V O L U M E 2 4 , N O . 6, J U N E 1 9 5 2
919
Table 111. Analyses of Mixtures of Streptomycin and Mannosidostreptomycin Sulfates Streptomycin, llg. Taken Found Average 1 0 1 06 0.9; 0 88 0 97
Jlannosidostreptomycin, hIg. Taken Found Average 4 0 3.94 4 03 4 12 4 03
2.0
2.22 1.93 1.94
2.03
3.0
2.78 3.07 3.06
2.97
3.0
3.31 2.94 2.88
3.04
2 . CI
1.69 2.06 2.12
1.96
4.0
4.12 3.98 4.22
4.11
1 .@
0.88
0.89
1.02
0.78
manganese, bismuth, cadmium, and strontium ions produced no color change after a sample of streptomycin was treated with periodic acid. It is possible that streptomycin after a short periodic acid treatment may provide a specific color reaction for lead. ACKNOWLEDGMENT
The authors nish to thank the Heyden Chemical Corp. for supplying the samples of' the streptomycins used in this investigation and also for the biological assays that are reported. The assistance of X. James Sage, x h o ran some confirmatory experiments on the determination of dihydrostreptomycin, is also gratefully acknowledged. LITERATURE CITED
Bricker, C. E., and Johnson. H. R., ISD. ENG.CHmI., A s z ~ . used in place of the wat'er blank for reading the optical densities. The very exacting conditions that niust be f o l l o ~ e d the , lengt,h of time required for the determination, the comparatively low wnsitivit,y, and the necessity of solving simultaneous equations are all disadvantages of the method. On the other hand, as contrasted to other chemical methods for determining mannopidost,reptoniycin (6, 10, 11 ), sugars and carbohydrate material do not interfere. Furt,hermore, this method provides an entirely new method for the simultaneous estimation of tiyo streptomycins in the presence of each other. Lead ion appears to be the only cation that gives a yellow color with streptomycin and t'he rose color with mannosidostreptoniycin after periodate treat'nient. Although a study over t,he entire pH range was not made, ferric iron, aluminum, calcium, magnesium, mercuric, mercurous, thallic, silver, beryllium, zinc,
ED.,17, 400 (1945).
Bricker, C. E., and Roberts, K. H., 4 x 1 ~CHEM., . 21,1331 (1949). Bricker, C. E., and Vail, W.-4.. Ibid., 22, 720 (1950). Colon, A , Herpich, G. E., Johl, R. G.. Keuss. J. D., and Frediani, H. A , , J r A m . Pharm. Asaoc., 39, 335 (1950). Eisenman, W,,and Bricker, C. E., ANAI.. C H m r . , 21, I507 (1949).
Emery, W.B., and Walker, =i. D., AnciZyst, 74,455 (1949). Fleury, P., and Boisson, R., C o m p t . rend., 208, 1509 (1939); J . pharm. chim., 30, 145, 307 (1939).
Garlock, E. A , Jr., and Grove, D. C.,J . CEin. Invest., 28, 843 (1949).
Hiscox, D. J.,AKIL. CHEM., 23, 923 (1951). Kowald, J. A., and AlcCormack, R. B., Ibid., 21, 1383 (1949). Perlman, D., J . Bid. Chem.. 179, 1147 (1949). Winsten, IT. A., and Eigen, E., J . A m . C h m . Soc., 70, 3333 (1948).
RECEIVED for review October 2 , 1951. Accepted March 26, 1952.
Improved Ferric Chloride Test for Phenols SAUL SOLOWAY AND SAMUEL H. WILE" The City College, College of the City of New York,New York,N . Y. The ferric chloride test for phenols as described in standard works on organic analysis fails in many cases. This study shows that this qualitative test can be greatly improved if carried out in organic solvents using small amounts ofpyridine as an alkalinizing agent. Some of the best tests are obtained in chloroform solution, a medium which had been described previously as giving negative results. All phenols, with some minor exceptions, give the test using these modified procedures. The chemistry of the test is discussed, showing the important role plaved by the solvent and an added base.
F
ERRIC chloride in solution give? colors with a number of organic derivatives. The common ones are phenols, enols, oximes, h>droxamic acids, and some carboxylic acids. Howc~vei,the appearance of a color change on the addition of ferric chloride depends to a great extent on such common factors as solvent, acidity, and concentrations of reactants. This study shows that a consideration of these factors leads to a greatly improved qualitative test for the phenolic group. The recipe given for applying the ferric chloride test to phenols is the solution or suspension of the unknown compound in water, methanol, or ethanol, to which is added a solution of ferric chloride in the same solvent ( 5 , 8, 9, 1 1 ) . If a marked color change does not result, some investigators have recommended the addition of solid or aqueous sodium bicarbonate. This procedure was parried out in water and methanol with 78 phenols (Table I). 1 Present address, Department of Chemistry, University of Kansas, Lawrence, Kans.
Among other phenols cited in the literature, Table I shows that such common compounds as p-benzylphenol, trans-diethylstilbestrol, meso-hexestrol, ethyl p-hvdroxgbenzoate, etc., gave negative tests with the usual procedure. -4consideration of the chemistry of this color test led to its trial in other solvents. Kesp and Brode ( 1 4 ) noted that color changes were obtained for many phenols in oxygenated or nitrogenated solvents, but not in hydrocarbons or their halogenated derivatives. Preliminary experiments verified thr observations of these authors. Holyever, the addition of relatively small amounts of pyridine to solutions of anhydrous feiric chloride and phenol in such solvents as benzene, toluene, o-xylene, chloroform, chlorobenzene, but>-lbromide, and ethylene dibromide, gave deep purple colors. It is the authors' experience that these solvents are superior to the ones usually used in this color test. With the exception of 2,6-di-fert-butyl-p-cresol, a highly hindered phenol that does not even react with metallic sodium in anhydrous ether
ANALYTICAL CHEMISTRY
980
Materials and Reagents. The reagents used were made up from the best grades of materials commercially available. The phenols and the few other derivatives listed in Table I were also commercial chemicals, many of them obtained gratis from the manufacturer. SOLUTION 1. Ten grams of anhydrous ferric chloride aere dissolved in 200 ml. of water and the solution was filtered. The color of the solution waa orange. SOLUTION 2. Ten grams of anhydrous ferric chloride were dissolved in 200 ml. of 99.9% methanol and the solution was filtered. The color of the solut,ion was orange. SOLUTION 3. Anhydrous ferric chloride (1.5 grams) was dissolved in 100 ml. of diethylene glycol diethyl ether and the solution was filtered. The color of the solution wm orange. SOLUTIOX 4. One gram of anhydrous ferric chloride was dissolved in 100 ml. of chloroform, 8.0 ml. of pyridine were added
(6),and many phenolic carboxylic acids, which provide a successfully competing group, every phenol derivative tried gave a color test with one of the revised procedures using an organic solvent and pyridine. EXPERIMENTAL
Three solvents were used to see the effect of adding pyridine to the ferric chloride-phenol reaction-methanol, chloroform, and diethylene glycol diethyl ether (DGDE). The addition of pyridine or other organic bases of comparable strength to the ferric chloride-phenol solution in water destroys the color because enough hydroxide ion is formed to cause the precipitation of ferric hydroxide. Diethylene glycol diethyl ether was chosen as one of the media because of its good solvent properties for anhydrous ferric chloride and for diversjfied types of phenols.
Table I.
Results of Color Tests in Four Solvents
All colois (listed) represent distinct color changes when compared to the colors of the solutions of cqinpounda in the a p ropriate solvents. t h e test is considered positive. I n all other cases the test is considered negatlve and is i n d i c a t d b y Seg. .4n arrow indicates a change in color upon addition of more reagent.
In these cases
Abbreviations B. Blue B1. Black Br. Brown G. Green Gr. Gray 0. Orange 01G. Olive green t (after a color). Transient Seg. Negative
Conipound NO.
1 2
3 4 5 G 7 8 9 10
11 12
13 14 15 16 17 18 19 20 21 22 23 24 25
26 27 28
29 30
Water Solution 1 solution 5 Solution 1
Name of Compound I. Phenols 2-Amin0-5-nitrophenol Seg. 4-Amino-3-pentadecylpheno1 Keg. m- Aminophenol Br a-Aminophenol RBr p-Aminophenol V Aminosalic lic acid R gena yl-p-hydioxybenzoate Neg. p-Benzylphenol Neg. Bis- (3-amino-4-hydroxyphenyl) sulfone RBr 4-tert-Butyl-2-(a-methylNeg. benzyl) phenol n-But 1 salicylate cateci'ol 0 Chlorohydroquinone 2-Chloro-&hydroxytoluene B a-Chlorophenol Pd Neg. 4-Chloro-o-phenylphenol m-Cresol BVd B + Gd o-Cresol Bd p-Cresol Seg. p-a-Cumylphenol 2,5-Di-fertamylhydroquinone Seg. Seg. 4,B-Di-lerf-butyl-o-cresol 2,G-Di-tert-butyl-p-cresol Neg. 2,5-Di-tert-butyl hydroquinone Seg. 4 .G-Di-tert-butvl-o-isouropylphenol Seg. trans-a a'-Diethylstilbestrol Neg. 2,4-Dih'ydroxybenzoic acid RV
GNeg,
fnne .".._
sul-
Solution 2
+
T.
Purple Pink Red Tan Violet White Yellow
2 +Solution pyridine
Chloroform Solution DGDE ex4 cess Solution 3 Solution 4 pyridine Solution 3 solution G
+
+
Bra
Br
R
R
G
Br
0
Br
Neg. Br RBr Br R Neg. Neg.
RO Neg. Br Br V h7eg. Neg.
RO b Keg. RBr Br
nv
R G
R
R OBr R RBr R Neg. Keg.
Rb
RBrb
V + BrC
Neg. Neg. V+R 0 Neg. Neg. Neg. Seg. Neg. Neg. Seg.
Keg V G Neg.
Neg. Neg. Seg.
Seg.
RBrb Br R
V
B1
P
G
R Br P
P. Br
P
R G
v B
Vb
Br
Br
Neg.
Br
R
Br
Neg. R
1-I' R
Neg.
Neg.
BBr Neg. Seg. Neg. Seg. Keg. Keg. Neg.
G OBr 0 RO T T OBr T
Xeg. Neg. Seg. Neg. Neg. Neg. Neg.
B-G R B B1 V PnJ' OBr V BV+ G
Seg. Keg. Neg.
Neg. Keg. Neg.
G: G Yep.
Neg. Neg. Seg.
G G Neg.
P. P. 01G G
Beg. Keg.
V+R
G
V
G
Seg.
Ge G Neg.
Seg.
Gi
h-eg.
R G
P
v
Keg.
Seg.
Xeg.
G1
Gt
Neg.
Seg.
G
Neg. Neg.
Keg.
n
reg. Neg. V
Pg.
T -+ 01G Gh
R
G B
V
Gb G
v
Neg. Neg. RBr
G G RBr
RBr
RBr
Br
R
n
Br
Br
R
B
1-d
Yb
Br
Bra
R Br
V
v
Br
R+
Vb 0 R Neg. Vb R Br Neg. Seg. Vb Ob Vb,0 Neg. Neg.
01G 0 R 2eg.
Brb R
R R 0
v
T R
Neg.
Rb
2,5-Dihydroxy-l,4-benzoquinone 2,4'-Dihydroxydiphenyl
+
hlethanol Solution 2 solution 5
P. Pn. R. T. V. W.
4,4'-Dihydroxydiphenyl sulT'd fone 2.4-Dinitrophenol 0 R 3.5-Dinitrosalicylic acid Keg. Ethyl-p-hydroxybenzoate V m-Ethylphenol R Guaiacol H acid Br meso-Hexestrol Yeg. teg.
31 32 33 34 35 36 37 38 39 Od 40 Vd3Q 41 Neg. 42 43 Neg. 1' 0 \b,g 44 8 Color is t h a t of a precipitate. b Color disappeared upon addition of excess reagent. C Color changed while standing. d Solution heated and subsequently cooled t o dissolve a s much
G Neg. Neg. Neg. Yep. Neg. T Gh G Neg.
Rf
Xeg. Gb Rb Br Neg. Neg. Neg. Neg. BrdRO Gb,h Gb Neg.
OT P Br Gh
R R V BY+G B Neg. B -
R T Br Neg.
Neg. V BV B Neg.
G+K
P. RO
R Vb Gb G
Geg. Neg. Vb
of compound as possible prior to addition of reagent.
Neg. Vb
G G Neg.
geg.
Xeg. RBrh V Seg. Neg. Neg. Neg. Ne& Neg. Neg. Neg. Neg.
R
R OBr Neg. BG zeg. B +G
R RO
OBr V B Neg.
BR
V O L U M E 24, NO. 6, J U N E 1 9 5 2
981 pended in 1 ml. of each of four solvents: water, methanol, chloroform, and diethylene glycol diethyl ether. At least 2 drops of ferric chloride in the same solvent were then added. To the aqueous solution or more usually the suspension, and to the methanol solution, one or more drops of sodium bicarbonate solution were added. In the case of the methanol solution, addition of pyridine was also tried, fractions of a drop to several drops of pyridine thus being substituted for the sodium bicarbonate. As a diluted solution of pyridine wp&9found to be more convenient, it was adopted for the test in diethylene glycol diethyl ether.
and the solution was filtered. In this case, a considerable fraction of the ferric chloride was precipitated as a pyridine complex, so that the resulting solution was saturated. The reagent, whose color was red, waa made up in this manner in order to show that the test can be carried out with a single solution. However, during the course of the investigation it became evident that the complexes formed with phenols are very soluble in chloroform, 80 that if pyridine were added after the phenol to a solution of ferric chloride in chloroform, a higher concentration of iron and hence a more intense color might be possible. SOLUTIOX 5. Half molar aqueous sodium bicarbonate. SOLUTION 6. Dilute pyridine-diethylene glycol diethyl ether solution waa made up by adding 25 ml. of pyridine to 75 ml. of diethylene glycol diethyl ether. Procedure. In each test approximately 30 mg. of solid or 1 drop of liquid of the substance t o be teeted was diesolved or sus-
Table I.
In many cases, the colors obtained were very sensitive to slight variations in the concentrations of the reactanta. In such cases the symbolism B+G (blue+green) was used in Table I.
Results of Color Tests in Four Solvents (Continued)
All colon (listed) represent distinct color changes when compared t o the colors of the solutions of compounds in the appropriate solventa the test is considered positive. In all other casea the test is considered negative a n d is indicated by Neg. An arrow indicates a change in color upon addition of more reagent
In these caaee
Abbreviations
P.
B. Blue BI. Black Br. Brown G. Green Gr. Gray 0. Orange 01G. Olive areen t (after a col6r). Transient Keg. Negative
Compound
No. 45
46 47
48
49
50 51 52 63 54 55 50 57 58 59 60 61 62 63 64 65 60 67 68 69 70 71 72 73
74 75 76
77 78 79 80 81 82 83 84
Water Solution 1 Solution 1 solution 5
Kame of Compound I. Phenols (Contd.) p-Hydroxypropiophenone A-Hvdroxvauinoline Stethyl sdic’ylate ,?-Methyl umbelliferone 4- (a-Methylbenzyl) -2-phenylphenol 2.2’-Methylenebis(4-methyl6-te~t-butylphenol) hi uno-6-tert-butyl-m-cresol 1-Naphthol 2-Naphthol 0 - Nitrophenol 2-Sitroresorcinol I-Nitroso-2-naphthol p-Nonylphenol Orcinol 3-Pentadecylphenol 5-Pentadeo ylresorcixiol Phenol Phenyl salicylate Phloroglucinol Picric acid n-Propyl gallate n-Propyl-phydroxybenzoate p-(n-Propyl) phenol Pyrogallol Resacetophenone Resorcinol Resorcinol monoacetate Salicylaldeh de Salicylic a d Sulfosalicylic acid Thymol 2,4,6-Tribromophenol 2,4.6-Tricblorophenol 1-Tyrosine 11. Enols Diethvl acetvl succinate nimedon (5:5’-dimethylcyclohexane-l,3-dione) E t h y l acetoacetate Ethyl benzoyl acetate III. Oximes Acetoxime n-Butyraldoxime
+
Keg. BGd
Solution 2
Methanol Solution 2 solution 5
+
0
B B
G Ga
Seg.d Keg.
P BC;1
Keg.
G+B
B
Bf
Keg.
v
Keg. Brb G ORb RO G Neg. Keg. Neg. Neg. Gb
G
Neg. Br BG Keg. Neg. Br Neg. Neg. Keg. Neg. Keg.
h’eg. Keg. R
OT Sen. P
G B1 G RBr Br 01G G G G Gb G V
G BI
G Brh G Gbvh R
B G B RBr Br Br-G B B B -+ G G P
%eg. V
v
G G BV R G
V
Keg.
Teg.
Keg. Pnb Keg. Keg.
Seg. Br G T Seg. G Neg. Keg. Neg
PnT‘ B+BG R R BY
Keg. BGb Br+P R BVb
Seg.
RV
v
v v v
G G
Vb
Br G Br
R
1-
v
R Xeg. Seg. Keg. Keg.
G reg. Neg. Seg.
Vd
Brb
Y+Br
Vd Pn+R ORd
Vb Rb
V Neg. Keg. Seg. Neg.
qeg.
R
Neg. Gb
P--+ R R Neg. ORb R
R R
Gb Neg. Rb Keg.
R
R OR
Br
v
RBr G
G
R
OR
R B+BT R Rb R R R + RV R T R
R
Neg.
v
Seg. Neg.
v v
B BY R BY B1
B BY Y Seg.
v
T’
v
G Neg. Keg. Nee.
c: R
G R
Br
G
B -+ G G
T
BGr V R Br OR V
7,
Br
Seg. Keg. B r 7,
OHr G B R G T
BV V Neg.
Neg. Keg. Neg. Neg.
G R 1Rv Neg.
K K
v
Rb
R
RV + Br
Br
T
Ti,
OTb
T+Br R R-LRTI
T
R
R
Keg. OT-R R-RV
RO R R
RO Neg.
Neg. Neg.
Keg Rh
Oa
;egv R
:;,
R R R
Nea.
Teg. Seg.
Seg. Seg.
OR
Sen.
Rh
reg.
vi,
+
RBr
Keg. Pna Keg. Keg. Br Br Neg. V Neg. Keg. P Neg. V Neg. B
Pg.
DGDE Solution 3 Solution 3 solution ti h-eg. G
Keg.
G Neg. B+G
+
+
G G
h-eg. Keg.
:eg. V
Solution 2 pyridine
Chloroform Solution 4 excess Solution 4 pyridine
v v
Neg. G R Pn Keg.
Br Keg. Vb Neg. Neg. Pb Keg. Xeg.
Y.
G
Y Seg. Seg.
R
T. V W.
Purple Pink Red Tan Violet White Yellow
G
Keg. GrBl T’ Neg. h7eg.
v
Pn. R.
b
IV. Thiols G G Seg. Ga C7 2-Aminobenzenethioi 85 Neg. Keg. Seg. B11 Blt Thio-Znaphtbol 86 Caing suspension of anhydrous ferrio chloride i n chloroform a n d adding a small drop of pyridine. / Color decreased in intensity b u t did not disappear upon addition of excess reagent. 9 Test r e r y weak. I, Concentration of phenol must be high.
G Seg. G GBll GBlt Keg. Regular procedure gave negative test.
G5
G
ANALYTICAL CHEMISTRY
982 Table 11. Effect of Variation of Solvent Compound in Chloroformo
+
solutr
4b Blue Color"
(Divide in Half)
\
/
Controld NO
change
Addition of Methanol
Addition of Chloroform
Blue color disappears
Blue color remains (decrease in intensity)
Controle KO
change
Four to 5 drops of liquid (or 60 mg. of solid) in approximately 2 ml. of chloroform. Compounds tested were p-benzylphenol, mono-6-tert-butyl-mcresol, a-chlorophenol, and p-nonylphenol, b Ferric chloride and pyridine in chloroform. C Violet in case of o-chlorophenol. d Methanol added t o Solution 4 in chloroform. e Excess chloroform added t o Solution 4 in chloroform.
RESULTS
'
was developed in chloroform with several phenols, and the solution was divided in half and diluted with equal volumes of chloroform and methanol. In the latter case, methanol changed the blue color to a light yellow, vihereas the additional chloroform merely lightened the color. The addition of more of the phenol and ferric chloride to the half containing methanol again produced a color test, obviously showing that this test is more sensitive in chloroform solution. This confirmed the authors' impressions, in preliminary experiments as well as in those from which Table I was compiled, that the aromatic hydrocarbons and halogenated aliphatic and aromatic hydrocarbons are superior to methanol and diethylene glycol diethyl ether as solvents for this test. The colors obtained in the former solvents were generalIy more intense.
Table I is a summary of the colors obtained with 78 phenols. Comparison of results with those obtained by other investigators under approximately the same conditions show differences in color. These apparent discrepancies were found to be due to slight differences in the experimental environment, such as the use of ferric chloride hexahydrate instead of anhydrous material in making up an alcoholic solution, concentration differences, etc. For example, such minor color changes were obtained with the cresols, o-chlorophenol, resorcinol, etc., in methanol without the addition of pyridine, that the test was originally noted as negative in Table I. When, however, higher concentrations of these phenols were used, very definite color changes were noted. The use of ferric chloride hexahydrate in methanol gave distinct color changes with the same phenols a t the same concentrations. With the exception of salicylic acid and its derivatives, the only class of phenols which gave doubtful or negative tests were those that contained a carboxylic acid group. A few thiophenols, enols, and oximes were included in the table to indicate the applicability of the revised test to other types of derivatives. The case of the hydroquinone derivatives is rather unusual. Whereas the monophenolic derivatives, which are not so easily oxidized, gave stable colors, the hydroquinones gave fleeting color changes. In water, the oxidation of these compounds with ferric ion is so rapid that any interpretable color change escapes the observer. Apparently, the oxidation rate in organic solvents is lower, as is the case with permanganate as an oxidant in pyridine and acetone as against water as a medium. ,4few experiments were carried out in which other amines were used instead of pyridine. In these experiments the color variation using m-cresol in benzene and methanol with anhydrous ferric chloride in the same solvents was determined. The amines compared were pyridine, X,X-dimethylaniline, 2-ethyl hexylamine, 8-dimethylaminopropylamine, @-dimethylaminopropionitrile, and diisopropylamine. In benzene solution, pyridine was far superior to the other amines in giving a more radical color change-e.g., to a blue. The other amines, in benzene, gave solutions that varied from light red to practically no change, with the formation of precipitates. In methanol solution, dark tan colors were obtained with pyridine and dimethglaniline, while the aliphatic amines gave red colors with precipitation. Table I1 shows clearly how the color obtained in this test varies with the solvent and the phenol concentration. The color
DISCUSSION
The question of what reaction actually occurs when ferric chloride in a solvent gives a color with a phenol has been discussed by many authors (2,3, 10, 13,14). It was Raschig (10) who first asserted that the color obtained was due to salt formation and not, as previously thought, to an oxidation product. Incidentally, Weinland and Binder ( 1 3 )found that catechol did not give a color with ferric chloride in acetophenone solution until pyridine was added. Wesp and Brode ( 1 4 ) postulated that the color was due to an anionic iron complex of the formula [Fe(OR)$]---, where OR is the conjugate base of the parent phenol. Banerjee and Haldar ( 2 ) , in a preliminary report, have recently proposed the complex Fe[Fe(OR)6]3as the color agent. None of these formulations has properly considered the role of the solvent. Ferric chloride crystallizes from water around room temperature as the hexahydrate, from the alcohols as alcoholates, and is solvated by many other solvents. Hence, it is reasonable to assume that in solution it exists as a solvated molecule, or dissociates to give solvated ions. The addition of a phenol must cause a displacement of solvent in order to get an iron-phenol complex of the anionic type. IYesp and Brode found that in an electrolysis the colored complex migrated to the anode. It is understandable that in solvents like xater and alcohol, the medium serves as the proton acceptor, so that it is phenoxide ion which can displace a molecule of solvent from the solvated ferric ion. It also becomes understandable why solvents like the hydrocarbons and their halogenated derivatives, which are particularly poor proton acceptors, would not permit the color test without an added base. On the other hand, the fact that these solvents are such weak bases is advantageous in the test, because they will probably form weaker solvated complexes with a Lewis acid like ferric chloride than water or alcohol, and hence be displaced more easily by relatively stronger bases like phenoxide. This idea may be formulated in terms of the following equilibria for the reaction in water: +OH
+ [ F e ( H 2 0 ) ~ ] ~d +H 2 0 + [Fe(H~0)5(+-0H)]+++ T
1
TT
11
111
-
Although the phenols are relatively weak acids (pK, lo), their strength should be greatly increased when complexed with triply charged ferric ion. The question of whether the solvent molecules are replaced in steps or are all replaced to give a coinplex of the type suggested by Kesp and Brode remains to be investigated. A spectrophotometric study varying concentrations might show the coexistence of various species. In other solvents such as the hydrocarbons and their halogenated derivatives, the concentration of ions is probably minute,
V O L U M E 2 4 , NO. 6, J U N E 1 9 5 2 so that any formulation should involve solvated ferric chloride molecules. In the few experiments cited using benzene as a solvent, the addition of pyridine gave good colors, whereas the other amines gave little color change, precipitation taking place instead. In methanol on the other hand, color formation was observed with all the amines added, the colors differing in some cases with the amine. These results indicate that the added amine may play a more complicabed role than merely that of a proton acceptor. It probably becomes part of the complex. It is difficult to underst’and TVesp and Brode’s interpretation of their own results on t’he variation of the absorption maxima obtained with the salicylic acid-ferric chloride reaction in various solvents. Their results show that the total spread obtained in varying the solvent is about the same as that obtained in varying the phenolic derivative in a given solvent. From these result,s one ~ - o u l dreasonably conclude that if color formation is due to an iron-phenol compound, such a combination must include the solvent as part of it. Yet they concluded that the solvent played no part in the reaction. Recently, Babko ( 1 ) in a spectrophotometric study of the ferric thiocyanate problem, has postulated the stepwise formation of various complexes [Fe(SCN) to Fe(SCN)a---] TFith increasing concentration. In a similar study of the cobalt t’hiocyanate problem, Lehn6 ( 7 ) has explained his results in terms of hydrated complexes from Co(H20)6++to C O ( H ~ O ) ~ ( S C N ) ~ - . The most recent x-ork done along these lines was a spectrophotometric investigation of the ferric chloride complexes of m-cresol, salicylaldehyde, and ethyl acetoacetate by Herbst et al. (4). Maxima were obtained at concentrations corresponding to 1 FeCI3:3 m-cresol, 1 FeCI:! :3 ethyl acetoacetate, and 1 FeCla : 1 salicylaldehyde. These workers, therefore, question the structure Fe(OR)e--- as that of the colored ferric chloridephenol complexes. Xesp and Brode ( 1 4 ) proposed this latter structure on the basis of the simi1arit.yof the absorption spectra they obtained, to those observed for other iron complexes such as thiocyanate and cyanide complexes, whose structures, a t the time, were thought to be Fe(SCS)ti--- and Fe(CN)G---, respectively. .Iceording to the formulation proposed in the above equilibria, the new findings would be interpreted by the formulas +-
[
1
F e ( H 2 0 ) 3 ( O a ) i (and similarly fortheethyl acetoacetate CH,
complex) and [fi/o‘Fe(H~Oh
4
the salicylaldehyde
983 type given in the above equations. The temperature shifts the equilibrium, thus resulting in a different distribution of colored solvated complexes-Le., not only is the degree of solvation changed, but so may be the number of OR groups in the coordination sphere of the ferric ion. In the course of this work, it was striking to note the great differences in colors as well as the intensities of the colors obtained in water versus methanol as a solvent. For example, the cresols and o-chlorophenol gave excellent tests in water but poor ones in methanol, in spite of the fact that the compounds were totally soluble in the organic solvent but insoluble for the most part in water. These results can be readily explained on the basis of the fact that water is a better proton acceptor than methanol, and hence yields a greater concentration of phenoxide, allother environmental factors remaining constant. Of course, the color differences of the aquo and the methanolic complexes may also be important factors. The phenolic carboxylic acids present a case of competition between two groups for the solvated ferric ion or molecule of ferric chloride. This competition m-as demonstrated by bleaching to yellow the deep blue color of the m-cresol-ferric chloride comples in benzene upon addition of acetic, benzoic, or p-hydroxybenzoic acids in amounts much less than the equivalent amount of pyridine present. The addition of a large excess of pyridine drop by drop did not cause the blue color to reappear. Also, the addition of these carboxylic acids to the solution of m-cresol and ferric chloride in benzene prevented any significant color change from taking place on dropwise addition of excess pyridine. Similar results were obtained in methanol and chloroform. Interestingly enough, this effect was not observed with salicylic acid. The use of carboxylic acids as phenol competitors in the formation of iron complexes may serve to distinguish chelated (salicylic acid and other derivatives) from nonchelated iron complexes. This question is no\%under investigation. The problem of the relation of the size and shape of the phenol-Le., steric effect-to complex formation is illustrated with some of the dialkylated derivatives-for example, 2,4-di-tert-butyl-o-cresol and 2,4-di-tert-butyl-o-isopropyl phenol gave positive tests, n hereas 2,6-di-tert-butyl-p-cresol did not. ACKNOWLEDGMENT
The authors acknowledge with thanks gifts of experimental samples from the American Cyanamid Go., Dow Chemical Co., Edwal Laboratories, Inc., Heyden Chemical Corp., Irvington Varnish and Insulator Co., Koppers Co., Inc., Monsanto Chemical Co., Rohm and Haas Co., and Tennessee Eastman Corp.
L
complex, assuming that the complex is a chelated one. In the case of the neutral complexes, if a fourth phenol molecule should complex-Le., displace a water molecule-the resulting complex has no charge. Hence, the loss of a proton from [Fe(H20)2 (+O),(+OH)] should be very difficult in the absence of added base. There may be other makima at different concentrations and wave lengths, so that the case is far from closed. The similarity of these reactions to the one under discussion in this paper warrants citing them as analogous situations. A reversible thermochromic effect was observed in some cases. For example, the color of the 2,4,6-tribrornophenol-ironcomplex in toluene and 1-butanol is lightened and changed on boiling and returns on cooling to room temperature. This is not true for o-cresol. In this case, boiling the solution probably accelerates an oxidation reaction. 4 similar reversible effect was observed by Soloway and Lipschitz ( 1 2 )when they studied the formation of ferric hydrovamates in propylene glycol. Aqueous solutions of cobalt salts show this effect. The e\planation usually given for this latter case is a change in the degree of solvation with temperature. In the case of the iron complexes, the result can be explained on the basis of the interlocking set of equilibria of the
LITERATURE CITED
Babko, 4. K., J . Gen. Chem. (U.S.S.R.), 16, 1549-60 (1946). Banerjee, S., and Haldar, B. C., Nature, 165, 1012 (1950). Claase, M., Arch. Pharm., 253, 360 (1915). Herbst, R. L., Jr., Close, R. H., Massacua, F. J., and DTyer, R. F., J . Am. Chem. SOC.,74, 269 (1952). Kamm, O., “Qualitative Organic Analysis,” 2nd ed., p. 63, Xew York, John Wiley & Sons, 1932. Koppers Co., Inc., Pittsburgh 19, Pa., BUZZ. C-9-115. LehnB, M., Bull. SOC. chim. France, 1951, 76-81. Meyer, H., “Analyse und Konstitutionsermittlung organischer Verbindungen,” 6th ed., pp. 375-9, Vienna, Julius Springer, 1938. Openshaw, H. T., “Laboratory Manual of Qualitative Organic Analysis,” pp. 41-4, London, Cambridge University Press, 1946. Raschig, F., 2. angem. Chem., 20, 2065 (1907). Shriner, R. L., and Fuson, R. C., “Systematic Identification of Organic Compounds,” 3rd ed., p. 98, Sew York, John Wiley &Sons, 1948. Soloway, S., and Lipschite, A., ANAL.CHEY.,in press. Weinland, R. F.. and Binder, K., Ber., 46, 874 (1913). TTesp, E. F., and Brode, W. R., J . Am. Chem. Soc., 56, 1037 (1934). RECEIVED for reriew February 15, 1952. Accepted Aprll 19, 1952.
