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

Canadian Journal of Chemistry (Impact Factor: 1.01). 02/2011; 57(22):2960-2966. DOI: 10.1139/v79-481

ABSTRACT 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 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.

  • [Show abstract] [Hide abstract]
    ABSTRACT: It has been shown recently that most ethers hydrolyze in aqueous acid media not by the traditional A1 or A2 process, but by a mechanism involving rate-determining proton transfer to the substrate, concerted with C-O bond cleavage. The reactions of azoethers are more complicated, because the azo group can be protonated in the acid reaction medium as well. This protonation has to be accounted for in the kinetic analysis. Often it simply ties up the substrate in an unreactive form; the hydrolysis reaction slows down as a result of the azo-protonated compound not being the reactant in the hydrolysis. However, there are other possibilities. If the ether group is suitably located in the substrate the azo-protonated compound can react with three water molecules (a "water wire") in a fast reaction, and the alkoxy group is lost as a result. Depending on the acidity, in this mechanism either the initial three-water attack, or the breakup of the resulting intermediate, can be rate-determining, and both of these were observed. A third possibility is that ring protonation of suitable substrates can occur, giving delocalized carbocations that can form a hydrolysis product in subsequent fast reactions. Thus, three different hydrolysis mechanisms for azoethers in acidic media can be observed. Six azoethers were studied, one of which contained two methoxy groups. Both of these hydrolyzed, but by different mechanisms.
    Canadian Journal of Chemistry 10/2012; 90(10):791-797. · 1.01 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The disappearance of 3- and 1-acetoxynortricyclanes (1 and 2) in aqueous perchloric acid was followed by capillary gas chromatography at different temperatures and acid concentrations. According to the activation parameters, solvent deuterium isotope effects and parameters of excess acidity equations, the AAC2 ester hydrolysis with two water molecules in the transition state is dominant at the lower acid concentrations studied (1–5.5 M HClO4) and the AdE2 hydration of the cyclopropane ring is dominant at higher acid concentrations (6–8 M HClO4) at 298 K. 3-Nortricyclanol (3) is formed via hydrolysis from 1, whose hydration products were not analyzed. 2-Norbornanone (4) is formed via both hydrolysis and hydration from 2. Copyright © 2000 John Wiley & Sons, Ltd.
    Journal of Physical Organic Chemistry 02/2000; 13(2):133-138. · 1.23 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The mechanism of acidâcatalyzed hydrolysis of a series of pâsubstituted N,Nâ²âdiarylsulfamides was investigated in aqueous mineral acid solutions. Rate profiles, reaction activation parameters, catalytic order of strong acids, solvent isotope effects, and analysis of the kinetic data by the excess acidity method suggest a change in the mechanism from A2 to A1. While the hydrolysis proceeds with an A2 mechanism in low acidity regions, an A1 mechanism takes place in high acid concentrations. Copyright © 2013 John Wiley & Sons, Ltd. The mechanism of N,Nâ²âdiarylsulfamides was investigated in high aqueous mineral acid solutions at elevated temperatures. Various criteria, such as the excess acidity method, the values of the reaction activation entropies, and the kinetic solvent isotope effect were applied to the kinetic data obtained by using the UVâ“Visible method. The data consistently suggest an A2 mechanism in low acid concentrations (up to 13â‰M) and an A1 mechanism in high acid concentrations (above 13â‰M).
    Journal of Physical Organic Chemistry 02/2014; 27(2). · 1.23 Impact Factor


Available from