Chlorophenols constitute a series offive basic compounds of covalently bonded 1-5 atomic chlorine to the fivecarbons of phenyl ring (except for the carbon atom an OH group attached it). Ifthe positional isomerism guesstimates this number reaches to 19 differentcompounds consisting of mono-, di-, tri-, and tetrachloroisomers and onepentachlorophenol. At room temperature, excluding 2-chlorophenol which isliquid, all of the chlorophenols are crystalline solids. Chlorophenols at leastwith two chlorine atoms could be used as insecticides, herbicides, fungicides,and precursors of other pesticides.1 It would definitely be said one of theimplications for the worthier recognition many fundamental reactions inchemistry and biochemistry are acid dissociation constants.2-4Therefore finding a good and easy procedure to specify the pKavalue of the compound is so important.
Two very common methods used for pKaascertainment are potentiometric titrations or spectrophotometric measurementsin aqueous media. However, these methods require a large amount of pure sample.Consequently, they cannot use for the acidity constant determining of compoundsin the small amounts of samples.5 Given that the determination ofacidity by experimental methods found to be difficult and may not even bepossible sometimes; therefore providing a computational method comparable toexperiment can be of great importance.In recent years, sweeping efforts havebeen conducted to determine the acidity constant by computational approaches.6-10Mora-Diez et al. have used the theoretical method to ascertaining of pKavalue of benzimidazoles.11 The acidity constant of the number oforganic compounds, especially carboxylic acids have determined by the Namazianand his coworkers.
12-15 Quina et al. have used DFT method for the pKacalculation of substituted flavylium salts and their quinonoidal conjugatebases.16Despite numerous experimental work inthe field of study of chlorophenol acidity,5,17-19 computationalcalculation still has not been done in this case. Therefore, the main target ofthis work is determining the pKa values of chlorinatedphenols using density functional theory methods. Fortunately, the resultsobtained from theoretical method were entirely comparable experimental finding.
The difference between measured pKa value by experiment andtheory was less than 0.4 pKa unit. Consequently, it can besaid this level of computation is suitable for auspicate relative pKavalues of chlorophenols.