IONIZATION OF PHENOLS IN AQUEOUS SOLUTION
March 20, 1964
1003
[COSTRIBUTIOX FROM DEPARTMENTS OF CHEMISTRY OF CARNEGIE IXSTITUTE OF TECHNOLOGY, PITTSBURGH, PEXNA , A N D THE COLLEGE O F THE HOLYCROSS, WORCESTER, MASS.]
Ionization of Substituted Phenols in Aqueous Solution B Y H.
c. K O , w.F. O'HARA,' T. HU, A N D L. G. HEPLER RECEIVED ACGCST12, 1963
Ionization constants and heats of ionization have been determined for 2,4,6-, 3,4,5-, 2,4,5-, and 2,3,5-trimethylphenols, m- and p-cyanophenols, and m- and p-hydroxyphenyltrimethylammonium chlorides in aqueous solution a t 25' I t is found t h a t differences in acidities of several of these compounds are due t o entropy effects Effects of substituents on acidity are discussed in terms of A H n n t , which refers t o the part of A H o t h a t is not due t o solute-solvent interactions
Introduction Ionization constants and heats of ionization of a number of substituted phenols in aqueous solution have been determined a t 25'. T h e results of these determinations have been used for calculation of AF', AH', and AS' of ionization of these acids. Substituent effects on acidity are discussed. Experimental Thermodynamic ionization constants were determined as previously described.2 Heats of ionization a t 26.0 f 0.2' were determined calorimetrically in apparatus also previously described.3,' All calorimetric measurements were made with 950 ml. of water or solution in the dewar. All trimethyl- and cyanophenols were obtained from Aldrich Chemical Co. All trimethylphenols were recrystallized from alcohol-water mixtures and had the following melting points: 2,4,&trimethyIphenol, 218-220", 3,4,5 trimethylphenol, 108110", 2,4,5-trimethyIphenol, 228-231 ', 2,3,5-trimethylphenoI, 93--94'. Both m- and p-cyanophenols were recrystallized from carbon tetrachloride and melted a t 112-113' and 111-112', respectively. XI1 melting points were in fine agreement with literature values. Both m- and p-hydroxyphenyltrimethylammonium iodides were prepared according t o t h e procedure of Hunig.j T h e iodides were converted t o t h e chlorides by dissolving them in water and treating the solution with an excess of solid AgC1. T h e resulting chlorides were purified by dissolving them in hot alcohol and adding ether to precipitate the desired crystals. The compounds were analyzed gravimetrically for C1-. T h e m-HOCfiHIN( CH3)3 +CI- and p-HOCsH4N( CH!)a+ CI- were found t o contain 18.50 and l8.75yG CI-, respectively, as compared t o the calculated value of l8.89yG.
Results The results of the determinations of the ionization constants of the various phenols in aqueous solution a t 25' are given in Table I TABLE I THERMODYNAMIC IOXIZATION CONSTAXTS OF SUBSTITUTED PHENOLS IN AQUEOUS SOLUTION AT 25" Acid
Phenol 2,4,6-TrimethylphenoI 3,4,5-TrimethyIphenol 2,4,5-TrimethylphenoI 2,3,5-TrimethylphenoI
Acid
PK
m-Cyanophenol p-Cyanophenol m-HOC6H4S(CH,),'CI p-HOCsHaN(CHa)a+Cl
8 57 7 9i 8.06 8 35
PK
10 10 10 10 10
00 89 25 57
67
Heats of ionization were all determined by measuring the heats of neutralization of substituted phenol solutions by aqueous sodium hydroxide. Ten-ml. aliquots of aqueous NaOH (4.980 -If for some measurements and 4.535 M for the others) were caused to react with 950 ml. of solution containing a known amount of substituted phenol. A general reaction for this process is HA(aq) iOH-(concd) = A J a q )
+ H20(liq)
(1)
(1) Chemistry Department, College of the Holy Cross, Worcester, Mass F O'Hara. T H u , and 1,. G Hepler, J Phys C h e m , 67, 1933
(2) W.
(1963) (3) R I,. Graham and I. G Hepler, J A m C h e m Soc., 78, 4846 (1956). (4) W . I.' O ' H a r a . C. H Wu, and I, G. Hepler, J . C h e m E d u c . , 38, 512 (19611. ( 5 ) S . Hunig, B e r . , 85, 1066 (1952):
Separate determination of the total heat effect associated with breaking the bulb containing the NaOH and dilution of the NaOH permitted us to calculate AH,for the reaction H.l(aq)
+ OH-(aq)
=
h - ( a q ) = HXO(liq)
(2)
from the Q (calorimetric) for the process represented by eq. 1. The values of AH?for the various substituted phenols were extrapolated to infinite dilution yielding the values of AH,' listed in Table 11. Also listed in Table TI are values for AH;,,' for reactions of the type HA(aq) = H + ( a q )
+ .\-(as)
These values were calculated according to A H i o n ' = AHw' AHz', where AHw' = 13.50 kcal. 'mole for the ionization of water.6 Standard free energies of ionization, AFi,,', have been calculated from the p K data in Table I and are also listed in Table 11. Entropies of ionization have been calculated from
+
AS,,,"
=
(AH,,,O
- AF,,,")/298
TABLE I1 THERMODYNAMICS OF IONIZATION OF SUBSTITUTED PHENOLSIN .4QUEOUS S O L U T I O S AT 298K. Acid
AHSO"
Phenol' 2,4,6-TrimethylphenoI -8.06 3,4,5-TrimethylphenoI -7.82 2,4,5-TrimethylphenoI -7.10 2,3,5-Trimethylphenol -7.50 m-Cyanophenol -8 30 p-Cyanophenol -8.58 m-HOCsH4S(CH3)aCCI- -7 41 P-HOCsHaN(CHi)a'CI-8 04 * A H o and A F o in kcal./mole and
AHmn0"
5.65 5 44 6 68 6.40 6 00
5 20 4 92 6 09 5 46 A S o in
AFionoa
AS,onoa
13.64 -26.8 14 85 -31.6 13.98 -2i 9 14.41 -26.9 14 45 -28 7 11 69 -21 8 10 87 -20 0 10 99 -16.4 11.39 -19 9 cal./deg. mole.
Discussion Sprengling and Lewis* have reported pK 10.88 for 2,4,6-trimethylphenol, in good agreement with our value. O'Haragreported pK 10.87 for this compound, determined by spectrophotometry. Wheland, Brownell, and Mayo'O reported the pK for p-cyanophenol to be 7.95. Fickling, et al.," report pK values for 'm- and p-cyanophenols to be 8.65and 7.97, respectively, in agreement with our values. Oae and Price'? report pK for m- and p-hydroxyphenyltrimethylammonium iodide t o be 8.03 and 8.2 1, in fair agreement with our values for the chloride salt. I t is useful to discuss the effects of substituents on the thermodynamics of ionization of phenols in terms of reactions of the type shown in eq. 3 (6) H. M . Papee, W J . Canada?, and K . J Laidler, C a n J C h e m . , 3 4 , 1677 (19.56). (7) L P. Fernandez and I,. G Hepler, J . A m C h e m S o c , 81, 1783 (1959) (8) G. R Sprengling and C . W. Lewis, ibid., 76, 5709 (19.53) (9) W . F O'Hara, P h D Dissertation, University of Virginia, 1962. ( I O ) G W. Wheland, R . hf Brownell, and E C Mayo. J A m C h e m Soc 7 0 , 2192 (1918). (11) M 1 4 Fickling, A Fishcher, B K . hlann. J Packer. and J V a u g h n ibid., 81, 4 2 2 6 (19,59) (12) S. Oae and C C Price, ibid, 80, 342.5 (19.78)
J. C. POWERS, J R . , W. R. HELLER,J KUMA~VOTO, A N D l V E. DONATH
1004 HMaq)
+ -\"-(as)
=
%.-(as)
+ HL(aq)
(3)
where subscripts s and u indicate substituted and unsubstituted phenol Previous investigations'? have shown t h a t AH3' may be expressed as the sum of two terms representing solute-solvent interactions (AHext) and "internal" effects These considerations have led to the equation AH?' = A N , , t
+ j3AS3"
( B = 280")
(1)
for reactions of type 3 Experimental enthalpy and entropy changes are represented by AH3' and ASj' while A H , , t represents the part of the total AH3' that is associated with breaking and forming 0-H bonds as affected by resonance, inductive effects, etc Combination of 4 with AF,' = AH3' - 7'5S3' leads to AF3" = 1 + ( ~ 3 - T,AS," ~ _ _ and t.hen, because
(P
(5)
AHd
.1H,,t
-
7Xs3'
-----AHint
- be explained by assuming t h a t a Stark shift occurs. % . shift of 12 cm.-' a t 9 X 1Oj v./cm. is observed on the band edge a t 6800 b.,and this corresponds t o an actual shift of 15 cm - l when an intensity change correction is applied. A method for treating data t o determine true band shifts in the presence of a simultaneous change in intensity is derived.
Phenol blue (I) has been of interest for some time because of the sensitivity of the position of its absorption "CH, N q - J - N ~ O
I
maximum to the polarity of a solvent. Brooker' found the maximum to be a t 5520 fL. in cyclohexane and 6680 A. in water. He rationalized this effect as due to a lowering of energy of the charged resonance forms, 11, in a more polar solvent resulting in increased resonance stabilization of the molecule. Bayliss and McRae? and Platt3 have used this concept of solvent(1) L. G. S Brooker and R H . Sprague, J . A m . Chem. SOC.,63, 3214 (1941). (2) S S Bayliss and E. G . McRae, J , P h y s . Chem., 68, 1002 (1954). (3) J , R . Platt, J . Chem Phys., 2 6 , 80 (1956).
