Dec., 1962
HYDROGEN BONDIKG IN ,&NITROALCOHOLS
2643
11. ASSOCIATION IN
HYDROGEN BONDING IN fi-NITRO ALCOHOLS.
BY H. E. UNGKADE, E. D. LOUGHRAN, AND L. W. KISSINGER University of California, Los Alamcs Scientific Laboratorg, Los Alamos, New lllexaco Received April 28, 1066
Infrared absorption spectra of p-nitro alcohols RzC(NOz)CH20H (where R = CHs and N02) show them to be largely monomeric in dichloromethane but partially associated in carbon tetrachloride. The nitro group exerts a strong influence on the association, changing the association number from three in neopentyl alcohol to a value near two in the nitro alcohols. The association constants of the nitro alcohols in carbon tetrachloride increase with the number of nitro groups, indicating that the tendency for hydrogen bonding in these compounds is related to the electron deficiency on the hydroxyl carbon.
Introduction It has been established recently’ that p-nitro alcohols and diols have predominantly monomeric hydroxyl absorption in dichloromethane and carbon tetrachloride. The diols particularly, however, are so insoluble in the latter solvent that it has not been possible to investigate more concentrated solutions where bonded species might be e ~ p e c t e d . ~For this reason the more soluble monohydric p-nitro alcohols R2C(K02)CHzOH have been re-examined in dichloromethane and carbon tetrachloride in more concentrated solutions and compared with the structurally related neopentyl alcohol. Experimental4 Materials.-2-Methyl-2-nitropropanol was prepared from 2-ni tropropane and paraformaldehydeb and melted a t 9192’ (lii. m.p. SZ”).!;Anal. Calcd. for GH8N03: C,40.34; H,17.61; N, 11.76. Found: C,40.66, 40.60; H, 8.41, 7.69; N , 11.74. Commercial neopentyl alcohol melted at 52-53‘ and was sufficient1 pure for the spectral measurements. Anal. Jalcd. for C5HlzO: C, 68.15; H, 13.71. Found: C,67.96; H, 13.82 The other compounds have been described previously.’ Measurements.--Infrared absorption spectra were determined at 25’ with a Model 21 Perkin-Elmer infrared spectrophotometer with a sodium chloride prism in matched sodium chloride liquid sealed cells of 0.1 and 0.05 cm. length and quartz cells of 1 cm. length against pure solvent in the reference beam. The spectrophotometer was operated a t slit schedule 984, response 1, gain 4.5, speed 2 , and suppression 0 and the curves were recorded on the scale 1 p = 5 cm. from 2-15 p . Solrtions were made up from weighed amounts of alcohols and pure solvents in IO-ml. volumetric flmku and diluted by pi etting 5 ml. into 10-ml. flasks and filling to the mark with sofvent. The solutions were stable and the readings reproducible. Absorbance values were determined by the base line technique6 and limited to instrumental readings between 0.08 and 0.8. In view of instrumental limitations the e values should be regarded as “apparent” molar absorptivities.
Results The &nitro alcohols have little tendency to associate in dichloromethane, probably due to interaction with the solvent. 2,2-Dinitropropanol, (1) Paper I. H. E. TJnenade and L. W. Kissinger, Tefrchedron. in press. (2) This work %as performed under the auspices of the U. S. Atomic Energy Commission. (3) Z. Eckstein, P. Gluzinsky, W. Sobotka, and T. Urbanski, J . C h m . Soc., 1370 (19811. (4) Microanalyses by M. Naranjo. 411 temperatures are corrected. (5) L. Henry, Bull. aoc. chm. France, 131 13, 1002 (1895). (6) J. J. Heigl, M. F’. Bell, and J. U. White, A n d . Chem., 19, 293 (1947).
for instance, has a single strong band at 2.78 p. This band as well as the as-nitro band at 6.36 p remains unchanged when the solution contains one equivalent of concentrated sulfuric acid (Fig. l).7 CM-’ 4000
1600
3000
3
2
0.
0.L
A. 0.2
0.4
0.6
kOH 2 . 7 8 ~ 0.0
3
6
P. Fig. 1.-Infrared absorption bands for 2,Z-dinitro ropanol in dichloromethane: (1) 0.1 N alcohol; (2) 0.1 $alcohol and 0.1 N sulfuric acid; (3) 0.025 N alcohol; (4) 0.025 N alcohol and 0.025 N sulfuric acid, all in 0.05-em. cells.
(7) It has been proposed b y A. Nielsen and H. Feuer (Symposium o n Nitro Aliphatic Chemistry a t West Lafayette, Ind., May 26, 1961) that certain pnitro alcohols can form internally hydrogen-bonded protonated species in solutions. The present evidence would indicate that such bonding does not occur in 2.2-dinitropropanol in 0.1 N solutions.
2644
H. E. UNGNADE, E. D. LOUGHRAN, AND L. W. KISSINGER
Vol. 66
CM-' 5000
4000
3000
-3.0
--
0.1
0.0125M
r
0
a
L
I
r -2.0 0 a
u
0.2
d
0.025M
-
CT 0
A.
I.Me3C CH20H
2. M e 2 C ( N 0 2 ) C H 2 0 H
0.3
-1.0
3. Me C(N02)2CH20H 4. (NOp)sC CH20H
O.05M 0.4
O.IM
0
- 0.5
-I
0.6
-1.5
0
log AROH 2.0
-2 0
*
Fig. 4.-Plot of log of analytical concentration of alcohol minus monomeric alcohol concentration against log absorbance of monomeric hydroxyl for the four alcohols.
3.0
P. Fig. 2.-Infrared absorption bands of trinitroethanol in carbon tetrachloride (0.1-em. cells).
0.08
.
bonded hydroxyl in 0.1 M solution and is monomeric only in higher dilution. To account for this difference in the behavior of the nitro alcohols it is assumed that one or two nitro groups can inhibit association, as was observed previously. The magnitude of the absorption intensity for as-nitro groups is in agreement with published values for relat'ed compounds.* T.4BLE
j 0.06
W
INFRARED -4BSORPTION
J
0
1
BANDSOF ALCOHOLS I N DICHLOROMETHANE'
0.04
Alcohol
0.02
0
0.2
0.4
ABSORBANCE,
Fig. 3.-Absorbance of hydroxyl bands of alcohols in carbon tetrachloride (0.1-cm. cells).
Only trinitroethanol has a small band for bonded hydroxyl a t 2.97 p in more concentrated solutions (20.1 M ) . I n dilute solutions Beer's law holds for hydroxyl as well as nitro bands and it therefore is possible to calculate molar absorptivities (Table I). The structurally related neopentyl alcohol, on the other hand, has both monomer and
AOH
C
Xss-NO2
E
As-NO,
€
CMe3CH20H 2.77b 52 .. .. .. .. CMe?(P;O,)CHaOH 2.77 65 6.46 335 7.41 93 CMe(W02)aCH20H 2.78 90 6.36 840 7.54 160 C(?;O?)ICH~OH 2 . i 8 < 125 6.23 1060 i . t i i d 300 a Determined in 0.1-cm. cells except as noted. b Bonded hydroxyl absorption occurs in concentrated solutions at 2.86 1.1. Weak bonded hydroxyl absorption can be observed in 0.1 M solution a t 2.97 /I. d Because of solvent absorption the intensity of this band W M determined in thinner cells.
Carbon tetrachloride solutions of neopentyl alcohol exhibit the characteristic bands of primary alcohols in the OH-stretching region a t 2.74, 2.84, and 2.96 p , which have been assigned to monomer, trimer (or tetramer), and polymeric hydroxyl (8) C . E. Grabiel, D. E. Risgrove, and L. B. Clapp, .J. A m . Chem. soc., 7 7 , 1292 (1955).
Dec., 1962
HYDHOGEX Bosurm
ISP-KITROALCOHOLS
2645
TABLE I1 INFRARED ABSORPTIONBANDSOF ALCOHOLS IN CARBON TETRACHLORIDE Alcohol XOH t5 &OH 2 h.kNO2 t b-NO2 t .. .. CMeaCH2011 2.77 37.9 2.84 46.1 .. .. CMe2(NO2)C&0H 2.77 57.6 2.87 94 6.46" 280 7.42 90 CMe(N02)&HZOH 2.78 93.2 2.90 45.6 6.34" 835 7.54 190 C( N 02)aCEzOH 2.78 165.7 2.95 16.5 6.24 1030 7.67 316 Absorbance values were determined in 0.1-em. cells except ~ 1 noted. s The molar absorptivities E for monomeric hydroxyl were obtained from absorbance, cell length, and the concentration of monomeric alcohols, E = AROH/[ROH]L,which in turn wm determined from the association e uilibrium, the analytical concentration of the alcohol, and the association constant in the expression K = (CROH- [ROHq)/n[ROH! n, from which nK[ROH]* [ROH] CROH= 0. For a rough approximation the association constant could be determined from absorbance and concentration of the most dilute solution. In the case of dimers [ROHI = - 1 f dl 4-~ K C R O H / ~while K , the equation for trimers WEB solved by trial and error iteration for various CROHvalues. The numerical E values represent averages over the concentration range of 0.01-0.1 M . b The concentration of alcohol polymers was obtained from the monomer concentration, [(ROH),] = CROE- [ROH]/n, and also from the equilibrium equation K = [(ROH),]/(CROE - n[(ROH),])n. The two methods were in good agreement. c Because of solvent absorption the intensity of this band was determmed in thinner cells.
+
group^.^ In contrast the p-nitro alcohols have only bands in this region (Fig* 2> in the concentrations investigated. The more intense band occurs a t 2.77 p in the region of monomeric hydroxyl and increases in relative intensity with dilution. The weaker band a t 2.87-2.95 p (Table 11) decreases in relative intensity with dilution and is assigned to bonded hydroxyl (Fig. 3). The degree of association of these alcohols has been determined from the plot of log (CROH [ROH]) against log AROH using the assumptiOnS made previously (Fig. 4).1° The results (Table 111) are regarded as fairly reliable since only the most precise measurements, i.e., CROHand AROH, were used. Since the method assumes, however, that only cyclic polymers are present, this assumption has been further examined by determining a necessary condition for cyclic polymers : A (ROH),,/ ( P R O H ) n = K'. Over a limited range of concentrations the values for this expression were reasonably constant for neopentyl alcohol, 2,2-dinitropropano17 and trinitroethanol but showed large deviat,ions for 2-methyl-2-nitropropanol (Table 111). One would conclude that in the latter case the cyclic dimers probably are accompanied by dimers with terminal hydroxyl groups.. Association constants have been determined for the four alcohols from the intercepts of the finear (9) W. C. Coburn a n d E. Grunwald, J . Am. Chem. Soc., 80, 1318 (1958). (io) A. Ens a n d F. E. Murray, Can. J . Chern., 36, 161 (1957); R. Mecke, Discussions Faraday Soc., 9, 161 (1950). I n the present investigation ROH signifies monomeric alcohol, C the analytical concentration, and (ROHI, polymeric alcohol. The spectroscopic nomenclature correspond8 to the reconimendations of the Hughes Committee, 11. K. Hughes, ~f ol., Anal. Chern. 24, 1349 (11152).
-
TABLE I11 A~~OCIATION C O ~ ~ T AOFN ALCOHOLS T~ IN CA~B,-,X TETRA CHLORIDE
A(RoH)"~ Alcohol
~ R O H ) ~no
(ARoH)"
KC
CMeaCHzOH 2.84 3 . 0 2 . 9 7 i 0 . 7 30.0 CMe2(X02)CH20H 2.87 1 . 5 0.57 f 0.4"! 0.9 CMe(X02hCH20H 2.90 2 . 0 .40f .07 5.9 C(N02hCHzOH 2.95 2 . 0 .21* .07 33.5 = Association number = slope of linear plot of log(CR0E[ROHI) us. log ARoa. Average value from seven solutions including the concentration range of 0.02 to 0.1 M. The intercept from the linear plot of log (CROH- [ROH]) against log AROHis log nK - n log E R O H ~ . The large deviations from the constant indicate that terminal hydroxyl probably is present also. Since A ( R O E ) J ( A R O H=) ~1.03 f 0.08 the main species may be
Plots Of log (CROH- [ROE]) against 1% AROH." The value for 2-methyl-2-nitroPropanol (Table 111) should be regarded as approximate because the mOnOmer hydroxyl band Probably n~aSureSnot only monomeric alcohol as in the other cases but also terminal hydroxyl Polymers. "he order of magnitude for the constant probably is correct. The replacement of one methyl in neopentyl alcohol by a nitro group has a remarkable effect. It reduces the association number and thus changes the type of bonded species. Simultaneously it causes a large decrease in the value of the associatiOn Constant, i.e., it affects also the degree of association, which is regained only by further introduction of nitro groups. The formation of cyclic dimers is promoted by an electron deficiency on the hydroxyl carbon.
