4850 \CONTRIBUTION FKUM
THE
DEPARTMEST 0 1 4 C U E M I ~ I KUNIVERSITY Y, UP PENNSYLVAXIA~
The Electric Moments of Organic Peroxides.
11. Aliphatic Peracidsl
The electric dipole riioiiieiits of five loiig-chaiu a1iI)Iiatic peracids have Leeii tiieasured i i i beiizeiie. W t h i i i esperiineiital error, all five liavc tlie saiiie inoiiient, 2.32 D. The molecular weights o f two of these acids in beiizene have been detcrinined cryoscopically arid slioiv t h a t these acids are iiionomeric iii solution. The results indicate cousiderable puckeriiig in the fiveirietiibered chelate riiig foriiied by hydrogeti boiidiiig betweeii the hydroxyl oxygen and the carbonyl oxygen atoms.
Recently, Davison3 discovered strong infrared spectroscopic evidence that perbenzoic acid exists normally in monomeric form due to an intraniolecular hydrogen bond between the carbonyl oxygen and the hydroxyl oxyperi atoms of the percarboxyl group. Gigu&reand Olmos' found the same chelation in aliphatic peracids by their ineasurenients of the infrared spectra of performic and peracetic acids. They estimated that the attraction of the carbonyl oxygen atom for the hydroxyl hydrogen atom is sufficient to overconie most of the potential barrier restricting rotation about the 0-0 bond. Their data were not sufficient to decide whether the chelate ring formed by this hydrogen bonding is planar or puckered. In addition to the spectroscopic information they pointed to the greater volatility and more nornial entropy of vaporization of the peracids as compared to the ordinary carboxylic acids in support of the chelation. They also suggested that the instability of these short chain aliphatic peracids may be due in part to the strain caused by the chelation. Exnanuel and co-worker$ also obtained cheiiiical and infrared spectroscopic evidence that the lower aliphatic peracids are strongly chelated a t room temperature or below, the chelation rendering these substances more readily combusted. Their explanation of the role of the hydrogen bonding in combustion recalls the mechanism proposed by Bartlett6 for the reaction of perbenzoic acid with olefins. More recently, Swern, Witnauer, Eddy and Parker' studied the infrared adsorption of longchain aliphatic peracids in solution and in tlie solid state atid also the X-ray diffraction patterns of these acids in the solid state. Their data showed that in solution the peracids exist nearly exclusively as monomers, being strongly chelated by the hydrogen bonding proposed earlier. In the solid state, these (1) A r e p o r t of wurk d o n e u n d e r c o n t r a c t with t h e U. S. L ) e p a r t m e n t of Agriculture a n d a u t h o r i i e d by the Kcsearch a n d h l a r k e t i n g Act uf 1'311j. T h e c o n t r a c t is being supervised b y t h e E a s t e r n Utilizatiun llcbcarcli a n d D c v c l o p m e u t Division uf tlie Agricultural R e s e a r c h Servicc. ( 2 ) l i a s t e r n Regional licsearcli 1,aboratory. (3) W. l i . T. D n v i w n , J . C h e w . Soc., ?-Isti(1931). 1 I ) 1'. A . Gigubre a n d A . W.~ l n wC~~,J JL. .C h c m , 30, 8 2 1 (1!)32) %. K. hluizus. G. \ ' a . T i i n < > f w v aa n d N. J f . E n i a n u e l , L L / c i u ( f y (.i) ' . hf. Chcrcdi l f c u d .Vouk S . .S.Y.K., IO, (i33 ( l ! l X ) ; %. K . hlalzus, 1 niclir.nl\u i i i i i l N . h l . E i ~ a t i ~ibici.. ~ l , 70, (l!).jU); 13. S X r p u r e n t . 1 . 1s. l'avlcivskayu, N . hl. l ~ u ~ ~ nand ucl G. Yaruslavskii, ibid., I O , IO25 (1!)50): I). G. Knirrrc a n d N. $1. L;matiucl, Zhuu. Piz. Khiiiz., 26, 423 (1952). ( 0 ) P. L). U a r t l e t t , R e c . C / i c m , I ' r u g v . (iiresjie-Hooker Sci. I.ibrary1, 11, x o . 1, 47 (1Q.50). ( 7 ) D. S w e r n . L. 1'. W i t n a u e r . C. I. S>bcrii, i b i d . , 7 1 , 4037 (19%). (9) W Lr,bunez. J . 11. R i t t e n h o u s e a n d .I G. Miller, ihid., 80, 230.7 (1938).
ELECTRIC MOMENTS OF ALIPHATICPERACIDS
Sept. 20, 1958
ment value 2.91 D, showing that the pr-electron repulsive forces of the two peroxy oxygen atoms combined with the hydrogen bonding prevent such rotation. If rotation of the acyl group is tried with 4 fixed, the values 2.51 D for 4 = 0" and 2.96 D for qi = 100" result. This indication that rotation of the acyl group is restricted would be anticipated from the effects of the hydrogen bonding, the double bond character of the C-0 link, and, for the case of qi = O o , on steric grounds. With w fixed a t 0", the moment is 3.23 D for free rotation about the 0-0 axis, showing not only that this type of rotation is excluded but also that the dihedral angle 4 must be less than 90". GiguPre and Olmos4 made their calculations for the model for which w = 0" and qi = O", but with 02 = 100" instead of 105". For this planar model we calculate the moment 1.83 D, which is nearly 0.5 D less than the observed value. No reasonable changes in the bond moments for the 0-H and C=O links or in the bond angles will bring the moment of this model close to 2.32 D. The observed value would require that, taken separately,
The following reference diagram is shown for the analysis of the moment values
The bond moment values may be symbolized as H-C = a, C=O = b, C-0 = c, H-0 = d, and the bond angles, C-0-0 = 4,H-0-0 = &, C-C-0 = 90" +a, O=C-0 = 90" /3. I n view of the pla-
+
0
485 1
I1
narity of C-C-0 groupings, only two dihedral angles are needed. The first, w , is taken in a clockwise direction looking along the C-0 axis from C to 0 for measurement of rotation of the acyl group out of the plane containing the 0-0 axis. The other dihedral angle, 4, is measured in a clockwise direction looking along the 0-0 axis from right t o left for rotation of the 0-0-H plane out of the C-0-0 plane. The reference diagram shows w and #I both
TABLE I THEDIELECTRIC POLARIZATION, MOLAR REFRACTION AND DIPOLEMOMENT O F ALIPHATICPERACIDS 610
a
VI 0
Perpelargonic (C,)
30 30
2.2629 2.2629
3.175 3.146
1.1528 1.1529
Percapric ((210)
30 30
2.2630
2.664
1.1526 1.1519
30 30 50
2.2624
Peracid
Perlauric ( C ~ Z )
....
....
...
2.285
...
2.2231 2.227
B
-0.0759 - .0745
1.1527 1.1519 1.1817
-
30 30
2.2626
1.978
...
1.1526 1.1522
-
Perpalmitic (C10)
30
2.2630
1.888
1.1531
-
....
+
= [ ( b COS 0 a COS a) COS w - d COS (180 - 02) (0, - 90) - d sin (180 - 02) sin (01 - 90) cos +;'
+ +
+
'+
COS
+ [d
sin (180 - 8 2 ) sin d ( b cos 0 a cos CY)sin w ] [d cos (180 - 0,) sin (e1 - 90) - d sin (180 - 02) cos (0, - 90) cos - (c - b sill p a sin CY)]
+
,0742 .0665
*1110
.... .... .... 2.2330
.0467 2.2355 .0402 2.2329 .0614 ....
.
.0357 ... -.0378 2.2334
Permyristic (C14)
a t zero. The following formula then applies to the molecular moment of the aliphatic peracids. p2
IN
BENZENE
MRD
Temp., OC.
To study the puckering, ;.e., the efTect of w and 4 on the moment, the bond angles and bond moments can be taken the same as in diacyl pero x i d e ~ ,dialkyl ~ peroxides and alkyl hydropero x i d e ~ , ~namely, ~'~ a = 0.4, b = 2.3, c = 0.62, !.c = 1.71, a = 20", p = 35" and = O2 = 106". The formula then becomes
- 0.4275 (1 + cos +)] * + (1.6617 sin + + 2.2599 sin w ) * + (0.67695 - 1.5954 cos +)*
p 2 = [2.2599 cos w
According to this formula, free rotation simultaneous about both axes gives the molecular mo(10) M. T. Rogers and T. W. Campbell, THISJOURNAL, 7 4 , 4742 (1 952).
.0748 2.2325
PPD
Obsd.
Theor.
fi
160.9 160.0
... ...
47.49 47.49
2.38 2.37
51.93 51.93
52.11
,.
61.34
2.28
Y
..... .. . . . .....
155.6
...
-0.196
- .223 - .239 . . .. .
165.0 60.5 . . . 59.5 165.9 60.5
..... 172.8 - ,224 . , . - ,152 184.9
2.27
.. 2.36
68.8 68.8
70.59
2.27
78.2
79.81
2.30
the H-0 moment must go up to 2.9 D or the C=O moment to 3.2 D or O2 must be reduced to 65". We may conclude that this model in which hydrogen bonding has completely overcome the p r electron repulsion is not correct. Apparently, with w = 0", qi lies somewhere between 0 and 90" and, using the formula above, we find that the spread of 4 values accounting for the range of observed moments is rather narrow. The mean value corresponding to the mean moment 2.32 f 0.04 D is 72 f 4". This configuration with w = 0", C$ = 72O, will be pictured as model I R--C=O /
:
/
I
0-0
H '
.
Model I appears to be a reasonable one in terms of the various factors known to be active in determin0
I1
ing the structure of peracids. In it, the -C-0-0 links are coplanar as expected from the double
4852
J. R. RITTENHOUSE, W. LOBUNEZ, DANIELSWERN AND J. G. MILLER
bond character of the bond joining the carbonyl carbon atom to the peroxy oxygen atom, as confirmed by the earlier results for diacyl peroxides. The 72" bending of the 0-H link out of this plane represents a reasonable result of the effect of the hydrogen bonding on the barriers to rotation ordinarily present in peroxides. This relocation of the position of minimum potential energy should
be accompanied by a lowering of the cis barrier and a heightening of the trans barrier. Using the bond lengths and bond angles proposed by Gigusre and Olmos, but with Bz = 105", the 0 . .. . . H distance is 2.53 A.in model I. Using the formula, only two other models, I1
VOl. 80
and 111, were found to have molecular moments equal to the observed value. In 11, both the acyl group and the 0-13 link are twisted out of the C-0-0 plane, and" if 4 = loo", then w = 345". In 111, the acyl group alone is twisted out of plane, w being 70°, 4 = 0". I t is possible that the opposition between the hydrogen bond and the pn-electroil repulsion might cause the small amount of twisting pictured for the acyl group in model 11, but it is hardly likely that a t the same time 4 would be unaffected. On the other hand, model 111 can have no reasonable basis. 15-e may conclude from the measurements reported here that the actual structure is a puckered one, rather close to model I, with 4 less than 100" anti (LI very nearly 0". PHILADELPHIA
4,P A .
(11) E a c h of tliese three 1,uckered models has a n equivalent superimposable mirror iniagc, t h e one in this case has 0 = L'(iO', 15'.
1101101
=
