Ionization constants of 4 dinitrophenols in water at 25 C

Abstract

Therlllod vnami c ionizaL ion con sLanLs of 2,3-, 2,5-, 3, ,1-, and 3,5-dini LrophcnoIR in aq ueous solution aL 25° C have beeJl deter min ed by a sp ec trophotome tric me thod. The r e p ectivc valucs fouIld, exprcssed as pK, a rc 4..959, 5.21 0, 5.4.22, and 6.69 2, pK has also been detefm in ed p otenLiometricall y fOf 2,3-and 3,fi-dinitrophenols; t he resp ective valu es obtained a re 4.98 and 6.66 . The experimental pIC \'alues for a ll six dinitroph enols a re lower than Lhe calcu lated valu es based on pK data for pheno l and t.h e mononi trophenols.Spectral absorption curves ar e prese nted for the ionized and uniol1l zed form s of t he four dinit rophellols.

The dissociation constants of some disubstituted anilines and phenols in aqueous solution at 25 C

“…Thus th e pK value of phe nol itself, 9.998 , [2] is raised to 10_287 in o-cresol, [6] th e subs titution e ffec t of th e o-methyl group bein g -0.289 ; th e pK valu e of pc hlorophe nol is 9.4h [2] so th a t th e sub stitution effect of the p-chloro group is 0.580, If th ese e ffects are strictly add iti ve, the co mbin e d e ffects of a-methyl and p -chloro s ubstitution should be 0.29 1 giving pK = 9 .707 for 4-chloro-2-methylphenol, which agrees very we ll with the observed value of 9.706• Substituent effects were calculated from the pK values of a-nitro phenol [7], m-nitrophenol [8], and m-nitroaniline [10] ; for other groups the data of Biggs [6] or of Biggs and Robinson [2] were us ed.…”

“…It is possible that the additivity rule fails when bulky ort ho substituted groups are present beca use these can produce s teri c inhibition of resonance. Thus the rule fails badly with 2,6-dibromoaniline and 2,6-dibromophe nol as it does with the corresponding dichloro compounds [1] and with 2,6-dinitrophenol [10]. Perhaps th e mos t remarkabl e failure of additivity is found with 4-methyl-2-nitroaniline; th e additivity rule holds for the corresponding ph e nol with only moderate s uc cess.…”

Dissociation constants of some substituted nitrophenols in aqueous solution at 25 C

“…The determination of the dissociation co nstants of a number of substituted phenols by the spectrophotometric method has been described rece ntly [1,2,3,4,5,6,7). 1 The purpose of this paper is to present data for twelve substituted phenols with an indicator range pH 2.1-5.5.…”

“…1 The purpose of this paper is to present data for twelve substituted phenols with an indicator range pH 2.1-5.5.…”

Dissociation of some substituted phenols in 50-percent aqueous methanol as solvent

“…The joint effects of P·CH 3 and 0·N0 2 are, therefore additive only to a poor approximation. The same has b een shown to be true, to a greater degree [11], for 2,4· 6' dinitrophenol with pK = 4.09 [12]; here the substitution of a p-N0 2 group in o·nitrophenol has lowered the pK value by 3.1 4 whereas the lowering would have been 2.84 if additivity held.…”