STUDIES I N AUTO-OXIDATION. I11
THEINITIAL ACT IN AUTO- OXIDATION^ H. N. STEPHENS School of Chemistry, University of Minnesota, Minneapolis, Minnesota Received October 18, 19%
Although peroxide formation is quite generally accepted as the initial step in auto-oxidation, much debate has centered around the question as t o whether the isolable peroxide is actually the primary addition product or whether there is first formed a more active and unstable substance, which rearranges into the isolable compound. This question was first raised by Engler and Weissberg (l),following the observation that benzaldehyde, dissolved in benzene together with indigo, and exposed to oxygen, decolorized the indigo much more rapidly than the same concentration of benzoylhydroperoxide, in the same solvent, exposed to an atmosphere of carbon dioxide. This observation seemed to indicate that, during the process of auto-oxidation, there was formed a more active intermediate than the isolable benzoylhydroperoxide. At the time when this observation was made, it was, of course, impossible to draw the necessary distinction except on the basis of two distinct compounds. In view of our present knowledge of activated molecules, however, it seems unnecessary to assume any difference, other than one of energy content, between the freshly formed peroxide and the known compound. At the moment of formation the energy-rich molecules would be expected to be much more efficient in the destruction of the indigo than would the normal molecules of the same substance. A conclusion similar to that of Engler and Weissberg was reached later by Staudinger in connection with his work on the ketenes and, in particular, on the basis of some work of Erdman (2) upon which Staudinger (3) commented. According to the latter, the addition of oxygen to the ethenoid linkage yields, as a primary product, a highly active, unsymmetrical peroxide, or moloxide (I), which may later rearrange into the more stable, symmetrical compound (11). R-CH=CH-Ri
0 2
-+
R-CH-CH-RI
+ R-CH-CH-Rr
\ / 0 II
l l
M I1
0
I ~
Presented a t the 15th annual Canadian Chemical Convention a t Hamilton, Ontario, June 1 to 3, 1932. 209
210
H. N . STEPHENS
In support of this viewpoint, it is pointed out that the products obtained by Erdman from trichloroethylene can be most simply accounted for by assuming that the “moloxide” (Ia) is, in part, reduced to the oxide and, in part, isomerizes to the normal peroxide (IIa). CCla
cc12
o=o CHCl
cc12-0
I
0
4
4
CHClzCOCl
CHCl
1
3
COClz
+ HCOCl
CHC1-0
IIa
This argument, however, appears unconvincing to the present writer, as the oxide would be a normal product of the interaction of peroxide with the original substance (4). Quite recently, Jorissen and Van der Beek (5) have revived the question of the nature of the initial peroxide as a result of an observation on the oxidation of benzaldehyde in the dark, presumably at room temperature. Under these conditions a strong peroxide test was obtained, although it had been shown previously (6) that in direct sunlight no peroxide survived. The authors state that their peroxide could not have been benzoylhydroperoxide, otherwise it would have reacted with the benzaldehyde. Finally, the conclusion is drawn that the substance is probably the primary addition product of oxygen and the aldehyde, which may later revert to the isolable benzoylhydroperoxide. The interpretation given by these authors to their experimental evidence seems to the present writer to be inconsistent with one of their own observations, namely, that exposure to direct sunlight of a mixture partially oxidized in the dark resulted in the destruction of the peroxide in a few minutes. The fact that the peroxide survived in the dark need only be ascribed to the difference in its rate of reaction with benzaldehyde in the dark and in direct sunlight, as Backstrom (7) has already shown that the reaction between benzoylhydroperoxide and benzaldehyde is sensitive to light in the near ultra-violet. There seems every indication, therefore, that the peroxide obtained by the dark reaction is benzoylhydroperoxide. Again, the assumption that the primary product is a substance of weaker oxidizing ability (lower energy content) than benzoylhydroperoxide is one which can hardly be justified thermodynamically, as the primary product should be richer in energy than the substance into which it rearranges.
STUDIES IN AUTO-OXIDATlON.
I11
211
The most recent attempt to revive the conception of a structural difference between the primary peroxide and the isolable product is found in the recent papers of Milas ( 8 ) , whose theory is based on his interpretation of recent work in band spectroscopy by Mulliken (9), Birge (lo), and others. This evidence indicates clearly that diatomic molecules of the type of CO, CO+, BO, BeF, CN, etc., possess electrons in excess of eight which behave essentially as the valence electrons of atoms. The explanation advanced by Mulliken and by Birge (11) is that the two nuclei, together with their K electrons, are enclosed in a common shell of eight, with the remaining electrons in an outer shell. Such outer electrons might properly be designated as ‘Lmolecularvalence electrons.” To outline briefly some of the main assumptions underlying Milas’ theory, the following might be mentioned: (1) Auto-oxidation is assumed to be possible “only when the auto-oxidants possess unshared or ‘exposed’ electrons.” These unshared electrons are referred to as “molecular valence electrons,” the inference being that they are, in all cases, essentially similar to the valence electrons of diatomic molecules of the type mentioned above (12). (2) These unshared electrons are assumed to have their spins unpaired and to be more loosely bound to the molecule than shared pairs (reference 12, p. 299). (3) The first change assumed to take place in any auto-oxidation is a change in energy level of these “molecular valence electrons.” That is, electronic activation is assumed to be a necessary preliminary t o all autooxidations (reference 12, p. 299). (4) A pair of such electrons already raised t o some higher energy level is then assumed to be donated to the oxygen molecule, with the formation of a “dative” peroxide. (5) The “dative” peroxide rearranges into the more stable peroxide or undergoes other reactions. For the sake of brevity and convenience may we examine these assumptions in order. (1) The present writer knows of no evidence from band spectroscopy or from any other source that would indicate that all unshared electrons may be properly regarded as LLmolecular valence electrons.” To take one typical example as an illustration, that of aldehydes, the carbonyl group might be considered as a pseudo-atom, having its valence electrons shared with hydrogen and organic radical, respectively. However, there seems no reason for assuming, with Milas, that unshared oxygen electrons in such molecules are capable of behaving as valence electrons. (2) The assumption that the spins of such unshared electrons are not paired would seem to require some justification in order to carry weight against the current opinion that they are paired. Likewise, there seems
212
H. N. STEPHENS
some need of substantiation for the assumption that these electrons are more loosely bound to the molecule than others. (3) Most, if not all, auto-oxidations are thermal reactions. It is true that light is apparently universally effective in accelerating these reactions, but they are not exclusively photochemical. It seems entirely improbable, then, that electronic activation is a necessary preliminary to autooxidation, for electronic excitation levels lying below the visible part of the spectrum are very rare. In any event, excitation of oxygen electrons in ethers and aldehydes (reference 12, pp. 352-5) seems quite impossible in view of the fact that the lowest excitation level of oxygen is of the order of 8 volts.2 I n spite of this fact, however, Milas assumes an excitation of oxygen electrons in the water molecule by supersonic vibrations (reference 12, p. 317). For the frequency in question, 750,000 cycles, the corresponding energy of activation would be 3 X volts, or approximately calories per mole! 7X (4) The donation of a pair of “molecular valence electrons’” to the oxygen molecule is represented through the use of the following electronic structure for oxygen
.. ..
0.. : o.. : As oxygen is known to have zero electric moment (13), this cannot represent the normal oxygen molecule. If it represents an electronically activated state there seems no possible way of accounting for the energy of activation in thermal reactions. It should be pointed out further in this connection, that the electronic structures used for some of the donor molecules are subject to similar criticisms, For example, the structure for ethylenic compounds R : C : C : R1
cannot be considered as representing the normal molecules, as the electric moment of ethylene is known to be zero (14). If it represents an electronically activated state there seems to be no place for such a structure in the representation of auto-oxidations which are thermal reactions. ( 5 ) In the foregoing pages it has been shown that there exists no acceptable evidence in favor of the existence of two structurally different forms of the peroxide; therefore the conception of the “dative peroxide,” “moloxide,” or any other hypothetical intermediate seems to serve no useful purpose. 2 This, I think quite justifiably, leaves out of the question the metastable level at 1.65volts, which apparently has a very low probability of formation by direct absorption of light.
STUDIES I N AUTO-OXIDATION. I11
213
THE MODE O F ACTIVATION I N AUTO-OXIDATIONS
In view of the fact, already mentioned, that auto-oxidations in general are thermal reactions, it seems necessary to exclude any such energetic activation as that postulated by Milas. Vibrational activation appears to be quite sufficient to account for experimental facts. It cannot be denied, of course, that when such reactions are hastened by photochemical means electronic activation must occur. However, as there is no evidence indicating that the use of light changes the primary step in the reaction, it seems safe to assume that the accelerating effect of light is due to the increase in vibrational energy following the absorption of a quantum of light. It will, therefore, be assumed in this paper that all auto-oxidations involve the primary addition of oxygen to a linkage which has been activated by vibration, whether the necessary vibrational level has been attained purely by thermal means or by absorption of light.
Ethenoid compounds In the case of compounds containing the ethenoid linkage it is quite easy to understand, on the basis of conventional structures, the vibrational weakening of one of the bonds between the two carbon atoms and the pairing of the two odd oxygen electrons a t this point. The preferred structure for the oxygen molecule is that of G. N. Lewis (15), since it is the only one which is in agreement both with the paramagnetism and the absence of electric moment. Using- electronic structures the addition of oxygen would be represented in the following manner. H .. H.. R:C::C:Rl
+
. .
: o,. : o.. : The addition of oxygen to the C-C manner.
H H ---f
R :C .. : C.. : R*
: ..o : .. o:
bond may be considered in a similar
Ethers It is well known that ethers, on pyrolysis, usually undergo rupture adjacent to the ether oxygen. It is evident then, that the C-0 linkage is the weak point in the molecule. Addition of oxygen to ethers would, therefore, be represented as follows:
.. .. .. .. .. ..
R:O:R1 --+R:O:O:O:Rl
+
. . : ..o : ..o :
214
H. N. STEPHENS
In this case t,he original linkage consists of only one electron pair instead of two, as in the case of ethenoid compounds. Hence the addition of oxygen a t this point effects a complete separation of the atoms sharing that pair,
Aldehydes I n the case of aldehydes, the facility with which dehydrogenation takes place justifies the assumption that the weakest linkage in simple aldehyde molecules must be the C-H bond. Therefore, the addition of oxygen would be expected to take place a t this point through pairing of the odd oxygen electrons with carbon and hydrogen respectively.
: ..o :
: ..o :
. . .. .. .. ..
R:C:H--tR:C:O:O:H
+
. .
: o.. : ..o : Saturated hydrocarbons The fact that saturated hydrocarbons are auto-oxidized offers an instance of the complete failure of the general theory of Milas, as these compounds possess no “molecular valence electrons.” In his most recent publication Milas recognizes this fact and proposes a special mechanism to fit this particular case (reference 12, page 346). This mechanism is essentially the one which follows from the viewpoint developed in the present paper. In the case of the paraffins, it is evident from the work of Pope, Dykstra, and Edgar (16) that oxidation tends to take place on the terminal carbon atom of the longest open chain. On the other hand, the oxidation of the alkyl benzenes has been shown by the writer to take place a t the carbon atom alpha to the ring (17). The apparent discrepancy in these results, is, of course, due to the well-known fact that hydrogen attached to the alpha carbon atom in the alkyl benzenes is abnormally reactive. I n each of the above cases, as well as in other recent investigations (18), the oxidations have been thermal reactions, which fact necessarily precludes electronic activation. According to the theory developed in this paper the oxygen molecule would be expect’edto attach itself a t the bond most susceptible to vibrational activation. At the time of writing, the evidence seems to indicate that the C-C bond has a 1owe.renergy of dissociation than the C-H bond; therefore the former would be expected to be preferentially attacked. However, recent work (19) indicates that the difference in the above dissociation energies isvery small; therefore, it seems quite possible that the weakest C-H bond in a given hydrocarbon molecule might have a lower energy of dissociation or activation than the weakest C-C bond. In any event, it
STUDlES I N AUTO-OXIDATION. I11
215
is quite obvious that C-H bonds are usually attacked in preference to C-C bonds, The initial act in the auto-oxidation of saturated hydrocarbons would then be represented by the following scheme:
. .
Ri R:C:Rz+:O:O:
.. ..
..
H
% ..
+R:C:Rz
:o: : ..o : H
where R1 and Rzmight represent hydrogen and / or hydrocarbon radical, depending on whether the hydrocarbon was a paraffin (16), an alkyl benzene (17)) or an alkyl cyclane (18b) 18c).
Miscellaneous compounds The foregoing illustrations deal with only a limited group of compounds, but it seems to the writer unnecessary to multiply examples by discussing in detail other types. However, brief mention might be made of two additional examples, which are of some interest. One of these compounds, thiobenzophenone, has been the subject of recent controversy as to the point in the molecule a t which oxygen adds (20; and 10, p. 328). Thiobenzophenone is an unstable substance which decomposes at 160170°C. in the following manner: 2(CsHs)zC-S
-+
(CBHB)Z*C(CBH~~ 4-2s
The oxidation products obtained by Staudinger and Freudenberger were benzophenone, sulfur and small amounts of sulfur dioxide; the fact that the latter appeared only in small amounts led them to the belief that the oxygen added to the carbon atom. Milas, in dissenting from this viewpoint, represented the initial act as the addition of oxygen to the sulfur atom, implying, as in other cases, an electronic activation. It seems evident, from the ease with which thiobenzophenone is decomposed thermally, that the assumption of electronic activation of this substance is unnecessary to account for its auto-oxidation. From the point of view developed in the present paper the oxygen would be expected to add a t the linkage of least thermal stability, which is obviously the C=S linkage, (C&)zC=S
+
0 2
-+ (CsHdzC-S
I I
0-0
However, in this case, the complete rupture of the double bond in the C=S group takes place so easily that it seems probable that the oxygen mole-
216
H. N. STEPHENS
cule obtains both the necessary electrons from the carbon atom, forming an alkylidene peroxide and liberating sulfur, 0 (CsHdzC-S
+ 02
--+
I’
(C6Hb)zC
\0
+s
The appearance of only traces of sulfur dioxide would indicate such a small amount of side reaction that we need not concern ourselves with its nature. The second compound of interest belongs to a general class mentioned by Milas (21) as typical examples of compounds possessing “molecular valence electrons.” Triethylphosphine possesses an unshared pair of phosphorus electrons and Milas assumes, with other previous workers, that the addition of oxygen takes place a t that point. However, the experimental evidence on the subject seems entirely ,at variance with this viewpoint (22). Leaving aside the reactions that take place in the presence of water, it has been found that the main isolable product of the oxidation of the dry substance is the compound, (CzHJzPO. OCzHs
Now, the simplest explanation of the origin of this compound is on the basis of initial formation of the peroxide (CzHs)zp .o.0. CzHb
which might rearrange in a very obvious manner to give the above diethyl phosphinic ester. Therefore, it seems reasonable to conclude again that the oxygen attaches itself a t a vibrationally activated bond, in this case the carbon-phosphorus bond. A comparison of the mechanism of auto-oxidation reactions suggested in the present paper with those presented by Milas reveals the fact that the initial stage of the present writer corresponds with the second stage of Milas in all cases except the last two mentioned. The formation of the intermediate peroxide in auto-oxidations is thus considered as a single act rather than as consisting of two stages. As has already been implied, this does not mean that no distinction is recognized between the freshly formed peroxide molecules and the normal molecules of isolable product. A difference in energy content admittedly does exist, but there appears to be no reason for associating this with a structural difference which can be represented by any present system of notation.
STUDIES IN AUTO-OXIDATION. I11
217
REFERENCES (1) ENGLER AND WEISSBERG: Kritische Studien uber die Vorgange der Autoxydation, p. 90. Braunschweig, 1904. (2) ERDMAN: J. prakt. Chem. 86,78 (1913). (3) STAUDINGER: J. prakt. Chem. 86,330 (1913). (4) LENHER:J. Am. Chem. SOC.63, 3737 (1931). (5) JORISSEN AND VANDER BEEK: Rec. trav. chim. 49,138 (1930). (6) JORISSEN AND VANDER BEEK: Rec. trav. chim. 46,245 (1926). VANDER BEER: Rec. trav. chim. 47,286 (1928). (7) BACRBTROM: Medd. Vetensk. Acad. Nobelinst. [6] 16,15 (1927). (8) MILAS: J. Phys. Chem. 33,1204 (1929); Chem. Rev. 10,295 (1932). (9) MULLIKEN:Proc. Nat. Acad. Sci. 12,144 (1926). (10) BIRGE:Nature 117,300 (1926). (11) See also LANGMUIR: J. Am. Chem. SOC.41,868 (1919); GRIMM:Z. Elektrochem. 31,474 (1925); GLOCKLER: Proc. Nat. Acad. Sci. 12,522 (1926). (12) MILAS: Chem. Rev. 10,298-9 (1932). (13) SMYTH: Dielectric Constant and Molecular Structure, p. 85. Chemical Catalog Co., New York (1931). (14) SMYTH A N D ZAHN:J. Am. Chem. SOC.47,2501 (1925). (15) LEWIS,G. N.: Chem. Rev. 1, 243 (1924). (16) POPE,DYKSTRA, AND EDGAR: J. Am. Chem. SOC.61,2203 (1929). (17) STEPHENS:J. Am. Chem. SOC.48,1824,2930 (1926); 60,186,2523 (1928). AND RIDEAL:J. Chem. SOC.1928,2324. (18) (a) BRUNNER (b) CHAVANNE: Bull. soc. chim. Belg. 36,206 (1927). AND BODE:J. Am. Chem. SOC.62,1609 (1930). (e) CHAVANNE (d) MONDAIN-MONVAL AND QUANQUIN: Compt. rend. 191, 299 (1930); Ann. chim. [l]16, 309 (1931). (19) MECKE:Nature 126,526 (1931). (20) S T A U D I N G E R AND FREUDENBERGER: Ber. 61,1836 (1928). (21) MILAS:J. Phys. Chem. 33, 1211 (1929). (22) ENGLER AND WEISSBERQ: Ber. 31,3055 (1898).
THE JOURNAL OF PHYSICAL CHEMISTRY, VOL. X X X V I I , NO.
2
