H. H. G. JELLINEK AND J. R. URWIN
548
Vol. 58
ULTRAVIOLET ABSORPTION SPECTRA AND DISSOCIATION CONSTANTS OF PICOLINIC, ISONICOTINIC ACIDS AND THEIR AMIDES BY H. H. G. JELLINEK AND J. R. URWIN The Johnson Laboratories, University of Adelaide, Adelaide, Australia Received February 20t 1964
Ultraviolet absorption spectra of nicotinic acid and its amide as a function of pH values were presented in two previous communications.ls2 Dissociation constants were derived from the experimental data. This paper brings an extension of the work to picolinic, isonicotinic acid and their amides. When the work comprising this paper was actually completed, a paper by Evans, Herington and Kynastona was published dealing with the absorption spectra and dissociation constants of the pyridine monocarboxylic acids.
Herington and co-workersa and we have found no essential differences. The s ectra of picolinic acid Materials.-Isonicotinic acid, picolinic acid, isonicotinpmide and picolinamide were the same samples as described show maxima a t about 2660 . and a shoulder, which in a previous paper.4 All other chemicals were of Analar shift with the pH values in the region of about 2700 grade. Apparatus.-Measurements were made with a Hilger A. At very low pH values the maximum absorption Uvispek spectrophotometer. pH values were determined shifts to longer wave lengths. There are indicaby means of a lass electrode. tions of two pairs ,Of isosbestic points in the region Solutions.-i!folutions were prepared by adding requisite amounts of hydrochloric acid solutions and solutions of the of 2350 and 2750 A. Picolinamide shows absorption maxima a t 2655 amide or acid to 0.05 M sodium acetate and making up the volume to a total of 50 ml. The concentrations of the re- A. and a definite isosbestic point a t 2400 A., the s ective acid or amide ranged from 1 X to 5 X 10-4 M. $he ionic strength of the solutions down to about pH 3.5 absorption curves for pH values 0 and lower do not had a constant value of 0.01. The measurements were pass through this isosbestic point. carried out at room temperature, 20 f 2'. Isonicotinic acid presents a picture of greater complexity. The absorption maxima shift appreExperimental Results and Discussion ciably with pH values. The absorption spectra The absorption spectra as a function of pH values of isonicotinamide, on the other hand, are again for picolinamide and isonicotinamide are shown in simpler, The absorption maxima shift mooderately Figs. 1 and 2. The spectra of the respective acids will not be presented, as they have been given by with pH values from about 2680 to 2630 A. There are indications of two isosbestic points a t about x 10' 2800 and 2340 A.
Experimental
w
.
0.50
1
.
h 0
2400 WAVE
Fig. 1.-Absorption
2500
LENGTH
2600
1
1700
(1) E. B. Hughes, H. H. G. Jellinek and B. A. Ambroae, THIS JOURNAL, 6S, 414 (1949). (2) H. H. G. Jellinek and M. G. Wayne, ibid., 65, 173 (1951). (3) R. F. Evans, E. F. G. Herington and W. Kynsston, Trans. Fara-
J. R. Urwin, THIBJOURNAL, Vol. 68, 168
2400 WAVE
A.U.
spectra of picolinamide at various pH values.
d a y Soc., BD, 1284 (1953). (4) H. H. G. Jellinek and (1954).
Po0
Fig. 2.-Absorption
2590
2600
no0
LENGTH A.U.
spectra of isonicotinainicle at various pH values.
It was ascertained that Beer's law was obeyed at constant pH values for the ranges of concentrations of acid and amide used in this work. Molecular extinct,ion coefficients for these compounds as function of pH values for constant wave
July, 1954
549
SPECTRA OF PICOLINIC, ISONICOTINIC ACIDSAND THEIR AMIDES
lengths are shown in Figs, 3 and 4. The theoretical curves are drawn through the experimental points and will be discussed below. Picolinic acid shows two distinct sections and the points appear to pass through a maximum at low pH values. The points for picolinamide also pass through a maximum a t low pH values. Isonicotinic acid shows a very distinct maximum at intermediate pH values, whereas isonicotinamide passes through a maximum at low pH values. The acids have an acid group and a basic group each, whereas the amides have two basic groups. The respective equilibria can be written as follows
+ Ha0 eNRCOOH + H30+ NRCOO- + HaO+ NRCOOH + H10 and +HNRCONHc+ + HrO e H+NRCONH1+ HsO+ NRCONHn + HsO+ +HNRCONH* + Ha0 +HNRCOOH
Calculations were carried out to fit the experimental points in Figs. 3 and 4 by choosing suitable pKa values and pH values for the isoelectric points. The calculations were carried out either according to the first method described in the previous paper on nicotinic acid‘ or that given in the paper on nicotinamide,2 respectively. The curves drawn through the experimental points in Figs. 3 and 4 were obtained in this way. Picolinic acid could be fitted by two constants, picolinamide by one constant; the decrease in extinctions at low pH values may be an indication of the equilibrium pertaining to the amido group. Isonicotinic acid could be fitted very well by two dissociation constants. The extinction values for the species +HNRCOOH are here smaller than those for NRCOOH for this particular wave length in contrast to nicotinic acid where the
.I
0
1
1
i
b
4
PH
6
f
B
Q
I
Fig. 3.-Molecular extinctions for picolinic acid and picolinamide a8 a function of pH values. Curves are calculated, points are experimental (A -2650 and 2600 A., respectively).
PH
Fig. 4.-Molecular extinctions for isonicotinic acid and isonicotinamide as a function of pH value!. Curves are 0 calculated, points are experimental (A ~ 2 6 0 A.).
reverse was the case.’ Isonicotinamide could be fitted by one p K a value and there is an indication of an equilibrium pertaining to the amido group at IsoNitroelectric Carboxyl gen Amido low pH values. Substance point group ring group Ref. All P K a values are comprised in Table I. The Picolinic 3.48 5.40* 1.60 ... This values marked by asterisks are thermodynamic diswork acid sociation constants, where the ionic strength has 3.21 5.40* l.OS* ... 3 been taken into account according to the DebyeIsonicotinic 3.45 4.95* 1.70 ... This Huckel theory. Table I contains also values obacid work tained by Herington and co-workers3 and values 3.31 4.84* 1.77* ... 3 for ben~oicacid, pyridine and acetamide. Nicotinic 4.24 4.95* 3.55* ... 1 It can be seen that the K . values belonging to 3.42 4.83* 2.08* acid ... 3 the carboxyl groups are very similar, though somePicolin... ... 2.10
