Calculations of steric hindrance in ester hydrolysis based on estimation of van der Waals strain energies of alkanes. [Gas phase; 298/sup 0/K]
Journal Article
·
· J. Am. Chem. Soc.; (United States)
Steric hindrance is traditionally defined by the Taft E/sub s/ values, which are derived empirically from rates of acid-catalyzed hydrolysis of esters: E/sub s/ = --log (k/k/sub 0/), where k is the rate constant for any ester RCOOEt and k/sub 0/ is for ethyl acetate. In this paper, the utility of hydrocarbon models for calculating E/sub s/ for R alkyl and cycloalkyl is explored. The postulate is that E/sub s/ is proportional to ..delta delta..H = (..delta..H/sub f//sup 0/ (neoalkane) -- ..delta..H/sub f//sup 0/ (isoalkane)), where neoalkane is RC(CH/sub 3/)/sub 3/. The neoalkane is intended to reflect the degree of steric strain in the tetrahedral intermediate and the isoalkane that is in the ester. Empirical procedures for estimating ..delta..H/sub f//sup 0/ prove relatively useless for these calculations. However, ..delta..H/sub f//sup 0/ estimated from molecular mechanics does provide good values: for example, E/sub s/ = 4.419 + 0.552..delta delta..H has a standard deviation of 0.4 for a total range of 4, and the correlation coefficient is 0.95 for 24 esters.
- Research Organization:
- Florida State Univ., Tallahassee
- OSTI ID:
- 7267349
- Journal Information:
- J. Am. Chem. Soc.; (United States), Journal Name: J. Am. Chem. Soc.; (United States) Vol. 98:15; ISSN JACSA
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400301* -- Organic Chemistry-- Chemical & Physicochemical Properties-- (-1987)
ACETIC ACID ESTERS
ALKANES
ATOMS
BUTANE
CALCULATION METHODS
CARBOXYLIC ACID ESTERS
CHEMICAL BONDS
CHEMICAL REACTION KINETICS
CHEMICAL REACTIONS
COMPUTER CODES
CYCLOALKANES
DECOMPOSITION
DISTRIBUTION
ENTHALPY
ESTERS
ETHANE
FORMATION HEAT
HEPTANE
HEXANE
HYDROCARBONS
HYDROLYSIS
KINETICS
LYSIS
M CODES
OCTANE
ORGANIC COMPOUNDS
PENTANE
PHYSICAL PROPERTIES
PROPANE
REACTION HEAT
REACTION KINETICS
SOLVOLYSIS
SPATIAL DISTRIBUTION
STRAINS
STRUCTURAL MODELS
THERMODYNAMIC PROPERTIES
VAN DER WAALS FORCES
400301* -- Organic Chemistry-- Chemical & Physicochemical Properties-- (-1987)
ACETIC ACID ESTERS
ALKANES
ATOMS
BUTANE
CALCULATION METHODS
CARBOXYLIC ACID ESTERS
CHEMICAL BONDS
CHEMICAL REACTION KINETICS
CHEMICAL REACTIONS
COMPUTER CODES
CYCLOALKANES
DECOMPOSITION
DISTRIBUTION
ENTHALPY
ESTERS
ETHANE
FORMATION HEAT
HEPTANE
HEXANE
HYDROCARBONS
HYDROLYSIS
KINETICS
LYSIS
M CODES
OCTANE
ORGANIC COMPOUNDS
PENTANE
PHYSICAL PROPERTIES
PROPANE
REACTION HEAT
REACTION KINETICS
SOLVOLYSIS
SPATIAL DISTRIBUTION
STRAINS
STRUCTURAL MODELS
THERMODYNAMIC PROPERTIES
VAN DER WAALS FORCES