within a laboratory was 0.72 mg. per gallon, not significantly different from the repeatability found for sample A in Table I. However, the standard deviation of reproducibility between laboratories was 2.98 mg. per gallon, compared t o the value of 0.89 (sample A, Table I). This difference is highly significant a t the 95% confidence level. Boltz and Mellon pointed out that lead interferes with the heteropoly blue method by forming a precipitate of lead sulfate. This procedure avoids interference from tetraethyllead by removing most of that compound by evaporation. The amount of lead remaining after acid oxidation of the
residue is so small that it remains in solution and thus does not interfere with measurement of the color. The method is now in routine use in a number of Shell laboratories and is considered t o be thoroughly reliable. Analysis of a single sample requires about 1 to 1.5 hours’ elapsed time, while about 1.5 determinations can be handled by a single analyst in an 8-hour day. ACKNOWLEDGMENT
The authors are pleased to acknowledge the assistance of the members of the Shell Standardization Committee
and the laboratories which they represent in the development and testing of the present method. LITERATURE CITED
( I ) Am. Soc. Testing Materials, Philadelphia, Pa., “ASTM Standards
on Petroleum Products and Lubricants,” D 1091-54T, 1954. (2) Boltz, D. F., private communication. (3) Boltz, D. F., Rlellon, M. G., ISD. ENG.CHEIU., h A L . ED. 1 9 , 873-7 (1947). (4) Jordan, T. E., “Vapor Pressure of Organic Compounds,” pp. 9, 251-2, Interscience, New York, 1954. RECEIVED for review October 17, 1957. Accepted February 19, 1958.
Analysis of Polyhydric Phenol Mixtures WlLLlS BECKERING and WALTER W. FOWKES Lignite Experiment Station, Region 111, U. S. Bureau of Mines, Grand Forks, N. D. ,A gravimetric method is described for determining pyrocatechol and the 4-alkyl-substituted pyrocatechols using lead acetate as the precipitating agent. 3,6-Disubstituted pyrocatechols do not form an insoluble salt under the experimental conditions used in this work. The 3-monosubstituted compounds do not precipitate when in a mixture with other phenolic compounds. However, in an aqueous medium, they precipitate .from solution.
A
examination of tar acids recovered during low-temperature carbonization of lignite indicated the presence of pyrocatechol and a number of its alkyl-substituted homologs. The distillate fractions containing these substituted pyrocatechols also had several high-boiling complex phenols, making infrared analysis for the individual pyrocatechols extremely difficult. N
INFRARED
SEPARATION
OF O R T H O DIHYDROXY AROMATICS
A number of metallic ions react with o-dihydroxy aromatics to form insoluble salts or complex ions: aluminum ( 6 ) , barium (15, 21), calcium ( I @ , iron (1, 6), mercury ( 7 ) , gallium (17), germanium (17), magnesium (15), lead (2, 9, 11, 12, 14, 21), antimonj- (8, 20). tin (19),tantalum (16),titanium (IO,16), zinc ( l a ) , and zirconium (17). Lead was considered the most promising precipitating ion. It reacts rapidly with pyrocatechol over a wide range of experimental conditions and can be readily converted back to the pyro1336
ANALYTICAL CHEMISTRY
catechol, by using concentrated hydrochloric acid (21). Most investigators used the lead-salt method as a qualitative test for o-dihydroxy compounds, and only a few applied it as a semiquantitative method for separating the pyrocatechols from a mixture containing other phenolics. Whether used for quantitative estimation or as a qualitative test, no mention was found in the literature of the fact that substituents ortho to the hydroxy groups might interfere with the formation of a lead salt and, hence, separation of the substituted pyrocatechol. Such interference did occur in the present work, which led t o more extensire examination of substitution effects and the ability of such compounds to form insoluble lead salts. EXPERIMENTAL
Quantitative characterization of the lead salts was carried out by the Bureau of Mines with pyrocatechol and then with alkyl-substituted pyrocatechols. An excess of 0.5M lead acetate was added to a weighed quantity of resublimed pyrocatechol, precipitating the lead salt, CsH402Pb. The precipitate was washed tlvice with a solution containing equal volumes of acetone and distilled water, followed by two washings with acetone alone. This treatment removed excess lead acetate and other phenolic compounds, if present. The lead salt, recovered by centrifuging the solution, was dried for a minimum of 1 hour a t 106’ C. Results of several applications of this treatment are given in Table I. The
lorn percentage yields were attributed to the increase in hydrogen ion concentration resulting from acetic acid formed during the reaction.
+ P b + +%
CeHa(OH)*
CeHaOzPb H+
+ OAc-
+ 2Hf
HOAc
(1) (2)
Table I. Yields of Lead Salt Using Pyrocatechol in Unbuffered Solution
Pyrocatechol, Gram
Lead Salt, Gram
Yield,
0 28.51
n. 8026
98.31 98 00 97.94 96 33 98 76 98.77
0 is93
0 1563 0 2055 0 1640
0,1390
3629 4384 5668 4638 0.3931 0 0 0 0
%
As the hydrogen ion concentration increases in Equation 1, the reaction shifts to the left in favor of undissociated pyrocatechol. Reaction 1 can be driven t o the right (formation of the insoluble salt) by decreasing the hydrogen ion concentration. This may be accomplished by adding a solution of sodium acetate to the mixture. The resulting buffer effect will increase the p H of the reaction mixture, thus favoring formation of the CJ3402Pb. Table 11, showing the results obtained in treating weighed quantities of pyrocatechol or substituted pyrocatechols with a solution of 2 parts of 0.5M lead acetate and 1 part of 5.OM sodium acetate, demonstrates that the reaction is quantitative if the p H is properly adjusted. Furthermore. the
presence of monohydroxybenzenes or nonortho dihydroxy compounds does not interfere in the separation. The reason for the conversion being slightly above 100% is not understood clearly. It could be due to the occlusion of lead acetate or sodium acetate in the crystalline structure of the lead pyrocatecholate. As satisfactory results were obtained with pyrocatechol, the method n-as applied to a few substituted compounds. 3-Methyl-, 4-tert-butyl-, and 4-methylpyrocatechol n ere recrystallized several times from high-boiling petroleum ether (60 to 110" C.). The 4-methylpyrocatechol \vas dried under vacuum in a desiccator for 18 hours. The lead salts nere then formed by the same method used for pyrocatechol. Results (see Tables I1 and 111) indicate that these compounds are precipitated completely from solution. Furthermore, the presence of a substituted resorcinol or hydroquinone does not affect the completeness of precipitation of the 4methylpyrocatechol. APPLICATION T O TAR ACIDS
Procedure. T h e method was applied t o a gross sample of mixed t a r acids recovered from a commercial lignite creosote-oil b y t h e usual alkali extraction. It \\-as assumed t h a t alkyl-substituted pyrocatechols would precipitate quantitatively from solution, regardless of substituent position on the ring.
A 1500-nil. sample of tar acids was treated, in 126-1111 portions, with an aqueous solution containing 2 parts of 0.5M lead acetate to 1 part of 5.031 sodium acetate, bringing the total volume to 260 ml. The lead pyrocatecholates were removed by filtering the solution and freed of other tar acids by washing the precipitate several times rrith acetone. The lead salt was dried and then treated with concentrated hydrochloric acid, forming lead chloride and the free pyrocatechol. The lead chloride mas filtered off and the pyrocatechols were extracted from the filtrate with ether. After evaporation of the solvent, the extracted p)-rocatechols were fractionated a t reduced pressure through a 4foot packed column. d total of eight fractions as collected, ranging in boiling point from 106" to 130' C. at 6 mm. of mercury pressure. The individual fractions were esamined spectrophotometrically, using a single-beam, Perkin-Elmer Model 112 spectrophotometer. Results. Infrared analysis of t h e distillate fractions revealed t h e follorving pyrocatechols present: pyrocatechol, -l-methylpyrocatechol, 4-ethylpyrocatechol, a n d a trace of 4-npropylpyrocatechol. T h e identity of these conipounds n-as established
chiefly b y comparison of t h e spectra of t h e distillate fractions with the infrared spectra of t h e pure materials. All of these compounds, except pyrocatechol, n-ere prepared in pure form in this laboratory. Previous work with the tar-acid fractions boiling in this range had established the presence of 3,6-dimethylpyrocatechol and, from the infrared spectra of some fractions, 3methylpyrocatechol was strongly indicated. The reason for these latter two pyrocatechols not precipitating from solution in the lead acetate treatment was uncertain. I n pursuing this problem several 3-substituted pyrocatechols were synthesized in the laboratory. ORTHO-SUBSTITUTED COMPOUNDS
Source of Pyrocatechols. 3-?\Iethylpyrocatechol was purchased from t h e dldrich Chemical Co. and recrystallized from high-boiling petroleum ether. 3-Ethyl- and 3-n-propglpyrocatechol were prepared from o-veratraldehyde, according t o Haryorth and Woodcock's method ( I S ) . Baker and coworker's procedure (4) Tvas followed for the synthesis of 3,6-di-
Table 111.
Compound Pyrocatechol 3-Methylpyrocatechol 4-Methylpvrocatechol 4-tert-But ylpyrocatec hol
Table 11.
Compound Pyrocatechol
0
b E
Weight, Grain 0 1826 0 2565 0 1480 0 1532 0 152% 0 1439 0,1559
Salt, Yield, Gram % 0 5114 100 29 0 7305 99 46 0 $245 100 17 0 4397 100 23 0 4377 100 44 0 4147 100 63 0 4481 100 38
4-tert-But j-1pyrocatechol 0 1031 0 2347 101 87 0.1260 0 2861 101 60 3-Methylpyrocatechol O.li05 0 4518 99 89 0.1419 0 3811 101 22 4-llethylpyro0 0730 0 1958 101 08 catechold e 0 0766 0 2077 102 21 a Mixed with 0.2853 gram of resorcinol. * Mixed with 0.0821 gram of phenol and 0.3261 gram of p-cresol. Xlixed with 0.2159 grain of phenol and 0.2410 gram of p-cresol. d Mixed nith 0.0767 gram of %methyl resorcinol and 0.0-127 gram of 2-methyl hydroquinone. e Xixed with 0.0813 gram of 3-methyl resorcinol and 0.0448 gram of 2-methyl hydroquinone.
Purity of Standard Samples
Literature 1-alues N.P. H, % C x 65 14 5 49 105 68 67 7.3 6 50 65 6 50 67 7:; 56-7
Yields of Lead Salt in a Buffered Solution
8 48
methylpyrocatechol from 2,5-dimeth-lphenol. Lead Salts. T h e following esperiment was performed t o determine whether ortho-substituted pyrocatechols form insoluble salts when treated n-ith lead acetatr. Equal quantities of pyrocatechol, 3-methyl-, 3-ethyl-, 3-n-propyl-, and 3,B-dimethylpyrocatechol !\-ere placed in separate vials, and 2 ml. of 0.5.V lead acetate was added t o each. All of t h e pyrocatechols except 3,B-dimethylpyrocatechol formed insoluble precipitates; a large excess of lead acetate failed to precipitate the 3,6-dimethyl compound. ,4 second experiment was carried out similar to the first except for the addition of 2 ml. of acetone before lead acetate was added. I n this case only the salts of pyrocatechol and 3-methyl3pyrocatechol were precipitated. Ethyl- and 3-n-propylpyrocatechol gave clear solutions, which d o d y turned green upon standing, and the solution 3,6-dimethylpyrocatechol containing turned yellow on standing. Further investigation revealed that acetone,
7 2 25
Observed Values M.P. (corr.) H, % C, 5 63 65 104 5 6 54 67 66 6 53 67 64 5 8 54 71 55
% 31 43 21 81
or ether, when a d d d to an aqueous suspension of the indi\-idual insoluble lead pyrocatecholates, dissolved the lead salt of 3-~~-propylpyrocatecliol. The 3-ethyl salt, when once formed. nould not redissolve in acetone solution. To determine whether the 3-n-propyl compound dissolved in organic nicdia as the undissociated lead qalt or dissociated and dissolved as the pyrocatechol, an aqueous suspension of the insoluble lead salt n a s prepared and estracted withether. After severalextractions the precipitate diqappeared and the tn-o layers were separated. The aqueous phase n-as tested for the lead ion by adding dilute sulfuric acid, yielding lead sulfate. Recovery of the lead was 96 to 98% of theory. Evaporation of the ether phase yielded the original pyrocatechol. The lead pyrocatecholate, then, must be insoluble in the organic phase but very slightly soluble and dissociated in the aqueous phase, permitting solution of the pyrocatechol. The 3-ethylpyrocatecho1, on the other hand, nould appear t o be insufficiently soluble in the aqueous phase to permit detectable dissociation. VOL. 30,
NO. 8, AUGUST 1 9 5 8
1337
DISCUSSION
I t is evident that pyrocatechol and the 4-substituted pyrocatechols will form insoluble lead salts n-ith lead acetate, irrespective of the size of the alkyl group or the solvent employed (3, 6 ) . Thiq result can be extended safely to include the4,j-disubstituted pyrocatechols. Some qualifications are necessary for precipitation of 3-monosuhstituted coni3-;\letliylpyrocatecliol prepounds. cipitated under the same experimental conditions as the 4-alkyl substituted compounds and therefore could be c*laqqifieti n ith them; hon ever, in treating a mi-4 tar acid fraction it did not qeparate out n ith the 4-sulistituted compounrls. The 3-ethyl- and 3-npropyl compounds formcd lead salts only in an aqueous medium. Khen acetone ~ a added 3 t o the mixture before addition of lead acetate, no precipitate formed. The 3-n-propyl lead salt appears to be less stable than the correy~onding3-ethyl salt, hecause the former redissolved when aretone or &her \vas added, n-hile the latter remained once the precipitate x-as formed i n aqueous medium. There appears, then, to be a gradation in properties as the size of the alkyl group in the 3-position increaseq. Finally, the 3,G-dimethylpyrocatechol failed to precipitate under any of the conditions used in this work. One can safely extend this to include all 3.6disubstituted compounds, irrespective of the size of the substituent groups. The behavior exhibited by the different pyrocatecliols probably can best be explained as a steric hindrance effect. The alkyl group in the 4subitituted pyrocatechols is renioved sufficiently from the hydroxy groups so that its effect on the ability to form lead salts is negligible. These compounds react like pyrocatechol and can be classified as sterically unhindered pyrocatechols. K i t h the alkyl group in the 3-position partial hindrance occurs, the degree of steric hindrance depending on the size of the alkyl group. Finally, the 3,G-disubstituted compounds can be classified as sterically hindered pyrocatechols, because they are unable to form insoluble lead salts, irrespective of the size of the alkyl group or the solvent nied iurn , Steric hindrance is not the whole answer to the behavior of the lead pyrocatecholates. Ionization of the pyrocatechol in the different solvents, as well as the distribution of the pyrocatechol in the aqueous and nonaqueous phases, must also be considered. This can be seen more clearly by examining the following equations
1338
ANALYTICAL CHEMISTRY
+ KI
R(OH)2 (aqueous) R(OH), (nonaqueous) (3) K2
R(OH), (aqueous) a , RO?-- + 2H+ (4) lis
R(OH)>inonaqucous) k==y ROA-ROrPb
lia
RO2--
+ 2HH- (5) + Pb-+ (6)
n here K l represents the distribution coefficient of the pyrocatechol betn een the aqueous and nonaqueous phases. IC2 is the ionization constant of the pyrocatechol in the aqueous phaqe. This represents the over-all reaction, hecause the actual dissociation takes place in several qtages. ICs is the ionization constant of the pvrocatechol in the nonaqucouq phaw. I I G ) . Consequently, some 3-substituted pprocatechols do not precipitate from nonaqueous solution but will precipitate from aqueous solution. Furthermore, the concentration of RO,-- is controlled by the magnitude of the diswhich is greater tribution roefficient, k'], than 1. This explains why the lead salt of 3-n-propylpyrocatechol dissolves when acetone, or ether. is added to an aqueous suspension containing the precipitate. That is, the concentration of RO,-- in the original aqueous phase, determined by the value of &, n hen a nonaqueous solvent is added, is depressed because of the increased solubility of undissociated pyrocatechol in the organic phase. This decreases the concentration of R(OH)*in the aqueous phase. which in turn decreases the concentration of RO,--, shifting the equilibrium in Equation 6 to the right and dissolving the salt. This also explains the failure of the 3-substituted compounds to precipitate
from a mixture of the tar acids. awmiing then the monohydroxy compounds present serve as the organic solvent phase in this case. The quantitative nature of the reaction n ith unhindered pyrocatechols can be utilized in the deterniining of molecular weights. The lead salt, after drying and neighing, can be decomposed by concentrated nitric acid, then coni-erted to lead sulfate by the addition of sulfuric acid. If the weight of the lead sulfate formed and the iveight of the lead salt before decomposition are knon n. the molecular neight of the pyrocatechol can be readily calculated. This method n as ujetl in the 11 ork in the characterization of 4methylpyrocatecliol. I t i i limited in the sense that true molecular n eights can be determined only nhen a qingle unhindered pyrocatechol 14 present in the mixture. If more than one unhindered pyroc:itechol is preqent, the average molecular weight of those present n ill be obtained. LITERATURE CITED
Babko, .I.I
