SANJINISHIMURA AND NORMAN C. LI
2908
Infrared Studies of Amine Complexes with Some
Organophosphorus Compounds1 by Sanji Nishimura and Norman C. Li Department of ChEmistTV, Duqueene University, Pittsburgh, Pa 16910 (Received February 14, 1068)
The thermodynamic properties of the hydrogen-bonded complexes of aniline and N-methylaniline with tri-nbutyl phosphate (TBP) and tri-n-octylphosphine oxide (TOPO) in cyclohexane solution were obtained by measuring the effects of solvent composition and temperature upon the first overtone N H stretching bands of the amines. Formation constants of the 1 :1 complexes were calculated using a computer program. The heats of formation of the aniline complexes with T B P and TOPO are -3.83 and -4.52 kcal/mol, respectively; the values for the corresponding N-methylaniline complexes are -4.15 and -4.80 kcal/mol. In addition, the formation constants and the heat of formation of the chloroform-TBP complex were determined by measuring the effects of solvent composition and temperature upon the first overtone N H stretching band of anilinein aniline-TBP-chloroform-cyclohexane system. The heats of formation of the N-methylaniline-TBP and chloroform-TBP complexes are equal, and the proton-donating properties of N-methylaniline and chloroform are similar.
Introduction Although hydrogen bonding by aromatic amines is a very important consideration, only a few thermodynamic data have been reported for systems in which aromatic amines serve as proton donor^.^-^ Recently the thermodynamic properties of the hydrogen-bonded complexes of aniline and N-methylaniline with a variety of oxygen-containing bases (n-propyl ether, tetrahydrofuran, anisole, ethyl acetate, n-butyl acetate, and N,Ndimethylacetamide) were determined by infrared spectroscopic method^.^ In the present paper, formation constants and heats of formation in cyclohexane solution are reported for the complexes of aniline and N-methylaniline with tri-n-butyl phosphate (TBP) and tri-n-octylphosphine oxide (TOPO) . Formation constants were determined from the measurements of the effects of complex formation upon the first overtone NH stretching bands of the proton donors. The proton acceptors were chosen because of their widespread use in the extraction of metal ions from aqueous to organic phase. In addition, since chloroform has been used as an organic medium in solvent-extraction studies, we have carried out infrared studies on aniline-TBP-chloroform-cyclohexane systems and have obtained formation constants and the heat of formation for the complex of chloroform with TBP. Experimental Section Equipment and Materials. All of the spectra in the first overtone NH stretching band region were obtained with a Cary Model 14 spectropkotometer, using 5-cm cells. The scan speed was 5 A/sec and the spectral width was about 3 cm-'. The sample and reference cells were placed in jacketed cell holders connected to a The Journal of Phgaical Chemistry
constant-temperature bath. The solution temperature was determined by inserting an iron-constantan thermocouple into the cell and was controlled to *0.5". TBP and chloroform were purified in the manner previously described.8 TOPO, an Eastman Organic chemical, was used without further purification. Aniline and N-methylaniline were distilled twice and the fractions boiling at 184 and 86" (at 15 mm) were collected. Cyclohexane was a Fisher Spectroanalyzed reagent. Sample Preparation and Measurement. Stock solutions of aniline and N-methylaniline were prepared, and the final concentrations were approximately 0.09 M . The concentration ranges covered by the other compounds were approximately as follows: TBP, 0.080.44M ; TOPO, 0.06-0.12M ; and chloroform, 7.5-10 M . All concentrations were corrected for the variation of their densities with temperature. The spectra of samples were measured vs. reference solutions of cyclohexane containing equivalent amounts of the proton acceptors and a volume of carbon tetrachloride equal to the volume of proton donor in the sample solution. Although the complete NH bands were always scanned, the absorbance values used in the calculations were obtained by running the chart at selected fixed fre(1) This investigation was supported by the Atomic Energy Commission through Contract No. AT(30-1)-1922, Paper No. NYO1922-44. (2) F. Takahashi and N. C. Li, J. Phys. Chem., 69,2950 (1965). (3) J. H. Lady and K. R.WhetseI, ibid., 68, I001 (1964). (4) K. B. Whetsel and J. H. Lady, ibid., 68, 1010 (1964). (6) K. B. Whetsel and J. H. Lady, ibid., 69, 1596 (1965). (6) J. Wimette and R. H.Linnell, ibid., 66, 546 (1962). (7) J. H. Lady and K. B. Whetsel, ibid.,71, 1421 (1967). (8) S. Nishimura, C. H. Ke, and N . C. Li, ibid., 72, 1297 (1968).
INFRARED STUDIES OF AMINECOMPLEXES WITH SOMEORGANOPHOSPHORUS COMPOUNDS
2909
Table I : Absorptivities, Dimerization Constants, and Formation Constants" 7 - N - M e thylaniline7--
-Aniline"-----
Temp, ' C
1.902 1.884 1.870 1.852 1.841 1.818 1.806
Interpolated values.
This research.
Ir
M
+ h!I
1.240
0.407
1.221
0.350
1,204
0.309
0.348 0.330 0.319 0.298
' ED = 0.80.
ED
=
I/tenr.
D (KD = CD/CM') (1) TBP = C (Kc CC/CMCTBP) (2)
=
( K c , = CC,/CMCS) (3) (Kc,) = CCI,/CTBPCS) (4)
where M and D are the aniline monomer and dimer, respectively, and S is CHC13. If CAO, CTB~", and Cso represent the total initial concentrations of aniline, TBP, and chloroform, respectively C A o = CM
Aniline-TBP*
0.501
8.45
0.448
6.83
0.392
5.40
Kcf
0.289
=
+ N ! + S = C' C" TBP + S
Aniline-CHCls4 Kde
0.485
0.385 0.365
quencies for several seconds. In agreement with the values chosen by Lady and Whetsel,' the analytical frequencies were 6734 and 6696 cm-1 for N-methylaniline and aniline, respectively. Background absorbances were taken at 6496 and 6211 cm-', respectively. Formation Constants. Formation constants, KC,and absorptivities of the 1: 1 complexes, BC, in the anilineTBP-cyclohexane and the N-methylaniline-TOPOcyclohexane systems were calculated as described by Whetsel and Lady.6 Values for aniline-TBP are summarized in Table I. The absorptivities of the free monomer and dimer species of the proton donors, E M and BD, respectively, and the dimerization constants for the donors, KD, were interpolated from the data of Lady and Whetsel? and are listed in Table I. The absorptivity values taken from ref 7 are consistent with measurements taken with our instrument. Included in Table I are the formation constants, KO!,and the absorptivity, eat, of the aniline-chloroform comple~.~ In the aniline-TBP-chloroform-cyclohexane system, we have assumed that the significant interactions are the dimerization of aniline and the formation of the 1 : 1 aniline-TBP, TBP-chloroform, and anlinechloroform complexes. The dimerization of chloroform may be neglected because of its small dimerization constante ( K = 0.013 l./mol in cyclohexane solution, 2 5 ' ) . The systems can be described by the equations
M
K D ~
rM
1.259
18 23 29 30 34 40 42 44 52 53 56 a
KD'
fM
+ 2CD f CC + CCI
(5)
= 0.63.
e E C ~
'
EC
=
CS'
0.30 =t0.01.
=
cs + CC! + CC"
(6)
+ + CC"
CTBP' = CTBP Cc
(7)
Under the condition that chloroform is present in large excess so that Cs0 ~ i :CS, the combination of eq 1-7 gives
+ [(2KD + KC) + Cs0(2KDKCrl+ KcKc~)]CI+ M ~[1 + KC(CTBP'- CA') +
2KDKCCM'
CsO(Kc,,
+ Kc,) + KC~!KC~CS~~]C~I c A o ( l + KC!d?So) 0 (8) =
If Beer's law holds for the individual species
A/Z
=
+
+
~ M C M EDCD ecCc
+ ECCCT(9)
where A is the measured absorbance, 1 is the path length in centimeters, and eCt is the molar absorptivity of the aniline-chloroform complex. Equations 8 and 9 were solved for Kcft by the method of least squares using a computer program. The values of KD, KC, Kc!, eM, BD, EC, and E C ~used for the solution of these equations are summarized in Table I. The program was operated by substituting an assumed value of KC,,and the appropriate values of KD, KC, and Kc, into eq 8 to obtain a set of values of CMfor a series of solutions containing known total concentrations CA', CS', and CTBP'. Corresponding values of CD,Cc, and CC
