The hydrolyses of benzamides, methylbenzimidatium ions, and lactams in aqueous sulfuric acid. The excess acidity method in the determination of reaction mechanisms

1981 ◽  
Vol 59 (19) ◽  
pp. 2853-2863 ◽  
Author(s):  
Robin A. Cox ◽  
Keith Yates
Keyword(s):  
Sulfuric Acid ◽  
Water Molecule ◽  
Reaction Pathway ◽  
Sulfuric Acid Concentration ◽  
Strong Acid ◽  
Hydrolysis Rate ◽  
Reaction Conditions ◽  
Acid Media ◽  
Hydrolysis Of

The excess acidity method has been applied to hydrolysis rate data for a number of benzamides, methylbenzimidatium ions, and lactams, obtained as a function of sulfuric acid concentration and temperature. All of the substrates studied except β-propiolactam (8) and methyl-2,6-dimethylbenzimidatium ion (7) were found to follow the AOT2 mechanism at all acidities. The excess acidity method provided considerable mechanistic detail; in dilute acid the transition state contains O-protonated (or methylated) substrate and three water molecules (large negative ΔS≠), but in more concentrated solutions a one-water-molecule mechanism takes over (smaller negative ΔS≠). In strong acid bisulfate ion acts as the nucleophile (positive ΔS≠). N-protonated intermediates are not involved for "normal" substrates, being observed in this work only for 8, which follows the AND1 pathway. Observed differences between benzamide and methylbenzimidatium ion (4) hydrolyses are due to their differing activity coefficient behaviour, the mechanism being the same for both. The hydrolysis of 7 involves a one-water-molecule SN2 displacement at the O-methyl group. Comparison with 7 shows that this displacement is not likely to occur under the reaction conditions for 4; however, for the N-methyl and N,N-dimethyl derivatives studied it is probably an important reaction pathway. A comprehensive mechanistic framework for amide hydrolyses in strong acid media is given.

Hydrolysis of MgH2 in MgCl2 solutions as an effective way for hydrogen generation

2021 ◽  
pp. 38-52
Author(s):  
V. Berezovets ◽  
◽  
A. Kytsya ◽  
Yu. Verbovytskyy ◽  
I. Zavaliy ◽  
...  
Keyword(s):  
Hydrogen Generation ◽  
Strong Acid ◽  
Magnesium Hydride ◽  
Hydrolysis Reaction ◽  
Hydrolysis Rate ◽  
Reaction Yield ◽  
Buffer Solution ◽  
Passivation Film ◽  
Products Of Reaction ◽  
Hydrolysis Of

Magnesium hydride (MgH2) has a high hydrogen storage capacity (7.6 wt%) and the Mg element is abundant on the earth. Due to its strong reduction ability, even at room temperature it can provide the hydrogen yield reaching 15.2 wt% H (1703 mL/g) when interacting with water, which makes it very attractive for the application in supplying hydrogen for autonomous H energy systems. However, the hydrolysis reaction is rapidly inhibited by the Mg(OH)2 passivation layer formed on the surface of MgH2. In order to remove the passivation film and improve the efficiency of the MgH2 hydrolysis process, several methods including alloying, ball milling, changing the aqueous solution, have been successfully utilized. In this paper the process of hydrolysis of magnesium hydride in aqueous solutions of MgCl2 used as a promotor of the interaction has been studied in detail. It was found that the initial hydrolysis rate, pH of the reaction mixture, and overall reaction yield are all linearly dependent of the logarithm of MgCl2 concentration. It has been shown that pH of the reaction mixture in the presence of MgCl2 is well described by considering a system “weak base and its salt with strong acid” type buffer solution. Reference data for this hydrolysis reaction were also carefully analyzed. The mechanism and the kinetic model of the process of MgH2 hydrolysis in water solutions involved passivation of the MgH2 surface by the formed Mg(OH)2 precipitate followed by its repassivation have been proposed. The obtained after the hydrolysis reactions precipitates were studied using XRD and EDS. It was found also that the final products of reaction consist of Mg(OH)2 (brucsite type) and remaining MgH2. This fact shows that the formation of solid species such as MgCl2 xMgO yH2O at the studied conditions is unlikely and decreasing of pH the reaction mixture has a different nature.

A comparison of the mechanisms of hydrolysis of benzimidates, esters, and amides in sulfuric acid media

2005 ◽  
Vol 83 (9) ◽  
pp. 1391-1399 ◽  
Author(s):  
Robin A Cox
Keyword(s):  
Sulfuric Acid ◽  
Isotope Effects ◽  
Ester Hydrolysis ◽  
Water Molecules ◽  
Acid Media ◽  
Tetrahedral Intermediate ◽  
Amide Hydrolysis ◽  
Solvent Isotope ◽  
Reaction Media ◽  
Hydrolysis Of

The mechanisms given in textbooks for both ester and amide hydrolysis in acid media are in need of revision. To illustrate this, benzimidates were chosen as model compounds for oxygen protonated benzamides. In aqueous sulfuric acid media they hydrolyze either by a mechanism involving attack of two water molecules at the carbonyl carbon to give a neutral tetrahedral intermediate directly, as in ester hydrolysis, or by an SN2 attack of two water molecules at the alkyl group of the alkoxy oxygen to form the corresponding amide, or by both mechanisms, depending on the structure of the benzimidate. The major line of evidence leading to these conclusions is the behavior of the excess acidity plots resulting from the rate constants obtained for the hydrolyses as functions of acid concentration and temperature. The first of these mechanisms is in fact very similar to one found for the hydrolysis of benzamides, as inferred from: (1) similar excess acidity plot behaviour; and (2) the observed solvent isotope effects for amide hydrolysis, which are fully consistent with the involvement of two water molecules, but not with one or with three (or more). This mechanism starts out as essentially the same one as that found for ester hydrolysis under the same conditions. Differences arise because the neutral tetrahedral intermediate, formed directly as a result of the protonated substrate being attacked by two water molecules (not one), possesses an easily protonated nitrogen in the amide and benzimidate cases, explaining both the lack of 18O exchange observed for amide hydrolysis and the irreversibility of the reaction. Protonated tetrahedral intermediates are too unstable to exist in the reaction media; in fact, protonation of an sp3 hybridized oxygen to put a full positive charge on it is extremely difficult. (This means that individual protonated alcohol or ether species are unlikely to exist in these media either.) Thus, the reaction of the intermediate going to product or exchanged reactant is a general-acid-catalyzed process for esters. For amide hydrolysis, the situation is complicated by the fact that another, different, mechanism takes over in more strongly acidic media, according to the excess acidity plots. Some possibilities for this are given.Key words: esters, amides, benzimidates, hydrolysis, excess acidity, mechanism, acid media.

Mechanistic studies in strong acids. VIII. Hydrolysis mechanisms for some thiobenzoic acids and esters in aqueous sulfuric acid, determined using the excess acidity method

1982 ◽  
Vol 60 (24) ◽  
pp. 3061-3070 ◽  
Author(s):  
Robin A. Cox ◽  
Keith Yates
Keyword(s):  
Sulfuric Acid ◽  
Proton Transfer ◽  
Strong Acid ◽  
Weak Acid ◽  
Medium Composition ◽  
Hydrolysis Rate ◽  
Water Molecules ◽  
Aqueous Sulfuric Acid ◽  
Ion Formation ◽  
Rate Determining Step

The excess acidity method has been applied to hydrolysis rate data, obtained as a function of medium composition, for four thiobenzoic acids, thioacetic acid, eight ethyl thiolbenzoates, and eight ethyl thionbenzoates in aqueous sulfuric acid. The mechanistic behaviour thus revealed has both similarities to and differences from that of a typical ester like ethyl benzoate, which gives benzoic acid by an A-2 reaction involving two water molecules in weak acid, and by A-1 acylium ion formation in strong acid. The thioacids follow this behaviour, except that the A-2 process involves three water molecules, and that the mechanistic changeover occurs in 60% rather than 80% acid. The A-2 process for the ethyl thiolbenzoates is slow; the major hydrolysis mechanism is acylium ion formation, not in an A-1 reaction but by a concerted A-SE2 process involving both proton transfer to sulfur and carbon–sulfur bond breaking. The major proton transfer agent is the undissociated sulfuric acid molecule. The thionbenzoate esters, in contrast, undergo very fast A-2 hydrolysis; so fast, in fact, that the initial protonation of sulfur is the rate-determining step in acids more dilute than about 62% w/w. It appears that proton transfer to sulfur is a comparatively slow process.

scholarly journals Bacterial cellulose production by Acetobacter xylinum CGMCC 1.2378 using coconut shell acid hydrolysate as carbon source

BioResources ◽  
2020 ◽  
Vol 16 (1) ◽  
pp. 1042-1062
Author(s):  
Yu Liu ◽  
Fangfang Wang ◽  
Yangyang Sun
Keyword(s):  
Sulfuric Acid ◽  
Carbon Source ◽  
Bacterial Cellulose ◽  
Sulfuric Acid Concentration ◽  
Potential Method ◽  
Reducing Sugar ◽  
Acetobacter Xylinum ◽  
Coconut Shell ◽  
Acid Hydrolysate ◽  
Hydrolysis Of

Bacterial cellulose (BC) was synthesized by Acetobacter xylinum using a carbon source of coconut shell hydrolysate, which was treated with an ultra-low concentration of sulfuric acid. The coconut shell was found to contain 57.13% holocellulose and 27.42% lignin. The effect of sulfuric acid concentration, reaction temperature, and reaction time on hydrolysis of coconut shell were evaluated by response surface methodology. The reducing sugar concentration was 8.39 g/L under the predicted optimum treatment at 200 °C for 32 min with a solution of 0.07% sulfuric acid. The holocellulose conversion rate was 56.1% at this condition. In a detoxification process using calcium hydroxide and activated carbon, furfural and hydroxymethylfurfural in the hydrolysate were almost completely removed, whereas formic acid and acetic acid levels decreased by 30%. After cultivation for 7 days at the reducing sugar status of 5 g/L, the BC production in medium with the detoxified hydrolysate could reach 1.66 g/L. After fermentation for 21 days, BC yield in medium using composited carbon source (20 g/L) of glucose and hydrolysate was 5.30 g/L. Structural analysis showed that BC obtained from medium of control and detoxified hydrolysate exhibited similar properties. This work provided a potential method for BC production.

The hydrolysis of thioacetic, thiobenzoic, and three substituted thiobenzoic acids in perchloric and sulfuric acids

1978 ◽  
Vol 56 (7) ◽  
pp. 935-940 ◽  
Author(s):  
John T. Edward ◽  
Graeme Welch ◽  
Sin Cheong Wong
Keyword(s):  
Sulfuric Acid ◽  
Activity Coefficients ◽  
Acid Concentration ◽  
Perchloric Acid ◽  
Transition States ◽  
Sulfuric Acid Concentration ◽  
Thiobenzoic Acid ◽  
The Difference ◽  
Different Response ◽  
Hydrolysis Of

The rates of hydrolysis of thioacetic, thiobenzoic, and three substituted thiobenzoic acids increase with concentration of solvent sulfuric or perchloric acid to a maximum in 30–40% acid and then decrease. Yates–McClelland r, Bunnett–Olsen [Formula: see text], and Hammett ρ parameters, and entropies of activation indicate an AAC2 mechanism over this range of acid concentrations. In acid concentrations above 50–60% the rates increase sharply and the same mechanistic criteria now indicate an AAc1 mechanism. The difference between the rate–acidity profile of thiobenzoic acid and that of ethyl thiolbenzoate can be explained by the different response of the activity coefficients of their transition states to increase in sulfuric acid concentration.

Lactams in sulfuric acid. The mechanism of amide hydrolysis in weak to moderately strong aqueous mineral acid media

1998 ◽  
Vol 76 (6) ◽  
pp. 649-656 ◽  
Author(s):  
Robin A Cox
Keyword(s):  
Sulfuric Acid ◽  
Water Molecule ◽  
Kinetic Method ◽  
Activation Parameters ◽  
Mineral Acid ◽  
Medium Effect ◽  
Acid Media ◽  
Unknown Mechanism ◽  
Reaction Rate Constants ◽  
The One

Reaction rate constants obtained in moderately concentrated sulfuric acid for the hydrolysis of simple lactams of ring sizes five, six, seven, and eight as a function of acidity and temperature have been analyzed using the excess acidity kinetic method. The basicity constants for these substrates have been recalculated; the 13C NMR spectra used to obtain these values are very sensitive to medium effects. It was found that the basicities of the lactams at 0.003-0.1 M lactam concentration were over half a pK unit more basic than they were at 0.5 M lactam, presumably because of the medium effect. Apart from this, the rate constant results obtained at different times by different groups using different techniques for monitoring the kinetics are in adequate agreement. The excess acidity analysis showed that the kinetics could be fitted according to the "three-water-molecule followed by one-water-molecule" mechanistic scenario previously found, or could just as well be fitted by a "one-water-molecule followed by unknown mechanism" scenario, with the mechanistic change taking place at 50 wt.% sulfuric acid for all the substrates. Other evidence makes the latter seem the more likely possibility of the two, and activation parameters based upon the "one-water-molecule" process were determined. Sufficient data points to enable the unknown mechanism to be established were not present; possible mechanisms applicable in media more concentrated than 50 wt.% sulfuric acid are discussed. Previously obtained values of the parameter r, the number of water molecules involved with the substrate in A2 processes, are now questionable.Key words: amides, lactams, excess acidity, hydrolysis, mechanism.

Determination of Sulfuric Acid Concentration for Anti-Cavitation Characteristics of Al Alloy by Two Step Anodizing Process to Forming Nano Porous

2014 ◽  
Vol 14 (12) ◽  
pp. 9572-9578
Author(s):  
Seung-Jun Lee ◽  
Seong-Kweon Kim ◽  
Jae-Yong Jeong ◽  
Seong-Jong Kim
Keyword(s):  
Sulfuric Acid ◽  
Acid Concentration ◽  
Sulfuric Acid Concentration ◽  
Al Alloy

The hydrolyses of some sterically crowded benzoate esters in sulfuric acid. The excess acidity method at different temperatures

1979 ◽  
Vol 57 (22) ◽  
pp. 2960-2966 ◽  
Author(s):  
Robin A. Cox ◽  
Malcolm F. Goldman ◽  
Keith Yates
Keyword(s):  
Sulfuric Acid ◽  
Activation Enthalpy ◽  
Sulfuric Acid Concentration ◽  
Hydrolysis Rate ◽  
Water Molecules ◽  
Steric Strain ◽  
Methyl Substitution ◽  
Different Temperatures ◽  
Acid Catalyzed ◽  
Aqueous Standard

The excess acidity method has been used to analyze the observed acid-catalyzed hydrolysis rate constants for methyl benzoate, methyl para-toluate, methyl ortho-toluate, and methyl 2,6-dimethylbenzoate, over a wide sulfuric acid concentration range, at several different temperatures. Enthalpies and entropies of activation in the aqueous standard state are reported, with slope parameters m≠ also given are the [Formula: see text] and m* values found for the protonation of these compounds. The mechanistic changeover from AAc-2 to AAc-1 hydrolysis occurs at lower acidity with increasing methyl substitution, mainly due to the decrease in activation enthalpy in the transition state for the AAc-1 process, caused by release of steric strain and increased mesomeric interaction. The AAc-2 hydrolysis involves two water molecules, and is energetically favourable and entropically unfavourable. The AAc-1 reaction is difficult energetically, but this is offset by the large positive activation entropies found.

Evaluation of the effect of substrate amount, sulfuric acid concentration and reaction temperature on acidic hydrolysis of cellulose casings applying a central composite design

2012 ◽  
Vol 29 ◽  
pp. S205-S206
Author(s):  
Jesús J. Vidal-Vidal ◽  
Guillermo Arzate-Martínez ◽  
Lizzette Moreno-García ◽  
Oscar M. Portilla-Rivera
Keyword(s):  
Sulfuric Acid ◽  
Central Composite Design ◽  
Acid Concentration ◽  
Reaction Temperature ◽  
Sulfuric Acid Concentration ◽  
Acidic Hydrolysis ◽  
Hydrolysis Of Cellulose ◽  
Hydrolysis Of ◽  
Central Composite

Process’ Optimization of Nanocrystalline Cellulose and its Properties

2012 ◽  
Vol 200 ◽  
pp. 373-376 ◽  
Author(s):  
Ping Ping Zou ◽  
Ping Zhang ◽  
De Gao ◽  
Qiao Na Xia
Keyword(s):  
Thermal Properties ◽  
Sulfuric Acid ◽  
Microcrystalline Cellulose ◽  
Freeze Drying ◽  
Sulfuric Acid Concentration ◽  
Nanocrystalline Cellulose ◽  
Electronic Microscopy ◽  
Hydrolysis Time ◽  
Reaction Conditions ◽  
Hydrolysis Temperature

This paper concerned nanocrystalline cellulose(NCC)’s isolation from microcrystalline cellulose(MCC) by sulfuric acid hydrolysis, meanwhile centrifuging, ultrasonic treatment and freeze-drying were carried out afterwards. It covered the effect of reaction conditions on the yield of NCC. It indicated that hydrolysis time was the factor that matters most and it was possible to obtain the largest yield with hydrolysis time being 108min, hydrolysis temperature being 43oC and sulfuric acid concentration being 33%. It also investigated the microcosmic morphology of NCC by Transmission Electronic Microscopy(TEM). MCC and NCC’s thermal properties were studied further.

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