May, 1963
1065
S T E P W I S E HEATS O F r\JEUTRALIZATION O F PIPERAZIKE
calcium employed but do state that the calcium was doubly dist.illed. Bronsted3 measured the dissociation pressure of calcium hydride in the temperature range 641-745'. The pressures obtained by Bronsted agree quite well with those obtained in this investigation. However, the slope of the line obtained from a plot of log PH%(atm.) vs. 104/T0K. from Bronsted's data is slightly different from that obtained in this investigation. The fact that the metal used in his investigation only analyzed 97.3% calcium could have caused this deviation. Appendix 1 According to Sievert's law diatomic gases dissolve atomically in metals. In the case of the dissolution of hydrogen in calcium Treadwell and Stecher's data2" indicate that this is true. This being the case it is possible to relate the equilibrium hydrogen pressure, mole fraction of hydrogen, N H , jn the calcium-rich phase and the activity of hydrogen as aH2
=
P H , = ICNH~
Then according to the Gibbs-Duhem equation
Nca d In aca =
-NH% d In aH2 = - N
H d~ In L V H ~
At a given temperature k remains constant and N E in terms of the variables Nca and is given by the relation
NH=
~NH, NE% 1
+
The Gibbs-Duhem equation can therefore be written as d In acs.=
d ln
-2NH,
1 - NH%
2Na2 N E % +1
Integration of this relation tvith the condition that aca 1 as N~~ 1 yields the relation --+
+
aca
=
Nca
2 -
Nca
Utilizing the data, in Table I1 the activity of calcium can be calculated.
THERMOCHEAlICAL STUDIES. V1I.l HEATS AND ENTROPIES OF STEPWISE NEUTRALIZATION OF PIPERAZINE AND TRIET YLENETETRAMINE B Y P I E R 0 PAOLETTI,
MARIOCIAMPOLIKl,
A S D A L B E R T 0 V-4CCA
Istituto d i Chimica Generale e Inorganica dell' Universitd d i Fireme, Firenxe, Italy Received November 1, 1962 The results of a calorimetric investigation of the stepwise heat of neutralization for piperazine and triethylenetetramine in 0.1 M KC1 a t 25" are here reported and discussed. The pK values of the piperazine were also measured under the same experimental conditions. For some polyethylenepolyamines containing secondary and primary nitrogen atoms, a linear relationship is found between the heat or entropy of the first neutralization stage and the percentage of secondary nitrogen atoms.
Introduction As a part of a calorimetric investigation on the heats of reaction of some polyethylenepolyamines with hydrogen ion2 or with transition metal ions,' we have now measured the heats of stepwise neutralization of piperazine (pip) and triethylenetetramine (trien). For these two bases, values of the enthalpy changes derived from the variation of the basicity constants with the temperature are reported in the but it is recognized that this method can seldom give figures as accurate as those measured directly. The calorimetric measurements were carried out in 0.1 M KC1 a t 25'. Experimental Materials.-Commercially available piperazine (Fluka) was twice recrystallized from light petroleum. Pure triethylenetetramine was obtained as previously described.' These bases were analyzed by potentiometric titrations against ca. 1.5 N hydrochloric acid and proved to be 99.9 and 99.8% pure, respectively. This same hydrochloric acid was standardized gravimetrically as silver chloride and used also in the measurements of the heats of neutralization. The solution of the amines in a carbon dioxidefree 0.1 M KC1 were prepared according t o the procedure previously described.* (1) Part V I , L. Sacconi, P. Paoletti, and &I. Ciampolini, J . Chem. Soc., 5115 (1961). ( 2 ) RI. Ciampolini and P. Paoletti, J . Phys. Chem., 6S, 1224 (1961). ( 3 ) €1. B. Jonassen, R. B. LeBlanc, A. W. Meibohm, and R. M. Rogan, J . Am. Chem. Soc., '72, 2430 (1950). (4) J. M. Paaano, D. E. Goldberg, and W. C. Fernelius, J . P ~ ~ , JChem., s. 66. 1062 (1961).
Calorimetric Measurements.-The calorimeter and the experimental technique used has already been described.2 At t h e end of the reaction the temperature was in the range 24.824.9" and no attempt was made to correct the heat value t o 25'. The ionic medium used was the 0.1 M KCl solution. Acid Dissociation 1Constants.-pH measurements were made using a Radiometer Model 4 p H meter equipped with saturated calomel and glass electrodes; 1.5 hi hydrochloric acid was added to the solution of the base. The solution was stirred continuously and the temperature was kept constant at 25.0 =t0.1'.
Results The stepwise heats of neutralization were determined by measuring the heats evolved for different hydrochloric acid-amine ratios and by calculating the exact amount of the protonated forms of the polyamines before and after the reaction. The concentration equilibrium constants used with triethylenetetramine were those of Schwarzenbach6 in 0.1 M KC1 a t 20' corrected to 25' using our AH values. For piperazine a t 25' the values of pK1 (9.72) and pK2 (5.60) are in excellent agreement with those of Schwarzenbach, et aZ.,6(corrected to 25') in 0.1 M Na?;O3, pK1 = 9.69 and pK2 = 5.59, as well as with the thermodynamic ones of Pagano, Goldberg, and Fernelius,4 pK1 = 9.79 andpK2 = 5.59. The experimental details of the calorimetric measurements are reported in Table I. The third column lists (5) G.Sohwarsenbach, H e h . Chim.Acta, 33, 974 (1950). (6) G. Sohwarzenbaoh, B. Maissen. and H. Ackermann, ibid., 36, 2333 (1953).
P. PAOLETTI, &I. CIAMPOLIKI, AND A. VACCA
1066
the heat evolved in each run, corrected for the heat of dilution of hydrochloric acid. The last column reports the calculated correction for the heat effect due to the neutralization of hydroxyl ions which arise from the basic dissociation of the amines. CALORIMETRIC DATAFOR
TABLE I THE SYSTEMS POLYAMINEHC1
+
Base,
mole
HC1. 10-5 mole
cal.
cal.
2696 2723 2895 3031 2900 3262
2692 2718 2892 1516 1469 1634
275.7 279.6 296 9 263.8 254.2 283 6
12.0 12.1 12 5 9 0 8.8 9.3
10-6
Base
Piperazine (vol., 946 ml.)
Q>
Triethylenetetramine (vol., 928 ml.)
2917 2925 2952 2960 2910 2905 1475 2960 1449 2905 1468 2907 967 2914 988 2970 986 2973 749 3942 716 3902 709 3736 a This term is to be subtracted from the value.
QOOP,"
326 4 16.1 330 2 16.1 324 3 16.1 329.4 11.7 324.6 11.6 324.9 11.6 309.2 9.3 316.4 8.7 314.7 9.6 287.0 8.3 275.4 8.1 272.6 8.0 corresponding Q
Table I1 reports the values of the thermodynamic functions - AF, - AH, and AS relative to the successive reaction stages. TABLE I1 THERMODYKAMIC FUNCTIONS FOR THE SUCCESSIVE NEUTRALIZATION STAGESOF POLYAM~NES AT 28"
-
AF, kcal./ mole
+
pip H + + pipH+ pipH+ H + + pipHn*+ trien H + + trienHf trienH H + + trienH22+ trienHzz+ H + + trienH8+ trienH2 * H + trienH44f
+ + + +
+
+
-AH, kcal./ mole
13.26 7.64 13.34 12.36 8.93 4.42
10.17 7.12 11.01 11.27 9.53 6.83
AS, e.u.
10.3 1.8
7.8
-
-
3.7 2.0 8.1
The values of AF have been calculated from the formula AF = -RT In K , where K is the concentration equilibrium constant. This procedure involves the assumption that the ratios [YH Y B H $ L ~ \ + / Y B H ~ + ] are taken as unity with respect to standard states in 0.1 M KC1. For piperazine this procedure is valid, as is shown by the extremely good agreement between the concentration equilibrium constants and the thermodynamic values. For triethylene tetramine this may not be the case especially for the last stages of protonation. Therefore AF and A S thus calculated may be affected by an error of uncertain size. Nevertheless it is likely that for the first stages of neutralization the uncertainities are negligible with respect to experimental error. The -AF3 and - A F 4 values as well as the corresponding AS3 and AS4 values may therefore he too high. For example applying the DebyeHiickel theory to the results obtained for the systems ethylenediamine-H+ and hexamethylenediamineH + by Everett and Pinsent' a value of ca. 3 A. for the + a
(7) D. H. Everett and B. R . W. Pinsent, Proc. Roy. SOC.(London), ABM, 416 (1952).
Vol. 67
mean distance of approachs was obtained. Csing this value for triethylenetetramine the following values were obtained: AF3 = -8.28; AF4 = -3.40 kcal./mole and AS3 = -4.3; AS, = - 11.5 e.u. The accuracy of the over-all heat of neutralization is of the same order of magnitude as the reproducibility of the calorimetric measurements, ie., better than =t 0.2%. On the contrary the values of the stepwise neutralization are less accurate as they depend on the values of the basicity constants used.
Discussion The heat evolved in the first neutralization stage of piperazine (10.17 kcal./mole) is lower than that of ethylenediamine (11.92 k~al./mole),~ while the entropy change (10.3 e.u.) is higher than that of ethylenediamine (5.7 e . ~ . ) .This ~ fact falls in line with what has already found for the aliphatic monobasic amines, i.e., a primary amine exhibits a lower heat of neutralization but a higher entropy change than a secondary amine.lo For triethylenetetramine and diethyleiietriamine which contain both secondary and primary nitrogen atoms the thermodynamic quantities of first neutralization -AH1 and ASl appear to be a linear function of the percentage of secondary nitrogen atoms. To illustrate this we have plotted in Fig. 1 the values of - AH1 and A& for eth~lenediarnine,~ diethylenetriamine,2 triethylenetetramine and piperazine against the ratio of the number of secondary nitrogen atoms to the total number of nitrogen atoms; the points lie on two straight lines. Such a linear relationship leads to the conclusion that both the monoprotonated ions denHf and trienH+ exist in two tautomeric forms, one salted in the primary and the other in the secondary nitrogen atoms, in almost statistical ratio. It may be stated, therefore, that the basicities of the primary and secondary amine-groups in these neutral polyethylenepolyaniines are nearly the same, as happens with the aliphatic monoamines.10 For piperazine the -AHz value is 3.07 kcal./mole lower than the -AH1 value, whereas for ethylenediamine the difference is only 1.7 k~al./mole.~This fact may be regarded as due to the weaker electrostatic repulsion between the positive poles which in the pipH22+ ion (predominantly in the chair form) are closer than in the enHz2+ion (predominantly in the trans form). For triethylenetetramine - AHz is greater than -AH1 in analogy with what was found for diethylenetriamine. This may mean that in the second stage of neutralization the fraction of primary amino groups that are being protonated is greater than in the first stage with the consequent greater evolution of heat. The structures of the diammonium ions would be +H,W.CHe.CH2.SH.CH2CH2.T\JH.CHz.CH2;"1H3 + (1)
+H~N.CH~.CH,.NH.CH~.CH~.~TH~+.CH~.CHZ.~H~ (11) (8) H. S. Harned and B. B. Owen, "The Physical Chemistry of Electrolytic Solutions," 3rd Ed.. Reinhold Publ. Corp., New York, N. Y., 1958, Chapter 2. (9) T. Davies, S. S. SinKer, and L. A. K. Staveley, J. Chem. S o c . , 2304
(1954).
(10) Cf., D. H. Everett and W. F. K. Wynne-Jones, Trans. Faraday Soc.. 36, 1380 (1939).
Xay, 1963
IONIZATION POTEXTIALS OF
The structures with two adjacent charges would be much less probable due to electrostatic repulsion. Confirmation of the form (11) is supplied by the lower -AHl.-z value for the trien (22.30 kcal./mole) as compared to that of diethylenetriamine (23.16 kcal./mole). I n fact if only form (I) did exist one would expect a greater heat of formation for trienHzz+than for clenH2+, due t o the lower electrostatic repulsion. The low - A l l s value (9.50 kcal./mole) may be attributed to the preferential protonation of the secondary nitrogen and a greater electrostatic repulsion of the positive charges. These two factors may also account easily for the sequence --AHs (trien) > .AHB(den) > --AH, (trien) . The entropy changes for the first neutralization stage are positive whereas those for the successive steps decrease till they become negative. Such a trend has already been found for the neutralization of ethylenediamine and diethylenetriamine2 arid for other stepwise reactions as the formation of metal complexes with neutral’*l1or anionic ligands.12 (11) hl. Ciampolini, P. Paoletti, and L. Saci.oni, J . Chem. Soc., 4553 (1960). (12) J. K. Kury, -4.D Paul, L. C. Iiepler, and R. E. Comirk, J . Am. Chem. Soc., 81,4186 ( l e s s ) , P.K. Gallagher and E. L. King, tbzd., 82, 3510 (1960).
1067
AZINES
en
den
trien
Pi P
I 12
-AH,
(kcaljmole). 11
1
0
Ratlo Nsec./Ntotai,
Fig. 1.-Plot of -AH1 ( 0 )and A& ( A ) against the ratio of the number of secondary nitrogen atoms to the total number of nitrogen atoms.
Acknowledgmen ts.-The authors are indebted to Prof, L. Sacconi for helpful suggestions and criticism. Acknowledgment is made to the Italian “Consiglio Nazionale delle Flicerche” for the support of this research.
CHARGE TRASSFER IKTERACTION BETWEEN IODIXE AXD AZIXES : IONIZATION POTENTIALS OF AZINES1 BY V. G. KRISHNA~ AND MIHIRCHOWDHURY Whitmore Chemical Laboratory, T h e Pennsylvania State University, University Park, Pennsylvania Received November 1, 1962 Charge transfer data on four azine-iodine complexes are reported. Azines are shown to be n-donors toward iodine. The trend of the charge transfer maxima indicates that the experimentally observed ionization potentials of the azines correspond to r-electron ionization.
Introduction The following note will be concerned with the complexes formed between iodine and monocyclic azines : pyridine, pyrimidine, pyrazine and s-triazine. The aspects considered are (1) the perturbation of the visible band of iodine and the ultraviolet and infrared spectra of donor, (2) the equilibrium constants of the molecular complexes formed, and (3) the energy and extinction coefficient of the charge transfer (CT) bands. Previous studies3y5have indicated that the N-heterocyclics are n-donors toward iodine. I n the first ~ W O a thorough investigation of the pyridine-iodine complex and in the third5 the electron donating properties of substituted monoazines have been reported. The n-donor properties of monoazines are indirectly perturbed by the n-electronic structure.6 I n the present case of polyazines, in addition to the variation of the ~ ~ interaction of the nn-electronic s t r u c t ~ r e , ’ the (1) This investigation was supported by a grant from the Air Force Office of Scientific Research t o the University. (2) Institute of Molecular Biophysics, The Florida State University, Tallahassee, Florida; from whom reprints can be obtained. (3) C. Reid and R. S. Mulliken, J . A m . Chem. Soc., 7 6 , 3869 (1954). (4) E. K. Plyler and R. S. Rlulliken, ibid., 81, 823 (1959). ( 5 ) J. Nag-Chaudhuri and 5. Basu, Trans. Faraday Soc., 55, 898 (1959). ( 6 ) H. C. Longuet-Hipgins, J . Chem. Phys., 18, 275 (1950). (7) N. Mataga and K. Nishimoto, Z. physik. Chem. (Frankfurt), 18, 140 (19573.
orbitals also changes markedlyg and the n-donor properties of these molecules should reflect both these changes. An important aspect of this investigation is its relation to the ionization potentials of azines. The experimentally observedlO-ll ionization potentials of azines have received a varied interpretation by different authors as due to n-lo-l3and n-electron7-l4ionizations. A more unambiguous interpretation is needed before the ionization potential data could be used as a n ex~ perimental criterion for various theoretical models proposed for K-heterocyclics. I n the following investigation an attempt will be made to resolve this ambiguity through the use of charge transfer data. A linear relation between the charge transfer maxima and the ionization potentials of the donors in a series of molecular complexes with a common acceptor has been e~tablished.~5.’6 Since the azines are n-donors toward (London), 1170, (8) R. McWeeny and T. E. Peacock, Proc. Phys. SOC. 41 (1957). (9) L. Goodman, J . Mol. Spectry., 6, 109 (1961). (10) I. Oinura, H. Baha, K. Higasi, and Y. Kanaoka, Bull. Chcm. SOC. Japan, 30, 633 (1957). (11) K. Higasi, I. Omura, and H. Baba, J . Chem. Phys., 24, 623 (1956). (12) T. Sakajima and A. Pullman, J . chzm. phys., 66,793(1958). (13) IC Watanabe. T. Nskayama, and J. afottl, quoted in ref. 14. (14) &I. A. El-Sayed, M. Kasha, and Y. Tsnaka, J . Chem. Phys., 34, 334 (1961).
