Add time:07/19/2019         Source:sciencedirect.com

The partition coefficients between 0.001 M HCl and octan-1-ol at 25°C are reported for seventeen 5, 5-disubstituted 2-oxobarbituric acids (1). The mean deviation for duplicate values of the partition coefficients was ±0.56% (largest deviation ±0.008 log units). Both phases were analyzed and the mean calculated recovery was 99.2%. Most recoveries were in the range 97.5–102.5%. Increased α-branching at the C1' carbon of a side chain (by successive replacement of hydrogen atoms with methyl groups) did not produce a constant increment in the log P value. Log P values for derivatives with one highly branched substituent, such as t-butyl, should be used with care in estimating a π value for the other substituent. Methylene group contributions to partitioning were determined to be very close to those determined in other studies. Calculated log P values (using the additive-constitutive principle of Hansch) for derivatives with two electron-withdrawing (−I) substituents did not agree with the measured values, even after adjustment of some of the hydrophobic substituent constants. Intramolecular interactions between the two substituents are advanced as an explanation for these discrepancies. The NMR spectra of 5, 5-diphenylbarbituric acid and 5, 5-diphenylhydantoin are discussed in terms of the conformations of the geminal phenyl groups.

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