JOURNAL OF RESEARCH of t he Notional Bureau of Standards -A. Physics and Chemistry Vol. 7 1A , No.5, September- October 1967 . Ionization Constants of Substituted Benzoic Acids In Ethanol- Water* G. M. Brauer, George Durany, and Harold Argentar Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234 r (Ma y II, 1967) Tht' thermodynamic ionizali on conSlanls of mela and para subslituled allyl· and p ropenyl benzoic acids wer£' detNmined poten tiomelrically in aq ueous e thanol of varying elhanol concentration. The /)/\ val ups increasl' wit h increasing e thanol content. The relali ve acid strength does not vary wi th cita n!!e in solvenl concentralion. The !1pK/!1 percent ETO H increases with ethanol cont e nt of the medi ulll unlil 1I llIaxilllulll is reacllt'd around ·W weight-pcrcelll ('I hanoI. 1\ plot of pK + log [H 20l vcrsus Ill(' rccipro('al or Ihe dielectri(, conslant of the solve nt gi ves a nearly linear relationship to aboul 44 weight-percelll elhanol. ~~f() m "K val u,'s found in Ihe li terature a s imila r linear relationship e,isls for other b,' nzoic a('ids. W ilh the ex('ept ion of Ill(' fl-propenylbenzo ic acid Ihe a-substituenl cons lanls of t he Hamlllett equal ion do not ('hange g reat ly with etha nol ('oncentration. This behavior Illay Iw indicaliy,' Ihal III(' ,olvalioll , h('11 s UIToI II Hlin!! Ihe p-propt'llyl-aciti differs frolll Ihat of olher 1H'llzoi(' a('id~ . 1\,.\ \Vor,\': 1\11 , 1- alld prol,enyll)(' ll wi(' acids. ('hangc of pK wil h dieleclric conSla nl , Hamme ll Sit!IlHI constall t s. ion ization ('(lilsta llt s. pK va lu es in ethanol-wate r, s igma ('o nst an ts ill l't ba nol-,,,,I(' r. ,ubSlillll ('d "('nwi(' acids. 1. Introduction (-CH= CH-CHa) groupS. These co nstants may be very val uable in predicting rate or equilibrium param­ Previous results [U have shown tha t a relationship eters such as che late stability constant s of aromatic exists betw een the reactivity and the presence of compounds Gp ntaining allyl or propenyl groups. The substituent groups in phenols. This relationship ap­ results of this and previously publis hed works were pli es to the io nizati on constants of substituted 0- also used to calculate the substituent constants or methoxyphenols (substituted guaiacols) as well as to various functional groups III aqueous ethanol the rate of the setting reaction of 4-all yl-2-m ethoxy­ solutions. phenol (e ugenol) and 5-allyl-2-methoxyphenol (chavi­ betol) wi th zinc oxide. One of the objectives of this 2. Experimental Procedures study was to determine the ionization constants of meta- and para-allyl- and propenylbenzoic acids. These 2.1. Materials const ants were determined in aqueous ethanol solu­ tions of varying ethanol content to study the effect The benzoic acid and potassi um acid phthalate of the composition of the solvent medium on ioniza­ were standard samples of the National Bureau of tion constants, relative acid strength, so lv ent-solute S tandards. interaction, and the free energy of ionizati on. Data The allyl- and propenylbenzoic acids were syn· from this and other investigations were used to ex­ thesized as previously described [IJ. In addition, amine the validity of existing relationships between p-propenylbenzoic acid was prepared by isomeriza­ the dielectric constant of the solvent and ionization tion from p-allylbenzoic acid. To 15 ml of a saturated constants, and to calculate ion size-parameters. solution of KOH in 90 percent e thanol, 0.1 g of p­ Another aim of this study was to obtain accurate aJJylbenzoic acid was added in a 50 mt fl ask. The values for the substituent co nstants of t he Hammett solution was saturated with argon and refluxed for equation for allyl (-CH2-CH=CH2 ) and propenyl 48 hr, cooled a nd neutralized with 6N HCl solution. The precIpItate was filtered, washed with water, *Tlw dellial n·'wan·h pJ'lJ~raOm ttl the '1ational Bur('au of Siandards is carried on in Co­ ...,)crativil w-;th tlw COU Il('il on Dental Hc search of the Americ an Denial Association: the decolorized with activated charcoal, and recrystallized \Jal iunaJ In!"1 ilutc' for DenIal Research: Ill(> Army O(,ntal Corps; the Dental S<:iences Division of th(' S('hotJl (If t\c' J'I) s pace \r1 edicine. USAF: C:6nd the Veterans Administration. repeatedly from a 30 percent etha nol-water solution. This palwr was pre ... ented at Ih(' 152d Ilwelill).!. of Ihe American C hernieal Societ y. Nt·\\ Yield: 0.04 g (40%) of p-propenylbenzoic acid, mp YHI"k, September 19M. I Figures in brackets indicate the lit erature refe ren('es at the e nd of this paper, = 215°C. 379 267- 984 0 - 67 - 3 ---- -------- 2.2. Procedures The constants A and B whicll are de pende nt on th e ;;:~ lvent composition were calculated from the expres­ The thermodynamic ionization constants at 25.0 ,sions by Bates, Paabo, and Robinso n [3] A = 1.825 X 106 ± 0.2 °C were determined potentiometrically in an (ET)-3/2 and B = 1.5 (78_3 /E)I /2 where T =absoJute inert atmosphere with a carbonate-free aqueous te mpe rature and E = di electric cons tant of the solve nt, ethanol solution of sodium hydroxide in the buffer ~ btain ed by int erpolation from the values given by region where the degree of ionization is between Akerlof [4] . The calc ulations we re made using th e 25 percent and 75 percent. Further pH values were Nati onal Bureau of Standards Ol1lllitab program [5J. , obtained in the vicinity of the end point. A Radiometer , pH meter with scale expander was used. Before each titration the instrument was standardized with 0.05 M potassiu m acid phthalate (PH = 4.008) and 0.025 M 3. Results and Discussion KH2 P04 - Na2 HP04 (pH = 6.865). A 40 ml ethanol-water solution of the acid (2 to 3.1. Ionization Constants 4 X 10 - 3M) was used in the titrations with the excep­ tion of p-propenylbenzoic acid, where 100 ml of a The apparent thermodynamic pK values of benzoic nearly saturated solution was employed. acid and the substituted benzoic acids in 0 to 64.8 A slow stream of nitrogen or argon saturated with weight percent (0 to 70 volume percent) aqueous the solvent used in the titration was passed through ethanol solutions (including the number of runs anJ the liquid in the reaction vessel for at least 10 min. the standard error of the mean of the pK values) are The inert gas was also passed through the solution given in table 1. The pK values were corrected for before each pH reading. All solutions were protected liquid junction potential and mediu m effe c!. by graphic from carbon dioxide contamination by sodlum hydrox- ~ interpolation of the average valu es of 8 = (Ej -logIllY") ide-asbestos absorbent. Details of the experimental from data of Gutbezahl and Grunwald [61 and Bates, procedures are given by Brauer, Argentar, and Durany Paabo, and Robinson [3] where Ej is the liquid junc­ [1]- The precise end point was obtained by plotting tion potential expressed in pH units and lilY" , the ~pH/~ ml or its reciprocal versus milliliters of NaOH ' medium effect, measures the differe nce in the standard added and taking as the end point the number of milli­ free energy of the proton in water and aqueous ethanol. lite rs of NaOH at which ~pH/~ rnl is a maximum The estimated standard deviations for pK values ob­ or ~ ml/ ~ pH is a minimum [2]- From the law of tained during one run were about 0 .004 pK units. With mass action and assuming (1) the validity of the in creasin g e thanol concentration th e pK values in­ Debye-Hiickel equation, (2) that the activity coeffici e nt crease markedly from around 4.2 in wate r to 6_4 for of the unchanged, undissociated acid molecule is the 64.8 weight percent ethanol solution. As has been unity, and (3) that the hydroxyl ion concentration s hown by Grunwald [6] ~pK = log lilY" + log (Y 1- /YHA) may be neglected, the following equation that was where lilY 1/, Y 1_ and YHA are th e degenerate acti vity employed to calculate pK values can be derived. coeffici ents ot the proton, anion, and undissociated acid referred to the infinitely dilute aqueous state _ CA -([Na+] + [H+]) AV[Na+] + [H+] pK = pH + log . ' +---:----"-;====== Since ~pK is a function of both solv ent and acid [Na+] [H+] 1'+ BV rNa+] + [H+] structure and log YH is a function of solvent only, (1) log Yr/YHA must be a fun c tion at least of the acid where CA = [HAJ + [A- l = total concentration of all structure and possibly of the solv ent. On increasing acid species, the ethanol concentration the nature of the ionization [HA] ~ concentration of th e undissociated acid , equilibrium is changed, partially because the ethanol­ and solvated carboxylate moiet y diffe rs from the cor­ [A - ] = concen t ration of anion of th e acid . responding water-solvated group_ TABLE 1. pK Values of substituted benzoic acids in aqueous ethanol solution Temperature = 25.0 ± 0.2 0c. We ight percent ethanol in solution Suhstituent 0.0 12 .2 25.2 44. 1 64.8 II ' pKallP . S/v'~ " pKcor.c II pKapp. S l v'~ pKcor.