PROCEDURES
The ultraviolet absorption spectra of each aromatic amine and its acetylated product were determined with a Beckman DK-1 spectrophotometer. I n each case examined, a wave length at which only the free amine absorbed could be found, and this wave length was chosen for the titration. Weighed samples of amine were dissolved in 100 ml. of dry pyridine which was saturated with dry hydrogen chloride, and the titration was performed using the 0.00lJf acetic anhydride solution according to procedures published previously (6).
~
RESULTS AND DISCUSSION
The results of the titration of different aromatic amines are given in Table I. Tlventy-five different neighed samples of aniline were titrated over a period of 2 months n i t h a n average deviation of 0.6%. The precision obtained on the other aromatic anlines was never greater than 0.8%. A typical titration curve for aniline is shown in Figure 1. The weight of aniine taken for analysis can be as low as 1 nig. with no loss in accuracy or precision.
.---
I 3
2
Vo urne of
Figure aniline
1.
4
Ac.0
Photometric
5
(mu
titration
of LITERATURE CITED
(1) Baumgarten, H. E., J . Am. Chem. SOC. 75, 1239 (1953).
Table I. Results of Photometric Titration of Aromatic Amines
No. of Samples Run
Av. Size of Sample,
Amine
Wave Length, MI
Mg.
%
Aniline m-Toluidine p - Anisidine p-Chloroaniline m-Aminoohenol 2-Amino-bphenylphenol p-Aminoacetanilide 2-Naphthylamine N-Methylaniline
320 350 340 370 320
7 11 7 7
5 8 8 6 5
99.8 99.7 103.0 104.2 107.4
The reaction between acetic anhydride and a n aromatic amine was catalyzed by the presence of a strong acid, such as perchloric acid or hydrogen chloride. Unless a small quantity of such a n acid is added t o the pyridine solution of the aromatic amine, about 15 minutes are required before a constant absorbance reading can be made after each increment of titrant added. I n the presence of a n acid catalyst, a constant absorbance reading can be obtained immediately. This same behavior has been observed in the acetylation of alcohols ( 3 ) . Although perchloric acid was first used as a catalyst, high results were obtained with some amines using this acid. This error was traced to the mater added to the system with the perchloric acid. If dry hydrogen chloride is used instead, this error is completely eliminated. Aniline and aromatic amines substituted with groups such as alkyl, alkoxy, hydroxy, and halogen can be successfully titrated by this method.
984
ANALYTICAL CHEMISTRY
relatively large quantities of a n aliphatic amine, such as n-butylamine, causes no interference in the aromatic amine titration: as there is enough hydrogen chloride in the solvent to convert the aliphatic amine t’oits conjugate acid which does not acetylate. There is considerable evidence (1 2 , 5 ) that the mechanism of the acetylation reaction of aromatic amines is a n electrophilic substitution reaction. Acetyl pyridiniuni ion, the probable acetylating agent when pyridine is used as solvent, acts as a n electrophilic substituting agent which attacks the amine ortho or para to the amino group. If the attack is ortho, the intermediate adduct can rearrange to form the corresponding acetylabed amine! pyridine, and acetic acid. I n t,his work, evidence was obtained for the presence of this intermediate in the form of a transient absorption peak a t 315 mp when acetyl pyridinium acetat’e was first added to a pyridine solution of aniline.
7
Purity,
Attempts to titrate paminobenzoic acid, p-aminoacetophenone, and o-nitroaniline produced no results because of the very slow rate of reaction between these compounds and acetic anhydride. However, the presence of these amines substituted by electron-withdrawing groups apparently has no effect upon the titration results obtained with the amines listed in Table I. For example, the titration curve obtained when a n equimolar mixture of aniline and onitroaniline is titrated with acetic anhydride is exactly the same as if aniline alone were titrated. Because both alcohols and phenols interfere with standard acetylation procedures, equimolar mixtures of aniline and other amines with both n-butyl alcohol and phenol were titrated. The titration results were identical with those obtained when the amines alone were titrated, indicating that the rate of acetylation of the amines is considerably greater than the corresponding rate for alcohols or phenols. The presence of
(2) Doering, W. v. E., hlcEwen, W. E., Zbid., 73,2104 (1951). (3) Fritz, J. S., Schenk, G. H., Abstracts, p. l5B, 136th Meeting, ACS, Atlantic City, N. J., September 1959. (4) Hillenbrand, E. F., Jr., Pentz, C. A., “Organic Analysis,” Vol. 3, p. 162, Interscience, Yew York, 1956. ( 5 ) McEwen, W.E., Terss, R . H., Elliott, I. W., J . AWL.Chem. SOC. 74, 3605 (19521. \
I
TUlU7LkZ 1
,i t , ( l Y O 6 j .
( 7 ) Mitchell, J., Jr., Hawkins, W., Smith, D. >! JI .. Am, , Chem. SOC. 6 6 , 782 (1947). RECEIVED for review January 20, 1960. Accepted .4pril11, 1960. Work supported in part by the United States Air Force under Contract AF49(638)-472 monitored by the Air Force Office of Scientific Research of the Air Research and Development Command. One of the authors, Sister Francis Hugh, was supported by a National Science Foundation summer research institute.
Correction
Gas Chromatography I n this review article by Stephen
Dal Nogare [ANAL.CHEM.32, 19R (1960)], on page 21R, column 1, line 33, t h e efficiencies for t h e capillary column described b y Lipsky, Lovelock, and Landowne [ J . A m . Chem. SOC. 81, 1010 (1959)], were erroneously quoted as 21 t o 200 theoretical plates per foot. The correct efficiencies ranged from 100 t o 1000 theoretical plates per foot.
