AARONN. FLETCHER
3686
The Effect of Hydrogen Bonding upon the Autoxidation Kinetics of Tetrakis(dimethy1amino)ethylene by Aaron N. Fletcher Chemistry Division,Research Department, Naval Weapons Center, China Lake, California 95666
(Received January 8,1969)
The autoxidation of tetrakis(dimethy1amino)ethylene has an apparent negative, zero, or positive Arrhenius activation energy depending upon the concentration of the hydrogen bonding catalyst, 1-octanol. The kinetics are both first and fourth order with respect to the 1-octanolmonomer concentration. When the concentrations of the alcohol self-association species are considered, it is found possible to explain these results by a ~ e r oArrhenius activation energy for both the acyclic self-association tetramer and the monomer. These results suggest that the acidity of the acyclic self-association species may be the important parameter in high-order alcoholysis rather than “solvation” of the reaction intermediate.
Introduction Conflicting of kinetics and mechanism usually result when alcohols serve as chemical reactants. For example, Fagley, et a1.,3interpret the alcoholysis of acid chlorides as being second order with a push-pull5 mechanism. Hudson and Loveday,2on the other hand, interpret the same reaction as being first, second, and third order with different alcohol self-association species serving as the reactants. Furthermore, Hudson and Loveday found second order only over a very limited concentration range. This writer wishes to report results on the autoxidation of tetrakis(dimethylamin0) ethylene (TMAE) , where similar high-order alcohol kinetics have been observeds (and with similar conflicts with respect to the kinetics and mechanism).lotll It will be shown that the effect of the alcohol, 1-octanol, can be explained solely on the basis of the acidity of the alcohol self-association species without having to consider that the high kinetic order is due to solvation of the reaction intermediate as reported earlier.s
Experimental Results The experimental techniques have been previously described.9 A new automated system was constructed around a Turner 210 spectrofluorometer.12 The solvent was n-decane. The rates of reaction a t 1-octanol concentrations below 10-2 M were found to be sensitive to the degree of agitation of the solution (Figure 1). This is attributed to catalysis by 0-H groups attached to the glass walls of the reaction vessel and will be designated the wall effect, W . The kinetic parameters were related bys
- d(TMAE) dt
=
(TMAE) X 0 2 X F(A)
(1)
where O2 represents the partial pressure of oxygen in The Journal of Physical Chemistry
Torr and F(A) represents some function of the alcohol concentration. An iterative procedures was used to determine constants, Ci in the expression F(A) = W
+ CiAi + C2Ai2 + C4Ai4
(2)
where A I is the concentration of the 1-octanol monomer calculated from the equilibrium quotients reported by Fletcher and Heller.13 Values for W were determined with no added alcohol; values for C1 (Figure 2) were determined at very low alcohol concentrations; values for C4 were determined at very high alcohol concentrations; both C1 and C4 were corrected for the effect of each other. It was not found possible to determine a reasonable value for C2 (consistent with subsequent activation energies and enthalpy changes for hydrogen-bond formation). The values for W (static), CI, and C4 are shown in Table I. Plots of the data at the two extremes of temperature, 5 and 45”, are shown in Figure 3. As indicated by Figure 1 and Table I, an average value for CI was taken from the results of all four temperatures.
Discussion The Effect of Alcohol Self-Association.
With in-
(1) R.F.Hudson, J. Chem. SOC.,761 (1966). (2) R.F.Hudson and G. W. Loveday, ihid.,766 (1966). (3) T.F. Fagley, G. A. von Bodungen, J. J. Rathmell, and J. D. Hutchinson, J . Phys. Chem., 71, 1374 (1967). (4) A. E.Oberth and R. S.Bruenner, ihid., 72,845 (1968). (5) C.G.Swain, J . Amer. Chem. SOC.,70, 1119 (1948). (6) E.D.Hughes, C. K. Ingold, S.F. Mok, and Y. Pocker, J . Chem. SOC.,1238 (1967). (7) E.A. Cave11 and J. A. Speed, ihid., 226 (1961). (8) A, J. Parker, Quart. Rev. (London), 16, 163 (1962). (9) A. N.Fletcher and C. A. Heller, J. Catal., 6, 263 (1966). (10) A. N. Fletcher and C. A. Heller, J . Phys. Chem., 71, 1507 (1967). (11) J. P. Paris, Photochem. Photobiol., 4, 1059 (1965). (12) G.K.Turner, Science, 146,183 (1964). (13) A. N. Fletcher and C. A. Heller, J. Phys. Chem., 71, 3742 (1967).
3687
EFFECT OF HYDROGEN BONDING 10
1
I
I
I
0.9
w
0.8
U
z Q m
I I :
20
0.7
4
0.6
I
I
I
I
I
I
20
40
60
80
100
120
0.5 0
140
T I M E lHOURSJ
Figure 1. Reaction kinetics under quiescent and agitated conditions. Agitation was performed from P to P'. Experimental conditions: 1200 Torr of 02;5" temperature; zero added 1-octanol; absorbance measurements were performed a t 350 nm.
I
I
I
1
I
I
I
I
I
Table I : Average Values for the Terms in P(a) 0-
1-
a t Various Temperatures
V Temp, O C T
O
5 15 30 45
wx
109,
Torr-'
sec-1
0.82 0.92 1.02 2.48
CI x 108, mol-' Torr-1 sec-1
0.85 0.85 0.85 0.85
C4,
mol-4 Torr-1 sec-1
4.75 1.82 0.485 0.134
monomer and the acyclic tetramer (Figure 4). The concentrations of the acyclic tetramer at 5 and 45" undergo a reversal in their relative amounts in the same manner as the over-all reaction rate function, F(A). Using the data of Table I, specific rate constants, k(Ai) can be calculated for one of the two tetramer species if the remaining species has a negligible kinetic effect. The rate constants also can be calculated as if both species had the same effect O
V i1
1
2
-1
3
L
4
5
6
7
8
9
10
MONOMER CONCENTRATION IMOLE L I T E R ' l ) x l O 3
Figure 2. Determination of specific rate function for the monomer. All points corrected for wall effect, W. Solid points show tetramer correction in contrast to measured open points: A, 45"; 0, 30"; 0, 15"; V, 5". Solid line is average of all four temperatures for solid points.
creasing alcohol concentration, the observed rate of autoxidation is found to be independent of temperature, to decrease with increasing temperature, and then to increase with increasing temperature (Figures 2 and 3). This type of anomalous behavior can be readily explained by the variation in the concentrations of the
where Ai may be the concentration of the acyclic tetramer, Ada, the cyclic tetramer, Ah, or their sum, Ad. Figure 5 shows the results of these calculations in an Arrhenius plot. Either the acyclic or the cyclic tetramer give a linear Arrhenius plot leading to activation energies of 0.0 and 3.8 kcal/mol, respectively. An averaging of both acyclic and cyclic species leads to a nonlinear plot (Figure 5). It would be unreasonable for the monomer to have a zero activation energy while the cyclic tetramer had a thousandfold larger specific rate constant with a positive activation energy. It appears far more likely that the acyclic species, with its availVolume 78, Number 11
November 1960
AARONN. FLETCHER
3688 able 0-H and zero activation energy, is the reactive t e tramer. Thus it is possible to write
F(A)
=;
W
+ 0.85 Ai + lo-' + 2 . 9 Ads X
(4)
L
which is valid from 5 to 45".
The Reaction Mechanism. Fletcher and Heller considered that the initial step in the autoxidation of TMAE was the formation of a contact donor-accaptor complex between TMAE and o ~ y g e n . The ~ donor-acceptor complex in turn was considered ta react with each alcohol species. Their m e c h a n i ~ m , ~however, J~ does not account for the markedly different rates of reaction observed between the monomer and the polymeric forms. The initial step proposed by Urry and Sheeto14would be preferable CH,
I
CH3-N
I CH3
CH3
I
N-CH3
I
gl
CH3 TMAE
$3 cH3-N\ CH3-N
ij
F>H
'iu-cH3 ?=f
-t RO-
(5)
I
I
followed by
CH3 I1
since a steady-state treatmentgleads to
- d(TMAE) = Klh(ROH)(TMAE)(O2) dt
(7)
where the differing acidity of the alcohol species, ROH, is reflected in the value of the equilibrium constant, Ki. The subsequent kinetic steps are identical with those previously described.'" I n eq 5 and 7, R represents either an aliphatic or an alcohol self-association acyclic chain. Since the "free" end of acyclic self-association polymeric species should be more acidic than the monomer,15 this mechanism can account for the reaction rate constants having the relationship acyclic tetramer
> monomer >> cyclic tetramer (8)
The Journal of Physical Chemistry
ix10.3
I
I A I
Ill 1210.2
I
I1111111
I
I11l111
1x10.'
1x10.0
Figure 3. Determination of specific rate constant for the tetramer. Lines are calculated from eq 4.
CH,
I
&-I3
1x10.8
FORMAL CONCfNTRATlON OF 1 OCTANOL (MOLE LITER.II
N-CH,
kH,
4
/I
Thus it is not necessary to assume that the solvation of the reaction intermediate by alcohol is the cause of the observed high order kinetics. If the cyclic tetramer is completely nonreactive, then "solvation" of the reaction intermediate by the catalyst is a very unlikely argument in the autoxidation of TMAE. Conflicts in the Kinetic Order of Alcohol Reactions. A primary cause for conflicting rmults for reactions involving alcohol has been the practicea-'tl6 of using the formal "added" alcohol concentration as if it was the molar concentration of the monomer. Since nonsterically hindered alcohols self-associate, the formal concentration is approximately equal to the molar concentration only a t low concentrations in a nonreactive solvent-and even carbon tetrachloride can serve as an acceptor for hydrogen bond formation.'' How low a concentration depends primarily upon the solvent and the temperature (different nonhindered alcohols appear (14) W. H. Urry and J. Sheeto, Photochem. Photobiol., 4, 1067 (1966). (16) L.J. Ballamy and R. J. Pace, Spectrochim. Acta, 22, 526 (1966). (16) F. W. Balfour and T. F. Fagley, J . Phya. Chem., 72, 1300 (1968). (17) A. N.Fletcher, ibid., 73, 2217 (1969).
EFFECT OF HYDROGEN BONDING
3689
E
1x10
l
I
‘
1 0 0 3 ~ T(K 1 )
Figure 5. Determination of Arrhenius activation energies for a specific polymer species. Acyclic tetramer, Ad.; cyclic tetramer, Ado; total tetramer, Ad.
1 x 1 0 ~
i
1
1xlO.l’ 1x10’
I
I I
1. I I I I I l l 1XlOZ
I
I IIlllIl
I
I I I Ill
1x10 1
1x10”
F’3RkOAL COYCENT9ATION OF 1 OCTANOL { V O L E LITER I )
Figure 4. Calculated concentration of 1-octanol monomer, and At, acyclic tetramer, Ada, a t 5 and 45”.
to self-associate to about the same degree.”) An example of the effect of plotting formal concentrations can be seen in the work of Fletcher and Hellers where they found that the autoxidation of TMAE gave a very reasonable third order in formal “added” 1-octanol over a tenfold range (up to 0.1 F ) , whereas the order became fourth from the same data when the molar concentration of the monomer was plotted. Any overestimation of the monomer concentration in effect causes a reduction of the apparent kinetic order in alcohol. This is a particular problem for high order reactions where the limited range of concentrations (either due to the speed of the reaction or else due to the possibility of reaching concentrations greater than
that of neat alcohol) prevent the observation of rate constants over a range of values that is large enough to truly distinguish a third order from a fourth. Since the only positive18-20evidence for particular self-association polymeric species of alcohols are those for dimers21s22 and tetramers,13 with only the tetramer being found in high concentrations of nonhindered true evidence for a reaction being third order in alcohol monomer over a large concentration range is probably an indication of an increased complication of the mechanism rather than a simple reaction with an alcohol trimer. (18) By positive, I eliminate evidence from two-parameter power series that have been fit to experimental evidence. It has been demonstratedl*,lg that numerous different models of self-association can be fit to these equations. Only very small differences in error functions (which very often have not been properly “weighted”)20 must be used as evidence of one model in preference to another. These power series are useful only after the correct model has been determined by positive evidence for the existence of the species used in the mathe matical relationship. The most positive evidence of which I know is the variation of the 2.86-pm band with the second power of the moncmer band at 1.41 pm as evidence for the dimer21122 and the variation of the 1.43-1.65-pm region to the fourth power of the 1.41-pm monomer band as evidence for the tetramer.18 (19) H. Dunken and H. Fritzsche, Spectrochim. Acta, 20, 786 (1964). (20) W. E. Denning, “Statistical Adjustment of Data,” John Wiley and Sons, Inc., New York, N. Y., 1943. (21) R. M. Hammaker, R. M. Clegg, L. K. Patterson, P. E. Rider, and 8. L.Rock, J . Phys. Chem., 72,1837 (1968). (22) A. N. Fletcher and C. A. Heller, ibid., 72, 1839 (1968).
Volume 73,Number 11 November 1969
