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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 16, Number 12
BKCSDE 16(12)
December 20, 1995

Title	Acidities of Benzyltetrahydrothiophenium Halides in Water. A Simple Method of Estimation
Author	Bong Rae Cho*, Yong Kwan Kim, Man So Han, Kwang Jin Oh
Keywords
Abstract	The pKa values of benzyltetrahydrothiophenium halides 1a-f in water have been estimated by measuring the absorbances of the solution in aqueous hydroxide ion solution. Assuming that the ratios of the activity coefficients remains close to unity, the absorbance of the solution can be expressed as A/[SH]o=(εSH+ε S-K[OH-])/(1+K[OH-]), where A, [SH]o, K, εSH, and εS- are the absorbance of solution, the initial concentration of 1a-f, the equilibrium constant, and the extinction coefficients for SH and S-, respectively. The εS- and K values that best fit with this equation were calculated by a nonlinear regression analysis with a large number of absorbance data determined at different [OH-] and [SH]o. The pKa values of the SH were then calculated with the relationship Ka=-log K+14. The validity of this method has been demonstrated by the excellent agreements between the experimental and literature pKa values of three organic acids. The pKa values of 1a-f estimated by this method are in the range of 12.5-15.3 and correlate well with the Hammett equation. The large negative deviation for the pKa values of 1e and 1f from the Hammett plot has been attributed to the extra hydrogen bonding between the phenyl group and water molecules attracted by the hydrophilic substituents.
Page	1218 - 1222
Full Text	PDF

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