667
V O L U M E 25, N O . 4, A P R I L 1 9 5 3 Table I.
sible conclusion that zwitter ion formation occurs in this particular case. On the other Exptl. Calcd. hand, p-cresol and hydroxybiphenyl containing the nonpolar 0.17 0 02 0 02 0.145 methyl and phenyl group, re0.00 0.145 0 45 0.33 spectively, move faster than 0.33 0 40 the parent compound. 0 33 0.18 0 11 0.164 Still another factor which 0 64 0.45 0.32 0.45 must be considered in attempt0 02 0 13 ing t o explain the anomalies 0 00 0 13 0 01 0.11 in the adsorption behavior of 0 00 0.11 these disubstituted compounds is the possibility that, in some, the geometrical arrangement of subqtituent proups on the ring is such as to allow a close fit with the adsorption centers on silicic acid, while in others spatial arrangement may tolerate the adsorption of only one group a t a time. Zechmeister ( 3 ) mentions the “general shape of the molecule” as one of the two primary factors by which the adsorption behavior of carbon compounds is determined I t is entirely possible that some differences in experimental and calculated R values presented here are due to just such a phenomenon as this.
Compounds Studied with Calculated and Experimental R Values R Values R Values
Coinpound Phenol Aniline o-h-itrophencl p-Nitrophenol 2 4Dinitrophenol 2’4 6-Trinitrophenol oliitroaniline p-Nitroaniline 2 4-Dinitroaniline 2:4,6-Trinitroaniline o-Chlorophenol p-Chlorophenol o-Hydroxyphenol
Exptl.
Calcd.
0 0 0 0 0 0 0 0 0
Compound p-Hydroxyphenol o-Arninophenol p-.4minophenol o-Cresol p-Cresol o-Anisidine
0.29
2; 21 82 09 62 35 48 18 21 0 55 0 61 0 44 0 05
0.21
0.29 0,254
...
0’ 20 0.19
...
0:33 0.29 0.18
o-Aminobenzoic acid p-Aminobenzoic acid
Tahle 11. Characteristics of Adsorbent, Solvent, and Adsorptives Silicic acid Benzene A i i n o nitrogen Phenol or alcohol oxygen Hydrogen (alcohol, amine, phenol) Nitro group Aromatic ring Keto or acid oxygen
A
D
DH
1.550
...
145
,.
.. .. ..
.. ..
..
5 8
1 0
0 17
...
0.04 0 002
0.20
... ,..
... ... ... ,.. ...
H ...
4.5
... ... ... ... ...
1.0
strongly adsorbed principal group. The ortho isomers of nitrophenol. nitroaniline, and chlorophenol, for example, move a t a rate which is not approximated by the LeRosen equation. A highly electronegative group, such as the nitro group, adjacent to the hydroxy group of phenol, would tend to draw electrons from the ovygen of the hydroxy group, decreasing the electron density in the neighborhood of that oxygen and thereby lessening its donor strength. At the same time and as a direct result of the inductive effect of the nitro group, the bond between the oxygen and hydrogen atoms of the hydroxy group would be weakened, thus increasing the adsorption strength of the hydrogen atom. There is reason to believe that simultaneously the adsorption affinity of the nitro group, normally weak, is increased by its inductive effect on the phenolic oxygen. Assuming these three effects take place in o-nitrophenol, the result would be a strong hydrogen bond between the hydroxy hydrogen and nitro group, cancelling any effect either of these would normally have on adsorption, and a t the same time there would be a decrease in the adsorption affinity of the phenolic oxygen. A very high R value would be expected for this isomer, and experimentally this prediction was verified. When the highly electronegative nitro group is substituted para to the hydroxy group, approximately the same effect is expected as when it is substituted ortho t o the hydroxy group, but because the distance between the groups in para disubstitution prevents internal hydrogen bond formation, one would expect a low R value for the para isomer. For p-nitrophenol the R value is very low, which would point to a greater increase in nitro group and hydrogen adsorption than the corresponding decrease in adsorption of the phenolic oxygen. A similar treatment of nitroaniline and chlorophenol can be made to account for the high R value of their ortho isomers. Whenever two strongly adsorbed groups, such as the amino or hydroxy group, are substituted in benzene there appears to be a supporting effect betveen them which results in very strong adsorption, much stronger than would be expected if each group were considered as acting independently. The effect of the solvent on adsorption behavior would best be considered on the basis of polarity of the adsorptive and the bolvent. For such compounds as p-nitrophenol, p-nitroaniline, p-hydroxybenzoic acid, and p-aminobenzoic acid one would expect an incompatibility between the highly polar adsorptive and the nonpolar solvent, which would result in more time spent on the adsorbent and consequently a lower R value. In paminobenzoic acid the effect is very marked and leads to the pos-
CONCLUSIONS
A systematic study of the ortho and para isomers of several disubstituted benzene compounds indicates that the adsorption behavior of the ortho isomer might be explained by combinations of internal hydrogen bond formation] steric hindrance, inductive effects, solvent effects, and the spatial arrangement of the substituent groups around the benzene ring. The adsorption behavior of the para isomer can be explained by the same factors except for internal hydrogen bonding. The degree of steric hindrance depends on the position and the size of the secondary group: the magnitude of the inductive effect depends upon the electronegativity of the secondary group; and solvent effects depend on the polarity of the adsorptive and of the solvent. When two strongly adqorbed groups are present in the molecule, the effect is to support each other in adsorption, with the result that such compounds exhibit very low R values. This study also revealed the limitations of the LeRosen equation as applied to the calculation of R values for these compounds. The influence eyerted by the second substituent group is such as to enhance or impair the normal adsorption affinity of a particular functional group. -4113’ attempt to correlate R value with the modifying effects discussed above would entail independent evaluation of the various effects by some means other than chromatographic adsorption. 4CKNOWLEDGMEYT
The authors wish to express their appreciation to the Office of Naval Research for financial assistance making this work possible. LITERATURE CITED
(1) LeRosen, A. L.,llonaghan, P. H., Rivet, C. ,4,and Smith, E. D., A x ~ LC.H E Y . , 23, 730 (1951). (2) Schroeder,W. 4., J . Am. Chem. Soc ,73,1122(1951). (3) Zechmeister, L.,Am. Scientist, 36,No.4 (1948). RECEIVED for review September 17, 19.52. Accepted February 2 , 1953.
Automatic Titrator Using Electrolvticallv. Generated Titrant sLCorr e cto in I n the article on “Automatic Titrator Using Electrolytically Generated Reagents” [ANAL. CHEM.,25, 226 (1953)l I references 3 and 4 of the Literature Cited should be: (3) Carson, W. N., Jr., ANAL.CHmr., 23, 1019 (1951). (4) Ibid.,25, 466 (1953).
W. N. CARSON,JR.
