5537
ALIPHATICPERACIDS IN SOLUTION AND SOLID STATE
Nov. 5 , 1955
yielded a 2,4-dinitrophenylhydrazone, m.p. 122.%I23 ', undepressed by admixture with an authentic sample.29 The 2,4-dinitrophenylhydrazone of isovaleraldehyde, m.p. 125.5126", strongly depressed the melting point of the above sample. Neopentyl Iodide with Bromine.-Neopentyl iodide (92.0 mmoles) reacted with bromine (93.3 mmoles) in petroleum ether for 1.25 hours a t - 78' and 1.5 hours a t 0' gave a very small amount (0.57 g., 4y0) of a n impure low boiling fraction, n % 1.4448, b.p. 55-58' (120 mm.), micro b.p. 106"; reported% for t-amyl bromide, b.p. 108" (765 mm.), n Z o 1.4421. There was not enough to refractionate. Analysis was obtained on a similar material, probably tamyl bromide, from an earlier run, b.p. 50.5-52' (115 mm.), n * 5 1.4370. ~ Anal. Calcd. for CsHllBr: C, 39.75; H, 7.34. Found: C, 40.18; H, 7.45. A higher boiling fraction on refractionation gave 2,3-dibromo-2-methylbutane, b.p. 74-75" (36 mm.), n% 1.5020-
1.5050, 7.04 g. (33% yield). Analysis was obtained on a similar material from an earlier run, micro b.p. 168-169', n Z 61.5000. ~ Anal. Calcd. for CaHl0Brz: C, 26.11; H, 4.38. Found: C, 26.52; H, 4.32. The structure was demonstrated by making the 2,4dinitrophenylhydra zone of 3-methylbutanone-2 formed after first treating the dibromide with water in a sealed tube at 100' for six hours. No depression was obtained either with ~ an authentic sample or with that obtained from the 2,3dichloro-2 methylbutane (see above). Reaction of t-Amyl Chloride with Chlorine .-&Amyl chloride was chlorinated under the conditions described above for neopentyl iodide with the inclusion of an equivalent of iodine monochloride. Besides 44% of recovered starting material there was obtained 29% of 2,3-dichlor0-2-methyl~ The butane, b.p. 60-67" (104 mm.), l t Z 6 1.4338-1.4415. infrared spectrum of refractionated material, b.p. 66-67' 1.4405, was identical with that of authentic (102 mm.), 2,3-dichloro-2-methylbutane(see above) and with the re(29) A similar conversion of 2,3-dibromo-2-methylbutane t o 3arrangement product from the reaction of neopentyl iodide methylbutanone-2 has been reported by F. C . Whitmore, W. L. and chlorine. Evers and H. S. Rothrock, "Organic Syntheses," Coll. Vol. 11, John t-Amyl chloride, chlorinated without iodine monochloride, 1943, p. 408. Wiley and Sons, Inc., New York, N. Y., was recovered almost unchanged, ( 3 0 ) K . A. Lange, "Lange's Handbook of Chemistry," Handbook 1, N. Y . Publishers, Inc., Sandusky, Ohio, Sixth Edition, 1946. BROOKLYN
[CONTRIBUTION FROM THE
Peroxides.
EASTERN REGIONAL RESEARCH LABORATORY*]
III.2 Structure of Aliphatic Peracids in Solution and in the Solid State. An Infrared, X-Ray Diffraction and Molecular Weight Study3
B Y DANIEL SWERN, LEE P. WITNAUER,
c. ROLANDEDDYAND WINFRED E. P A R K E R
RECEIVED APRIL 22, 1955 Infrared absorption spectra have been obtained on long-chain aliphatic peracids in solution (CB-CIO and in the solid state Infrared studies show t h a t in solution the peracids exist exclusively as intramolecularly chelated monomers containing a five-membered ring (I). In the solid state the peracids, like the corresponding n-aliphatic acids and alcohols, occur as dimers (11) in which two of the monomer units are linked through intermolecular hydrogen bonds. The dimers, unlike those of the corresponding n-aliphatic acids and alcohols, dissociate completely to monomers when they are dissolved in a solvent. Molecular weight determinations in benzene support this last conclusion. In solution, peracids show a broad band of medium intensity at about 865 cm.-' ( 1 1 . 5 6 ~ )attributed , to skeletal vibrations of the 0-0 bond. The peracids, unlike the even and odd numbered naliphatic acids, crystallize with the same end packing and angle of tilt, so that a plot of long spacings against number of carbon atoms yields a single straight line. ( C14, CIS),and X-ray diffraction patterns have been obtained on the solid peracids (Cg-Cla).
Although organic peracids have been known for about fifty year^,^ their structure (usually written
//O
as RCOsH or R-C-02H) has not been worked out unequivocally. Within the past five years several investigators have applied physical methods, particularly infrared (which has proved to be the most fruitful), to the solution of this problem.6-11 These workers studied only the lowest aliphatic peracids7-' (performic, peracetic, perpropionic and ( 1 ) A laboratory of t h e Eastern Utilization Research Branch, Agricultural Research Service, U. S. Department of Agriculture. Article not copyrighted. (2) P a r t 11, THIS JOURNAL, 77, 4037 (1955). (3) Presented a t t h e Fall Meeting of t h e American Chemical Society, Minneapolis, Minn., September 11-16, 1955, (4) D. Swern, Chem. Revs., 46, 1 (1949). (5) E . Briner and P. de Chastonay, Compl. rend., 2S8, 32 (1954). (6) W. H. T.Davison, J . Chem. Soc., 2456 (1951). (7) P . A. GiguPre and A. Weingartshofer Olmos, Can. J . Chem., SO, 821 (1952). ( 8 ) D. G. Knorre and N. M. Emanuel, Zhur. F i z . K h i m . , 26, 425 (1952). (9) 2. K. MaIzus, V. M. Cherednichenko and N. M . Emanuel, D o k lady A k a d . N ~ u kS . S . S . R . ,7 0 , 855 (19.50). (10) G . J . Minkoff, Pvoc. Roy. SOC.(London), 8 2 2 4 , 176 (3954). (11) B. S. Neporent, T. R. Pavlovskaya, N. M. Emanuel and N. G . Yuroslavskii, Dokludy A k u d . N a u k S .'Y..S.R., 7 0 , 1025 (I95OY.
perbutyric acids) and perbenzoic acid!,% The short-chain peracids are corrosive liquids and they are difficult to obtain and keep in a pure state completely free of water, hydrogen peroxide and/or the corresponding carboxylic acid. They are quite unstable (performic acid is explosive') and readily attack the cells used in determining their infrared spectra. Perbenzoic acid although i t is a solid which can be readily purified, is a more complicated compound to study because of the absorption characteristics of the phenyl group. I n spite of these difficulties certain conclusions have been reached. Gigusre and Olmos7 reported t h a t performic and peracetic acids exist exclusively as hydrogen bonded, intramolecularly chelated monomeric substances (I), both in the liquid and
I
I1
vapor states. These investigators described an approximate configuration of the percarboxyl group and also gave interatomic distances. T h e
5538
3.SWERN, L. P.D*ITNACER, C . R.EDDYAXD I\-. E. P.'IRKER
spectroscopic results did not permit deciding between a planar or slightly puckered five-membered chelate ring. Davison,Gon the basis of an infrared study and the melting point, concluded that perbciizoic acid, m.p. 41",is normally mononieric in solution and t h a t chelation is dimensionally possible if the skew configuration of the peroxide bond is somewhat nearer to a cis configuration. Minkoff l o has stated that from his infrared studies alone it cannot be decided whether the hydrogen bonding is inter- or intramolecular, b u t Davison6 and Giguh-e and Olmos' give considerable evidence that chelation is more important than dimerization. I n our previous paper,2 we described the preparation of crystalline, relatively stable aliphatic peracids, containing up to 1s carbon atoms. Riost of these (C*-C1e) were obtained in extremely high purity, completely free of the contaminants usually found in the lower molecular weight peracids studied by previous investigators. The availability of pure solid peracids prompted us to conduct an infrared examination of thein in solution and in the solid state using a high-resolution infrared spectrophotometer. X-Ray diffraction powder patterns and molecular weights in benzene also mere oh-tained. Experimental
Vol. 77
different for the two different chain lengths. Since most of the work was done on perpalmitic acid, this is the only peracid whose spectra are reported (Figs. 1 and 2). For comparison, the spectra of palmitic acid and cetyl alcohol also are given. As Fig. 1 shows, the hydroxyl stretching vibration of the peracids (curves A and B) is found a t 3280 cm.-' (3.05 b ) in carbon tetrachloride solution. The position of this band shows no change with concentration and its absorptivity shows only slight change. For all the peracids studied (except percaprylic acid) the average of all molar absorptivities a t all concentrations is e = 31.6 liters per mole per centimeter. The maximum variation from this a t any one concentration was only 370, although each compound was examined at concentrations from 0.006 to at least 0.3 mole per liter. The position of this band compared to that of the unassociated fatty acid a t 3530 cm.-' (2.83 p ) and of the unassociated alcohol at 3635 cm.-' (2.75 p ) (curve F), shows that the peracids are hydrogen bonded. The insensitivity of the band to dilution indicates that in solution the hydrogen bonding is intramolecular. At as low a concentration a5 O.0OG M, peracids show no evidence of an unassociated hydroxyl band; only the band of associated hydroxyl can be seen. On the other hand, palmitic acid a t O.OOG -1I shows not only the strong, broad dimer band but Starting Materials.-The preparation of pcrcaprylic, also a very sharp, unassociated-hydroxy1 band a t perpelargonic, percapric, perhendecaiioic, perhuric, pertridecanoic, permyristic mid perpalmitic acids nlready ha.; 3530 cm.-l ( E = 22.2). Davies13 l 4 states that carbeen described.2 boxylic acids begin to show a weak but definite Infrared Absorption Spectra.-These were obtained with band due to unassociated hydroxyl a t 0.01 to 0.02 a Beckman IR-3 spectrophotometer using sodium chloride prisms. The peracid solutions decomposed in the amalga- M. Cetyl alcohol shows both associated and unmated cells producing a green color and a sharp band a t associated ( E = 27.6) bands a t as high a concentra2335 cm.-I first noticeable after about 15 minutes. On tion as 0.33 llf (curve F). Since an equilibrium exists the other hand, the solutions showed no change in spectrum between monomeric and dimeric alcohols and beon standing several hours in glass flasks at room temperature. The spectra, therefore, were run in sections each re- tween monomeric and dimeric carboxylic acids in quiring no more than 12 minutes, the cells being rinsed and solution, with the extent of dimerization varying filled with fresh solution for e:xh section. The mull5 ap- with the concentration, it must be concluded that peared to be stable. For the inulled snmples, a demount- the peracids exist in solution solely as intramolecuable cell with a mont-type chnnnel iii (me wititlow was u';ed,I2 with a spacer approximately 0.03 mm. thick. The mineral larly hydrogen-bonded monomers. The structure proposed by Gigu6re and Olmos7 (I) fcr performic oil absorption was partially compensated by a tape-recorded blank using pure mineral oil iii the same cell with :I spacer and peracetic acids is a reasonable one for the entire approximately 0.02 mm. thick. This method catinot ccm- homologous series of aliphatic peracids. pensate exactly for the three strongest al~sorptionlxmds o f I n the solid state, however, the hydroxyl band of the mineral oil because of the noli-qu:intitative nature of a the peracids (Fig. 2, curve A) is shifted to even mull. X-Ray Diffraction Patterns.-Powder patterns were rc- lower frequencies (3200 cm.-', 3.13 p ) and is broadel corded with a powder camera, 14 cm. in diameter, using than in solution. This broadening and larger CrKa radiation. Specimens wcrc prepared by pressing shift of frequency suggests that in the solid state a finely ground sample on a glass slide and cutting from it a the peracids, like the carboxylic acids, are intermonarrow section 0.3 mm. thick. lecularly hydrogen bonded. (Intermolecular bondMolecular Weights.-These were tlcteriniiietl cryoscopically in benzene by the usual procedure. ing in the solid state is discussed it1 more detail later.) I n comparison, cetyl alcohol in the solid Results and Discussion state (curve C) has its hydroxyl band a t about 3250 Infrared Spectra.-In solution, the infrared cm-1 (3.1 p ) , and palmitic acid (curve E) has the spectra of all the aliphatic peracids examined are usual broad band centered roughly around 2900 basically the same, showing only minor variations cni.-* (3.4 p ) . Thus, the hydroxyl band of the with chain length. The only two peracids examined solid peracids is between that of alcohols and carin the solid state (C14and Cia) show no significant boxylic acids, but is much nearer to that of the differences in spectra for wave numbers greater alcohols. The solid peracids, therefore, arc slightly than 1300 cm.-l. I n the 700-1300 c m - l region, more strongly intermolecularly hydrogen bonded these two solids show many bands not found in the than are the solid alcohols but not nearly so strongly solution spectra. These bands appear to form as the solid acids. wave-number progressions that are distinctly (13) M M D a x i e s and G I3 B RI S u t h e r l ~ n r i .I. f i i p m P h y ? , 6 , ~
(12) J. S.Ard. Aizal. C h e m . , 23, OR0 ( l M l ) , and private communicntioa.
I~
756 (1938). (14) M . hI. navies, ; b i d , , 8 , ,577
(Inlo).
Nov. 5 , 1955
WbVES
20
=
25
45
1000
PER
5.5 WAVELENGTH.
CENTIMETER.
6
50
WAVES 1100
Ln
5539
L4LIP€IATIC P E R A C I D S I N SOLUTION AND S O L I D S T A T E
900
MICRONS
PER C E N T I M E T E R . 800
700
IPERPALUlTiC
bClG
~
A - 2 0 0 G I L iN C C l r 8 - 8 0 0 G I L I N CCI. C-400 G I L I N C I C L D H E X ANE PbLMiTlC ACID
i
G-I99 G I L
i
~
1 I
IN CCI,
E - 4 0 0 G / L IN C Y C L O H E X .
)CETYL ILCOHOL F-790 GIL
'~
I N CCI,
1
CELL LENGTH
--0 4 8 m m
Fig. I.-Infrared
spectra of perpalmitic acid (A, B, C), palmitic acid (D, E), and cetyl alcohol (F) in solution, WIVES
5000
3000
4000
2500
PER
CENTIMETER.
I500
2000
1100
1400
1100
I00
80
60
40
20 #'
E.
0
20
25
30
35
40
4 5
50 5.5 WAVELENGTH. WAVES
PER
60 MICRONS.
65
75
70
06
SO
9.0
9 5
CENTIMETER.
700 COMPOUNDS
A-PERPLLMITIC bClD B-PLLMITIC ACID C-CETYL ALCOHOL
1
!
STATE
'SOLIDS. MULLED
IN
T E UP i RATURE
30. C.
20
0 , 40
95
Fig 2 -Infrared
10
105
ll
ll 5
12 12 5 WAVELENGTU
I3
I3 5
14
14 5
15
MICRONS
spectra of perpalmitic acid (A), palmitic acid (B), and cetyl alcohol (C) in the solid state
Peracids have a strong, single carbonyl absorption band a t 1747-1748 cm.-' (5.72 F ) both in solution (Fig. 1, curve A) and in the solid state (Fig. 2, curve A), although in the latter case the band is slightly less sharp. Carboxylic acids in solution (Fig- 1, Curve D) have a strong absorption band due to the carbonyl group a t about 1708-1712 cm.-l
(5.85-5.84 p)15 which shifts to slightly lower frequencies16 (1698-1705 em.-', 5.89-5.57 p ) in the solid state (Fig. 2 , curve B). I n solution (curve D),
Af';; FQ&
A ~ ~ ~ ) c ~ e m ~
Heether,
(16) R G s l n c ~ a t rA , F McKay and R N 74,2570(1952)
Knight and Jones.
Swern,
THISJ O U R N A L ,
D. SWERN,L. P. WITNAUER, C . R. EDDYAND IT.E. PARKER carboxylic acids also show a weak carbonyl band at about 1760 cm.-' (5.68 p), attributed to the monomeric carbonyl group, which is not present in the solid state spectra.13 The peracids, however, show no evidence of unbonded carbonyl group under any conditions which we have studied, which is in accord with the conclusion that they are completely chelated. The shift of the carbonyl frequency parallels the shift of the hydroxyl frequency. If one assumes that the frequency of a n unbonded carbonyl lies near 1760 cm.-' (5.GS p) in peracids (as it does in carboxylic acids13 1 6 ) , then the shift of the peracids to 1747-1743 cni.-l (5.72 p) on forming the chelate is less than the shift to 1705-1723 cm.-l (5.87-5.50 p) in the carboxylic acid dimers." As with the hydroxyl band, the carbonyl frequency thus also suggests that the hydrogen bonding is less strong in the peracid chelate than in the carboxylic acid dimer. The failure of peracids to produce an unassociated carbonyl (or hydroxyl) absorption band on high dilution, however, is a consequence of a favorable steric situation. Like the fatty acids,'* the peracids show the methylenic deformation vibration a t 13.69 cm.-' (6.81 p) and the symmetric methyl deformation a t 1379 cm.-' (7.25 p) the latter increasing in strength as chain length decreases. The asymmetric methyl deformation appears as a weak, resolved peak a t 1460 cm.-l (6.85 p) for Cloarid lower, increasing ill strength as chain length decreases. The 1329 cin.-l (7.00 p) band is present in all the peracids examined, with molar absorptivity in the range 244 to 267. A band in this vicinity was discussed by Minkoff.lo Although it is found in a wide variety of hydroperoxides and peroxides, it does not seem justified to assign it to any particular vibration a t present I n the 1100-1150 cm.-' (0.00-8.X p) region, the peracids have two moderately strong bands, varying in location and contour with chain length. These may possibly be related to the bands found near 1235 (S.10p) and 1283 crn.-I (7.78 p) in the fatty acid series.'" The strong broad absorption a t about 940 ern.-' (10.61p) (Fig. 1, curve D) occurs in the solution spectra of many carboxylic acids, and may arise from vibrations of the 0-H group in and out of the plane (deformation mode) of the carboxyl group.I320 The peracids in solution (Fig. 1, curve .I)show much weaker absorption in this regioii (947 cm.-', 10 36 p ) . In the solid state, however, this band is sharpened considerably and, in thc peracids, is shifted to lower frequency (Fig. 2 , curve -4) whereas that of the carboxylic acids (Fig. 2, curve B) is essentially unchanged. As Bellarny points out,?' the band in this region would be expected to show marked changes associated with changes of state which may alter the degree of hydrogen bonding. This supports the conclusion suggested earlier from other spectral evidence that (17) L J Bellamy ' 7 he Infrared Spectra i,f Complex \f
