V O L U M E 25, NO. 4, A P R I L 1 9 5 3 One of the authors (P.S.F.) wishes to acknowledge the assistance of the Merck Graduate Fellowship in Analytical Chemistry
(1949-50).
595 Latimer, R. M., “Oxidation States of the Elements,” p. 53, New York, Prentice-Hall, 1938. McConnell, H., and Davidson, K.,J . Am. Chem. S O C ,72, 3164 (1950).
LITERATURE CITED (1) Buck, R. P., and Swift, E. H., ANAL. CHEM.,24, 499 (1952). (2) Farrington, P. S., J. Am C h m . Soc., 74, 966 (1952). (3) Farrington, P. S., and Swift, E. H., ANAL. CHEM.,22, 889
(1950). Fenwick, F., J . A m . Chem. Soc., 48, 860 (1926). (6) Foulk, C. IT., and Bawden, A. T., Ibid., 48, 2045 (1926). (6) Kolthoff, I. hl., and Lingane, J. J., pp. 455-8, “Polarography.” Yew Tork Interscience Publishers, 1946. (4)
hleier, D. J., hlyers, R. J., and Swift. E. H., Ibid., 71, 2340 (1949). Myers, R. J., and Swift, E. H., Ibid.,70, 104T (1948). Ramsey, W. J.,Farrington, P. S., and Swift, E. H., ANAL. CHEM.,22, 332 (1950). SeasP, J. W., Kiemann, C., and Swift, E. H., Ihid., 19, 197 (1947). R E C E I V ~for D review August 27, 1951. Accepted January 24, 1953. Contribution No. 1623 from the Gates and Crellin Laboratories of Chemistry, California Institute of Technology, Pasadena, Calif.
Differentiation of Chelating from Nonchelating Phenols SAUL SOLOWAY AND PERRY ROSEN The City College, College of the City of New York, New York, N. Y . As a result of previous observations in connection with a modified ferric chloride color reaction for phenols, a test has been developed to differentiate chelating from nonchelating phenols. This test is based on the fact that ferric complexes of chelating phenols have a wider range of pH stability than the others. It is shown that the stability of the phenolic-ferric ion complexes varies considerably with the solvent. Results indicate that acetic acid shows promise as a competitive agent against certain types of weakly chelating phenols, and hence may be used to differentiate members of the chelating class from one another. The reason for the differences in chelating strength of some of the common orthosubstituted phenols with ferric ion is discussed briefly.
A
LTHOUGH it is well known that certain ortho-substituted phenols form stable heterocyclic structures with many metallic ions, no simple chemical procedure is available to distinguish such chelating phenols from those not capable of chelation. Practically all phenols give color reactions with ferric chloride under certain conditions (8). However, the chelating types yield complexes which are more stable over a wider range of environment. This fact serves to differentiate the chelaters from the nonchelaters. I t was recorded (8) that acetic, benzoic, and p-hydroxybenzoic acids could inhibit the formation of the typical color reaction between m-cresol and ferric chloride in some solvents, but not with salicylic acid under the same conditions. This observation led to the thought that carboxylic acids might form complexes with ferric ion of a stability intermediate between those of chelating and nonchelating phenols. The procedure developed on the basis of this idea proved to be only partially successful-that is, acetic acid apparently can compete against any nonchelating phenol for ferric ion, but this was also the case for some wellknown chelating derivatives. A second scheme based on the greater p H range stability of chelated phenol-ferric ion complexes proved to be successful for carrying out the differentiation. This test used in conjunction with the first one allows not only the differentiation of chelaters from nonchelaters, b u t also the distinction of certain chelaters from one another on the basis of their relative stabilities toward acetic acid. EXPERIMENTAL
The tests using acetic acid as a competitive agent against phenols for ferric ion were carried out in five solvents: water. methanol, benzene, diethyl ether, and 2,2’-dichlorodiisopropyl ether. This variation served a twofold purpose: to provide at least one good solvent for all the compounds tested and to furnish some information on the variation of the stability of the complexes with solvent. I n the second set of tests a phenol was added to a suspension of
ferric hydroxide in water and methanol. These suspensions were prepared by adding aqueous ferric chloride to dilute solutions of pyridine in water and methanol. The pH, as determined with a set of pHydrion indicator test papers, was found to be in the range 4.6 to 4.8. It was noted (Table 111) that under these acid conditions most of the chelating phenols gave stable colors with the ferric ion present, whereas none of the nonchelaters did. On further acidifying the solutions to the extent of dissolving the excess ferric hydroxide, it was noted that all the chelaters gave colored solutions and/or precipitates easily distinguishable from a blank of ferric ion a t the same p H (1.7 to 1.9). The nonchelaters gave the same color as the blank. These results, which are given in Table 111,demonstrate the greater pH range stability of chelating phenol complexes with ferric ion. MATERIALS AND REAGENTS
The reagents were made up of the best grades of materials commercially available. All the organic solvents were anhydrous. The phenols and other derivatives tested were mostly commercial chemicals. Some few were synthesized. I n such cases the purity as judged from physical constants was at least the equal of the materials described in the literature. Solution 1. Ten grams of anhydrous ferric chloride were dissolved in 100 ml. of water. The solution was filtered. I t s color was orange. Solution 2. Ten grams of anhydrous ferric chloride were dissolved in 100 ml. of absolute methanol. The solution ~ - y & 9filtered. I t s color was orange. Solution 3. One-half gram of anhydrous ferric chloride was suspended in 100 ml. of anhydrous benzene. After the suspension was shaken for a short time, the solution was allowed to stand and then was separated from the undissolved material by decantation. The color of the solution was dark brown. The solution was not filtered because a trial run gave a filtrate with a muddy brown appearance, presumably due t o the uptake of a small amount of water from the paper with the possible formation of some insoluble hydrate. Solution 4. Five grams of anhydrous ferric chloride were dissolved in 100 ml. of peroxide-free diethyl ether. The solution was filtered. I t s color was dark brown.
ANALYTICAL CHEMISTRY
596
Table I.
Results of Color Tests in Five Solvents
(A11 colors listed represent distinct color changea,when compared t? the colors of,the solutions of compounds in the appropriate solvent. When no distinctive color change takes place, the test is considered negative and is indicated b y Neg. An arrow indicates a change of color on standing.)
Abbreviations R. B1. Br. C.
Compound
NO.
Name of Compound
1 2
2-Amino-5-nitrophenol m-Aminophenol o-Aminophenol p-Aminosalicylic acid n-But 1salicylate catecxol 2,4-Dihydroxybenzoicacid 2,5-Dihydroxy-1,4-benzoquinone 4,4 -D1hydroxy-3,3'-diaminodiphenyl sulfone 4,4 '-Dihydroxydiphenyl sulfone 2,4-Dinitrophenol 3,5-Dinitrosalicylicacid Gallic acid o-Hydroxyacetophenone p-Hydroxyacetophenone m-Hydroxybenzaldehyde p-Hydroxybenzaldehyde m-Hydroxybenaoicacid p-Hydroxybenaoicacid o-Hydroxybenzylalcohol 2-Hydroxy-3-methoxybenzaldehyde 1-Hydroxy-2-naphthoic acid 3-Hydroxy-2-naphthoic acid 2-Hydroxyquinoline 8-Hydroxyquinoline o-Kitrophenol Orcinol Phenol Phenyl salicylate 5-Phenylsalicylic acid n-Propyl gallate Pyrogallol Salicylaldehyde Salicylic acid Sulfosalicylic acid Xylenol
Blue Black Brown Colorless
Water Solution 1 pyridine acetic acid
G. Green Gr. Gray S e g . Segative 0. Orange P. Purpie
Pn. Pink
R.
15..
Y.
Methanol Solution 2 pyridine acetic acid
++
Red White Yellow
Benzene Solution 3 pyridine acetic acid
++
Diethyl E= Solution 4 pyridine acetic acid
++
+
+
Dichlorodiisopropyl Ether
+
Solution 5 ,pyridine acetic acid
+
I. Phenols 3 4 5 6
7 8
9
10
11 12 13 14 15 16 17 18 19
20 21
22 23 24
25 26 27 28 29
30 31 32 33 34 35 36
11.
YBrQ Seg. Bra B r P Re G" P
Bra ROb Bra R Rob Ba RBr
Br 0 Br R 0 BP" RBr
RBr G R
P
Br Br RBr R Br BP RBr
RBr
RBr
RBr
RBr
RBra Bra
Br
Pn Neg. R B P
0 0 0
Rob ROb ROb P ROb P Rob 8 RO* P P ROb Keg. ROb Neg. ROb B RBra
P 0 0 0 0 0 0 0
Br Br RBr
0 Br RBr R RO YG R
Neg. Neg.
P
R Bra BrPa Br R B Br
Pa
T
BrC B Neg.
Neg. Neg. Neg. Neg. 0 Wa 0 0 R YC B B RO RO
P Bra Br Bra
RBrd Yf Ba
RBr" Bra
0
RO
RO
Neg.
RBra R B P
T
Bra
Bra
P
Bra
Xeg.
Neg.
Neg.
Xeg.
GrO
Y
Y
'E
T
BrC
R 0 OBra Y
Yf I*f Y Ba
BrC Seg. Neg.
RBr
RBr
R GrG YGr
Y YG YGr Y Y
E9 G
YO"
P
Y Neg. GC Keg.c 0
C
G
P C
Br
0
B P B P R R e -+ 0 OBr' 0 Xeg. Neg. R YO Neg. Neg. R R P RBr+ Y
Neg.e P C Neg. R Y cc Y a Ya Ge P P Y R e Y Neg. Neg. Keg. S e g . Seg. Keg. Pn Y heg.C Neg. Seg. Neg.C Neg. Seg. Pne Pe C
Br
G
Y
Br
B
B
Neg. Neg. Keg. Neg. Neg. Neg. YGa G G Keg. Neg. Seg. Keg.< GrG Y Neg.c P Pne C Neg.C R k Neg. B C Ge P P Seg. P P RO R Y Keg. ROd P Y Y Y Y BG Y
Br Keg. GrGa Keg. YG YG RBr Br Br GrBl Br
Ge
Br Neg. GrG Seg. Neg. Seg. Seg. Neg. Neg. h-eg.
B Seg. GrG Br YG P Br B BPQ P R R RO
YBr Pieg. GrG Y Y
R Y Y
BGa Seg. GrGa Br Ga BrQ YBra Ba Ba Ba Bra Pa RPa GB1
Keg.
G h-eg. GrBl Neg. BrC$e YG Br B G Y G Br
P
P Neg. GrBl
P P P Y
RBra R B I Rob 0 GB1a GBla Rob 0 Rob 0 Rob 0 ROb 0 a R a P a p a B " B RE Rd RBr R B r RBr RBr ROb 0 Rob O
B
TI
G Y Y YI Y
Y
0 0
Diethylacetylsuccinate Ethyl acetoacetate
P P
Br R
39 40
111. Oximes p-Aminobenzamidoxime p-Aminophenylacetoamidoxime-dihydrochloride Benzamidoxime
Br
R
RO
RBr
RBr
R
Br OBr
R
RO RO
RBr
B
RBr
R
R
R
R
-L
O
0
Y
T
RPa
?"". Y
Pn-+Br
GrG
YG BP" P O Y Y Y
Y Yf
beg. Br
geg.
2-w
Ba Ba OBr
Yf
Yf
Enols
37 38
Y
GrBl G r + YG
Yf
B
P
Y4
BGa
YGa
S e g . Pne
R
wa
Ya
G"
0 P
P
0". 0Y'G. G
BGa
RBr P
R R
Yf Y Yf Y
YBr Neg. RBr Br- OBr Y BGra T
Ba
G
G
Y Bre YO Y P B B RO RBr P RO
Br
Keg.
Br Br Xeg. Br BrC Neg. Neg.
See. RBr Br R Bo Bra
Br
Br B1
Neg. GrBl OBr Br Br P P B B RBr RBr
f:
TG
Y
RBr Br
BG h-eg. BP B K:g. B R B R
YG Brd Keg. Bra Yo ROa B1' Ba R Br $Bra Br Br 0 Brd Seg. RBrd Neg. Bra OBr GrPa
Y
Keg. Neg. RBr4 Neg. Neg. Keg. Neg.
Pn*
BGr BG
T
GQ I G GBI'" Bra
Xeg. Pn
Y
Neg.
Neg. R
Br RBr
Neg.
RQ
R
Brg
Gra
Xeg.
Neg.
Y
Y
Neg. 0"
Keg. Oa
Neg. C
Keg. RQ
Brb Bra
Yf Y
Neg. Ra
Y
Y
Y
R P P 0 41 Color is t h a t of precipitate. b Precipitate is t h a t of ferric hydroxide. c Concentration of phenol was increased. d Appearance of oil. e Weak color. I Method of test will give yellow precipitate in diethyl ether without addition of phenol. IColor of undissolved material.
Bra
Y
a
Solution 5. Ten grams of anhydrous ferric chloride were dissolved in 100 ml. of 2,2'-dichlorodiisopropyl ether. The solution was filtered. I t s color was dark brown. Solution 6. TKOmolar aqueous hydrochloric acid. Solution 7. Dilution of 12 M aqueous hydrochloric acid with methanol to yield a 2 M solution. This solution is referred to as 2 M methanolic hydrochloric acid. PROCEDURES
use of Acetic Acid as Competitive Agent, Approximately 20 mg. of solid or 2 drops of liquid were dissolved or suspended in 1.5 ml. of each of five solvents: v-ater, methanol, benzene, diethyl ether, and 2,2'-dichlorodiisopropyl ether. To these solutions 1 drop of ferric chloride solution in the same solvent was added, followed by 1 drop of pyridine which developed the color of the ferric-phenol complex, and finally 2 drops of glacial acetic acid, The presence of a color distinctly different from that of the blank was interpreted to indicate the presence of a group. The change for each at each stage of this procedure is recorded in Table I. Effect of Change of pH on Color of the Ferric-Phenol Cornplex. One drop of aqueous ferric chlpride and 1 drop of pyri-
dine were added to 1.5 ml. of each of two solvents: water and methanol. The precipitation of ferric hydroxide in the aqueous solution was immediate, whereas it was necessary to heat the alcoholic solution to boiling in order t o get rapid precipitation. The methanolic suspension was cooled to room temperature. Approximately 20 mg. of solid or 2 drops of liquid were then added, followed by the dropwise addition of 2 ;If hydrochloric acid in the same solvent until the number of droas added was 1 in excess of the number just required to dissolve the excess ferric hydroxide. The presence of a color dietinctly different from that of a blank was taken t o mean the presence of a chelating group. Table I11 records the colors obtained a t each stage of this procedure. RESULTS
The data of Table I show that acetic acid competes successfully against all nonchelating phenols for solvated ferric ion or solvated ferric chloride. However, in the cases of those compounds normally thought of as chelaters, some do not show up as such in any of the solvents tried (see Table 11). These types are the salicylate esters, enol esters, and the o-hydroxy carbonyl com-
597
V O L U M E 25, NO. 4, A P R I L 1 9 5 3 pounds. From Table I11 I t is seen t h a t these same compounds. like the simple nonchelating phenols, gave no color with the suspension of ferric hydroxide bcfore acidification. This behavior is an indication of a weakly chelating or nonchelating phenol. The addition of acid, however, followed by the dissolution of the ferric hydroxide and consequent increase of ferric ion conceritration yields colors with these weakly chelating phenols b u t not the nonchelating ones. The strongest chelaters of the compounds tried are the odihvdric phenol derivatives. It is seen in Table I1 that these corripounds shon chelation in all solvents. The fact t h a t the two o-hydroxynaphthoic acid isomers show chelation in all solvents but benzene, n hercas salicylic, 2,4dihydroxrbenzoic, and 5phenylsalicylic acids shoiz .chelation in water and nqethanol only, ma\ be due to the higher solubilities of the iron chelates of the former in nonhydroxrlic solvrnts. It was the inability of acetic arid t o distinguish clearly many of the chelating from nonchelating ferric-phenol complexes which led t o the trial of hydronium ion or, more specifically, an acid buffer as competitor. Table I11 shov s that in this way it was possible to obtain a clear distinction between the two classes of phenols, One exception was encountered-namely, o-nitrophenol. The reasons why this potential chelater does not give a positive test are given in the discussion. On standing in the aqueous solution for a niinute or longer the methyl salicylate test mixture developed a purple color. This behavior was probalily due to the liberation of some salicylic acid as a result of the hydrolysis of the ester. Although too felv compounds have been tried, Table I1 indicates that further study might show that chelaters can be divided qualitatively in groups by the use of acetic acid and other comprtitive agents for the ferric-chelate compleues. DISCUSSION
The structures of the compounds formed between ferric ion and chelating phenols in aqueous solution have been investigated by various authors ( 1 4 ) . This work indicates thattheratio of the number of moles of ferric ion to simple phenol, chelating phenol, or enol varies with pH. Babko ( 1 ) found that a t a low p H the ferric ion : salicylate ratio was 1 to 1. This result was substantiated recently by Broumand and Smith (4). The colored ion present may be formulated as
Table 11.
1
2 3 4 5 6
7
8 9 10 11
1” 13
14
15 16
17 18 19 20 21 22 23 24 25 26 27
+-
The ratio could be lon-wed t o 1 :2 and 1:3 with increased pH, according to Babko. Siniilar results were obtained by Foley and -4nderson (6) with ferric ion and sulfosalicylic acid in acid solution, and by Banks and Patterson ( 3 )in alkaline solution. Complexes of ferric ion n i t h ethyl acetoacetate investigated by Herbst (6) and by Brouniand and Smith also gave similar variation with pH. The addition of acetic acid to the ferric-phenol complex in aqueous solution produced only a negligible change of pH in these euperiments, so that any color change is due t o specific anion competition. Tables I and I1 show that this competition is dependent on solvent. One of the prime factors in stabilizing these ferric complexes is the ability of a given solvent to take up protons from the complexing agent. Hence, the solvents methanol and water, which gave a greater number of chelation results, might have been expected to do so because of their greater basicities and dielectric constants. Benzene, the weakest base, gave the least number of chelation reactions whereas the ethers occupied an intermediate position. Table I1 shows t h a t the weakest chelaters are those phenols having strong electronegative groups such as nitro, carbonyl, and carbalkoxy in the ortho position. These groups can be represented in resonance structures as containing a positive atom Tyhich would compete with ferric ion for an electron pair, and hence weaken the ferric oxygen bond. Since the nitro group is the strongest electronegative group of those mentioned, it might be expected t h a t o-nitrophenol Lyould give the weakest test. This idea is contained in the following structures for the chelates containing the three groups mentioned:
Suinmary of Results of Table I Showing Variation of Chelation with Solvent
Compound p-Aminohenzamidoxime 2-Amino-5-nitrophenol o-hminophenol p-Aminophenylacetoamidoximedihydrochloride p-Aminosalicylic acid Benzamidoxinie n-Butyl salicylate Catechol Diethyl acetyl succinate 2,4-Dihydroxyhenzoic acid 2,5-Dihydroxy-l,4-benzoquinonr 4,4’-Dihydroxy-3,3’-diaminodiphenyl sulfone E t h y l acetoacetate Gallic acid o-Hydroxyacetophenone 2-Hydroxy-3-methoxybenzaldehyde 1-Hydroxy-2-naphthoic acid 3-Hydroxy-2-naphthoic acid 8-Hydroxyquinoline o-Nitrophenol Phenyl salicylate 5-Phenylsalicylic acid n-Propyl gallate Pyrogallol Salicylaldehyde Salicylic acid Sulfosalicylic acid Indicates presence of chelation. Indicates absence of chelation.
Diethyl E t h e r
Dichlorodiisopropyl Ether
Benzene
RIet hanol
Water
Xumh+r of Solvents in Which Chelation Takes Place
ANALYTICAL CHEMISTRY
59% Table 111.
Summary of Chelating Derivatives as Shown by Change of pH
(All colors listed in this table represent distinct color changes when compared t o the colors of the solutions of compounds in the appropriate solvent. When no distinctive color change takes place the test is comidered negative and is indicated by Neg. An arrow indicates a change of color on standing.) Abbreviations Gr. Gray P n . Pink B. Blue B1. Black S e g . Segative H. Red 0. Orange W, White Br. Brown P. Purple Y. Yellow G. Green ComMethanol Water pound Ferric hydroxide 3Iixture -t solution 7 Indication Ferric hydroxide Mixture solution 6 Indication N0 S a m e of Compound I. P h e n o l s Y Nonchelater Nonchelater Y Neg. 1 Benzyl-p-hydroxybenzoate Neg. Y Nonohelater Neg. Nonchelater Y Neg. 2 p-Benzylphenol B r Chelater Br Nonchelater Y Neg. n-But 1 salicylate 3 G Chelater Chelater GrBl B Br 4 CatecEol Y Nonohelater Neg. Y Seg. Nonchelator Chlorohydroquinone 5 Y Nonohelater Y Seg. Neg. Nonchelater 2-Chloro-5-hydroxytoluene 6 Chelater Br Br R Chelater Br 2 4-Dihydroxybenzoic acid 7 2:5-Dihydroxy-l,4-benzoqui8 RBr Chelater Chelater R R B r none 4,4‘-Dihydroxy-3 3’-diamino9 Br Chelater Br Chelater Br RBr diphenyl sulfon’e Nonchelater Y Nonchelater Y 4,4’-Dihydroxydiphenyl sulfone 10 Chelater G C h d a t e r g. GB1 11 Gallic acid P Chelater RBr Chelater P Br Neg. o-H droxyacetophenone 12 Nonchelater Y Neg. Nonchelater Y Neg. o-ddroxvscetoohenone 13 Nonchelater Y Y Neg. Sonchelater ;n-~ydrol;ybenzaldehyde Neg. 14 Nonchelater Y Y Neg. Xonchelater p-H droxybenzaldehyde Neg. 15 Nonchelater 0 Seg. Y Nonchelater Neg. m-&.droxyhenzoic acid 16 Y Sonchelater Y Neg. Nonchelater Neg. p-Hydroxybenzoic acid 17 Chelater G Br Br-B Chelater RBr o-Hydroxybenzylalcohol 18 Nonchelater Y Neg. Nonchelater Y Neg. p-Hydroxydiphenyl 19 2-H~droxv-3-methoxvbenzal20 Chelater Chelater Br G B C; Br deh deChelater BG Br Pn* -+ B Chelater Br 21 l-Hydroxy-2-naphthoic acid Chelater G Chelater Br Gr Br 22 3-Hydroxy-2-naphthoic acid Nonchelater Y Nonchelater ?;eg. Y Seg. 2-Hydroxyquinoline 23 Chelater GrB1a Chelater Br G Br 8-Hydroxyquinoline 24 P Chelater Seg. Chelater Y-Pb Seg. Methyl salicylate 25 Nonchelater Y Seg. Y Nonchelater Neg. 26 2-Naphthol Nonchelater Y Neg. Nonchelater Y Neg. 27 o-Nitrophenol Nonchelater Y Nonchelater Seg. Y Neg. 3-Pentadecylphenol 28 Nonchelater Y Nonchelater Neg. Y-YG Neg. 29 Phenol Chelater P-R Nonchelater Br Y 4eg. Phenyl salicylate 30 Chelater G Chelater? Br W GrQ BrP 5-Phenylsalicylic acid 31 Chelater Sonchelater Y Nonchelater Y-YG Neg. 32 n-Propyl-p-hydroxyben: zoate g* G B Chelater Br 33 n-Prop 1 gallate Chelater Br P Chelater R P 34 PyrogJol Chelater Br P RBr Chelater Neg. Resorcinol monoacetate 35 Chelater Rr Br P Chelater Br 8-Resorc lic acid 36 Chelater P R Br P Chelater Br 37 Salicyladehyde Chelater Br R Chelater P“ R Salicylic acid 38 Chelater R P Br Chelater R Sulfosalicylic acid 39 Nonchelater Y 1‘ Neg. Nonchelater Seg. 2,4,6-Tribromophenol 40 Nonchelater Y Neg. Nonchelater Y Neg 2,4,6-Trichlorophenol 41 II. E n o l s Chelater Br Neg. 0 Pb Chelater Neg. Diethyl acetyl succinate 42 Chelater R Chelater Neg. Br Keg. Ethyl acetoacetate 43
+
-
p””
Pg*
-
44 45 46 47
111. Miscellaneous 2-Aminopyridine p-Sitrohenzhydroxamic acid IV. Oxime8 p - Aminobenzamidoxime p-Aminophenylacetoamidoximedihydrochloride Benzamidoxime p-Nitrobenzamidoxime
48 49 Color is t h a t of precipitate. b Weak color.
P
Sonchelater Chelater
Neg. Br
Br
Neg.
Pg*
Sonchelater Chelater
RBr
0 + Br
Chelater
Br
Br
C he later
R
Chelater Nonchelater Nonchelater
Br OBr Neg.
RBr R RBr Y
Chelater Chelater Sonchelater
Br Neg. Neg.
0
Y
-
Although only one case of a hydroxamic acid derivative is included among the chelaters, this group is known to form ferric hydroxamates which are stable in acid solution ( 7 ) . ACKNOWLEDGMENT
Obviously an analogous explanation can be given for the enol esters being weak chelaters. The carboxyl group is also electronegative. However, because it is capable of proton release and hence conversion to the electron donating carboxylate ion, it serves as a strong chelating group. The o-dihydric phenol derivatives are the strongest chelaters for a similar reason. Not only is the second hydroxyl capable of giving up a proton but in so doing a strong basic group, phenoxide, is formed. The case of resorcinol monoacetate is an interesting one. Table I11 shows it to be a chelater. It is felt this result is probably due to the presence of some 2,4- and/or 2,6-dihydroxyacetophenone which formed by rearrangement in the preparation of the compound.
The authors acknowledge with thanks gifts of experimental samples from the American Cyanamid Co., The Don, Chemical Co., Edwal Laboratories, Inc., Heyden Chemical Corp., Koppers Co., Inc., Monsanto Chemical Co., and Rohm & Haas Co. LITERATURE C I T E D
(1) Babko. -1.K., J . Gen. Chem. (U.S.S.R.),15, 745 (1945). (2) Banerjee, S., and Haldar, B. C., Nature, 165, 1012 (1950). (3) Banks, C. V., and Patterson, J. H., J . Am. Chem. SOC.,73, 3062 (1951). (4) Broumand, H., and Smith, J. H., Ibid., 74, 1013 (1952). (5) Foley, R. T., and Anderson, R. C., Ibid., 70, 1195 (1948); 72, 5609 (1950). (6) Herbst, R. L., etal., Ibid., 74,269 (1952). (7) Soloway, S., and Lipschitz, A., ANAL.CREM.,24, 898 (1952). (8) Soloway, S., and Wilen, S. H., Zbid., 24,979 (1952). RECEIVED for review November 7 , 1952.
Accepted January 15, 1953.
