catalyzed decompositions of N-methyl-,N,N'-dimethyl-, and N,N',N'-trimethyl-N-nitrosourea in aqueous solution have been studied. Below pH 2, the N-methyl compound undergoes both denitrosation and hydrolysis to yield methylurea, nitrous acid, methylamine, nitrogen, and carbon dioxide. The acid-catalyzed denitrosation and hydrolysis of the trimethylnitrosourea are somewhat more rapid than the corresponding reactions of N-methyl-N-nitrosourea. The solvent isotope effect, k/sub H/sub 2/O//k/sub D/sub 2/O/ = 1.3, and the absence of chloride ion catalysis suggest that the denitrosation reaction proceeds by a rate-determining proton transfer which is followed by the rapid loss of the nitroso group. The results for the hydrolysis reaction are compatible with a formulation in which a hydrate of the nitrosourea is protonated in a rate-determining step to form a tetrahedral intermediate which subsequently decomposes to yield methyldiazonium hydroxide and a carbamic acid derivative. The base-catalyzed reactions of the mono, di-, and trimethylnitrosoureas are first order in hydroxide ion over a broad pH range. The hydrolysis of N-methyl-N-nitrosourea yields methanol and derivatives of carbamic acid. Salt effects on the reaction rate are negligible except for the influence of lithium ion. The rate constants for the hydrolysis of the mono- and dimethyl compounds depend upon the buffer concentrationat pH 9.5.more » The rate constants for the hydrolysis of the trimethyl compound also depend upon the buffer concentration, but a limiting value is not achieved. The solvent isotope effect for the base-catalyzed reaction, the exchange reaction of water-/sup 18/O with the carbonyl group of the urea, and the fact that N-methyl-N-nitrosourea is hydrolyzed about 2.2 x 10/sup 4/ times more rapidly than N,N',N'-trimethyl-N-nitrosourea suggest that the hydrolysis occurs by a mechanism in which a tetrahedral intermediate is formed.« less

ChemInform Abstract: THE HYDROLYSIS OF α‐ACETOXYSTYRENES. KINETICS AND INVESTIGATIONS OF OXYGEN‐18 EXCHANGE

The alkaline hydrolyses of ..cap alpha..-acetoxystyrenes 1a-f and 1-acetoxy-1-ethoxy-2-phenylethylene (2) have been shown to proceed by the same mechanism which has been demonstrated for the hydrolysis of alkyl and aryl acetates. Among the pieces of experimental evidence leading to this conclusion are the inverse solvent deuterium isotope effects (0.74 + 0.07 for 1c, 0.80 +- 0.09 for 2), the kinetics of hydrolysis, which are first order in hydroxide ion, and the absence of general base catalysis of hydrolysis. In mildly acidic solution, however, the hydrolysis of 2 proceeds exclusively by a mechanism involving a rate-determining proton transfer to the leaving group double bond, a mechanism which was previously demonstrated for ..cap alpha..-acetoxystyrenes in strongly acidic solution. Carbonyl labeled ..cap alpha..-acetoxystyrene-/sup 18/O as synthesized, and /sup 18/O exchange from the carbonyl position during alkaline hydrolysis was investigated; no /sup 18/O exchange was observed. This behavior is similar to that observed for aryl esters, and contrasts with that observed for hydrolysis of esters with less acidic leaving groups. These observations support our contention that acetophenone enols are about as acidic as phenols, a conclusion which, along with the fraction enol in acetophenone, leads to a carbon pK/sub a/ for acetophenone of 15.8more » +- 1.0.« less

A kinetic study of the hydrolysis of 2-germaacetate in neutral and alkaline solutions

In neutral and alkaline solutions, the 2-germaacetate ion decomposes to give germane and bicarbonate (or carbonate): GeH/sub 3/CO/sub 2//sup -/ + H/sub 2/O yields GeH/sub 4/ + HCO/sub 3//sup -/. The rate of this decarboxylation is pH independent and is first order in the germaacetate ion. The rate constant at 60/sup 0/C and ionic strength 1.0 is 1.62 x 10/sup -5/ s/sup -1/; the activation energy is 28.6 kcal/mol. In strongly alkaline solutions (especially at hydroxide concentrations above 1 M), a second decomposition reaction, producing hydrogen and the insoluble brown polymer H/sub 2/Ge/sub 2/O/sub 3/ . xH/sub 2/O, competes with the decarboxylation: 2GeH/sub 3/CO/sub 2//sup -/ + 2OH/sup -/ + (3 + x)H/sub 2/O yields 6H/sub 2/ + H/sub 2/Ge/sub 2/O/sub 3/ . xH/sub 2/O + 2CO/sub 3//sup 2 -/. The rate of the latter reaction is first order in germaacetate ion and is inversely proportional to the Hammett acidity function h, i.e., first order in hydroxide ion; -d ln (GeH/sub 3/CO/sub 2//sup -/)/dt = k(OH/sup -/). At 60/sup 0/C, k = 2.0 x 10/sup -5/ M/sup -1/ s/sup -1/; the activation energy is 20.8 kcal/mol. Probable mechanisms of the reactions are discussed.

The hydrolysis of .alpha.-acetoxystyrenes. Kinetics and investigations of oxygen-18 exchange

The alkaline hydrolyses of ..cap alpha..-acetoxystyrenes 1a-f and 1-acetoxy-1-ethoxy-2-phenylethylene (2) have been shown to proceed by the same mechanism which has been demonstrated for the hydrolysis of alkyl and aryl acetates. Among the pieces of experimental evidence leading to this conclusion are the inverse solvent deuterium isotope effects (0.74 + 0.07 for 1c, 0.80 +- 0.09 for 2), the kinetics of hydrolysis, which are first order in hydroxide ion, and the absence of general base catalysis of hydrolysis. In mildly acidic solution, however, the hydrolysis of 2 proceeds exclusively by a mechanism involving a rate-determining proton transfer to the leaving group double bond, a mechanism which was previously demonstrated for ..cap alpha..-acetoxystyrenes in strongly acidic solution. Carbonyl labeled ..cap alpha..-acetoxystyrene-/sup 18/O as synthesized, and /sup 18/O exchange from the carbonyl position during alkaline hydrolysis was investigated; no /sup 18/O exchange was observed. This behavior is similar to that observed for aryl esters, and contrasts with that observed for hydrolysis of esters with less acidic leaving groups. These observations support our contention that acetophenone enols are about as acidic as phenols, a conclusion which, along with the fraction enol in acetophenone, leads to a carbon pK/sub a/ for acetophenone of 15.8more » +- 1.0.« less

Kinetics and mechanism of the alkaline hydrolysis of guanidine, hexamethylguanidinium perchlorate, and tetramethylurea

The kinetics of the alkaline hydrolysis of tetramethylurea and hexamethylguanidinium perchlorate show terms second order in hydroxide ion. The mechanism probably involves the hydroxide ion catalysed decomposition of a tetrahedral intermediate. The rate of the alkaline hydrolysis of guanidine reaches a plateau at alkali concentrations >1M. Mechanisms involving either attack of hydroxide ion on the guanidinium cation or unimolecular decomposition of the free base are consistent with the kinetic data but unimolecular decomposition of the guanidinium anion, as observed for some carbamic acid esters and a guanidine derivative, is not. The attack of hydroxide ion on the guanidinium cation is thought unlikely as it cannot explain the 750-fold increase in rate over the hexamethyl derivative. This rate difference can be accounted for if hydrolysis proceeds through the free base as this route is inaccessible to the hexamethyl derivative. A unimolecular decomposition of guanidine free base would result in the production of cyanamide but this could not be detected, consequently a mechanism involving the attack of water on the free base is preferred.