1768
J. Phys. Chem. 1981, 85, 1768-1769
Interaction of Primary Aliphatic Amines with Chloranil. Kinetic and Spectroscopic Studies P. C. Dwlvedl" and Anll K. Banga Department of Chemistry, Harcourt Butler Technological Instltute, Kanpur 208002, India (Received: May 9, 1980; In Flnal Form: December 31, 1980)
Interactions of primary aliphatic amines, n-propylamine, n-butylamine,n-hexylamine, and n-heptylamine,with chloranil have been examined by ultraviolet spectroscopy. The results indicate immediate formation of a monosubstituted product between the amine and chloranil, which subsequently decomposes to give a disubstituted product. Pseudo-first-order rate constants for the disappearance of the monosubstituted product as well as the formation of the disubstituted product have been determined. Solvent effects on the rates are studied in detail. Possible participation of an electron donor-acceptor complex formed between the amine and chloranil in the reaction is indicated.
Introduction The role of electron donor-acceptor (EDA) and u complexes as reaction intermediates in the nucleophilic substitution reactions of aromatic amines with chloranil (CA) has been reported Although primary and secondary aliphatic amines are known to form both the mono- and disubstituted products with CA,5-10kinetic studies of these reactions are scanty. The only paper in this direction is that of Lautenberger and Millerlo for the reaction of n-butylamine with CA. They found the rate of disappearance of the monosubstituted product to be the same as the rate of formation of the disubstituted product, indicating that no intermediate of appreciable stability exists in the step monosubstituted product disubstituted product of the reaction. Das and Majee5 have prepared the disubstituted products of a few primary aliphatic amines with CA and identified them as 2,5-dichloro-3,6bis(amino)-p-benzoquinones.Yamaoka and Nagakurall studied the interaction of n-butylamine with CA by a rapid scan spectrophotometric method and presented evidence for the formation of the chloranil radical anion. They have determined the rate of this ionization reaction by employing a conductometric technique. In all of these studies, evidence for the EDA complex formed between the primary amine and CA as reaction intermediate is lacking. In view of the close similarity of the final reaction product of primary aliphatic amine + CA5J0with that of aniline + CA4 (which is known to proceed through the EDA complex4),it was thought worthwhile to examine whether the EDA complex makes any contribution in the reaction of a primary aliphatic amine with CA. We have undertaken to study in detail the interaction of CA with npropylamine, n-butylamine, n-hexylamine, and n-heptylamine for this purpose.
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(1) Z. Rappoport, J. Chem. SOC.,4498 (1963). (2) T. Nogami, T. Yamaoka, K. Yoshihara, and S. Nagakura, Bull. Chem. SOC. Jpn., 44, 380 (1971). (3) T. Yamaoka and S. Nagakura, Bull. Chem. SOC.Jpn., 43, 355 (1970). (4) T. Nogami, K. Yoshihara, H. Hosoya, and S. Nagakura, J . Phys. Chem., 73, 2670 (1969). (5) B. K . Das and B. Majee, J. Indian Chem. SOC.,45, 1054 (1968). (6) L. F. Fieser, J . Am. Chem. Soc., 48, 2936 (1926). (7) N. P. Buu-hoi, R. Royer, and B. Eckert, Recl. Trau. Chim.PaysBas, 71, 1059 (1952). (8) D. Buckley, H. B. Henbest, and P. Slade, J. Chem. SOC., 4891 (1957). (9) R. Foster, R e d . Trau. Chim.Pays-Bas, 83, 711 (1964). (10) W. J. Lautenberger and J. G. Miller, J . Phys. Chem., 74, 2722 (1970). (11) T. Yamaoka and S. Nagakura, Bull. Chen. SOC.Jpn., 44, 1780 (1971).
0022-3654/81/2085-1768$01.25/0
Experimental Section Cyclohexane was purified by shaking with concentrated H2S04and washing with distilled water, then with sodium carbonate solution, and again with distilled water. It was then shaken with anhydrous calcium chloride for several hours and was then refluxed and distilled over Na pieces. Chloroform (BDH, Analar) and dichloromethane (E. Merck, LR) were dried over anhydrous calcium chloride and distilled. The amines employed in this investigation were available commercially and were purified by distillation over potassium hydroxide. Chloranil was purified by recrystallizing 4 times from benzene to give yellow platelets (sealed-tube mp, 289 "C). Electronic spectra were measured on a Beckman DU spectrophotometer fitted with a variable-temperature cell compartment using matched silica cells of 1-cm path length. Freshly prepared stock solutions were used for the measurements. Pseudo-first-order rate constants, k (in the presence of a large excess of donor), based on the measurements of the intensity of the mono- and disubstituted product were evaluated by employing eq 1 and 2, respectively. Here, 2.303 Do k=log t Dt
Do, Dt, and D , are the absorbances at time 0, t , and at the end of reaction, respectively. Results and Discussion All of the primary amines used in this study possess negligible absorbance above 250 nm. The ultraviolet absorption spectra of CA and a solution of the amine + CA (with the amine in large excess) were initially of identical form in all cases. Some typical spectra are shown in Figure 1for the n-propylamine + CA system in cyclohexane solution at 299 K. However, the absorbances of the mixed solutions change rapidly during measurements. Hence, the absorbances in Figure 1 are not to scale. An absorption band a t 289 nm (attributed to the monosubstituted productlo) appears in the fresh solution. This band diminishes rapidly, and a growing band at -354 nm, caused by the disubstituted product, appears in the spectrum. The 354-nm band attains its maximum absorbance in 11 min, indicating that the monosubstituted product changes into the disubstituted product in a relatively slow step. It may be mentioned here that CA itself exhibits an ab-
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0 1981 American Chemical Society
The Journal of Physical Chemistry, Vol. 85, No. 12, 198 1
Interaction of Aliphatic Amines with Chloranil
1769
TABLE I: Kinetic Dataa on the Reaction of Chloranil with Primary Aliphatic Amines at 299 K at the A,, of the disubstituted product ( 3 5 4 nm)
at the h,, of the monosubstituted product (289 nm)
amine
solvent
dielectric constant
n-propylamine
cyclohexane CHC1, CH,Cl, cyclohexane CHC1, CH,Cl, cyclohexane CHC1, CH,Cl, cyclohexane CHCl, CH,Cl,
2.02 4.5 9.1 2.02 4.5 9.1 2.02 4.5 9.1 2.02 4.5 9.1
n-butylamine n-hexylamine n-heptylamine
a
The concentrations of the amine and CA were 6.7 X
E,,
As*,
J K-' mol-'
103k,
s-'
kJ mol-'
3.65 4.03 4.79 4.22 4.56 5.37 5.95 6.33 7.10 6.33 6.71 7.87
26 21 16 20 17 14 15 12 9 12 10 8
- 190 - 207 - 224 - 210 - 220 - 229 - 225 - 234 - 242 - 234 - 241 - 244
3.45 3.84 4.61 4.03 4.41 5.02 5.76 6.14 6.91 6.14 6.52 7.67
lO'k,
lo-, and 2.9 X
S-
30 0
3LO
2
380
X.nm
Figure 1. UV absorption spectra of CA (2.9 X M) (curve l), and M) in a mixture of n-propylamine (6.7 X lo-' M) and CA (2.9 X cyclohexane at 299 K (curve 2, fresh solution; curves 3 and 4, after 5 and 10 min, respectively).
sorption band at 289 nm. The zero-time absorbance of the mixed solution at 289 nm (obtained by extrapolation of log Dt vs. time plot) is greater than that of CA itself in all of the cases. This observation indicates that the 289-nm band in the mixed solution is not due to CA. It should be assigned to the monosubstituted product which has a larger extinction coefficient than that of CA. Lautenberger and Millerlo have reported the extinction coefficient of CA to be larger than that of the monosubstituted product. This result appears to be in error. The pseudo-first-order rate constants, k, were evaluated by employing both the 289-nm as well as the 354-nm bands. The results are shown in Table I and Figure 2 along with the energies of activation, E,, and the entropies of activation, A S . The k values follow the order n-heptylamine > n-hexylamine > n-butylamine > n-propylamine in all of the solvents studied. This is also the order of the donor strength of these electron donors. Thus, the k values increase with an increase in the donor strength of the primary aliphatic amines. Our k values for the n-butylamine + CA system are in good agreement with the reported data.1° The E, values were evaluated from the rate constants at various temperatures in the range 288-309 K. These values vary in the expected direction in all of the systems (E, decreases as k increases). The near equality of the rate constants (as well as the E, values) at both bands (289 and 354 nm) indicates that no intermediate of appreciable stability exists in step I1 (monosubstituted disubstituted product) of the reaction. The product rates are sensitive to the solvent, k values increasing and E, decreasing with an increase in solvent polarity. This
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'
As*,
J K-' mol-' - 189 - 204 - 222 - 206 -212 - 229 - 224 - 232 - 242 - 232 - 241 - 244
27 22 16 21
18 14 16 13 10 13 10 8
M,respectively, in all cases.
L
\
\i
I
I
2 60
Ea, kJ mol-'
6 8 Time,min.
1
0
Flgure 2. Pseudo-first-order rate plots for n-propylamine (6.7 X lo-' M) CA (2.9 X M) in cyclohexane at 299 K: at (1) 289 and (2) 354 nm.
+
solvent dependence of k values suggests that there may be some charge separation in the transformation of the EDA complex to the final product. The entropies of activation, AS*, are large and negative, the magnitude being higher in more polar solvents. These AS* values indicate that the transition state in step I1 is more polar than the initial state (which is likely to be the monosubstituted product). The dependence of the rate constants and activation energies upon the electron donor ability of aliphatic amines supports the assumption that the reactions of primary aliphatic amines with CA proceed through an EDA complex. The amine being present in large excess, all of the CA in the mixed solution is converted into an EDA complex. Thus, the possibility of free CA in the mixed solution is negligible. This observation provides further support to the assignment of the 289-nm band to the monosubstituted product. Based on our kinetic data, the reaction of a primary aliphatic amine with CA may be formulated as follows: amine + CA
very fast
EDA complex
monosubstituted product
slow
very fast
step I
disubstituted product
+
Unlike Nagakura and co-workers4 for the aniline CA system, the mechanism of final product formation in the aliphatic amine + CA system does not involve c complexes as reaction intermediates. Acknowledgment. The award of a senior research fellowship to A.K.B. and financial assistance to P.C.D. by the UGC, New Delhi, is gratefully acknowledged.
