First-order kinetics esters

In one of the earliest kinetic studies of an organic reaction earned out m the nine teenth century the rate of hydrolysis of ethyl acetate m aqueous sodium hydroxide was found to be first order m ester and first order m base... 

Bobrowski and Das33 studied the transient absorption phenomena observed in pulse radiolysis of several retinyl polyenes at submillimolar concentrations in acetone, n -hexane and 1,2-dichloroethane under conditions favourable for radical cation formation. The polyene radical cations are unreactive toward oxygen and are characterized by intense absorption with maxima at 575-635 nm. The peak of the absorption band was found to be almost independent of the functional group (aldehyde, alcohol, Schiff base ester, carboxylic acid). In acetone, the cations decay predominantly by first-order kinetics with half life times of 4-11 ps. The bimolecular rate constant for quenching of the radical cations by water, triethylamine and bromide ion in acetone are in the ranges (0.8-2) x 105, (0.3-2) x 108 and (3 — 5) x 1010 M 1 s 1, respectively. 

Explain, why hydrolysis of an ester follows first-order kinetics in presence of an acid and it follows a second-order kinetics in presence of dilute alkali. 

The L-dopa esters were also examined for their enzymatic hydrolysis in human plasma and/or by purified pig liver carboxylesterase (EC 3.ELI Table 8.1). In human plasma under the conditions of study, hydrolysis again followed pseudo-first-order kinetics. In all but two cases examined, enzymatic hydrolysis was slightly faster than chemical hydrolysis. For the methyl ester, rates of chemical and enzymatic hydrolysis were comparable, whereas the /erf-butyl ester was not hydrolyzed in plasma and was protected from chemical hydrolysis presumably by becoming bound to plasma proteins. 

In previous chapters, we discussed the hydrolysis of a number of esters of A-(hydroxymethyl)phcnytoin, namely esters of organic acids (7V-acyloxy-methyl derivatives, Sect. 8.7.3) or inorganic acids (Sect. 9.3.2). Hydrolysis of these potential prodrugs released 3-(hydroxymethyl)phenytoin (11.45), whose breakdown to phenytoin and formaldehyde was also investigated per se [79], The latter reaction followed pseudo-first-order kinetics. At pH 7.4, the f1/2 values were 4.7 and 1.6 s at 25° and 37°, respectively. The tm values decreased tenfold for each increase of pH by one unit, which, together with the absence of any buffer catalysis, indicates catalysis by the HO- anion. 

Under these conditions we expected and observed pseudo-first order kinetic production of PNP from OP esters. Kinetic solutions also contained the following 0.1 x 10 mol dm dlsodlum ethylene dlammlnetetraacetate, 0.1 mol dm buffer (sodium borate unless otherwise specified), and NaClO added to bring the solution to Ionic strength 0.50 mol dm. ... 

The reactions obeyed pseudo-first-order kinetics consistent with a rapid reversible protonation of the substrate, S, at the ester carbonyl followed by a rate-determining decomposition to acetic acid and nitrenium ion according to Scheme 19. In accordance with equation 13, the pseudo-first-order rate constant, k, was shown to be proportional to acid concentration and inversely proportional to the activity of the water/acetonitrile solvent . 

Many drugs are quite stable but functional groups such as esters and lactam rings which occur in some drugs are susceptible to hydrolysis and functional groups such as catechols and phenols are quite readily oxidised. The most common types of degradation which occur in pure and formulated drugs obey zero or first order kinetics. 

It was proposed that propagation proceeds by reaction of carboxyl anion with epoxide to form an ester-alkoxide anion which can react in turn with anhydride to form ester and another carboxyl anion. The rate-determining step was thought to be the reaction of carboxyl anion with epoxide, which would lead to first-order kinetics with respect to epoxide concentration. 

The catalytic hydrogenation of linoleic esters has also been treated kinetically by Boelhouwer et al,88. If the velocity constants of the consecutive reactions are represented by kx and k2 and first order kinetics are assumed, the following equations hold ... 

The NH acidities of some sterically hindered ureas, namely the ureido esters (93), have been reported.81 The kinetics and mechanism of the alkaline hydrolysis of urea and sodium cyanate, NaCNO, have been studied at a number of temperatures.82 Urea hydrolysis follows an irreversible first-order consecutive reaction path. Tetrahedral intermediates are not involved and an elimination-addition mechanism operates. Sodium cyanate follows irreversible pseudo-first-order kinetics. The decomposition of the carcinogen /V-mcthyl-/V-nitrosourca (19) was dealt with earlier.19 The pyrolysis of /V-acctylurca goes by a unimolecular first-order elimination reaction.83... 

We shall return to this important mechanism in a moment after a brief mention of first-order kinetics. The reaction between the acid chloride and the neutral alcohol to give an ester may not have the bimolecular rate expression expected for this mechanism rate = /cfR COCIH OH]. 

In the aminolysis of esters 6, 8a, 8b, and 9a-9d with the amine 5a, no evidence was obtained for mieelle or complex formation (Bruice el al., 1968). The rate of disappearanee of the esters obeyed pseudo-first order kinetics at [5a] > [ester] and eonstant pH and could be expressed... 

The hydrolysis of steroid hydrogen sulphates, important to biochemists in view of the excretion of many steroid hormone metabolites as water soluble salts of hydrogen sulphates, has been studied in some detail [20,21]. It had been known for some years that ether extraction of the steroid hydrogen sulphate from acidified solutions is accompanied by hydrolysis of the sulphate in the ether phase, with liberation of the steroid alcohol. This hydrolysis was shown to follow first-order kinetics, but to be dependent in a remarkable way on the solvent. Ethers are the most effective solvents, followed by others of low polarity including benzene, esters and ketones, while water and alcohols have a strong retarding effect on hydrolysis. Neither acids nor bases produce significant catalysis, and the presence of did not result in incorporation of isotopic... 

The solvated ion pair [(C8Hi7)3NMe] [RhCl4]", formed from aqueous rhodium trichloride and Aliquat-336 in a two-phase liquid system, hydrogenates a,p-unsaturated ketones and esters selectively at the C==C double bond. The reduction of benzylideneacetone follows first-order kinetics in substrate below 0.2 M, and approaches second-order in hydrogen at partial pressures below 0.12 atm (1 atm = 101.3 kPa). The catalysis also depends on the nature of the solvent, the phase transfer catalyst and stirring rates. 

Decompositions ot benzyl acetate" , benzyl benzoate and a series of allyl acetate esters have been studied by the toluene carrier flow technique. First-order kinetics for reactant disappearance or for CO2 formation were assumed to apply to the initial bond rupture processes... 

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