Hydrogen-bonding studies—VII

SpectrocMmIca Acta, 1961, Vol. 17, pp. 369 to 378. PergamonPress Ltd. Printed In NorthernIreland

Hy~ogen-bon~g

studies--VII*

Near infrared spectroscopicstudies of the intermolecularhydrogenbonding of phenol, p-cm01 and p-chlorophenol MILDRED M. ~~~E~ and ROBERT WEST Department of Chemistry, University of Wisconsin, Madison, Wisconsin (Received 27 October 1960;

in revised form 20 December

1960)

Ab~~t-~e temperature dependence of the equilibrium constants of the ~termolecular hydrogen bonding of phenol, p-cresol and p-chlorophonol has been studied by measnring the variation with temperature of the molar extinction coefficients of the first overtone of the free O-H stretching vibration in the near infrared. The dimer is the predominant form for phenols in dilute carbon tetrachloride solution at room temperature. Polymeric forms arise only at phenol concentrations above about 0.2 IMat 30°C. The enthalpies of cyclic dimer formation are: p-chlorophenol, 3.78 ( k0.20); phenol, 5.12 ( f0.10); andp-cresol, 6.09 ( 10.18) kcal/mole dimer. The pK$,6’” values for the same phenols are 9.39, 10.0 and 10.17, respectively. Thus the heat of dimerization decreases as acid strength increases among the three phenols.

MEASWREMENTSof~the equilibrium constants of hydrogen bonding have been made on many systems by a number of methods [l]. The chief tool of many investigators has been infrared spectroscopy 12, 31 and most recently nuclear magnetic resonance [4, 51. LIDDEL and BECKER [6] have shown that methanol, ethanol and t-butanol are dimeric in dilute solution in carbon tetrachloride and have obtained equilibrium constants and enthalpies of hydrogen bonding for the cyclic dimers of methanol, ethanol and t-butanol by an infrared spectral study of the free O-H fundamental s~et~hing vibration near 3 p. SAUNDERS and HYNE have studied equilibrium constants of hydrogen bonding of t-butanol, phenol and methanol by nuclear magnetic resonance [7]. SAUNDERS and HYNE concluded by an interesting process of curve fitting that phenol and t-butanol exist as trimers in dilute carbon tetrachloride solution and that methanol exists as a tetramer. Their data for methanol and t-butanol therefore contradict the infrared findings of LIDDEL and BECKER. BECKER has shown, however, that the NMR data of SAUNDERS and HYNE are also consistent with dimer formation by these compounds in dilute solution [8]. By replotting SAUNDERSand HYNE’S NMR curves of frequency shift vs. concentration, he found a non-zero slope for the dimer which is not immediately apparent from the original plots. * Previous paper in this series: R. WEST and C. S. KRAIRANZEL, J. Am. Chem. Sot. In press (1961). t Present address: Research Laboratories, American Cyanamid Company, Stamford, Connecticut. [I]

;gf6,P~~~~~~~

[2] f3j [4] [5] [6] [7] [S]

J. N. H. C. U. M. E.

1

and A. L. MCCLELLAN, The Hydrogen

Bond pp. 348-356.

Freeman,

San Francisco

KR&TZEK and R. MECKE, 2. phytik. Chem. (Le
369

MILDRED M. MAUUIRE and ROBERT WEST Recent studies of the intermolecular hydrogen bonding of various aliphatic alcohols have been made in the near infrared by ENS and MURRAY [9]. They used the first overtone absorption of the free O-H stretching vibration to calculate equilibrium constants of trimerization and tetramerization of these alcohols. Similar work in the fundamental region was done by COIXJESHALLand SAIER [3] in calculating equilibrium constants for the intermolecular hydrogen bonding of alcohols and sterically hindered phenols. Equilibrium constants of association of some phenols have been determined by NMR but measurements are characterized by much uncertainty [5]. Controversy exists concerning the exact structure of the dimer of various alcohols and phenols. Much evidence for alcohols suggests the closed dimer structure (I) having “bent hydrogen bonds” [lo, 11, 61. Earlier workers postulated an open dimer structure (II) for some alcohols [ 121. In infrared investigations of the hydrogen-

bonding equilibria of hydroxylic molecules it is usually assumed that if the open dimer is present the free O-H of the open dimer does not contribute to the monomeric O-H absorption [13]. Some evidence indicates that the monomeric O-H absorption is near the polymeric free 0 -H absorption but is not coincident with it [la]. In this study we have used long path length cells to study the equilibrium constants of hydrogen bonding of phenol, jp-cresol and P-chlorophenol in the near infrared region. We have worked in dilute solution at controlled temperatures to determine if constant equilibrium constants for the dimer, trimer or the tetramer can be obtained and have used the temperature dependency of equilibrium constants to calculate enthalpies of hydrogen-bond formation for the cyclic dimers of phenol, p-cresol and p-chlorophenol.

Experimental Reagents The phenol was Merck reagent grade material, fractionally crystallized and distilled in vacuum. The fraction boiling at 85.5-86.O”C (20 mm), m.p. 43*O”C, was collected. A nitrogen stream was maintained over the system to prevent bumping. p-Chlorophenol was prepared from Eastman White Label material by two fractional crystallizations and then collection of the distilled sample boiling at 100°C (10 mm), m.p. 44~4°C. p-Cresol was prepared from Eastman Practical Grade by repeated [9] A. ENSand F. E. MORAY, Cam.J. Chem.25, 170 (1957). [lo] M. VANTHIEL,E. D. BECKER and G. C. PIMENTEL, J. Chem. Phys. 27, 95, 486 (1957).

rlll fl2j [13] [Ia]

E. L. R. F.

D. BECKER, U. LIDDEL and J. SHOOLERY,J. Mol. Spectfoosc. 2, 1 (1958). P. KUFIN, 3. Am. Chem. Sot. 74, 2492 (1952). - . MECKE, Discussions Faraday Sot. 9, 161 (1951). A. SMITH and E. C. CREITZ,J. Rec. Natl. BUT. Standards 46, 145 (1951).

370

Hydrogen-bonding studies-VII

fractional crystallization and collection of the distilled sample boiling at 86°C (10 mm), m.p. 35-l’%. The carbon tetrachloride used as a solvent for infrared study (Fisher Certified Reagent Grade) was placed in an open beaker in an evacuated desiccator over P,O, and allowed to stand at least 24 hr before use to remove all traces of moisture. All solutions were prepared immediately before use by successive dilutions of O-2 M solutions made from weighed amounts of the phenols. Solutions were prepared in a low constant humidity room to minimize evaporation and to insure dryness. Spectra A Cary model 14M double-beam spectrophotometer was used to determine the optical densities of the first overtone of the free O-H stretching mode in the near infrared. The scanning time was 5 A/set and nominal slit widths were 0.2-0.3 mm. The spectrometer was desiccated with P,O, to reduce water-vapor absorption. Matched pairs of silica absorption cells were used, with path lengths of 1 cm and 5 cm for concentrations from 0.080 to 0.20 M and from 0.010 to 0.050 M, respectively. Pure CCI, was placed in the reference beam. Spectra were determined with both sample and reference cells in a thermostated hollow brass cell block. Temperature control was accurate to f0.05”C above 0°C and f0*5”C below this temperature. The optical densities of each of about seven concentrations of the phenols between 0.010 M and 0.200 M were studied as a function of temperature. The optical densities were corrected for the variation of the number of molecules in the light path due to the density change of the solvent with temperature. An apparent molar extinction coefficient was calculated from

where c is the total phenol concentration of alcohols in moles per liter, and d is the cell length in centimeters. The molar extinction coefficients of the free O-H stretching mode were plotted vs. concentration at each temperature and extrapolated to infinite dilution to obtain the molar extinction coefficient at infinite dilution, &MO[6]. The values of the molar extinction coefficients at infinite dilution at each temperature were used to calculate equilibrium constants of dimerization of the phenols.

Results The plots of molar extinction coefficients of the first overtone of the free O--H stretching vibration of phenol, p-cresol and p-chlorophenol vs. concentration of the phenols in moles per liter are shown in Figs. l-3. Equilibrium constants for cyclic dimerization and trimerization of these phenols were calculated at each temperature for each point on the plots of molar extinction coefficients vs. concentration according to the following relationships :

371

MILDRED M. MAQUIRE and ROBERT WEST

372

Hydrogen-bonding studim-VII KB is the eq~librium constant of cyclic ~meriz~tion, X3 is the eq~lib~um constant of trim&&ion and C is the total concentrstion of alcohol [6]. If

then

where C,, C,, X, and XM are molar concentrations and mole fractions of dimer and monomer. Similarly, if .& = i+ ni

then

Kr=$e

<5x

where C,, C,, X, and XniI are the concentrations and mole fractions of trimer and monomer. K, and K, were calculated from the relationships log Li, = log Jr, + log (1000~~~~~) snd log K,

= log K, + 2 log (lOOOp,/M,)

where No is the molecular weight and p0 is the density of carbon ~tr~c~o~de at the temperature in question. Tables 1-3 show Kx for cyclic dimerization Table 1. Kx for din-&z&ion

-

Cont.

-17*ci”C

(MI

Kx

0.200 0,160 0.132 0.100 0.080 0.050 0.020 0.010

19*4* 22*6* -

& 683 1004 -

I

-9.5% Kx 17.3* 19.6* -

and KT for trim&z&ion 15.0%

O*O”C

Kx

K,

KT

14*4* 15+6* -

616 1090 -

17.6 16.6 19.4 19.1 19.2

-

512 684

1240 1620 2760 7320 15,000

* Points not used in calculations of average &

__

Kx IO.7 11.0 10.9 10.7 9.54 11.3 10.4 11.6

1

of phenol 3O*O”C

KT

Kx

379 495 591 755 831 1580 3530 7900

7.82 7.83 7.22 7.11 6.40 8.55 7.11 5.68

KF 270 333 375 477 590 1160 2440 3500

45*O”C Kx

fh

5.56 180 5.43 294 4.63 207 5.13 364 3.77 354 1200 2780 12,000

or AH.

and K, for trimerization of phenol, Ip-cresol and p-chlorophenol, respectively, at various concentrations and temperatures. The equilibrium constants for cyclic dimerization, K,, at each concentration at a particular temperature were constant to about &lO per cent. The equilibrium constants of cyolic dimerization of phenol and p-cresol at concentrations above about O-075 M at the lowest temperatures showed undesirable downward trends, indicative of the formation of polymers higher than dimer. Assooiated O-H bands at lower frequency than the dimer 373

_---_I- I- I__ I MILDRED M. MAQUIIWand ROBERTWEST

Table 2. Kr for dimerization and Kr for trimerization of p-ores01

Cont. (MI 0.190 0.152 0.095 0.076 0.047 0.019 0.009

0.5%

Kx

lO.O”C

Kr

156* 17.8* 18.7* 12.31 22.0 23.5 23.5

585 873 1450 1090 3640 8020 18,000

20.0%

30.0%

Kr

Kr

&

13.1* 14.2* 16.2 16.2 17.0 19.2 14.8

484 686 1210 1420 2850 6360 11,000

10.4 10.4 9.69 IO-5 7.51

Kr

Kx

&

384 486 669 732 1490 3760 5960

8.24 8.29 8.67 7.17 7.42 -

301 393 -

4O.O”C

Kp

K,

170 1140 2380 -

6.78 5.92 5.59 4.55 5.30 5.81 -

240 264 353 393 837 1860 -

* Points not used in calculation of average Kr or of AH.

Table 3. K, Cont. W) 0.196 0.157 0.098 0.078 0.049 0.019 0.009

for dimerization and KT for trimerization of p-ohlorophenol

-

O*O”C

Kx

Kr

11.4 11.8 9.98 10.0 13.7 11.9 16.3

Table 4.

Phenol p-Cresol p-Chlorophenol

416 520 718 941 2060 4220 12,500 -

2O*O”C

lO*O”C

9.33 9.52 8.34 8.81 11.2 11.7 8.08

30.0%

&

Kx

Kr

Kx

336 415 611 785 1580 4130 6000

8.10 7.88 7.16 6.65 -

286 343 515 594 -

9.05 7.77

3220 5960

6.07 5.78 5.61 5.51 7.57 6.87 -

Kx -~

Kr

214 246 391 565 1050 2360 -

Average equilibrium constants of cyclic dimerization K, p-oresol and p-chlorophenol at various temperatures - 17.5%

-9*5”C

36.6 -

24.5 -

0.0%

1

18.4 23.0 12.2

4O.O”C

Kz

4.82 5.01 4.23 3.78 5.73 6.42 4.53

Kr 172 217 298 335 814 2300 3840

for phenol,

10°C __-

15°C

1 20°C

30°C

40°C --

45°C

16.7 9.57

10.8 -

9.70 7.77

7.22 7.96 6.24

5.66 4.93

4.90 -

/ -

bands were also noted in the spectra of these phenol and p-cresol solutions. Accordingly, the points for some of the more concentrated solutions at low temperatures were not used in further calculations (Tables 1 and 2). At room temperatures polymer formation was noted in phenol and p-cresol solutions only near 0.2 M. Equilibrium constants for trimerization and tetramerization of phenol, p-cresol and p-chlorophenol were calculated over the same concentrations and temperatures for which dimer constants were calculated. The trimer constants agreed only to -&SO--l00 per cent and tetramer equilibrium constants were even more divergent, differing by about f300 per cent. Since dimer constancy was best for phenol, p-cresol and p-chlorophenol in dilute carbon tetrachloride solution near room temperature, it was concluded that the dimer is the predominating structure for the phenols under these conditions. 374

Hydrogen-bonding

studies-VII

Table 4 shows the average equilibrium constants for cyclic dimerization, K,, for phenol, p-cresol and p-chlorophenol at various temperatures. Table 5 shows AH, the intermolecular hydrogen-bonding energy of these phenols and some aliphatic alcohols calculated from the temperature dependency of the equilibrium constants, K,, of cyclic dimerization. l?ig. 4 show8 the plot of log Rx vs. l/Z x IO4 from which the enthalpies of dimerization may be derived. Our value8 for AH were I.6

1.4

I.2

.6

.6

I

I

36.0

I

I

I

I

34.0

38.0

‘/T

x

I

I

-

320

IO4

Fig. 4.

actually obtained by a least-squares treatment of the data, using each determined value of K, aa a separate point. The probable error in AH was eafimated using the formula of BIRGE [15]. Table 5 also shows the acid dissociation constants (pK,‘s) and AF and AS values for intermolecular hydrogen bonding of these phenol8 and aliphatic alcohols at 30°C calculated from the enthalpies, AH, and the average equilibrium constants of cyclic dimerization, K,. Data for the aliphatic alcohols were obtained from LIDDELand BECKER[6]. [I51 R.

T. BIRGE, Phya. Rev. 40, No.

2, 207(1932). 375

MILDREDM. MAQUIREand ROBERTWEST

The reproducibility of the molar extinction coefficients for various concentrations of phenol was determined using both the l-cm and 5-cm cells. Three separate lots of phenol were investigated and three values of cM were obtained at various Table 5.

Thermodynamic

-

-AH (kcal/mole dimerlI

I

5.12 (AO.10)

Phenol @Zresol p-Cl-phenol Methanol Ethanol t-Butanol

_1

functions of hydrogen bonding for phenols and aliphatic

6.09

(*O.lS)

3.78 9.2 7.2 4.8

( l 0.20) (&25) (fl.6) (-&l.l)

alcohols-

-

-AF (kcal/mole, 303°K)

-AS (Cal/V mole, 303°K)

1.19 1.25 l-10 0.6 1.1 1.3

13.0 16.0 8.8 28 20 12

-

-

25°C P%

10.0 10.17 9.39 -

concentrations at 30°C. Table 6 shows the reproducibility of molar extinction coefficients for the concentrations studied. Because of poor reproducibility of molar extinction coefficients for solutions more dilute than 0.010, our investigation was limited to the region above 0.010 M. Table 6. Reproducibility of molar extinction coefficients of the first overtone of the free O-H stretching vibration of phenol in carbon tetrachloride Concentration (M) 0.200 0.100 0.050 0.020 0.010 0.005 0.002 0.001

Average eN

2.47 3.05 3.22 3.43 3.45 3.47 3.73 4.46

Precision (%) fl.7 +0.6 f1.7 h1.4 h1.6 *4.9 A5.8 f6.9

Discussion and BECKER have stated that a non-zero slope of the plots of the molar extinction coefficients of the free O-H stretching vibration vs. concentration of methanol, ethanol and t-butanol is indicative of dimer formation by these alcohols. The equilibrium constants of dimerization, K,, of these alcohols can be calculated from the limiting slope of the plots of molar extinction coefficients vs. concentration according to the relationship [6] LIDDEL

376

Hydrogen-bondingstudies-VII

However, other workers have pointed out that trimer could contribute the limiting slope, since for the trimer [7]

somewhat to

-%l = -6K,C&& dC (The contributions due to high polymeric forms would be negligible.) Using the relationships between molar extinction coefficients and equilibrium constants developed by LIDDEL and BECKER [6], our experimental data show that constant equilibrium constants can be obtained only for dimerization, and not for trimer or tetramer formation (Tables l-3). Thus for these three phenols the dimer is the predominant form in the concentration range below O-2 M in carbon tetrachloride. Our conclusions that phenols are dimeric in dilute solution agree with similar findings of LIDDEL and BECKER for methanol, ethanol and t-butanol, but contradict those of SAUNDERS and HYNE [7] who interpret their NMR data as indicating that phenol associates as a trimer in carbon tetrachloride over the same concentration range which we have investigated. Trimer may be a predominant form in phenol solutions above O-2 M at room temperature, but our data indicate that it is The monomer-trimer equilibrium may also not important below this concentration. become more significant below 20°C since at lower temperatures equilibrium constancy for dimerization is poorer at the higher concentrations (0.0%0.2 M). In this investigation only the free O-H of the monomer has been assumed to contribute to the first overtone absorption. The unbonded O-H of the open dimer (II), if any were present, might also contribute to this first overtone free O-H absorption. However, we believe that no open dimer is present because the stretching frequencies of the fundamental O-H stretching vibration of the three phenols investigated are in the range of “bent” hydrogen bonds [ll], and suggest the closed dimer which has no unbonded O-H (I). Moreover, the first O-H overtone of the three phenols is sharp and symmetrical at all of the temperatures studied and suggests only a single free O-H species, the phenol monomeric O-H. In the event that open dimer is present, however, a major contribution of the open dimer O-H to the free O-H absorption is unlikely even at the highest concentrations investigated. At these concentrations open dimer O-H could contribute only about 15 per cent to the total O-H absorption in view of our experimental equilibrium constants. Values for AH and AS have been tabulated per mole of dimer (Table 5). These values would be those for the hydrogen bond if the dimer had the open structure. The values for AH seem too high for hydrogen bonds to weakly basic hydroxyl oxygens [l]. If the dimer has the closed structure (I) the AH values per mole of dimer should be halved to obtain the enthalpy per mole of hydrogen bonds. The tendency toward the formation of polymers higher than dimer increases from p-cresol to phenol to p-chlorophenol (Figs. l-3). The enthalpies of dimerization increase (become more negative) in the same order (Table 5). It is noteworthy that the hydrogen-bonding energy for these phenols increases with decreasing acid strength (Table 5). However, the basicity of the hydroxyl oxygen in the three phenols 377

MILDRED M. MADUIRE and ROBERT WEST

probably increases in the same order as the hydrogen bond strength.* Two possible explanations for the observed order of hydrogen bond strengths in phenols may be offered: (1) The basicity of the oxygen may be more important than the acidity of the hydrogen in determining the enthalpy of hydrogen bonding, or (2) the influence of the Para-substituent group on the basicity of the hydroxyl oxygen is greater than on the acidity of the proton. Explanation (2) is not unreasonable in that the hydrogen atom may be shielded by the oxygen from electronic effects on the aromatic ring. Confirmation of the order of hydrogen bond strengths is given by the previous infrared studies of MECKE [13] who found AH = -3.72, -4.35 and -4.4 for the hydrogen-bonding association of p-chlorophenol, phenol and p-cresol respectively. MECKE’S work was carried out in more concentrated solutions where considerable association to higher polymers takes place, and his values for AH therefore represent averages for association to give various polymeric species. Nonetheless, the same qualitative trend in AH is evident. Our heats of dimerization for phenols are somewhat lower than those calculated by LIDDEL and BECKER [6] for methanol and ethanol, and overlap their value for t-butanol (Table 5). Equilibrium constants for cyclic dimerization at 30°C taken from LIDDEL and BECKER’S paper are, for methanol K, = 2.6, for ethanol K, = 5.5 and for t-butanol K, = 8.5 [6]. Free energies and,entropies of dimerization for the aliphatic alcohols have been calulated from LIDDEL and BECKER’S data and are listed in Table 5 for comparison with our data for phenols. These quantities are of the same order of magnitude for aliphatic alcohols as for phenols. Acknozuledgntenta-Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, and to the Wisconsin Alumni Research Foundation for financial support of this research. The authors are indebted to Mr. DAVID L. POWELLfor assistancewith treatment of the data. * The relative basicity of the oxygen*in the corresponding methyl ethershas been determinedby measuring the shift of the free O-H stretching absorption of phenoluponhydrogenbondingto the ethers. Veluesfor Arcs, obtainedby MissKARENROBEIGTSON, are : p-chloroanisole, 146 cm-l; anisole, 159 cm-l; p-methylanisole, 166 cm-l, indicating that the basicity increases in the order given.

378