General Acid Catalysis in Acetal Hydrolysis

Nimmo, Keith (1971) General Acid Catalysis in Acetal Hydrolysis. PhD thesis, University of Glasgow.

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Recent advances in the study of acetal hydrolysis and the use of acetals in model systems for the investigation of enzymically catalysed hydrolysis are reviewed. The kinetics of the hydrolysis of benzaldehyde substituted (X) - phenyl methyl acetals and substituted (Y)-benzaldehyde phenyl methyl acetals were measured in pivalate , acetate, d1-deutero-acetate, beta-chloropropionate, formate and chloroacetate buffers, and in hydrochloric and deutero-chloric acids. The hydrolysis rates measured in buffer solutions, of the twelve acetals studied were found to be dependent on the concentration of undissociated acid and shown to be authentic examples of general acid catalysis, with reference to current opinions that such observations might be manifestations of solvent, electrolyte or buffer effects. The d1-acetic acid catalysed d4-methanolysis of benz-aldehyde phenyl methyl acetal was followed in a high resolution N. M. R. spectrometer and the initial bond fission found to occur between carbon and the phenolic oxygen. It seemed not unreasonable to assume that the same process occurs also in the hydrolysis reaction. Bronsted and Hammett linear free energy relationships were found to correlate the ca catalytic constants obtained from the hydrolysis of both (X) and (Y) substituted-acetyl series. The catalytic constants of the substituted (X)-phenyl acetals increased as the electron-withdrawing power of (X) increased, and the positive p value increased consistently as the catalytic power of the buffer decreased, viz. 0.43 in chloroacetate to 1.25 in pivalate. The P value for the hydronium ion catalysed reaction was -0.45. Similarly, the negative p values obtained from the substituted (X) - benzaldehyde acetals increased in magnitude from. -2.04 in chloroacetate to -2.34 in pivalate, with a value of -1.94 for the hydronium ion catalysed reaction. As the electron-withdrawing power of (X) increased, the Bronsted value decreased consistently from 0.96 for X= p-MeO to 0.49 for X = m - NO2. Conversely as the electron-withdrawing power of (Y) increased, the a value increased from 0.63 for Y=p - 0 - MeO to 1.05 for m - Solvent isotope effects k(H3O)/k(D3O+), indicated not only a consistency of mechanism in. the hydrolysis of all the substrates studied, but the trend in. values, from 0.62 for X = p-MeO to 1.1 for X = m - NO2, and 0. 83 for Y = n-MeO to 0.66 for Y = m - F, correlated with the a and p values in determining the relative amount of bond making and braking in the transition states. The results obtained were all indicative of an A-SE2 mechanism in the general acid catalysed hydrolysis of aromatic aryl methyl acetals, with concerted proton transfer to phenolic oxygen and carbon-phenolic oxygen bond fission in the rate limiting step and where incipient carbonium ion stability and leaving group ability, rather than a high relative degree of proton transfer in the transition state, were commensurate to faster hydrolysis rates. The comparison of this data with that obtained in. other reaction series indicates that incipient carbonium ion stability and leaving group ability are prerequisites for the observation. of general acid catalysis in acetal hydrolysis, although relief of steric strain through carbonium ion formation is also relevant. The hydrolysis of benzaldehyde methyl acetyl acylal, the possible intermediate of acetate anion nucleophilic attack on benzaldehyde phenyl methyl acetal, was found to be general acid catalysed, and is believed to be the first reported example of buffer catalysis in acylal hydrolysis. The hydrolysis of p-methylbenzaldehyde methyl S-phenyl thioacetal was carried out in weak acid buffers, and the results suggested, although not conclusively, that this substrate might also be catalysed by undissociated acid.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Organic chemistry
Date of Award: 1971
Depositing User: Enlighten Team
Unique ID: glathesis:1971-78568
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 30 Jan 2020 15:10
Last Modified: 30 Jan 2020 15:10

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