(8) Shiner, V. J., Smith, AI. L., ANAL. CHEM.28, 2028 (1956). (9) Shoemaker, C. E., Zbzd., 27, 552 (1955). (10) Simons, J. H., ed., “Fluorine Chemistry,” Vol. 2, pp; 102-57, Academic Press, n’ew 1 ork, 1954. (11) Sundareean, RI., Karkhanavals, M.
D., Current Sci. (India) 23, 258 (1954). (12) Willard, H. H., Diehl, H “Advanced Quantitative Analysis’,’” Van Nostrand, New York, 1954. (13) Willard, H. H., Winter, 0. B., IND. ESG. CHEJI., ANAL. ED. 5, 7 (1933).
(14) Zebroski, E. L., Alter, H. W., Heumann, F. K., J . Am. Chem. SOC. 73, 5646 (1951). RECEIVED for review October 14, 1957. Accepted January 4, 1958. Division of Analytical Chemistry, 132nd Meeting, ACS, New York, N. Y., September 195i.
The Functionality of Phenols by Bromination ARTHUR
K. INGBERMAN
Development laboratories, Bakelite Co., Division o f Union Carbide Corp., Bound Brook, N. 1.
b A solution of bromine in acetic acid, catalyzed by pyridine, provides a specific and quantitative bromination of the unsubstituted ortho and p a r a positions in phenol. The method is applicable to phenols and many monosubstituted phenols, 2,4,6-trisubstituted phenols, the dihydroxydiphenylmethanes, multiring phenols, bis(0-hydroxybenzyl)amine, phenols in the presence of hexamethylenetetramine, ethers, and hydroxymethyl groups.
P
Sovolak resins are generally regarded as linear (non-crosslinked) chains of phenolic nuclei, joined by methylene links with side chains. and very few, if any, methyl01 groups. The cure of such resins with additional formaldehyde or hexamethylenetetramine is probably analogous to a vulcanization process-i.e., the establishment of additional cross links as methylenes, or meth>-lenimine linkages, respectively. between adjacent chains that have unsubstituted reactive positions in the phenolic rings ortho or para to the phenolic hydroxyl groups. A method of determining the number of unsubstituted positions in phenolic nuclei ortho or para t o the phenolic hydroxyl therefore offers not only a means for analyzing a large number of phenols, but also a tool for following the kinetics of the phenol formaldehyde cross-linking reactions. The classical bromate-bromide method of Koppeschaar ( 5 ) has been used for the estimation of phenol and unreacted phenol in phenol formaldehyde condensates. The method can be extended only to meta-alkylated phenols. Variable nonstoichiometric results were obtained with ortho- or para-substituted alkyl phenols as well as with the dihydroxydiphenylmethanes ( I , 9). The bromate-bromide method also requires the use of aqueous systems, which limits its applicability for less soluble phenols. Alkaline iodometric techniques are more selective and useful for cresols as well as HEKOLIC
dihydroxydiphenylmethanes, but are still limited by the use of aqueous systems (8, 7). The variable results obtained with the bromate-bromide method are probably due t o the high reactivity of the brominating agent, H20Br*, which has a tendency to brominate side chains as well as nuclear positions. Experience in this laboratory with a milder reagent (iodine monochloride in acetic acid) was not too promising. Bromination with molecular bromine in acetic acid proved to be a somewhat more facile reaction, and phenol was completely brominated within 45 minutes a t 80” C. to yield 2,4,6-tribromophenol. These conditions were still somewhat too rigorous for the more reactive compounds-i.e., saligenin and p-phenylphenol. The use of pyridine as a catalyst, however, provided specific and stoichiometric bromination of the ortho and para positions of a wide variety of phenols, generally complete within a fen- minutes a t room temperature. It is probable that the active brominating agent is pyridinium perbromide. EXPERIMENTAL DETAILS
An amount of sample that consumed about three milliatoms of bromine was weighed into a 250-ml. iodine flask and treated with exactly 25.00 ml. of a 0.15M solution of bromine in acetic acid. After complete dispersion, a solution of pyridine in acetic acid mas added, the flask was stoppered, and the contents were permitted to stand at room temperature for a minimum of 2 minutes, but preferably not longer than 20 minutes. About 1 ml. of a 27y0 solution of pyridine in acetic acid was found effective. A minimum of 5 ml. of 50% aqueous potassium iodide was added, and the liberated iodine was titrated in the usual manner with standardized 0.15M sodium thiosulfate, A blank was run in the same manner. The pyridine used was Barrett Grade 2A, purified first by fractionation over barium oxide, followed by treatment with 10% of its weight of liquid bromine, fractionated, and finally refrac-
tionated over barium oxide. All other solvents and reagents were standard chemically pure grades. The phenols used for this study -sere all carefully recrystallized or redistilled to constant physical properties with the exception of 2,6-bis(o-hydroxybenzyl)phenoland 3,3‘- bis(o- hydroxybenzyl) -2,2’-dihydroxydiphenylmethane. These compounds were recrystallized only once. Some gelled phenol formaldehyde condensates that were difficultly soluble in acetic acid alone were frequently soluble in a 1 to 1 mixture of acetic acid and dimethylformamide. The use of dimethylformamide did not affect the results. With unknown materials it is advisable to run a series of determinations with varying reaction times to ensure complete bromination. CONSUMPTION OF BROMINE The bromine consumed was determined as the difference between the total bromine employed as measured by the blank determination and the residual bromine determined iodometrically. The detection of the end point using starch as an indicator was generally useful for pure compounds. K i t h polymers, the use of this indicator was complicated by the formation of colored or insoluble bromination products. I n such cases, the titration was carried out potentiometrically using platinum and calomel electrodes. Glass electrodes were not usable because of their high membrane resistance. Generally, it was faster and more convenient to perform a dead-stop end point titration. By this technique, no intermediate readings were taken. The end point of the thiosulfate titration was accompanied by a sudden depolarization of the electrodes system. These electrometric determinations were made with a Leeds & Northrup 7664 pH meter using platinum foil electrodes. The results obtained by the dead-stop end point technique, the usual potentiometric titration, and the visual titration n i t h starch all coincided. VOL. 30, NO. 5 , MAY 1958
1003
Table I.
Bromination of Phenolic Compounds
Compound Phenol p-Cresol m-Cresol p-lert-Butylphenol p-tert-Amylphenol p-Phenylphenol p-Cyclohexylphenol 3-Methyl-5-ethylphenol 0-tert-~4mylphenol 4-Indanol 5-Indanol Saligenin p-Hydroxybenzyl alcohol p-Hydroxybenzyl methyl ether 2,4,6-Tribromophenol 4,4’-Isopropylidinediphenol 2,2 ‘-Dihydroxydiphenylmethane 2,4’-Dihydroxydiphenylmethane 4,4’-Dihydroxydiphenylmethane 2,2‘-Dihydroxydibenzyl ether Bis (o-hydroxybenzy1)amine 2,6-Bis(2 ‘-hydroxy-5’-methylbenzyl)-p-cresol 2,6-Bis (0-hydroxybenzy1)phenol 3,3‘-Bis(o-hydroxybenzyl)-2,2 ‘dihydroxydiphenylmethane
2,4,6-Tris (2 ’-hydroxy-5’-methylbenzy1)phenol
Calculated Br Observed Br Sample Consumption, Consumption, Wt., G. Atoms/Mole Atoms/Mole 3.00 2.97 0.1001 2.00 1.96 0.1615 3.00 2.93 0.1101 1.98 2.00 0.2311 1.97 2.00 0.2395 2.00 2.00 0.2505 2.00 1.98 0.2706 2.93 3.00 0.1410 2.00 1.95 0.2430 2.00 1.99 0.2009 2.00 1.99 0.2000 2.00 1.95 0.1920 2.00 1.98 0.2127 2.05 2.00 0.1787 0.00 0.00 0.4006 4.00 3.91 0.1594 3.90 4.00 0.1512 3.92 4.00 0.1516 3.97 4.00 0.1530 3.99 4.00 0,1783 5.00 4.96 0.1185
5%
of
Theory 99.0 98.0 97.7 99.0 98.5 100.0
99.0 98.0 97.5 99.3 99.3 97.5 99.0 102.5 ... 07.8 97.7 98.0 99.3 99.8 99.2
0.4801 0.1804
2.00 5.00
1.99 4.84
99.5 96.8
0.2106
6.00
5.91
98.5
0.4577
3.00
2.96
98.6
ins has not been investigated; however, both p-phenylphenol and p-cyclohexylphenol brominate stoicliioAll the monosubstituted phenols tested metrically as shown in Table I. to date yield stoichiometric results The method appears to be applicable (Table I). The method does not appear to multiring phenolic compounds (Tato be applicable to 2,4,6-trialkylated ble I). This table lists a number of phenols when one of the alkyl groups is compounds that have been or could be 2,6-di-tert-butylphenol, tertiary-Le., and 2,6,2’,6‘-tetra-tert-butyl-4,4‘-dihy- isolated from both cured and uncured phenol-formaldehyde condensates. Of droxydiphenylmethane. There apparticular interest are the results obpears t o be a tendency for the bromine tained with 2>4,6-tris(2’-hydroxyto replace the tertiary alkyl group on 5’-methylbenzyl)phenol, which is the the ring and thus overbrominate. simplest prototype of a trisubstituted These observations agree with a n earlier phenolic moiety found in a cured pheconclusion (3) that exhaustive brominanol-formaldehyde condensate. I n this tion of alkyl-substituted benzene derivcompound, the meta positions and the atives results in replacement of all termethylene groups are subject to the tiary and secondary alkyl groups by maximum activation. The fact that bromine, leaving primary alkyl group the pyridine-catalyzed bromination is unaffected. not vigorous enough t o cause brominaThe method is probably applicable to tion in these positions indicates the phenol-formaldehyde condensates that applicability of this technique to highly do not contain more than one hydroxycross-linked systems. methyl group per phenolic ring. Thus, The probable nitrogenous reaction both saligenin and p-hydroxybenzyl products to be expected during cure of a alcohol were brominated stoichiometriphenolic Kovolak resin with hexacally, but 2,6-dimethylol-p-cresol and methylenetetramine are: hydroxyben2,4,6-trimethylolphenol consumed brozylamines (traces), bis(hydroxybenzy1)mine. Also, the method would not be amines, and tris(hydroxybenzy1)amines applicable to analysis or study of the cur(4). Shono and Takahashi (8) have ing reactions of phenolic resins derived shown that bis(o-hydroxybenzy1)amine from p-tert-butylphenol. The suitacan react with saligenin to yield tris(obility of the method t o study of phydroxybenzy1)amine. Thus, bis(o-hyphenylphenol or p-cyclohexylphenol resSCOPE A N D LIMITATIONS
1004
0
ANALYTICAL CHEMISTRY
droxybenzy1)amine behaves as if it were pentafunctional in the curing reaction, four for the ortho and para positions, and one for the amino hydrogen. Bis(o-hydroxybenzy1)amine was synthesized from saligenin and anhydrous ammonia in ethanol by the method of Paal and Senninger (6), and brominated in the usual manner (Table I). This compound consumed exactly five atoms of bromine per mole. Therefore, the bromination technique may also provide a measure of functionality of nitrogencontaining phenol-formaldehyde condensates. The reactions of other nitrogen-bearing intermediates with bromine in acetic acid catalyzed by pyridine have not been investigated; so the method should be restricted to the compounds discussed until other nitrogenous resin intermediates have been studied. The following compounds n-ere subjected to the action of the reagent from 1 to 16 minutes: 4,4‘-isopropylidenediphenol; 2,2’-dihydroxydiphenylmethane; 2,4’-dihydroxydiphenylmethne; hexamethylenetetramine; p-cresol; bis(0-1iydroxybenzyl)amine; 2,6-bis(o-hydroxybenzy1)phenol; and 3,3’-bis(ohydroxq-benzyl) - 2,2’-dihydroxydiphen ylmethane In all cases, the number of atoms of bromine consumed per mole of compound was equal to the number of unsubstituted ortho and para positions to phenolic hydroxyl groups, indicating little tendency to overbroninate. Hexamethylenetetramine did not consume bromine. LITERATURE CITED
(1) BeckurtE, H., “Die Methoden der
Massanalyse,” 2nd ed., pp. 578 ff.1 Vieweg, Braunschxeig, 1931. (2) Fox, J. J., Barker, bI. F., J. SOC. Chem. Ind. (London) 39, 169 (1920). (3) Hennion, G. F., Anderson, J. G., J . Am. Chem. SOC.68, 424 (1946). (4) Hultzsch, M.,“Chemie der Phenolharze,” 1st ed., pp. 94-101, Springer Verlag, Berlin 1950. (5) Koppeschaar, Id. F., 2. anal. Chm. 15, 233 (1876). (6) Paal, C., Senninger, H., Ber. 27, 1799 (1894). (7) Pence, C. >I,, J. Ind. Eng. Chem. 4, 518 (1912). (8) Shono, T., Takahashi, S., J. Chem. SOC. Japan, Ind. Chem. Sect. 56, 422 (1953). (9) Sprung, M. M., ANAL.CHEM.13, 35 (1941).
RECEIVED for review January 22, 1957. Accepted June 3, 1957. Meeting-inMiniature, North Jersey Section, ACS, January 30, 1956. Division of Analytical Chemistry, 133rd Meeting, ACS San Francisco, Calif., April 1958.
