Identification of Alcohols from Rates of Alkaline Hydrolysis of Their 3,5-Dinitro benzoate Esters Joseph R. Robinson School of Pharmacy, Unicersity of Wisconsin, Madison, Wis. 53706 CHEMICAL KINETICS has been utilized in the characterization of organic compounds in a general way for many years, for example, by measuring the time necessary to form sugar osazones. This rather limited approach can be expanded t o include many other classes of compounds, whose rate behavior under standard conditions could be used for identification purposes. Previous studies that demonstrated this suggestion in principle were designed with some other goal in mind ( I ) and/or included only a limited number of compounds (2). This study demonstrates that chemical kinetics can be applied to the identification of organic compounds as a n aid in the characterization process.
w
0 Z
EXPERIMENTAL Reagents and Apparatus. Acetonitrile (Fisher certified grade) was used without further purification. All other chemicals were of analytical or reagent grade quality. Water was redistilled from alkaline permanganate. 3,5-Dinitrobenzoic acid was recrystallized from anhydrous ethanol; m.p. 204-205’ C. All p H measurements and adjustments were made with a Corning model 12 p H meter with a n expanded scale utilizing a wide range type E3 Beckman p H glass electrode. The p H meter-electrode system was standardized against calcium hydroxide buffer as described by Bates (3). Water bath temperatures were maintained to i O . 1 ” C with Sargent Thermonitor electronic relays. Preparation of Esters. All esters were prepared according to the procedures outlined by McElvain (4), Cheronis, Entrikin, and Hodnett ( 5 ) , and Shriner, Fuson, and Curtin (6). Simple addition of 3,5-dinitrobenzoyl chloride t o the alcohol with subsequent boiling for a few minutes was employed for methyl, ethyl, n-propyl, n-butyl, n-amyl, benzyl, and allyl alcohols. All other esters were synthesized through the alternate pyridine procedure (4, utilizing 3,Sdinitrobenzoyl chloride as the acylating agent. The esters were recrystallized from various proportions of ethanol-water mixtures, and the melting points obtained were in all cases in good agreement with reported values (4-6). Kinetic Procedure for Recording Spectrophotometric Determinations. The standard buffer employed in all runs was prepared by mixing 60 ml of 0.05M dibasic sodium phosphate solution with 40 ml of acetonitrile (the total volume is somewhat less than 100 ml). The resultant solution is titrated to the final desired p H with 1 or 2 drops of concentrated N a O H solution. (1) G. G. Guilbault, D. N. Kramer, and E. Hackley, ANAL CHEM.,38, 1897 (1966). (2) T. Aung, E. A. Healy, and R. K. Murman, Chemist-Analyst, 49,73, (1960). (3) R. G. Bates, J . Res. Nut/. Bur. Std., 66A, 179 (1962). (4) S . M. McElvain, “The Characterization of Organic Compounds,” Macmillan, New York, 1953. (5) N. D. Cheronis, J. B. Entrikin, and E. M. Hodnett, “Semimicro Qualitative Organic Analysis-The Systematic Identification of Organic Compounds,” 3rd ed., Interscience, New York, 1966. (6) R. L. Shriner, R. C . Fuson, and D. Y. Curtin, “The Systematic Identification of Organic Compounds,” 4th ed., Wiley, New York, 1962. 1178
ANALYTICAL CHEMISTRY
m
a
0.0
I
240
260
280
300
320
340
360
380
40(
Figure 1. Absorbance spectrum of methyl, 3,5-dinitrobenzoate in pH = 12 acetonitrile-phosphate buffer at the beginning of the reaction ( t = 0) and after completion of reaction ( t = m ) Buffer solutions were equilibrated a t 25 i 0.1 ’ C prior to reaction. Six milliliters of buffer were introduced into a 2-cm absorption cell (or 3 ml of buffer in a 1-cm cell), together with 100 p1 of a 1 X lo-* molar solution of ester in acetonitrile. The solutions were mixed by inversion of the cell and the change in absorbance was recorded o n a Cary Model 15 recording spectrophotometer with thermostated cell compartments. The release of 3,Sdinitrobenzoate was followed a t 285 mp for all of the esters. (The molar absorptivity of 3,5-dinitrobenzoate in the acetonitrile-phosphate buffer was determined t o be czSj = 2.05 X lo4.) All experiments were carried out with a large excess of hydroxide ion and therefore pseudo first-order kinetics were observed. Apparent second-order rate constants were obtained by dividing each observed first-order rate constant by the apparent activity of hydroxide ion as calculated from the measured p H and the quantity pK, = 14.00. Firstorder rate constants were obtained by plotting log ( A , - A , ) (absorbance a t time infinity minus absorbance at time t ) against time or by the method of Guggenheim (7). F o r reactions where Guggenheim plots were applied, a t least one of the determinations were carried t o completion and plotted by the alternate procedure to ensure that the reaction followed pseudo first-order kinetics throughout its entirety. Acetonitrile purified with phosphorus pentoxide (8) gave (7) E. A. Guggenheim, Phil.Mag., 2,538 (1926). (8) G . L. Lewis and C. P. Smyth, J . Clzern. Phys., 7,1085 (1939).
Table I. Second-Order Rate Constants for the Alkaline Hydrolysis of Some 3,5-Dinitrobenzoate Esters
3-METHYL 2-BUTYL
g0.2
a
m
E:
Parent alcohol
Ester mp
2-Octyl n-Amyl Allyl 2-Heptyl n-Hexyl Dodecyl 2-Pentyl n-Octyl Isoamyl n-Butyl n-Propyl s-Butyl 3-Methyl-2-butyl
32 46 48 49
3-Hexyl
Isobutyl Ethyl
'""I
\
-
ISOPROPYL
1000
SECONDS
Figure 2. Semilogarithmic plot of hydrolytic rate against time for some 3,5-dinitrobenzoate esters a t p H = 12, 25°C in acetonitrile-phosphate buffer
3-Pentyl Methyl P-Phenethyl Cyclohexyl Benzyl Cyclopentyl r-Amyl Cinnamyl Isopropyl Z-Butyl Ethylene glycol a
a
the same rate data as commercially available (Fisher certified) acetonitrile. Kinetic Procedure for Non-Recording Spectrophotometric Determinations. The same volume of the 40/60 acetonitrilephosphate buffer (parts by volume) previously described was prepared. Fifty milliliters of the buffer was then placed in a 50-ml volumetric flask, the remaining buffer being retained for blank determinations. After equilibration to 25" C, 1 ml of 1 X molar solution of ester in acetonitrile was then introduced into the volumetric flask. Subsequently, a t various time intervals, 3 ml of solution were withdrawn and the absorbance was measured o n a Beckman model DU spectrophotometer. The change in absorbance as a function of time was treated as previously described. RESULTS
As shown in Figure 1, the 3,5-dinitrobenzoate esters have a n ultraviolet absorption spectrum much different from that of the parent acid anion. The time course for hydrolysis of the ester can be conveniently followed a t 285 mp. Apparent second-order rate constants for the esters are recorded in Table I. Hydrolytic rate determinations were carried out mainly a t p H = 12. However, a number of determinations were carried out a t p H values between 11 and 13. No apparent catalysis by the buffer was observed over this range. Above p H 13, tribasic sodium phosphate precipitates from the buffer. Below p H 11, the buffer capacity is quite low. The 3,5-dinitrobenzoate ester of ethylene glycol is the diester. At p H 12 and lower, only the monoester hydrolyzes to any appreciable extent, as determined from spectrophotometric data. The anion produced from saponification of the first ester apparently hinders attack a t the second ester function. No attempt was made to determine the rate constant for hydrolysis of the second ester group.
102k (Limole-sec)" Mean Av. dev.* 1.74 15.35 50.48 2.07 12.63 10.28 2.09 11.50 13.60 16.32 18.73 2.59 1.34 0.845 14.14 23.00 1.12 65.81 29.20 3.35 48.36 4.34 0.117 39.87 4.66 0,042 132.5
58
60 61 61 62 64 74 75
76 77 86 93 97 107 108 112 112 115 117 121 122 142 169
N ~of,
detns
0.04 0.04 0.12 0.03 0.05 0.05
0.02 0.02 0.03 0.02 0.06 0.03 0.04 0.02 0.03 0.07 0.05
0.08 0.04 0.03 0.14 0.02 0.008 0.15 0.04 0.001
0.30
4
7 6 4 5 4 4 6 6 6 8 4 4 4 6 9 3 8 3 4 9 4 2 5 4 2 5
In 40% acetonitrile, 0.05M phosphate buffer at 25" C. Average deviation = 2 1 xi -
Xi(.
n-1
DISCUSSION
I n applying accurate kinetic measurements t o qualitative analysis, two general approaches immediately suggest themselves. The compound to be identified is subjected to reaction under specified conditions and the rate constant obtained is compared with previously tabulated values. A derivative of the compound is prepared and the derivative is subjected to reaction, with subsequent calculation of the rate constant. In applying the method to alcohols, the derivative rather than the parent alcohol was chosen because the derivative can easily be obtained in pure form, while the parent compound may not always be readily purified, especially when only small amounts are available. To avoid possible catalytic effects, purity of all the reaction components is essential; from the derivative many other data can also be obtained, such as m.p., b.p., I R , and NMR spectra, etc.; other kinetic processes involving the parent alcohol, such as rate of esterification, are not easily measurable. The buffer composition was chosen in order to accommodate essentially all members of the class likely to be encountered. The relatively high concentration of acetonitrile should allow dissolution of most other 3,5-dinitrobenzoate esters. In addition, acetonitrile requires no further purification t o be of use spectrally. At p H values employed in this study, acetonitrile is stable for sufficient time to allow completion of the kinetic runs. The high sensitivity of spectrophotometric analysis allows measurement of very small quantities of ester. For example, in this study approximately 0.2 mg of material were required for each run on the normal chart scale. If necessary, the VOL. 3 9 ,
NO. 10, AUGUST 1967
1 1 79
expanded scale could be used to increase sensitivity and therefore only 0.02 mg would be required. I n many cases, the melting point of the 3,5-dinitrobenzoate ester would be insufficient to identify it. This is readily demonstrated with the n-propyl and sec-butyl esters, whose melting points differ by only one degree. In addition, the boiling points of the parent alcohols would be of little value in identification because they differ by only two degrees. However, as shown in Table I, the second-order hydrolytic rate constants for these two compounds differ by a factor of eight, making characterization quite simple. The sensitivity of a few 3,5-dinitrobenzoate esters toward alkaline hydrolysis is shown in Figure 2 (the plots included in this representation have been normalized for comparative purposes). Through a combination of rate constant and
melting point of the 3,5-dinitrobenzoate derivative, any alcohol in Table I can be identified. Other classes of organic compounds should lend themselves to characterization through such kinetic procedures. ACKNOWLEDGMENT The author expresses his gratitude to Kenneth A. Connors for suggesting the possible utility of precise kinetic measurements in qualitative analysis and for his help throughout the study. H e also thanks Jessie Armstrong for her technical assistance. RECEIVEDfor review March 24, 1967. Accepted May 26, 1967.
Application of Atomic Absorption Spectrometry to Extraction of Mercuric Iodide into Carbon Tetrachloride Michael D. Morris and Lee R. Whitlock’ Department of Chemistry, The Pennsylcania State Unifiersity, Unicersity Park, Pa.
INTHE STUDY of any solvent extraction system, it is necessary t o determine the concentrations of one o r more of the partitioning species in one or both phases. Such analyses are needed t o determine the equilibrium constants for the system. F o r this purpose, many standard analytical techniques have been employed, but only spectrophotometry and radiotracer monitoring are widely used ( I ) . The present communication describes the application of atomic absorption spectrometry to extraction studies. The virtues and limitations of this technique are well known to analytical chemists, and have been recently reviewed ( 2 , 3). Of particular importance in extraction studies are the excellent (almost perfect) selectivity of atomic absorption, its applicability to the determination of metals in almost any aqueous or nonaqueous medium, its experimental simplicity, and its good sensitivity. A major disadvantage of atomic absorption spectrometry is the sensitivity of the method to matrix effects (2, 4). It is necessary that calibration curves be made from standards whose compositions resemble as closely as possible those of the solutions to be analyzed. To test the applicability of atomic absorption spectrometry to extraction studies, we have chosen the relatively simple system, extraction of mercuric iodide from 0.5M sodium perchlo-
16802
rate into carbon tetrachloride. Marcus ( 5 , 6 ) has made extensive studies of the extraction of mercuric iodide from 0.5M sodium perchlorate into benzene and has verified that the only extracted species is HgI2. [For this system, the partition coefficient is 46 f 1 (5, 6).] Because mercuric ion forms several iodo complexes, the distribution coefficient depends o n the mole ratio 1:Hg. If this mole ratio is restricted to 2 :1 or less, then the formation of Hg13- and HgIIP2may be neglected. In this case the relation between the distribution coefficient, D = (Hg)o/(Hg),Q, the partition coefficient, P = (HgIz)o/(HgIz),, and the successive formation constants, KI and K2 for HgI+ and HgIz is given by Equation 1 (6).
[W(l K1 - Kz D[(1 -
+ D)P - 2D(1 + P)]Z (1
+ 0 )P + D(1 + P)1
(1)
For Equation 1 to hold, the average ligand number of mercuric ion in the aqueous solution, A, must be less than or equal t o 2. F o r the medium of this study, 0.5M NaC1O4 a t 25”C, Marcus (6) has found Kl = 8.9 X 10l2and Kz = 7.4 X 10’0. Because these constants are so large, virtually all (99+ %) of the iodide in solution is bound to mercuric ions and A is equal to the stoichiometric ratio, I :Hg. EXPERIMENTAL
Present address, Department of Chemistry, University of Massachusetts, Amherst, Mass. (1) V. V. Fomin, “Chemistry of Extraction Processes,” National
Science Foundation (U.S.A.) and the Israel Program for Scientific Translations, Jerusalem, 1962, pp. 28-50. (2) J. W. Robinson, “Atomic Absorption Spectroscopy,” Marcel Dekker, New York, 1966. (3) R. Lockyer, “Advances in Analytical Chemistry and Instrumentation,” C. N. Reilley, Ed., Vol. 3, Wiley, New York, 1964, pp. 1-29. (4) R. Hermann and C. T. J. Alkemade, “Chemical Analysis by Flame Photometry,” Wiley, New York, 1963, pp. 278-372. 1 180
ANALYTICAL CHEMISTRY
Mercuric nitrate, standardized against EDTA, was used as the source of the mercuric ion. Sodium perchlorate was prepared by neutralizing sodium carbonate with perchloric acid. All reagents were ACS reagent grade. Distilled water was used t o prepare all solutions. All measurements were carried out a t room temperature, 25” 3 ” C. The initial mercury concentration in the aqueous phase was kept constant a t 2.49 X 10-4M. Various amounts of po-
*
( 5 ) Y. Marcus, Acta. Chem. Scand., ll, 329 (1957). (6) Ibid., p. 599.
