TETROXIDE' Molecular Addition Compounds with Donor Molecules HARRY H. SISLER University of Florida, Gainesville, Florida
BEcausE of its convenient physical constants (melting point, - 12.3"C.,boiling point, 21.3"C.), its easeof preparation and purification, as well as its very interesting chemical characteristics, dinitrogen tetroxide has, during the past quarter-century, been the subject of considerable study as a solvent medium for chemical reactions, and as a reactant in the liquid phase. Much of the pioneering work in the use of this substance as a solvent has been done by Addison and his co-workers ( 1 ) . During recent years a number of studies in this field have been carried out in the author's laboratory (%st4 , 6 , 6 , 7 ) . The character of dinitrogen tetroxide as a solvent medium is largely determined by the following factors: ( a ) its low dielectric constant (2.42 a t 18OC.) which prevents it from acting as a good ionizing solvent, (b) the various dissociation equilibria into which it enters, (c) its ability t o act as a Lewis acid and thus to form addition compounds with donor molecules, and (d) its oxidizing properties. Dissociatia Equilibria. The equilibrium N z O g 2N02 in the liquid and gaseous states is well known. Since Not is an odd molecule and is intensely colored, this equilibrium has been studied magnetically, spectro~ h o t o m e t r i c a has well as tensimetricallv. I n the -
I Presented as part of the Symposium on Recent Advances in the Chemistry of Inorganic Nitrogen Compounds before the Division of Chemical Education at the 131st Meeting of the American Chemical Society, Miami, April, 1957.
VOLUME 34, NO. 11, NOVEMBER, 1951
pure, liquid state a t atmospheric pressure, dinitrogen tetroxide is composed of more than 99% of the species NzO4 (8,9). Though, as evidenced by its exceedingly low electrical conductance (1.3 X lo-* mho a t 17'C.), autoionization of this solvent is exceedingly small, the existence of the equilibrium NzO4 NO+ NC3- has been definitely established (10). The equilibrium NzOI $ KC2+ NOa- has also been postulated, but very little real evidence for it seems t o have been obtained. St~ucturr:of h e NzOp Molecule. The molecular structure of the dinitrogen tetroxide molecule was for many years the object of considerable scientific discussion and argument. This argument has been resolved by Broadley and Robertson (11) who studied crystalline dinitrogen tetroxide by the method of X-ray diffraction. Their results indicate the following coplanar structure:
=
+
+
It should he noted that the N-N distance (1.64 A,) i s considerably longer than the normal N-N distance in a molecule such as hydrazine. Smith and Hedberg (1%) recently published the following analogous coplanar structure for the gaseous phase.
Very active metals reart with liquid dinitrogen tetroxide in a manner analogous to their reactions with water. We may account for these structures in terms of a resonance hybrid of such bond structures as the following:
DINITROGEN TETROXIDE AS THE BASIS FOR A SOLVENT SYSTEM
Addison and his co-workers (1)have studied a number of reactions in liquid dinitrogen tetroxide from the point of view that dinitrogen tetroxide is the parent solvent for a "system of compounds" analogous to the "ammonia system" or the "water system." Thus the dissociation equilibrium mentioned above, N104$N0 NO,-, was considered to be analogous to the autoiouizat,ion reactions known or postulated for various other solvents. For example,
+
2Hn0 2NH, 2502
* HIO + OH+
= NH,+ + SH,-
* SO + SO1--
Just as the addition of hydrogen cbloride to water increases its reactivity toward metals so also does the addition of NOCl to liquid dinitrogen tetroxide increase its reactivity toward metals. Thus, metals such as zinc, iron, and tin react with solutions of SOCl in liquid dinitrogen tetroxide as follows. MI.,
N204
analogous to
Arnphoteric metals such as zinc behave toward liquid dinitrogen tetroxide solutions in a manner analogous to their behavior toward aqueous acids and hases. Thus, zinc metal reacts only very slowly ~ r i l hliquid dinitrogen tetroxide. However, zinc reacts rapidly with solutions of NOCl (see above) or [Et2NH1]NC3in this solvent. Zn
++
According to this idea, substances furnishing the NO+ group would be "solvo" acids in dinitrogen tetroxide and substances furnishing NO3- ions would be "solvo" bases. Such substances as NOCl and NORr should behave as solvo acids and the nitrates, e.g., [Et2NH2] NO2 should act as solvo bases. The low dielectric constant of dinitrogen tetroxide does not favor ionic reactions. However, a solution of NOCl in liquid dinitrogen tetroxide reacts with solid silver nitrate in aocordance with the following equation:
This reaction would be a typical neutralization reaction analogous to the following react,ions from the water and ammonia systems:
+ 2NOCI liq. + MCL + 2NO (11 = Zn, Fe, Sn, otc.)
+ z[EtnNH21NOa+ 2N1040j
arialogous to
+ 2NsOH + 2H20 Zn + 2KNH2 + 2NHa
Zn
--
+ 2x0
+
Sag[Zn(OHj4] H2 KI[Zn(SH2)41
+ Ha
Zinr nitrate is insoluble in liquid dinitrogen tetroxide but dissolves readily in solutions of diethyl ammonium nitrate in liquid dinitrogen tetroxide.
This reaction is analogous to the following: Zn(OH),,,
(NH4)C1
-
[EtAXH3],[Zn(N0,)..?) Diethyl ammonium nitratoeincate (~s.zetcomposition unknown)
H2O + ZNaOH -+ Sa+[Zn(OH)41
liq. + KNH? + KC1 + 2NH2 SH,
A number of solvolytic reactions in liquid diuitrogen tetroxide have been investigated. Thus the salt diethyl ammonium chloride undergoes solvolysis in accordance with the following equation:
+
+
The general type reaction MCI N204+ NCCI MNOa is fully reversible in liquid diuitrogen tetroxide depending upon the solubility of MCl in the system, and the removal of NOCI from the syst,em. The process for preparing NOCl from potassium chloride is in some ways analogous. I n this process gaseous dinitrogen tetroxide is passed through a column of potassium chloride moistened witha trace of water. The following reaction occurs:
However, interesting as these solvent system relatiouships may be, it is in the area of molecular addition compounds that much of the recent work in dinitrogen tetroxide chemistry has been done. It is well known, for example, that the course of nitration reactions in which dinitrogen tetroxide is used as a nitrating agent may be strongly modified by the choice of solvent for the nitration reaction. In general, it has been found that solvents which are strong electron donors give a distinctly different reaction. This led to speculation concerning the possible formation of addition compounds between dinitrogen tetroxide and Lewis bases. JOURNAL OF CHEMICAL EDUCATION
PHASE STUDIES OF ND-LEWIS BASE SYSTEMS
During the past ten years many hinary systems composed of dinitrogen tetroxide with a variety of electrondonor bnbstances have been investigated by Sisler and co-workers (e, 3, 4, 5, 6, 7) and by Addison's group (13, 14, 15). These inrestigations have involved, in most instances, the cryoscopic determination of the freezing point-composition curves for the systems. From the form of these curves the existence of various compounds can be detected. Dinitrogen Tetroxide-Ether Systems
Binary systems of dinitrogen tetroxide with a variety of different ethers have been studied (2, 4, 5, 6, 7). Most of these ethers form compounds which because of their low melting points and the very flat maxima in their freezing point curves were assumed to he of rather low stahility. I n general, there ~ v a as strong correlation between compound formation and t,he effect of the ether as a solvent for S204 nitration reactions. There is a tendency toward oxidation-reduction reactions between dinitrogen tetroxide and some of the ethers, particularly xhere addition compound format,ion is weak. The following Tahle 1 is a summary of the results of these studies. I t mill be noted that most of the compounds correspond to the formula N204.2R20; several ethers form, in addition, compounds of the formula NzOr.Rz0. The "dibasic" ethers, such as 1,4-dioxane and l,3-dioxane, form 1 : 1 compounds, but t,hese are equivalent to 1:2 compounds of "monobasic" ethers. TABLE 1 Molecular Comoounds of N90awith Ethers ratio
EBPTused
Ethyl ether n-Propyl ether i-Propyl ether n-Rntvl ether 1-l3ut;l ether (CICH,CH,),O Trimethylene oxide
fursn a-Methyltetrahydrofuran 2,1-Dimethyltetrahydrofman Tctrahydropyran 1,4-Dioxnne I ,3-Dioxane Trioxnne 13thyleneglycoldiethyl ether
.V?O,:ethe? Melting point of in comnm~nd comnound 1:2 1:2 (?) 1:2 ( ? ) 1 : 2 (?)
-74 8 -ii.5(incang.) -65.O(incong.) -79 5 (inconn.) .~..
...... ......
-53.5 -5B.4 -20.5 -4:3.0(incong.)
..
......
of compound at melling mint
Low Very !on Very law Verv low
... ...
Loa Low Moderate
. . ...
1:2
-50.5
Moderate
1:2 1:2 1:l 1:1 1 : (7)
-34.
Law Moderate Stable Moderate
-56.8
+4.5.2 2.0 -II.O(incong.)
+
l:L en. -60 ca. -60 1:2 ineong. = incongruent melting point.
...
Very low Very law
The failure of t-bntyl ether to form an addition compound whereas n-hutyl ether does form such a compound is best explained in terms of steric interference of the bulky tertiary hutyl groups with the ether oxygen. I11 the case of p,pl-dichlorodiethyl ether, the failure to form a rompound with dinitrogen tetroxide may he attrihuted either to electron mithdran-a1 from the ether
VOLUME 34, NO. 11, NOVEMBER, 1957
oxygen by the electronegative chlorine atoms or steric interference of the bulky chlorine atoms or perhaps a combination of the two effects. The change which occurs in the sequence, CH,-CH2
CHICH?
CHZ-0
CH,
I
I
I
LH,
\0/
/CH3\ CH, CH2 CH? I
CHr I
\o/ Trimethylene oxide
Tetrshydrofmxn
Tetrahydropyran
is interesting, but as yet unexplained. Trimethylene oxide forms 1: 1 and 1 : 2 rompounds with dinitrogen tetroxide, both of which have rongruent melting points and well-defined maxima in the freezing point conlposition curve of the binary system. Tetrshydrofuran forms a 1: 1 and a 1:2 compound but the 1 :2 compound melts incongruently. Tetrahydropyran forms only a 1:2 compound. It is also interesting to compare tetrahydrofuran vith perfluorotetrahydrofuran :
Perfluorotetrahydrofuran gives no evidence a t all for compound formation ~vithdinitrogen tetroxide. This is to he expected on the basis of the assumption that these Nz04-et,heraddition compounds result from Lewis acid-hase interaction, for the electron-withdrauing effect of the fluorine atoms robs the ether oxygen of any electron-donor capacity. Most striking, however, among these interactions, is the formation of the rather stable complexes of dinitrogen tet,roxide with l,Cdioxane, and to a slightly lesser degree with 1,3-dioxane. 1,4-Dioxane forms the white crystalline complex, N204.0(CH2CHz)z0,which melts a t +45.2OC., and, a t room temperature, is a white, crystalline, diamagnetic solid. (Remember that the boiling point of dinitrogen tetroxide is 21.3'C. and, a t this temperature, it is brown in color and is definitely paramagnetic.) The compound NzOcl,3-dioxane is similar in character though melting a t +2.0°. It is prohable that trioxane
behaves similarly but the high melting point of trioxane itself observes the maximum of the N20a-trioxane compound causing this compound to melt incongruently a t -11'. An estimated melting point of about -9' can be derived from the phase diagram. Any theory concerning the structure of addition compounds of dinitrogen tetroxide must account for the unusual stability and high melting points of the dioxane and trioxane compounds. Binary Systems with Other Oxygen Bases
Phase studies by the cryoscopic method have been carried out on other types of oxygen bases by Addison
and his co-workers (IS, 1 4 , l J ) . The results of some of this work are summarized in Table 2. TABLE 2 Molecular Compounds of N201with Various Oxygen Bases Stability
.,
1:l 1:l 1:l 1:l 1:2
nitrosominea
CHhNNO CIH~NNO I(CeHd(CHdNN0
(CeHd(CzHdNN0
+38" f14' +20Q -10" - V
1:2 1:2
+-38:3 -
1:s
-17 (~noonp.) -12-
1:2 (1)
,
Stahle Stable Stable Stshie Stable
Moderate Moderate
Mod. low
Addition products in which the mole ratio of KL?4 t o B is considerably greater than 0.5 were also found when a considerable excess of dinitrogen tetroxide was used. The product obtained with an excess of NzO4 on triethylamine is pink and highly explosive. All of these products are unstable a t room temperature, in fact a t any temperature above 0°, in some cases even lower. EVIDENCE CONCERNING THE STRUCTURE OF N20rADDITION COMPOUNDS
In view of the known equilibria in liquid dinitrogen tetroxide the question arises concerning these addition compounds as to whether they are compounds of NtO, molecules, compounds of NOz radicals, or solvates of nitrosyl nitrate:
In order to obtain evidenre concerning this point, Sisler and co-workers (2) measured the magnetic susceptiICIHI) (CH$CO 1 : 2 I?) - ~ l ~ ( i ~ ~ ~ ~ ~ .bilities ) of Nz04.2(CiH&0, N z O I . C ~ H ~ Oan , d NsO+.Medembe 1:2 CHCOOH Moderzte C.HCOOC~HI I:% 2C;Hlo0 a t temperatures well above their respective 1:2 -40' Moderate (CHI~CO 1:I -42" Moderate CaHGHO melting points. They were all found to be diamagnetic up to temperatures at which dinitrogen itself begins to he paramagnetic. Furthermore, it was found that Binary systems of some non-oxygen bases have been N404.0(CHzCH2)20 is diamagnetic a t room temperastudied by the crgoscopic method (IS, 15). These data ture. Since NOz is an odd molecule, these data indicate a.re summarized in Table 3. the addition compounds are not addition compounds of NO2, since such would be paramagnetic. This concluTABLE 3 sion is supported by the fact that all these addition comMolecular Addition Compounds of NzOl with pounds crystallize as whits solids. Non-Oxvoen Bases In an effort to determine whether or not the addition compounds are solvated nitrosyl nitrates, the Raman spectra of NzO4.2 (C2H6)20a t -72' and of Nz04.2 CsHloOa t -72' and -55' were studied (8). In addition the infrared spectrum of N Z O ~ . O ( C H ~ C H ~at) ~ O CaHrCHKN 1:2 -42" Low CHaCN 1:l -409 Low 25" was measured (2). The frequencies obtained in the 1:2 -4Z0 Low CICH*CHzCN 1:2 -02' Low various spectra were compared with known frequencies CIHSCN 1:1 -26' Low for NO+, NOa-, NOl+, and NOz-. The lack of corre1:l - 7O Moderate CCHI 1:l -18' Low CrHGHnh (Mesitylene) spondence served to eliminate the possibility of the presH C CHs ence of these ionic species in the systems studied. H,n\c/ \H. It was, therefore, concluded that the addition com1:1 -37Low AH, HA pounds with ethers are complexes of Nz04 molecules. C '' C \ H! In spite of the evidence obtained, however, Addison and his co-workers (16) a t first strongly questioned this conclusion. Their questions were based upon such data as the fact that the addition of certain oxygen bases to dinitrogen tetroxide greatly increases the rates at which = The oxygen ia n o t the basic center in this molecule. metals react with dinitrogen tetroxide ( I T ) , e.g.,
Corbonyl Com ounds
e
Molecular Compounds of N20, with Tertiary Amines
Sisler and co-workers (3) have found that when dinitrogen tetroxide is mixed with tertiary amines in ethereal solution at about -75", yellow precipitates of the general formula N z 0 4 . 2 B ,where B is pyridine, quinrr line, isoquinoline, acridine, a-picoline, B-picoline, or triethylamine, are formed. In contrast, the sterically hindered molecules 2,6-lutidine and 2-methylquinoline d o not form addition compounds under these conditions.
Addison interpreted this fact in terms of the assumption that the base reacted with Nz04molecules to form NO. B+ ions. More recently (IS), Addison has indicated that he no longer holds to this conclusion, for he has found that nitrosyl chloride does not yield addition compounds analogous to those of dinitrogen tetroxide. If the addition compounds are ionic as Addison had maintained, then it would he expected that nitrosgl chloride would form such compounds. It is now believed that the equilibrium represented by the following equation exists in the various binary systems:
JOURNAL OF CHEMICAL EDUCATION
Where B is a very strong electron donor, the equilibrium will be shifted t o the left and an ionic complex will be obtained. Thus Comyns (18) has shown that the R3N-N204compounds are ionic, for solutions of N2Cain R,N are good electrical conductors. For very weak electron donors the equilibrium will he shifted to the right and molecular complexes will be formed. I n terms of this concept the following quite reasonable classification of dinitrogen tetroxide addition compounds has been offered (IS). Addition Compounds of NzOl with Bases Donor
L i p i d phase
Strong RsN
Ionic
Solid phase Ionic
Medium-st~ength
R1O, R2N .NO, RCN
Ionic molecoln~." llolecular
0
R,C==O, R
l L
OR', RzSO .Molecular
Molecular
* Generally only very small concentrations of the ionic formprincipally molecular.
ELECTRONIC STRUCTURE OF DINITROGEN TETROXIDE COMPLEXES
If we refer t o the electronic formula for the NzOa molecule presented above, it is apparent that by the process of breaking the a bonds in the N2C4molecule, bonding orbitals on each of the nitrogen atoms may be made available for the acceptance of electron pairs from various donor molecules as indicated in the following diagram.
Since the coordination number of nitrogen atoms with respect to oxygen in nitric acid is limited t o 3 (presumably by steric factors) (191, it might he supposed that the acceptance of donor molecules by the nitrogen atoms in Nz04might he sterically hindered. However, it will be recalled that the N-N distance in N204is greater than normal and this would help to reduce the strain. The ionic structure involves the sharing of electrons from the donor molecule with the nitrosylium ion, NO+. This ion which contains only 10 valence electrons can upon saturation contain a total of 14 electrons. Thus, the NO+ has the possibility of accepting either one or two electron pairs. IB:
-
~ 4 +I [NOs-] :
or
-
p:
N-0: T "
+l[NO1-]
Since the first of these structures would be favored, the distinctly ionic dinitrogen tetroxide compounds are chiefly 1:1compounds. The addition compounds of dinitrogen tetroxide call be further classified in terms of the type of orbital which is involved in the donor molecule. Thus, the donor may contain an atom which has one or more unshared pairs of electrons available in s, p, or sp hybrid orbitals. Addison names such donors onium donors. In some instances, however, the donor molecule shares a pair of VOLUME 34. NO. 11, NOVEMBER, 1957
electrons in a molecular a orbital which can overlap with the bonding orbital on the N204molecule. Addison calls these donors a donors. We thus have four possible classes. (1) Molerular compounds with "onium donors": includes most oi the compounds studied. (2) Ionic compounds with "onium donors": includes compounds with the aliphatic tertiary amines. (3) Molecular compounds with "a donors": includes compounds with the aromatic hydrocarbons. (4) Ionic compounds with "a donors": no evidence for these. a Donors are too weak to form ionic compounds. The Dioxane Compounds. An explanation of the unusually high melting point and stability of the complexes with 1,4-dioxane and 1,3-dioxane remains to be considered. It was originally considered possible that the N,On molecule and the dioxane molecule, both being in a sense bifunctional, could interact so as to form a linear polymer which would result in a higher melting point. However, measurements of the viscosities of N&1,4dioxane mixtures (5) showed no important increase in viscosity of the mixtures over the pure components. Furthermore, cryoscopic measurements (5) on dilute solutions of dinitrogen tetroxide in l,4-dioxane indicate only one Nz04unit per solute molecule. The polymeric hypothesis is, thus, eliminated. A more attractive hypothesis is represented by the structure below in which the two oxygens of the dioxane molecule each share a pair of electrons with the same Nz04molerule.
The resulting tricyclic structure would approach a spherical shape. Such molecules tend to have relatively high melting points because of entropy considerationscamphor, for example. Furthermore, the fact that each pair of Np04and dioxane molecules are bound by two bonds which must he broken simultaneously in order for dissociation to occur creates an entropy factor which stabilizes the system. Molecular models of this postulated structure demonstrate the compatibility of this hypothesis with known bond distances and atomic radii. The following section serves to show that the structure is also compatible with modern chemical bond theory. Theoretical Consideration of the Biqclic Structure of the N204-1,4-Dioxane and Nz04-1,S-Dioxane Compounds. I n the past few years evidence from quantum theory studies by Coulson, Lennard-Jones, Mulliken, and their various co-workers, has accumulated which indicates that non-bonding electron pairs ("lone pairs" in the valence shell, e.g., the two lone pairs on the oxygen atoms in 1,4- and l,3-dioxanes, occupy directed orbitals orthogonal to the bonding orbitals. These lone pairs may make considerable contributions to the dipole moment of the molecule (because they are "off-center," so to speak). They also play a part in determining molecular configuration through the electron-electron repulsion energy. For example, in hydrazine they prohably make a contribution to the rotational barrier.
This view is of particular interest in the case of bidentate groups such as the dioxanes, and suggests that the effective distance separating the twocoordinatingatoms (i.e., the two oxygens) is the distance between the directed lone pairs (i.e., the centers of negative charge), rather than the distance between atomic centers. Thus, if 1,Cdioxane has the "boat" form, a lateral projection of its carbon-oxygen skeleton will be approximately as follows ( e = superimposed carbons; O = oxygen). o
Effective dirtonce between lone
0'
-0-0 - - -distonce -The lone pair directed orbitals (probably of the sp3 type) would make approximately tetrahedral angles with the two bonding orbitals on each oxygen. Thus, it appears quite reasonable that the distance between lone pairs is somewhat less than the oxygen-oxygen distance. The two orbitals, aa', could then form bonds with the two a orbitals of the nitrogens in N204,possibly with some change of hybridization in the latter.
The bonding would be more effective if rehybridization at the N occurred, ronverting N204to a non-planar configuration, thus: Superimposed 0-otoms
Superimposed 0-atam
orbitols on the N-atoms
This would not only give better "overlap," but would also direct orbitals on the N outward, further reducing discrepancy in N-iY and 0-0 distances. The directional effect of oxygen lone pairs in 1,3-dioxane would operate in essentially the same way. Thus, looking in this case at a different ("head-on") projection of the molecule, Effective lone pair distance
Here, however,it would takea morecareful calculation to decide whether the lone pairs, m',are directed slightly inward, or slightly outward. An interesting point is that the above analysis for l,3-dioxane is unchanged if the over-all molecular codguration is the "chair" rather than the "boat" form, since this change involves merely a shift of the 5-carbon atom above or below the plane of the 1,3,4,6-atoms, and does not affect the relative configuration of the 1- and 3-oxygen atoms, or the lone pairs, aa'. It is, further, interesting to note that the 0-0 distance in 1,3-dioxaneis only 0.1 A. less than the 0-0 distance in the boat form of l,4-dioxane. However, it is difficult to see how 1,4-dioxane could have the "chair" form and act as a bidentate coordinating group with NIOa. I n the "chair" form not only is the oxygen-oxygen distance greatly increased, hut the situation is still more unfavorable when the orientation of the lone pairs is considered, as the following figure shows.
b ("choir" form)
The coordination would have to operate through the ab' or the bat lone pairs. This strongly indicates that the 1,4-dioxane in the coordination compound is in the "boat" configuration. Yet, the electron diffraction data ($0, H),apparently indicate that the isolated 1,Cdioxane has the "chair" configuration. I n cyclohexane, the "boat" form probably has an energy about 6 kcal. (twice the 3 kcal. barrier for rotation about a carbon-carbon single bond) higher than the "chair" form, which is of course definitely known t o be the equilibrium form. I t seems likely that in 1,4-dioxane essentially the same factors operate, with the "boat" form about 6 kcal, higher than the "chair" for the isolated molecule, making the "chair" form the only form present to any degree, as the electron diffraction results indicate. However, it seems very probable that in the form* tion of the addition compound the "chair" form is converted to the "boat" form. This would require only a small bonding energy for the coordination bonds (i.e., the amount required t o reverse the 6 kcal. favoring the "chair" form in the isolated molecule). If the change from "chair" to "boat" form in the course of the addition reaction is to be plausible, the activation energy for this conversion must not exceed an upper limit set by the observed rate of reaction. From the low-frequency ring-bending modes in cyclohexane ("chair" form) it has been estimated (22) that there is an energy "pass" for conversion of the "chair" t o "boat" form lying about 14 kcal. above the energy of the "chair" form. This does not involve passage through the planar configuration of the ring, which they estimate requires an activation energy of about 31 kcal. As stated above, the energy of the "boat" form itself is about 6 kcal. above that of the "chair" form.
-
Thus in cyclohexane the path would be: t Activated
Choir form
--
%
Slote
16 Kcol.
-
- - - -- - v --
It seems unlikely the activation energy in 1,Cdioxane would be higher than this (i.e., 14 kcal.); in fact, it would probably be considerably lower because a considerable part of the 14 kcal. in cyclohexane is due to the potential harriers hindering rotation about the carboncarbon bonds connecting the six pairs of adjacent methylene groups, while in 1,Pdioxane there are only two pairs of adjacent methyleue groups. However, in order to obtain a lower limit for the rate, let us assume 14 kcal. also for 1,ldioxane. Then, neglecting the eutropy of activation, which should be unimportant, the expression for the specific reaction rate is AE*
KT - R r k = - e h
Taking T
= 273'K.
KT erg/deg. X 273 deg. - - 1.38 X h 6.624 X 10-27 erg see. AE* 14000 cal.
- 1.987 cal./deg.
= 5,69
X 273 deg.
=
10LPsec.-l
25.8
If the reaction is regarded as psuedo-unimolecular, i.e., if the conversion of the "chair" to the "boat" form is the rate-limiting step (assuming the rate of addition of NzO, to the "boat" form to be rapid), this specific rate corresponds to a half-life of 0.02 sec.
zation of the N orbitals with the resulting non-planarity of the N204molecule would autonlatically decrease the double bond content. However, even if the bonding in the addition compound is to a considerable extent electrostatic between the positive N atoms and the oxygen lone pairs, the directional properties of the latter as discussed above should be important in bringing the oxygen electrons as close as possible to the N atoms. ACKNOWLEDGMENT
The helpful assistance of Professor William Taylor of The Ohio State University in considering the theoretical aspects of the dinitrogen tetroxide-dioxane complexes is gratefully acknowledged. LITERATURE CITED (1) ADDISON,C., AND R. THOMPSON, J. C h a . Sac., 1950, 8211,218. J . Am. Chem. (2) RUBIN.B., H . SISLER,AND H. SCHECHTER, Soc., 74, 877-82 (1952). D., H. BURKKARDT, AND H. SISLER,J . Am. (3) DAYENPORT, Chem. Soe., 75, 4175-8 (1953). J., AND H. SI~LER, J. Am. Chem. Soc., 75,5188(4) WHLNGER, 91 (1953). (5) LING,H., AND H. SISLER,J. Am. Chem. Soe., 75, 5191-3 f~ l!XZi.~ ~ ~ , (6) GIBBINS,B., G. EICHHORN, AND H. SISLER,J . Am. C h a . Soe., 76, 4668-71 (1954). ( 7 ) SISLER,H., AND P. PERKING, J. Am. Chem. Soe., 78,1135-6 (1956). J. Chem. Phvs.. (8) STEESE.C.. AND A. WHITTAKER. . . 24.. 776-9 (1956). A,, J . Chem. Phys., 24, 780-3 (1956). (9) WHITTAKER, R., Ttans. Faraday Soe., 52, 1255 (1956). (10) BRADLEY, J., AND J. ROBERTSON, Natu~e,164, 915 (1949). (11) RROADLEY, (12) SMITH,D., AND K. HEDBEEG,J. Chem. Phys., 25, 1282 1195Ri \."--,. (13) ADDISON, C., AND J. SHELDON, J . Chem. Soc., 1956, 1941-9. (14) Ibid., 2705-9. (15) Ibid.,2700-12. C., AND J. LEWIS,Quart. Reu. (London) IX, [21, (16) ADDISON, 11549(1955). C., J. SHELDON, AND N. HODOE,J. Chem. Soc., (17) ADDIBON, 1956.3ROO-11. -~ (18) COMYNS, A., Natuw, 172, 491 (1953). (19) PAOLINO, L., J . Am. Chem. Soc., 55,1895 (1933). L., Rev. Mod. Phys., 8,231 (1936). (20) BROCKWAY, A&. C h a . S c a d . , 1, 149 (21) HASSEL,O.,AND H. VIERVOLL, ~
Thus, the rate of conversion appears to be adequate. It has been assumed above that the bonding is to the r bonds of N atoms in N20n. This would probably require some reduction of the participation of the latter in double bonds within the N201molecule; the rehybridi-
VOLUME 34, NO. 11, NOVEMBER, 1957
~
.~~~~ ,.
11067)
(22) BECKETT, C., K. PITZER, AND R. SPITZER, J. Am. C h a . Soe., 69,2488 (1947).
