1157
ASSOCIATION CONSTANTS OF AROMATIC AMINES
Spectroscopic Determination of Association Constants of Primary Aromatic Amines with Dimethyl Sulfoxide and
Hexamethylphosphorotriamidel by Christian Madec, Jacques Lauransan, and Pierre Saumagne* Laboratoires de Thermodynamigue Chimique et de Spectrochimie MolBculaire, FacultB des Sciences, 8QN-Brest, France (Received September 81, 1070) Publication costs assisted by Universitb de Bretagne Occidentale, &est
The formation constants of 1-1 complexes of substituted anilines, naphthylamines, aminopyridines, and aminopyrimidines with the proton acceptors DMSO and HMPT were determined at room temperature by infrared spectroscopy. These primary aromatic amines are classified according to their proton-donor power; the nature and position of the substituents are discussed and correlations are established between the values of the association constants and some parameters.
I. Introduction Hydrogen-bonded complexes in which aromatic amines act as proton donors have been studied extensively in recent years.2-6 It is well known that the infrared spectra of the stretching vibrations of the NH2 group of primary aromatic amines with proton acceptors demonstrate the presence of 1-1 and 1-2 complexes in which one of the donor molecules is bonded to one or two acceptor molecules, respectively.
I
I
N
N
/\
H
/ \
H
A/ 1-1 complex
H A
/
H '\\
A
1-2 complex
The work reported in this paper was undertaken in order to obtain the association constant of 1-1 complexes between various primary aromatic amines (substituted anilines, naphthylamines, aminopyridines, and aminopyrimidines) and the proton acceptors dimet hyl sulfoxide (DMSO) and hexame thylphosphorotriamide (HMPT) in dilute carbon tetrachloride solutions a t 25". The association between donor D, acceptor A, and complex C can be represented by the equilibrium A D C, if only 1-1 complexes are present. The association constant K , for such a system is given
+
by
where CD" and C A o are the initial concentrations of donor and acceptor, and CD, CA, and CC are the equi-
librium concentrations (in moles per liter). (In dilute solutions, activities are replaced by the corresponding concentrations.) Because of the various overlapping bands of donors and molecular complexes in the infrared spectrum between 3200 and 3600 cm-l, equilibrium concentrations cannot be obtained directly by application of the BeerLambert law. Therefore, the Kagarise method was used.' The absorbance per unit length is plotted as a function of Cc, the complex concentration, calculated from initial concentrations CD" and C A O and from an arbitrary value for K. For a constant value CD", when the arbitrary value of K is equal to the true equilibrium constant we obtain a linear correlation between D/l and Cc (as shown in Figure 1).* This method is applicable for any relative values of CD" and CA". When CA">> CD", the Basu-Chandra method was also
11. Experimental Section All spectra were obtained with a Perkin-Elmer 225 spectrophotometer. The spectral slit width was ap(1) Part of "ThBse de 38me cycle" of C. Madec, Brest, 1970. (2) K. 13. Whetsel and J. H. Lady, J . Phys. Chem., 69, 1596 (1965). (3) K. B. Whetsel and J. H. Lady, ibid., 71, 1421 (1967). (4) J. Lauransan, P. Pineau, and J. Lascombe, J . Chim. Phys., 63, 635 (1966). (5) J. Lauransan and P. Pineau, ibid., 65, 1937 (1968). (6) 8. Nishimura and N. C. Li, J . Phys. Chem., 7 2 , 2908 (1968). (7) R. E.Kagarise, Spectrochim. Acta, 19, 629 (1963). (8) D/I = fO(cDo - cc) -k icco, where CD and €0 are the molar absorptivities of donor D and complex C and 2 is the optical path
length in centimeters. (9) B. B. Bhowmik and S. Basu, Trans. Faraday SOC.,58,48 (1962). (10) A. K. Chandra and S, Basu, $bid., 56, 632 (1960). (11) A. B. Sannigrahi and A. K. Chandra, J . Phys. Chem., 67, 1106 (1963). The Journal of Physical Chemistry, Vola76, No. 8,1971
C. MADEC,J. LAURANSAN, AND P. SAUMAGNE
1158
Table I: Association Constants of Some Primary Aromatic Amines with Dimethyl Sulfoxide (DMSO) and Hexamethylphosphorotriamide (HMPT) in Carbon Tetrachloride Solution a t 25" No. of compound
Compound
1
Aniline 2-Nitroaniline 3-Nitroaniline 4-Nitroaniline 4-Bromoaniline 2-Nitro-4-chloroaniline 2 4 hloro-4-nitroaniline 2-Aminopyridine 3-Aminopyridine 4-Aminop yridine 2-Aminopyrimidine 4-Aminop y rimidine 5-Aminop yr imidine a-Naphthylamine @-Naphthylamine
2 3 4 5 6 7 8 9 10 11 12 13 14 15 5
-DMSOK, 1. mol-1
Av,
K,
Avl
cm-1
1. mol-1
cm-1
2.5 11 15 22.5 4.5" 30 35 5 8 20 2.5
44 76 67 76 55 81 87 61 54 65 110
7 35 65 85 14 130 140 18 27.5 65 9 40 50
66 112 90 99 76 116 114 88 76
-HMPT--
44 49
5 8
90 130 117 88
ua(NHz), cm-1
3482 3523 3497 3509 3488 3522 3516 3511 3485 3509 3543 3537 3487 3477 3484
vi( NHz)
,
om-1
3396 3400 3407 3416 3401 3401, 3414 3409 3398 3415 3430 3425 3400 3395 3397
Calculated by Lauransan.6
oi
DMSO was obtained from Laboratoires du Bois de Boulogne; HMPT, pyridine, and CCh, R P or "spectroscopic" grade, were Prolabo products. All solvents were dried by storing over Linde 4A molecular sieves and manipulated in a drybox to avoid atmospheric moisture.
0.8
I
1
0
1
I
2'
I
I
1
I
I
I
3
4
5
6
7
8
IO% ' ,&)
Figure 1. Evaluation by Kagarise method of the association constant of the 2-nitro-4-chloroaniline-HMPT system in carbon tetrachloride solution a t 25". D = absorbance of VI("*) at 3400 cm-1. The uncertainty in each value is represented by a vertical line. CD" = 7.9 X 10-4 M ; 6.5 X lobaM < CA" < 4 X lob2M ; 1 = 5 cm; the assumed value of K is 130 f 10 1. mol-'.
proximately 1 cm-l. The wave number precision was about 1 or 2 cm-I depending on band shapes, while the precision of measured absorbance was about 1%. Because of the weak solubility of amines in carbon tetrachloride, cells of 3- or 5-cm path length with CaFzwindows were used. The amines were commercial products from Prolabo, Aldrich, and Eastman Kodak; the aminopyrimidines were kindly provided by Mr. P. Dizabo. The Journal of Physical Chemistry, Vol. 76, N o . 8,1971
111. Results In each case a series of ternary mixtures consisting of amine, acceptor, and carbon tetrachloride, was prepared. The amine concentration was maintained at a constant low value (about M ) . The acceptor concentration which was always larger than that of the M . Under donor, was varied between lo-' and these conditions self-association of the amine is not observed, while the acceptor concentrations were chosen to avoid formation of 1-2 complexes, as confirmed by the infrared spectra. The determinations of the various constants were made by studying either the absorbances of the vl(NH2) bands or those of [vl(NHz)]1-1.12 The precision in the determination of absorbances of donor bands is often reduced by the presence of absorptions due to the acceptors. However, in some cases it was confirmed that the constants obtained from measurements of vl(NHz) or [v1(NH2)I1-1are equal. The Kagarise determinations are illustrated by the example of Figure 1. This graph allows the determination of uncertainty in the value of the constant K of about 10%. For any donoracceptor mixture the equilibrium constant, calculated using the Basu-Chandra method, is in good agreement with that obtained by the Kagarise method. (12) n(NH2) is the symmetrical stretching vibration of the free amine and [YI(NHz) 11-1 the symmetrical stretching vibration of the 1-1 complex.
ASSOCIATION CONSTANTS OF AROMATIC AMINES The experimental results are summarized in Table I. For each amine the frequencies of the vI(NH2) and V3("2), symmetric and antisymmetric stretching vibrations are listed as well as the value of the frequency shifts of the symmetric stretching vibration, Av = vl(NH2) - [ Y ~ ( N H ~ for ) ] ~each - ~ complex. The association constants obtained with HMPT are always approximately four times larger than those obtained with DMSO. Among the various acceptors generally studied, these two are among the strongest.
IV. Discussion The results obtained in the present work are compared with those reported for related hydrogen-bonded systems. The influence of substituents on the acidity of amines is discussed, and correlations between the values of association constants and other physicochemical quantities are established. 1. Comparison with Other Proton Donors. I n Table I1 are reported the values of association constants, in
Table I1 : Association Constants between Some Proton Donors and DMSO in Carbon Tetrachloride Solution a t 25' K, Compound
a-Naphthol Phenol Indole Pyrrole Benzylic alcohol Heptyl alcohol Butyl alcohol Chloroform
1. mol-'
275a 188.5," 230b 215 12.3," 15.60 15" 90 90 3.2 (mol fract)-'
5 J. P. Leicknam, Thesis, Paris, 1966. b T. Gramstad, Spectrochim. Acta, 19, 829 (1963). F. Cruege, 3rd cycle Thesis, Bordeaux, 1963. d Calculated from nmr studies of binary mixtures [W.-C. Lin and S.-J. Tsay, J. Phys. Chem., 74, 1037 (1970)l.
liters per mole, between several proton donors and DMSO. The comparison between our results (Table I) and those of Table I1 places the ability of the NH2 group to form hydrogen bonds with DMSO between that of chloroform and alcohols. Hence, from the values of association constants, we obtain the following order of proton donors by increasing acidity:13-16 chloroform < aromatic amines, pyrrole and alcohols < phenols. This classification can be changed by the presence of substituents. 2. EJ'ect of Substituents on the Aniline Aciditg. a. Nature of Substituent. The results of Table I show that p-bromoaniline and p-nitroaniline are more acidic than aniline. Indeed these electron-withdrawing substituents discharge the nitrogen atom and increase the acidity of the NH2 group, the charge density on the nitrogen atom varying from 1.729 for aniline to 1.694 for p-
1159 chloroaniline, whose acidity is equal to that of p br~moaniline,~ to 1.531 for the p-nitroaniline. l6 On the other hand, an electron-donating substituent decreases the acidity of the amine; this effect has been shown by Lauransan.* b. Position of Substituent. The association constants of the three nitroanilines, ortho, meta, and para, with DMSO are 11, 15, and 22.5 1. rnol-l, respectively. This order is the same as that of the melting points (71.5, 112.5, and 147.8") and the decrease in the solubility of these amines in carbon tetrachloride. I n p-nitroaniline, the contribution of the mesomeric effect +A4 of the amino group and -M of the nitro group decreases the charge density of the nitrogen atom the NH2 group and increases the acidity of this amine. As this type of resonance is weaker in the meta compound, the mesomeric effect is transmitted with more difficulty and this compound is less acidic. The charge density of the nitrogen atom confirms this result, being 1.651 and 1.531 for the m- and p-nitroanilines, respectively. The same calculations give a value of 1,502 for the nitrogen charge density in o-nitroaniline. This results seems to be in opposition to the value of the association constant. In this compound the influence of the ortho substituent is anomalous and will now be discussed. c. Ortho Compounds. I n these compounds interaction between one hydrogen atom of the NH2 group and the neighboring substituent have been considered. For these amines, the stretching frequencies ug(NH2) are raised and the differences V3("2) - vl(NH2) are increased, in comparison with the corresponding para-substituted anilines (see Table I). Recently Dyall'7, lShas assumed an intramolecular bond between one hydrogen atom of the NH2 group and one oxygen atom of the N O 2 group in o-nitroaniline. This intramolecular bond decreases the amine acidity. It was established by studying v3("2) and vl(NH2) frequencies of nitroanilines in various proton acceptor solutions that there were 1-2 complexes for the m- and pnitroanilines, whereas there is only formation of the 1-1 complex for o-nitroaniline.lg N-Methyl-2-nitroaniline was studied in carbon tetrachloride solution with weak DMSO concentrations (CA"< lo-' M ) . Under these conditions association is not observed. We assume that the intramolecular (13) The values of the association constants of amide-DMSO systems can be compared with the result of Table 11: rhodanine, K = 168 1. mol-'; succinimide, K = 65 1. mol-'; maleimide, K = 60 1. mol-1.1446 (14) L. Le Gall, J. Lauransan, and P. Saumagne, J . Chim. Phys., 66, 650 (1969). (15) L. Le Gall, A. Le Narvor, J. Lauransan, and P. Saumagne, C. R. Acad. Sci. Paris, 268, 1285 (1969). (16) B. R. Lynch, Tetrahedron Lett., 17, 1357 (1969). (17) L. K. Dyall, Spectrochim. Acta, 25, 1423 (1969) (18) L. K. Dyall, ibid., 25, 1727 (1969). (19) J. Lauransan, Thesis, Bordeaux, 1967. The Journal of Physical Chemistry, Vol. 76,No. 8 , 1971
1160
C. MADEC,J. LAURANSAN, AND P. SAUMAQNE
bond prevents association between the hydrogen atom and the oxygen atom of DMSO. Therefore, in DMSO Dya1l2O supposes that the hydrogen atom is bonded simultaneously to the substituent and the acceptor. The ortho substituent decreases the amine reactivity, which accounts for the fact that the association constant of o-nitroaniline with DMSO is weaker than those of the meta and para isomers. This weaker reactivity of ortho compounds has also been observed by Pineau2I for chlorophenols. d. Comparison of the Acidity of W-Chloro-4-Nitroand %-Nitro-4-chZoroaniEines. Our results (Table I) show that two electron-withdrawing substituents strongly increase the acidity of NH2 group. In these compounds an interaction between one hydrogen atom of the NH2 group and the ortho substituent is possible. This interaction is weaker in compound A than in compound B because of the different hydrogen-acceptor H
H
v N
H
H
(p0 vcl Y? I
NO2
C1
A
B
center distances. This observation can explain the larger acidity of 2-chloro-4-nitroaniline. e. Correlation between the Association Constant and the Hammett u Parameter. It is well known that in aromatic series the Hammett u constant characterizes the electronic effect of a substituent. The value of u depends on the nature and position of the substituent. Thus we have plotted log K as a function of u (Figure 222) for the substituted aniline-HMPT systems. The correlation obtained is reasonably good. Therefore the Hammett relations are suitable for describing complexation in nonionizing solvents. Similar correlations have been recently established for the substituted phenol-pyridine s y ~ t e m s . ~ ~ J ? ~ 3. Acidity of Naphthylamines, Aminopyridines, and Aminopyrimidines. The acidity of these amines is compared to that of aniline and the influence of the position of the NH2 group on the acidity of aminopyridines and aminopyrimidines is discussed. a. Naphthylamines. The acidity of these amines is somewhat larger than that of aniline (Table I). The conjugation of the lone pair of electrons of the nitrogen atom with the ar electrons of the aromatic rings seems to be increased slightly. b. Aminopyridines. The results of Table I show that the endocyclic nitrogen atom has qualitatively the same effect as an electron-withdrawing substituent The Journal of Phusical Chemistry, Vol. 76, N o . 8, 1971
-.-
0.5
1
1.5
6
Figure 2. Plot of log K vs. u (from ref 22) for substituted aniliie-HMPT systems. Numbers correspond to the donors listed in Table I.
(NO2, for instance). The order of the acidity of these amines is the same as that presented for the nitroanilines: 4-aminopyridine is more acidic than 3-aminopyridine, in good agreement with the values of the charge density of the exocyclic nitrogen atom (see Figure 3).26 As in o-nitroaniline, the stretching frequency Va("2) is raised and the difference va(NH2)v1(NH2) is larger for 2-aminopyridine, by comparison with 4-aminopyridine (Table I). The decreasing value of the association constant of compound IV (Figure 3) can be explained by an interaction between a hydrogen atom of the NH2 group and the lone pair of electrons of the endocyclic nitrogen atom. We note that the three isomers become more acidic as their dipole moments increase in the order 3.04, 3.12, and 3.95 D for compounds IV, 111, and 11, respectively (values were determined in benzene solution26). c. Aminopyrimidines. The tendency for these amines to form hydrogen-bonded systems with a proton acceptor has been studied qualitatively by JaqueLafaixmZ7The values of the association constants with HMPT confirm her conclusion that the 4-amino- and 5aminopyrimidines have an acidity of same magnitude, larger than those of 2-aminopyrimidine and aniline. For each aminopyrimidine the influence of the relative position of the NH2 group is discussed by comparison with the corresponding aminopyridines. i. 6-Aminopyrimidine (Compound V I ) , The presence of the two endocyclic nitrogen atoms in positions (20) L. K.Dyall, Spectrochim. Acta, 22, 467 (1966). (21) P. Pineau, Thesis, Bordeaux, 1961. (22) R. W. Taft, Jr., "Steric Effects in Organic Chemistry," M. S. Newmann, Ed., Wiley, New York, N. Y., 1956,pp 571-619. (23) T . Zeegers-Huyskens, Thesis, Louvain, 1969. (24) J. Rubin, B. 2. Zenkowski, and G. 8. Panson, J . Phys. Chem., 68, 1601 (1964). (25) 8. F.Mason, J . Chem. floc., 3619 (1958). (26) J. Barasain and H. Lumbroso, Bull. SOC.Chim. Fr., 492 (1964). (27) A.Jaque-Lafaix, A. Burneau, and M. L. Josien, J . C h h . Phys., 65, 345 (1968).
1101
ASSOCIATION CONSTANTS OF AROMATIC AMINES
q=2802
H$r,
0 I
a10
K-7
1-
aniline
Oe5-
H H
\" qd.774
e1
'lq=V42
0
11 K = 6 5
/
1.5-
I
I
1
40
50
60
I
70
80
D
90 AV(cm')
Figure 4. Plot of log K vs. AV for amine-DMSO systems. Numbers correspond to the donors listed in Table I.
4-aminopyridine
H H
Q
q=v45
I0
3
H H 'N'
q4.682
301
Figure 3. Values of the charge density of the exocyclic nitrogen atom of aniline, aminopyridines, and aminopyrimidines (calculated by Masonz6)and association constants with H M P T (in 1. mol-').
1 and 3 increases the acidity of this amine. The association constant with HMPT is approximatively twice that of 3-aminopyridine (compound 111). This result is in good agreement with the values of charge density of the exocyclic nitrogen atoms (Figure 3). ii. 4-Aminopyrimidine (Compound V ) . As in 4aminopyridine (compound 11) the presence of the 1nitrogen atom increases the acidity of this amine by comparison with aniline, but this effect is decreased because of interaction of hydrogen atom of the NH2 group with the lone pair of electrons of the 3-nitrogen atom as in 2-aminopyridine (compound IV). Here we can make the same comment about vs(NHZ) - vl(NH2) as in the case of the nitroanilines and the aminopyridines (Table I). The compound 4-aminopyrimidine is less acidic than 4-aminopyridine. iii. 2-Aminopyrimidine (Compound V I I ) , The reactivity of this amine is strongly decreased because of the interaction between the hydrogen atoms and the two nitrogen atoms 1 and 3. This effect is more important than in 2-aminopyridine (compound IV) ; the association constant with HMPT is twice as weak for 2-aminopyrimidine as for 2-aminopyridine. As in aminopyridines, we note that the acidity of the three isomers increases with their dipole moments,
6 OL
I
0.5
-
1
1
1.5
6
Figure 5 . Plot of AV us. u for substituted aniline-HMPT systems. Numbers correspond to the donors i i t e d in Table I.
which are 0.827, 2.623, and 2.625 D for compounds VII, V, and VI (values calculated by Kwiatkowski2*), respectively. We can conclude that the effect of an endocyclic nitrogen atom ortho to the NH2 group is to decrease the value of the association constant. This value is not, then, in agreement with the charge density calculated for the exocyclic nitrogen atom.
4. Correlation between Association Constants and Frequency Shifts. The frequency shifts, defined by AV = vl(NH2) - [ Y ~ ( N H ~ )of ] ~ the - ~ , symmetrical stretching vibrations are listed in Table I. For the same donor, the value of A V increases with the basicity of the acceptor. Larger values are found for HMPT complexes than for those of DMSO. This result is in good agreement with the fact that HMPT is more basic than DMSO, as shown previously. For the same ac(28) J. S. Kwiatkowski, Acta Phys. Polon., 30, 963 (1966).
The Journal of Physical Cher?kiatry,Vol. 76, Nb. 8, 1971
NOTES
1162 ceptor the value of Av increases with the acidity of the donor, except in the case of ortho-substituted amines. In Figure 4, log K is plotted as a function of Av for amine-DMSO We obtain a linear relationship between these two factors. The points off of the straight line correspond to the ortho compounds. The values of Av are plotted in Figure 5 as a function of the Hammett u parameter for the substituted anilineHMPT systems. The correlation between these two factors is good because of the relationships previously
established. Similar correlations have been found in the systems phenol-pyridine and phen0l--aniline.~3
Acknowledgments. We wish to thank Dr. G. Turre11 (Bordeaux) for valuable comments on the manuscript, and Dr. P. Dizabo (Paris) for the gift of aminopyrimidines. (29) A Fermi resonance is observed between n ( N H 2 ) u and the first overtone of the in-plane bending vibration at about 3200 cm-1. This resonance is weaker in the case of 1-1 complexes but can slightly perturb the value of the [vi(NHz) 11-1 freq~ency.'~
NOTES
The Far-Ultraviolet Spectrum of Ice by Allen P. Minton' Polymer Department, Weizmann Institute of Science, Rehovoth, Israel (Received October 16, 1970)
The following procedure was employed to prepare the sample for measurement. The end plate not containing the quartz window wm removed and a small aliquot (0.1-0.3 cc) of previously degassed doubledistilled water was introduced into the prerefrigerated
Publication costs borne completely by T h e Journal of Physical Chemistry
During a study of the relationship between optical properties and intermolecular interactions in the water substance, the absorption spectrum of ice in the fas-uv (180-190 mp) was considered to be of interest, especially when compared to those of liquid water and water vapor. The only previous quantitatively reported measurements of uv absorption in ice2 were limited to wavelengths shorter than 170 mp, and it was not clear whether these data could be safely extrapolated into the region of interest. The molar extinction coefficient B of liquid water increases exponentially with decreasing wavelength throughout this region and shifts to the blue with decreasing t e m p e r a t ~ r e . ~ It , ~was therefore assumed that the absorption of ice would be somewhat less than that of water at the same wavelength, and that correspondingly longer transmission path lengths would be required to bring the absorptivity of the sample into the region capable of accurate measurement.
Experimental Section The sample cell illustrated in Figure 1 was constructed of copper to ensure effective thermal connection between the sample and the thermostat (a refrigerated bath of ethylene glycol solution). The length of the path was determined by the thickness of the central spacer, and several of these were prepared with thicknesses ranging from 1to 10 mm. The Journal of Phyekal Chemistry, Vol. 76, No. 8,1971
A
6
A'
/
0
-4
cm ----I
Figure 1. Sample cell for the memurement of the far-uv spectrum of ice: A,A', end plates with cooling coils; B, central spacer; C, quartz window. (1) Chaim Weizmann Junior Postdoctoral Fellow; present address: Laboratory of Biophysical Chemistry, National Institute of Arthritis and Metabolic Diseases, NIH, Bethesda, Md. 20014. (2) K. Dressler and 0. Schnepp, J . Chem. Phys., 33, 270 (1960). (3) M. Halmann and I. Platzner, J . Phys. Chem., 70, 580 (1966). (4) D. P. Stevenson, ;bid., 69, 2145 (1965).
