Theoretical calculation of steric effects in ester hydrolysis

Journal Article · Wed Dec 08 04:00:00 EST 1976 · J. Am. Chem. Soc.; (United States)

DOI:https://doi.org/10.1021/ja00441a003· OSTI ID:7328431

DeTar, D F; Tenpas, C J

Steric effects in aliphatic reaction series may be represented quantitatively by E/sub s/ values derived empirically by Taft some 20 years ago. E/sub s/ = -log k/sub rel/ = -log k/k/sub 0/ where k is the rate constant for acid-catalyzed hydrolysis of an aliphatic ester RCOOEt and k/sub 0/ is the corresponding rate constant for CH/sub 3/COOEt. We find that log k/sub rel/ = 0.340--0.789 ..delta..SE where ..delta..SE = steric energy of RC(OH)/sub 1/--steric energy of RCOOH. Steric energies were calculated by molecular mechanics using minor modifications of the Schleyer 1973 hydrocarbon force field; ..delta..SE values closely parallel ..delta delta..H/sup 0//sub f/ values, the differences in the corresponding enthalpies of formation. The standard deviation of E/sub s/ is 0.24 and the correlation coefficient is -0.98 for 25 esters spanning a range of 4000 in relative rates. Rates have also been calculated for a further group of 20 esters for which only qualitative information is available, and these extend the range of predicted relative rates to about 800,000. While the Taft E/sub s/ values have been generally accepted as a measure of steric effects, there has always been a concern that this empirical dissection of steric and polar effects might not be clean. The success of the present theoretical calculations lends strong support to the hypothesis that E/sub s/ values do in fact measure steric effects for alkyl groups.

• Research Organization: Florida State Univ., Tallahassee
• OSTI ID: 7328431
• Journal Information: J. Am. Chem. Soc.; (United States), Journal Name: J. Am. Chem. Soc.; (United States) Vol. 98:25; ISSN JACSA
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400301* -- Organic Chemistry-- Chemical & Physicochemical Properties-- (-1987)
CALCULATION METHODS
CARBOXYLIC ACID ESTERS
CHEMICAL REACTION KINETICS
CHEMICAL REACTIONS
DECOMPOSITION
ENTHALPY
ESTERS
FORMATION HEAT
HYDROLYSIS
KINETICS
LYSIS
ORGANIC COMPOUNDS
PHYSICAL PROPERTIES
REACTION HEAT
REACTION KINETICS
SOLVOLYSIS
STEREOCHEMISTRY
THERMODYNAMIC PROPERTIES