Charge distribution in and dipole moments of some aliphatic alcohols

Charge distribution in and dipole moments of some aliphatic alcohols. Setharampattu S. Krishnamurthy, and Sundaresa Soundararajan. J. Phys. Chem. , 19...
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4036

S. S. KRISHNAMURTHY AND S. SOUNDARARAJAN

Charge Distribution in and Dipole Moments of Some Aliphatic Alcohols by S. S. Krishnamurthy and S. Soundararajan Department of Inorganic and Physical Chemistry, I n d i a n Institute of Science, Bangalore-12, I n d i a (Received November 1, 1968)

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Using the treatment of Smith, et uL,l charge distributions in several aliphatic alcohols and consequently their dipole moments have been evaluated. The dipole moments of trichloroethanol(2.04D) and 1,3-dichloropropan2-01 (2.11 D) have been measured in benzene solution at 35”. The results of evaluation and measurements are interpreted in terms of the occurrence of intramolecular interaction between the hydroxyl hydrogen and an acceptor atom X (halogen or oxygen) at the p-carbon atom.

Introduction The study of conformational equilibria has attracted the attention of several workers in recent years.2-s Krueger and Mettee2 have detected intramolecular hydrogen bonding in 2-cyano- and several nitroethanols while Urbanskig~lohas thoroughly investigated the ability of nitro groups to act as proton acceptors. Baitinger, et ala,’ have obtained unequivocal evidence for the formation of a six-membered intramolecular hydrogen bond in p-nitroalcohols and have refuted the earlier work of Ungnade, et al.,” which indicated the absence of any such intramolecular interaction. In their studies of conformational equilibria of 2,2-dihalo-, and 2,2,2-trihal~ethanols~ in dilute CCI, solutions, Krueger and Mettee have evaluated the thermodynamic parameters for the equilibria and have interpreted their results in terms of the stabilization of gauche structure by the formation of an X .H-0 intramolecular hydrogen bond. More recently, Buckley, et a1.,12 have made an ir study of rotational isomerism in ethylene glycol and concluded that ethylene glycol exists wholly in the gauche form. The stability of this gauche form is caused by the presence of two adjacent O-H groups leading to two equivalent intramolecular H-bonded structures. The present investigation attempts to analyze the available dipole moment data on aliphatic alcohols on the basis of the recently recognized notions of stabilization of intramolecularly H-bonded conformations in these systems. Since the inductive effects operating in the halo- and nitroethanols are considerable, me have calculated the theoretical moments of several aliphatic alcohols after allowing for induction by the method of Smith, et aZ.l Because of lack of data in the literature, we have determined the experimental moments of trichloroethanol and l,&dichloropropan-2-ol in benzene solution at 35”. Experimental Section MateTials. Benzene was purified as described in the T h e Journal of Physical Chemistry

literature. l 3 Trichloroethanol was donated by Cilag Chemie, Switzerland. 1,3-Dichloropropan-2-ol was distilled just prior to use, bp 169” (680 mm). Apparatus and Methods of Measurement. One of our earlier publication^'^ describes the equipment used for dielectric coiistant and density measurements and the method of computing the dipole moments from dielectric constant and density data. Table I summarizes the results of our measurements.

Results and Discussion The parameters used for calculating the formal charge distribution by the Smith, et aL,l scheme are taken from the l i t e r a t ~ r e . ’ ~ ’ ~Table ~ ’ ~ I1 shows the formal charge distributions in several aliphatic alcohols. The bond distances and the bond angles required for computing the dipole moments from charge distribution data are obtained from Sutton’s “Tables of Interatomic Distances.”lG Table I11 gives the experimental dipole (1) R. P. Smith, T. Ree, J. L. Magee, andH. Eyring, J. Amer. Chem. Soc., 73,2263 (1951). (2) R.J. Krueger and H. D. Mettee, Can. J . Chem., 43,2888 (1965). (3) P. J. Krueger and H. D. Mettee, ibid., 42, 326 (1964). (4) P. J. Krueger and H. D. Mettee, ibid., 42,340 (1964). (5) P. J. Krueger and H. D. Mettee, ibid., 42,347(1964). (6) P. J. Krueger and H. D. Mettee, J . Mol. Spectrosc., 18, 131 (1965). (7) W. F. Baitinger, P. von R. Schleyer, T. S. 8. R. Murthy, and L. Robinson, Tetrahedron, 20, 1635 (1964). (8) L.P. Kuhn and R. A. Wires, J . Amer. Chem. SOC.,86,2161 (1964). (9) T.Urbanski, Tetrahedron, 6, 1 (1969). (10) T. Urbanski in “Hydrogen Bonding,” D. Hadzi, Ed., Pergamon Press, New York, N. Y.,1959,p 143. (11) H.E.Ungnade and L. W. Kisainger, Tetrahedron, Suppl. 1, 19, 121 (1963). (12) P. Buckley and P. A. Giguere, Can. J . Chem., 45,397 (1967). (13) A. Weissberger and E. Proskauer, “Organic Solvents,” “Techniques of Organic Chemistry,” Vol. VII, Interscience Publishers, New York, N. Y., 1955. (14) S.S.Krishnamurthy and S. Soundararajan, Tetrahedron, 24, 167 (1968). (15) S.Soundararajan, ibid., 19,2171 (1963).

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CHARGEDISTRIBUTION AND DIPOLEMOMENTS OF SOMEALCOHOLS Table I : Dielectric Constants and Density Mole fraction

ti

x

Dielectric constant, e

104

Density, d

Trichloroethanol in benzene at 35 =k 0.02”” 2,25320 2.25672 2.26155 2.26522 2.26798 1,3-Dichlorohydrin in benzene a t 35 =k

0.000 8.786 15.04 22.19 28.57

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0.000 9.620 20.08 29.06 34.80 49.83

2.25005 2.25562 2.26194 2.26591 2.27188 2.27900

0.86282 0.86344 0,86390 0.86435 0.86485 OIO2Ob 0.86284 0.86332 0.86376 0.86425 0.86471 0.86531

2.4093, firnoan= 0,8118, PT- = 110.9 cc MRD = = 2.04. “ a r n e a n = 2.6050, fimesn = 0.5761, P T = ~ 116.7 CC, MRD = 27.1 cc, PO= 88.3 cc, PD = 2.11. (CY and p are Hedestrand’s constants, PT” = total polarization, MRD = molar refraction, PO= orientation polarization.)

the left. This implies a stabilization of the cis structure (I) by intramolecular hydrogen bonding X . .H-0. However, in tribromoethanol, the hydrogen-bonded form predominates to a greater extent than in trichloroethanol. From spectroscopic studies, Krueger and Mettee4 have found that intramolecular hydrogen bonding in trihaloethanols increases in the order F3C < CLC < Br3C, a trend which is the reverse of the normal accepted order of propensity of hydrogen bonding by halogens. These authors have listed a number of possible reasons for this unusual behavior. Spectroscopic studies have shown that 2-chloro- and 2-bromoethanols exist in trans and gauche forms in the gaseous state and in solution3 (structures I11 and IV),

‘amean=

27.2

CC,

PO = 82.3 Cc, /LD

moments and the moments calculated theoretically for various possible conformations of the aliphatic alcohols. In simple .unsubstituted alcohols, the observed moments agree well with the moments calculated assuming free rotation (Table 111). This suggests that the barrier to free rotation in these molecules is low and that conformational heterogeneity is not significant as revealed by spectroscopic studies. l7 I n 2,2,2-trihaloethanols, only two extreme conformations arise1*

I (cis)

I1 (trans)

One can compute the equilibrium constant for isomerization cis trans from a knowledge of the experimental and theoretical moments for the two conformers. For this computation we have used the equationls

+

pmean’

= (1

- x ) / ~ o i s+ ~ xpttans2

where pmesn is the “average” moment observed experimentally, p c Z aand ptransare the moments calculated theoretically for the cis and trans forms, respectively, and x is the mole fraction of the tmns conformer. The equilibrium constants (K,) for trichloroethanol and tribromoethanol a t 35” turn out to be 0.606 and 0.474, respectively. The corresponding standard free energies of isomerization ( A F = -RT In K,) are 307 and 458 cal/mol. The present results thus indicate that for trichloro- and Lribromoethanols, the equilibrium lies to

I11

Iv

V

the cis form (V) being ruled out.20 Taking the azimuthal angle to be 64OlZ1we have calculated the moments for the gauche form (IV) of 2-chloroethanol both when the H of the 0-H group points towards and away from chlorine. We obtain the values of 2.05 and 3.00, respectively. Similar calculation for 2-bromoethanol yields values of 2.04 and 2.93 D, respectively. The moments calculated for the trans conformations of 2-chloro- and 2-bromoethanols assuming free rotation of hydroxyl hydrogen, are 2.30 and 2.28 D, respectively. The experimental moments (1.96 for chloro- and 2.18 D for bromoethanol) can thus be reconciled with an equilibrium between the trans and gauche forms. Also the experimental value is much lower than the one calculated for the gauche form in which the hydroxyl hydrogen points away from halogen and, hence, rules out significant contributions from this structure. The spectroscopic results of Krueger, et aLj3which clearly indicate the presence of only one distinguishable gauche type conformer in 2-haloethanols with “bonded” OH, support our conclusion. In the gauche orientation the hydrogen bonds are strong enough to hold the H atom of the OH group in that position where it is nearest to the X atom.” (16) L. E. Sutton, “Tables of Interatomic Distances and Configuration in Molecules and Ions,” Special Publication No. 11, The Chemical Society, London, 1959. (17) E.B. Wilson, Advan. Chem. Phys., 2,367 (1959). (18) The cis and trans refers to hydroxyl hydrogen being near or away from halogen. (19) S. Mizushima, “Structure of Molecules and Internal Rotation,” Academic Press, New York, N. Y., 1954,p 34. (20) 8. Mizushima, T. Shimanouchi, T. Miyazawa, K. Abe, and M. Yasumi, J. Chem. Phys., 19, 1477 (1951). (21) P.Buckley, private communication. (22) K. Kojima, T. Tokuhiro, Y. Takeoka, and E. Hirano, Proe. Intern. Symp. Mol. Struct. Speotry. Tokyo, A222, 1 (1964); Chem.

Abstr., 60,157146 (1964). Volume 73, Number 19 December 1969

4038

S. S. KRISHNAMURTHY AND S. SOUNDARARAJAN

Table I1 : Formal Charge Distribution in Aliphatic Alcohols" No.

1 2 3 4

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5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 6'

e cor

CHaOH CHaCHzOH (CHa)zCHOH (CH3)sCOH ClCHzCHzOH ClzCHCHzOH ClaCCHiOH BrCH2CHzOH BrzCHCHzOH Br&CHzOH ICHzCHzOH IzCHCH20H IaCCHzOH FCHzCHzOH FzCHCHzOH FaCCHzOH HOCHzCHzOll HzC=CHCHzOH HCzCCHzOH CHaOCHzCHzOHb (NOz)(CH3)zCCHzOH (C1CHz)zCHOH

0.368 0.304 0.260 0.227 0.489 0.608 0.687 0.470 0.561 0.617 0.475 0.552 0.595 0.534 0.739 0.922 0.400 0.468 0.298 0.450 0.614 0.576

ec B

€Ha

...

0.0478 0.0395 0.0338

0.103 0.0886 0.0772 0.685 1.045 1.290 0.616 0.897 1.074 0.634 0.872 1.006 0.819 1.453 2.019

... 0.131 0 0847 0.555 1.063 0.708

...

0.0636 0.0880 0.0894 0.0611 0.0729 0.0802 0.0618 0.0718 0.0774 0.0695 0.0961 0.120 0.0520 0.0608 0.0387 0.0585 0.0798 0.0749

. ,. 0.0135 0.0115 0.0100 0.0891 0,1361

...

0.0801 0.1171

... 0.0824 0.1131 , ,.

0.1061 0.1891

... ...

0,0170

.,,

0.0721 , ,.

0.0921

ex

€0

-2.230 -2,245 -2.256 -2.256 -2.206 -2.181 -2.165 -2.211 -2.191 -2.180 -2.209 -2.192 -2.184 -2.197 -2.154 -2.114 -2.225 -2.505 -2.246 -2.217 -2.179 -2.187

fH(alo)

1.720 1.718 1.716 1.716 1.724 1.728 1.731 1.723 1.727 1.729 1.724 1.727 1.728 1.726 1.733 1.740 1.721 1.674 1.717 1.722 1.729 1.727

? . .

f . .

... ... -1.004 -0.748 -0.574 -0.882 -0.628 -0.467 -0.913 -0.607 -0.434 -1.235 -1.077 -0.935 . e ,

.-..

.-.. *..

-2.392 -0.987

denotes charge; carbon, hydrogen, etc., are designated a, p, y with reference to the hydroxyl group.

Table I11 : Calculated and Observed Electric Moments of Aliphatic Alcohols Compound

fiEXRt1

CHaOH CHaCHzOH (CH, )zCHOH (CHa)sCOH HzC-CHCHzOH HCECCHZOH ClCHzCHzOH BrCHzCHzOH C1033zOH Br3CCHzOH FsCCHzOH (C1CHz)zCHOH HOCHzCHzOH CHsOCHzCHzOH (NOz) (CH3)nCCHsOH

1.69 1.67 1.68 1.69 1.63 1.78 1.96 2.18 2.04h 1.73

a

rap

...

2.11* 2.27 2.22 3 .3,i

4oalod

1.61 1.80; 1.45; 1.64" 1.79; 1.49; 1.65" 1.65 1.95; 1.15; l.60a 1.90; 1.38; 1.66" 2.05; 2.30b 2.04; 2.28' 0.40; 3.28' 0.34; 3.01' 1.41; 1.81' 2.12d 2.29; 1.30; 1.64; 1.10' 1.94; 2.86' 3.82, 5.38,4.47, 4.30'

The values are in the order for the skeletons

H

H

/"-"\

Me

s

(MeIH-, \c- 0

(MeIHa

/

H

Me and for free rotation. The values are, respectively, for structure IV with intramolecular H bond and I11 with free rotation of 0-H. The values are for structures I and 11, respectively. For struoture VI. The values are for structures VII, VIII, gauche form with the hydroxyl hydrogens trans, and trans form with the hydroxyl hydrogens cis. For structures IX and X, respectively. ' For structure X I ; the other d u e s in the row are for structures without an intramolecular bond, 0-H. .OzN. Present work; other values in the column are taken from literature [A. L. McClellan, "Tables of Electric Dipole Moments," Freeman, London, 19631. The structures labeled I, 11, etc., are shown in the text.

'

H,

(CY

...

... ,

...

... ...

a

,,

.

.

.-..

*.. . ... .

*.*

,

... ... ...

.... ._.. .* .

...

.-..

.-,,

....

.-..

.-.,

I...

. .-..

.-. . .-.. .-.

-.

.-. .-.,

*

...I

.

I

0.0750 0.0606 0.555 0.362

._.

0.0097 0.0079 0.0721 0.0471 I.

-

b e ~ ( ~= ~ e ) 1.543.

For 1,3-dichloropropan-2-ol,several structures are possible. The experimental value (2.11 D) is close to the one (2.12 D) calculated for the structure VI, with the two C-C1 bonds tilted 60" in the opposite sense from the H-C,-CB plane. Once again, this structure is stabilized by intramolecular hydrogen bonding 0-H. .C1. Moreover, as the bulky chlorine atoms are away from each other, the steric repulsions are reduced to a minimum.

c1 VI

For ethylene glycol, the dipole moment (2.29 D) calculated for the gauche form (azimuthal angle 64") with the two 0-H groups cis-trans to each other (conformation VIIa or b) agrees well with experimental value (2.27 D), thus corroborating the spectral data which reveal a rigid gauche conformation for this molecule.12 The special stability, of the gauche conformation arises out of the presence of the two equivalent structures VIIa and b. The earlier work Krueger, et al.,6 have postulated two types of gauche

.

The JouTnal of Physical Chemistry

*

.-.,

,..

VIIa

VIIb

VI11

4039

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CHARGE DISTRIBUTION AND DIPOLEMOMENTS OF SOMEALCOHOLS forms, VI1 and VI11 for this molecule with the latter appreciably populated at low temperatures. Buckley, et have pointed out that the presence of species VI11 with two intramolecular H bonds is unlikely in view of considerable strain in such a cyclic structure. The moment calculated for structure VI11 turns out to be only 1.30 D different from the experimental value. Thus, the present results confirm the conclusions of Buckley, et a2.,l2 that ethylene glycol exists wholly in the conformation VII. For 2-methoxyethanol again, two conformations need be considered gauche and trans (IX and X).s In the gauche conformation (IX), the hydroxyl hydrogen will enter into hydrogen-bonding interaction with the methoxyl oxygen, while in the trans conformation (X), the 0-H will be “free.” The moment calculated for I X is 1.94 D when it is assumed that the H of the hydroxyl points towards methoxyl oxygen. The experimental value is 2.22 D, indicating that the predominant conformation is IX. The slightly higher experimental value can be explained on the basis of some slight contribution from the trans form (X) for which a moment of 2.86 D has been calculated. Spectroscopic studiessPzashow that 2-methoxyethanol exists

H

A

IXa

IXb X = -0CH3

predominantly

X

in the intramolecularly hydrogen-

bonded gauche conformation, with the trans conformation slightly populated. The stability of the gauche form is not only due to attractive force of the hydrogen bond, but also due to the loss of repulsive interaction compared to the trans forma8 The dipole moment (3.82 D) calculated for the intramolecularly hydrogen-bonded conformation of 2,2-dimethyl-2-nitroethanol (structure XI) is closer to the experimental value (3.35 D) than the moments

0---H XI

calculated for the other conformations (Table 111), indicating that for the nitroethanol, the preferred conformation is XI with a six-membered chelate ring resulting from intramolecular H bonding between hydroxyl proton and nitrooxygen. Spectroscopic studies by several authors, notably Baitinger, et ala,’ and U r b a n ~ k i , ~confirm ,’~ such an interaction. The experimental moments of trifluoro-, dihalo-, and iodoalcohols are not available in the literature. However, for the sake of completeness, the charge distributions in these compounds have been evaluated, and the values are shown in Table 11.

Acknowledgment. The authors thank Professor M. R. A. Rao for his keen interest and encouragement. S. S. E(.thanks U. G. C. (India) for a fellowship. (23) A. B. Foster, A. H. Haines, and M. Stacey, Tetrahedron, 16, 177

(1961).

Volume 78, Number I8 December 1969