NOTES
Jan., 1957 dium methoxide, was obtained by adding water vapor to a weighed sample of sodium methoxide in the vacuum system until the mole % of water was about 40. The solid was then evacuated. The vapors which came off were trapped and identified as methanol by infrared analysis. A chemical analyais of the solid residue confirmed that most of the sodium methoxide had been converted to sodium hydroxide.
123
Results and Discussion I1 gives the compositions Of the rium mixtures a t 25". The last column indicates the formula of the extrapolated dry solid. Figure 1 shows the phase diagram for the ternarysystemsodium oxide-water-methanol at 250. The two
TABLE I1
TERNARY SYSTEM SODIUM OXIDE-WATER-METHANOL AT 25' IN WEIGHT% ' COMPOSITION Satd li uid NarO'C%,OH
Wet solid Nan0 CHaOH
18.6 18.4 18.2 18.7 20.2 20.3 20.6
35.5 35.7 35.7 36.4 35.7 36.8 35.8
81.4 82.5 79.1 75.3 68.7 66.3 65.6
55.0 54.5 52.5 51.6 49.8 48.2
20.5 65.5
37.0 40.4
20.8 21.4 22.3 22.8 24.6 25.6 26.7 26.8 28.4 29.6 30.8 32.5 33.8 35.3 37.3 39.4 40.0
38.1 35.7
64.5 60.8 54.7 50.3 44.7 39.3 32.5 29.0 24.2 18.3 13.4 7.9 4.3 1.7 0.7 0.24 0.13
..
..
36.8 38.7 42.1 43.5 43.4 42.3 42.7 43.0 43.2 42.6 44.2 41.8 43.8 44.6 46.4
34.6 31.0 24.3 22.8 21.5 20.4 19.2 16.3 14.8 11.9 12.0 8.7 10.0 10.1 5.0
40.0
0.13 46.7
2.5
40.0
0.13 48.0
0.99
40.6
0
48.0
Extrapolated dry solid
,.
0
NaOHCHsOH NaOH CHsOH NaOHCHaOH NaOH42HsOH NaOH .CH30H NaOHCHsOH 3NaOH.2HzO .CHIOH NaOH.CH30H 3NaOH.2Hz0.CHaOH 3NaOH.2H20.CHaOH 3NaOH.2H20CH30H 3NaOH.2HzO.CH30H 3NaOH.2Hz0C H 3 0 H 3NaOH ,2HzOCH30H 3NaOH.2HzO.CH30H 3NaOH.2HzOCH30H 3NaOH.2HzO C H 3 0 H 3NaOH.2H20,CHsOH 3NaOH.2H20CH30H 3NaOH.2HzOCH3OH 3NaOH.2Hz0CH30H 3NaOH.2HzOC H 3 0 H 3NaOH.2H20CH30H 3NaOH.2HzOCH30H 3NaOH.2Hz0CHIOH 3NaOH.2H20CH30H NaOH.H20 3NaOH.2H20CH30H NaOH.HzO 3NaOH,2HzOCH30H NaOH.HzO NaOH.HzO
+ +
+ + +
TABLE I11 X-RAYPOWDER DIFFRACTION DATA Characteristic lines and relative intensities NaOHCHaOH ~N~OH.PHPO.CH~OH
4.92 ( 8 ) 4.11 (w) 3.58 (va) 3.30 (m) 3.11 (w) 2.86 (mw) 2.72 (a) 2.57 (vs) 2.40 (w) 2.10 (w) 1.93 (a) 1.81 (ms) 1.68 (w) 1.63 (w) 1.358 (w)
3.64 ( 8 ) 3.25 (w) 2.91 (m) 2.73 (m) 2.41 (vs) 2.13 (mw) 2.00 (m) 1.81 (m) 1.69 ( 8 ) 1.62 (m) 1.54 (vw) 1.444 (w) 1.335 (mw) 1.199 (mw) 1.073 (mw) 0.963 (vw)
Fig. 1.-Isotherm for system sodium oxide-water-methanol a t 25", composition in weight per cent.
principal solubility curves intersect a t the isothermally invariant point, 20.5 weight yo sodium oxide and 65.5 weight yo methanol. The greatest portion of the saturated liquid is in equilibrium with the solid 3NaOH.2€120.CH30H. A sample of this hydrate methanolate was prepared by the method of Smith,3and its X-ray powder diffraction pattern was compared with those of wet solid residues ohtained in the phase study. Another isothermally invariant point was found at 40.0 weight % sodium oxide and 0.13 weight % methanol, a t which the saturated liquid is in equilibrium with both 3NaOH.2H20CHsOHand NaOH. HzO. The characteristic lines and their relative intensities in the X-ray powder diffraction patterns for two of the equilibrium solids are given in Table 111. The sodium hydroxide methanolate has five strong lines, while the hydrate-methanolate has three different strong lines. With the aid of these standards an X-ray pattern of a wet solid residue could be used to confirm the identity of the extrapolated dry solid in equilibrium with saturated liquid a t any given point on the solubility curves. Acknowledgment.-The X-ray diffraction patterns were obtained by P. I. Henderson. (3) D. F. Smith, U. 5. Patent 2,418,372 (1947).
THE IONIC DISSOCIATION OF 5?,4-, 2,6- AND 3,4-DICHLOROBENZOIC ACIDS IN WATER' BY MARIONMACLEAN DAVISA N D HANNAH B. HETZER National Bureau of Standards, Washington 1 6 , D . C . Received September 17, 1966
No pK values are in the literature for dichlorobenzoic acids. Recently, in connection with stud( 1 ) This research was supported in part by the United States Air Force, through the Air Force Office of Scientific Research of the Air Research and Development Command under contract No, CSO-670. 65-21.
124
NOTES
ies of acidity in non-aqueous solvents, we needed approximate pK values for 2,4-, 2,6- and 3,443chlorobenzoic acids and obtained the respective experimental values 2.76, 1.82 and 3.64. The solubility of 2,6-dichlorobenzoic acid in water is great enough for satisfactory estimation of pK by potentiometric titration, but 3,bdichlorobenzoic acid is so difficultly soluble in water that a spectrophotometric procedure had to be used. 2,4-Dichlorobenzoic acid is intermediate in solubility, and its pK was measured by both methods. Values of pK can be calculated for the three acids by applying a generalization of Shorter and Stubbs.2 These authors showed that the change in the free energy of ionization of benzoic acid produced by two or more substituents is generally very close to the algebraic sum of the effects of individual substituents, except in cases where substituents are present in both the 2,6- or the 2,3-positions. The calculated pK values, based on ionization data for benzoic acid and for 0-, m- and p-chlorobenzoic acids, are 2.73, 1.68 and 3.65 for 2,4-, 2,6- and 3,4dichlorobenzoic acids, respectively.a The results of our measurements support the generalization of Shorter and Stubbs, inasmuch as the experimental and calculated pK values are essentially the same in the cases of 2,4- and 3,4-dichlorobenzoic acids, but not in the case of 2,6-dichlorobenzoic acid. Evidently the first ortho-substituted chlorine atom is more effective than the second ortho-chlorine in enhancing the strength of benzoic acid. Experimental acid, although of best available commercial grade, contained a reddish gummy impurity and had a strong odor. After three recrystallizations from aqueous ethanol, using decolorizing charcoal, and an additional crystallization from benzene, again using charcoal, the color and odor were removed, the melting point was 163-164',4 and the purity by potentiometric weight titrations in a ueoue ethanol was 99.9%. A single titration of a saturate1 aqueous solution a t 25' indicated a solubility equivalent to a little over 0.0025 M. 2,6-Dichlorobenzoic Acid.-cu,a,2,6-Tetrachlorotoluene, the starting material, was hydrolyzed to the corresponding aldehyde by the prolonged action of concentrated sulfuric acid a t about 55' .6 The aldehyde was oxidized to the acid by heating with alkaline potassium permanganate. The crude product was recrystallized from cyclohexane, yielding long, Aat, slender needles; after heating to 80' in a vacuum oven these melted a t 143-144°.4 The same melting point has been reported for specimens prepared from other starting materials.6J The purity by potentiometric weight titrations in water was 99.9%. 3,4-Dichlorobenzoic acid,a after recrystallization from aqueous ethanol, usin decolorizing charcoal, melted a t 206207°,4 and a purity 08s little over 99.8% was indicated by potentiometric weight titrations in aqueous ethanol. A single titration of a saturated aqueous solution a t 25' indicated a solubility equivalent to about 2.9 X lo-' M . Potentiometric Measurements of pK.-Values believed to approximate closely the thermodynamic dissociation
Materials.-2,4-Diclilorobenzoic
(2) J. Shorter and F. J. Stubbs, J . Chsm. Sec., 1180 (1949). (3) Using more recent lharmodynamic ionisation data (see J. F. J. Dippy, Chsm. Rm.. 26, 161 (193011, the corresponding oaloulated p K values are 2.72, 1.68 and 3.60. (4) Melting points were determined uaing an ASTM thermometer maintained a t 3-in. immersion. (5) H. E. Fierz-David and L. Blangey, "Grundlegende Operationen der Farbenchemie," Springer-Verlag, Vienna, Austria, 1943, p. 166. lithoprinted b y Edwards Brothers, Inc., Ann Arbor, Michigan, 1944. (6) J. F. Noms and A. E. B e a m , J . Am. Chsm. Sea.. 62,953 (1940). (7) 8. D. ROM.ibM., TO, 4039 (1948). (8) The authors thank the Heyden Chemical Corporation for the gift of this material.
Vol. 61
constants (expressed as p K ) of 2,4- and 2,6-dichlorobenzoic acids were obtained by aqueous titrations at 25 f lo,using glass and saturated calomel electrodese and standard sodium hydroxide approximately ten times as concentrated as the solution being titrated. 0.01 M solutions (100-ml. portions) of 2,6-dichlorobenzoic acid and 0.002 M solutions of 2,4dichlorobenzoic acid were titrated. In the titration of 2,6-dichlorobenzoic acid the p H meter was adjusted using NBS standard otassium tetroxalate buffer (pH. for 0.05 . hereas in the titraM solution equays 1.68 i 0.01 a t 25') ,w tion of 2,4-dichlorobenzoic acid the adjustment was made using NBS standard potassium hydrogen phthalate (pH, for 0.05 M solution equals 4.01 & 0.01 a t 25'). pH data at 0.5-ml. intervals, ranging from 0.5 to 5 ml., were used in the computations. The equation used in calculations of pK was1ot11
+-
IB-I [H+] 094 (1) pK = pH log [HB] [H+] -k 1 0+. 51.32dji The pK values obtained in two titrations of 2,b-dichlorobenzoic acid were 1.84 f 0.Ol6 and 1.82 f 0.01. The latter value is thought to be the better one. A titration of 2,4-dichlorobenzoic acid (120 ml.) using -0.023 M alkali yielded the pK value 2.76 f 0.03.12 Measurement of pK for 2,4- and 3,4-Dichlorobenzoic Acids by Combined Spectrophotometric and Potentiometric Measurements.-In calculating pK the following equation was used
-
PK pH - log ([Sl/[Al) (2) The measurement of pH waa performed potentiometrically. The pH meter was adjusted before measurements with the most suitable buffer standard (phthalate or tetroxdate), and the temperatures ,Of Eohltions during pH measurements Well-buffered solutions ave pH were usually 25-25.5 values reproducible to a t least 1 0 . 0 1 unit. &e term [SI/[A], which was determined. spectrophotomet$cally, represents the ratio of the equilibnum concentrations of ionized and non-ionized acid (in moles per liter) present in a dilute solution maintained a t a suitable pH by the addition of an appropriate buffer mixture of low ionic strength.'* Spectral absorption measurements were made with a Beckman DU quartz spectrophotometer, using 1-cm uartz cells in a cell box maintained at 25.00 f 0.05'. ;$though no activity corrections were made, the solutions were EO dilute that the results are believed to be close approximations to the thermodynamic pK values. In the case of 3,4-dichlorobenzoic acid, the stoichiometric concentration was 4 X 10-5M,I4and buffer mixtures of acetic acid and sodium acetate ranging from 5 X lo-' M to 2 X M in ionic strength were used. The [SJ/[A] ratios in these solutions varied from about 0.6 to 1.5. Table I summarizes the results for 3,4dichlorobenzoic acid. The average pK value is 3.64, either including or omitting the data for solutions having a temperature hgher than 25.5' a t the time of pH measurement. =i
.
(9) The titration apparatus was similar to the one described by C. J. Penther and F. B. Rolfaon, Ind. Eng. Chem., Anal. Ed., 16, 337 (1943). (10)All of the symbols have their usual, well-known significanoe. The third term on the right is. of course, equal to logf, which in dilute C E ' g p ) ; A and B are conatants of the solutions equals -A
