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INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY. VOL. 46, 159- 170 (1993) Ab Initio Investigation of Phloroglucinol KIM MANDIX Department of Electrophysics, The Technical University of Denmark DTH 322, DK-2800 Lyngby, Denmark and The Engineering Academy of Denmark, Department of Chemistry and Chemical Engineering, DIAK 375, DK-2800 Lyngby, Denmark ARNE COLDING, KNUD ELMING, LEIF SUNESEN, AND IRENE SHIM The Engineering Academy of Denmark, Department of Chemistry and Chemical Engineering, DIAK 375, DK-2800 Lyngby, Denmark Abstract All-electron ab initio Hartree-Fock (RHF) calculations have been carried out to investigate the ketoienol equilibrium of phloroglucinol. The calculations predict that the enol form of phloroglucinol, 1,3,5- benzenetriol, is by far the most stable of the two. This is confirmed by NMR spectra taken on phloroglucinol. A comparison of the keto enol form transformation of phloroglucinol with that of the phenol system shows that the keto form of phloroglucinol, 1,3,5-cyclohexanetrion, is more abundant in the phloroglucinol system, and the keto form of phenol, 2,4-cyclohexadien-l-on, in the phenol system. 0 1993 John Wiley & Sons, Inc. I. Introduction Aliphatic carbonyl compounds that have hydrogens on their a-carbon atoms are known to be in equilibrium with their corresponding enols, but the equilibrium is usually strongly shifted toward the keto form of the compounds. The enol-keto transition can be observed in the synthesis of acetaldehyde when water is added to acetylene; under acidic conditions, the primarily formed vinyl alcohol is immediately transformed into acetaldehyde. Aliphatic carbonyl compounds undergo numerous characteristic chemical reactions. Thus, with hydroxylamine, H2NOH, they form oximes; with alcohols under acidic conditions, they form acetals; and with Grignard reagents RMgX; they form secondary and tertiary alcohols. The a-hydrogens of ketones are acidic, and they can, e.g., be substituted by halogen atoms or alkyl groups. Phenol and related compounds are stable in the enol form. Phenol is a weak acid with a pK, value of 10.0, and in aqueous solution, phenol is in equilibrium with H30' and phenoxide anions, PhO-. In aqueous sodium hydroxide solution phenol forms salts. Furthermore, phenols participate in chemical reactions that are quite similar to those of alcohols. Thus, phenols, like alcohols, react with acid chlorides or acid anhydrides to form esters. Phenols also react with alkyl halides to form ethers, a reaction analogous to that of alcohols. 0 1993 John Wiley & Sons, Inc. CCC 0020-7608/93/0 10159-12

Ab initio investigation of phloroglucinol

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Page 1: Ab initio investigation of phloroglucinol

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY. VOL. 46, 159- 170 (1993)

Ab Initio Investigation of Phloroglucinol

KIM MANDIX Department of Electrophysics, The Technical University of Denmark DTH 322,

DK-2800 Lyngby, Denmark and The Engineering Academy of Denmark, Department of Chemistry and Chemical Engineering, DIAK 375, DK-2800 Lyngby, Denmark

ARNE COLDING, KNUD ELMING, LEIF SUNESEN, AND IRENE SHIM The Engineering Academy of Denmark, Department of Chemistry

and Chemical Engineering, DIAK 375, DK-2800 Lyngby, Denmark

Abstract

All-electron ab initio Hartree-Fock (RHF) calculations have been carried out to investigate the ketoienol equilibrium of phloroglucinol. The calculations predict that the enol form of phloroglucinol, 1,3,5- benzenetriol, is by far the most stable of the two. This is confirmed by NMR spectra taken on phloroglucinol. A comparison of the keto enol form transformation of phloroglucinol with that of the phenol system shows that the keto form of phloroglucinol, 1,3,5-cyclohexanetrion, is more abundant in the phloroglucinol system, and the keto form of phenol, 2,4-cyclohexadien-l-on, in the phenol system. 0 1993 John Wiley & Sons, Inc.

I. Introduction

Aliphatic carbonyl compounds that have hydrogens on their a-carbon atoms are known to be in equilibrium with their corresponding enols, but the equilibrium is usually strongly shifted toward the keto form of the compounds. The enol-keto transition can be observed in the synthesis of acetaldehyde when water is added to acetylene; under acidic conditions, the primarily formed vinyl alcohol is immediately transformed into acetaldehyde. Aliphatic carbonyl compounds undergo numerous characteristic chemical reactions. Thus, with hydroxylamine, H2NOH, they form oximes; with alcohols under acidic conditions, they form acetals; and with Grignard reagents RMgX; they form secondary and tertiary alcohols. The a-hydrogens of ketones are acidic, and they can, e.g., be substituted by halogen atoms or alkyl groups.

Phenol and related compounds are stable in the enol form. Phenol is a weak acid with a pK, value of 10.0, and in aqueous solution, phenol is in equilibrium with H30' and phenoxide anions, PhO-. In aqueous sodium hydroxide solution phenol forms salts. Furthermore, phenols participate in chemical reactions that are quite similar to those of alcohols. Thus, phenols, like alcohols, react with acid chlorides or acid anhydrides to form esters. Phenols also react with alkyl halides to form ethers, a reaction analogous to that of alcohols.

0 1993 John Wiley & Sons, Inc. CCC 0020-7608/93/0 10159- 12

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160 MANDIX ET AL.

Phloroglucinol, or 1,3,5-benzenetriol, reacts with other chemical compounds as hav- ing either carbonyl or hydroxy functional groups. In many respects, the chemistry of phloroglucinol is similar to the chemistry of phenol. Thus, phloroglucinol forms esters when reacting with acid chlorides or acid anhydrides. However, phloroglucinol also reacts as a ketone. With hydroxylamine, HzNOH, phloroglucinol reacts to form a tri- oxime [ l l , and with methyl iodide, CH31, it forms 2,2,4,4,6,6-hexamethylcyclohexane- 1,3,5-trione [l].

Based on this background, we ask the following question: To what extent is phloroglucinol a ketone? Because of the aromatic stabilizing effects, phloroglucinol is usually perceived as having three hydroxy groups, i.e., 1,3,5-benzenetriol. However, the chemical reactions of phloroglucinol mentioned above indicate that the molecule is in the keto form, i.e., 1,3,5-~yclohexanetrione. The two molecules are shown in Figure 1.

In the present work, we have undertaken theoretical investigations of acetaldehyde, phenol, and phloroglucinol to determine the relative stability of their keto and enol forms. The molecular systems have been investigated by performing all-electron ab initio Hartree-Fock (RHF) calculations.

The investigations of acetaldehyde and its enol form, vinyl alcohol, and phenol and its keto form, 2,4-cyclohexadien-l-one, have primarily been performed to establish the suitability of the RHF method for studying the relative stabilities of ketones and enols. Thus, it is well known that the stable form of acetaldehyde is the keto form, whereas the stable form of phenol is the enol form.

11. Details of Computations

The RHF calculations have been performed in the Hartree-Fock-Roothaan formal- ism [2]. The molecular integrals were calculated using the MOLECULEDNTEGRAL [3]; the RHF calculations were performed using the program system ALCHEMY [4].

The basis set used for carbon and oxygen is Huzinaga's (10,6) basis set [5]. The oxygen and carbon primitive basis sets have been contracted to (4,3), using a segmented contraction scheme. For the hydrogen atoms, we used Huzinaga's (4) basis [6], augmented with a set of p-functions with an exponent of 0.7. The resulting (4,l) primitive hydrogen basis was contracted to (2, l), also using a segmented contraction scheme.

111. Acetaldehyde

The geometries used for acetaldehyde and vinyl alcohol are idealized geometries, but we optimized the C-C and the C-0 interatomic distances. For acetaldehyde, the bond angles around the aldehyde carbon were fixed to 120.0", and around the a-carbon, to 109.5", the tetrahedral angle. One of the a-hydrogens were placed in the plane defined by the two carbon and the oxygen atoms. For both acetaldehyde and vinyl alcohol, the carbon-hydrogen distances were chosen as 2.04 ao. This value is the average of the standard carbon-hydrogen bond lengths [7]. Vinyl alcohol was assumed to have planar geometries and the bond angles around both carbon atoms were fixed at 120.0". The oxygen-hydrogen distance was set to 1.80 a0, and the

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AB INITIO INVESTIGATION OF PHLOROGLUCINOL 161

H

H

H

0 ( b)

Figure 1. The geometry of phloroglucinol: (a) in its enol form, 1,3,5-benzenetriol; (b) in its keto form, 1,3,5-cyclohc?xanetrione.

carbon-oxygen-hydrogen bond angles were taken to be 109.0'. Acetaldehyde and vinyl alcohol are shown in Figure 2.

From our investigations of acetaldehyde and vinyl alcohol, we obtained the total energies reported in Table I. It is noted that acetaldehyde has the lowest energy when the carbon-carbon distance is 2.86 a0 and the carbon-oxygen distance is 2.32 ao. For vinyl alcohol, we identified the geometry of the lowest energy as the one where the carbon-carbon bond length is 2.50 a0 and the carbon-oxygen bond length is 2.58 ao. The equilibrium geometry of acetaldehyde is found to be 0.008133 au or 21.37 kJ/mol lower in energy than the equilibrium geometry of vinyl alcohol. Consequently, according to the Boltzmann distribution, the vinyl alcohol/acetaldehyde

Page 4: Ab initio investigation of phloroglucinol

162 MANDIX ET AL.

/ H H

\

( b)

form, acetaldehyde. Figure 2. The geometry of acetaldehyde: (a) in its enol form, vinyl alcohol; (b) in its keto

ratio at room temperature is 15300. Thus, the HF calculations verify the experimental fact that acetaldehyde is more stable in its keto than in its enol form.

IV. Phenol

For phenol in its enol as well as in its keto form, the six carbon atoms were placed in the corners of a planar hexagon. The distance between neighboring carbon atoms were fixed at 2.64 ao, which is the average carbon-carbon bond length in phenol. As in acetaldehyde and vinyl alcohol, the carbon-hydrogen bond length was chosen to be 2.04 ao. The carbon-carbon-hydrogen bond angles were chosen to be 120.0". In the keto form of phenol, 2,4-cyclohexadien-l-one, the angle between the two hydrogens neighboring to the carbonyl group was fixed at 109.0" and the oxygen-carbon-carbon angle was fixed at 120.0". The carbon-oxygen distance was optimized. For phenol, we kept the carbon-oxygen distance constant at 2.58 UO, which is the experimental carbon-oxygen distance for phenol [8]. However, we optimized the oxygen-carbon-carbon angle. In addition, we have performed calculations on phenol where the hydrogen atom of the hydroxy group and the neighboring hydrogen atom was placed in an out-of-plane position. The two molecules are shown in Figure 3.

The total energies obtained in our RHF calculations of the ground states of phenol and 2,4-cyclohexadien-l-one are shown in Table 11. For phenol, we found that adjusting the in-plane oxygen-carbon-carbon angle was more effective in reducing the total energy of the molecular system than moving the hydroxy hydrogen or one of the hydrogens that are neighbors to the hydroxy group out of the plane of the benzene ring. The equilibrium geometry of phenol was determined to be a planar structure

Page 5: Ab initio investigation of phloroglucinol

AB INITIO INVESTIGATION OF PHLOROGLUCINOL 163

TABLE I. Total energy of the RHF ground state of acetaldehyde and its enol form, vinyl alcohol

Molecule

Distance Distance c-c c-0 (a01 (a01

Total energy

(au)

Acetaldehyde

Vinyl alcohol

2.64 2.80 2.86 3.08 2.86 2.86 2.86 2.42 2.50 2.53 2.64 2.86 2.50 2.50 2.50

2.32 2.32 2.32 2.32 2.21 2.43 2.58 2.58 2.58 2.58 2.58 2.58 2.32 2.50 2.65

- 152.873284 - 152.880860 -152,881301 - 152.874537 - 152.877946 - 152.874656 - 152.855083 -152.869731 - 152.872727 - 152.872643 - 152.867781 - 152.842924 - 152.848585 - 152.869945 -152.872714

The rest of the geometries were chosen as the following: Acetaldehyde: R(C-H) = 2.04 ao, (OCH = 120.0', (HCH = 109.6". Vinyl alcohol: R(C-H) = 2.04 ao, (OCH = (HCH = 120.0"; R(0-H) = 1.80 ao, (HOC = 109.0". no = 52.92 pm.

with an oxygen-carbon-carbon angle of 122.2', which is in accordance with the experimental data for phenol [8]. The equilibrium geometry of phenol is found to be 0.076017 au or 199 kl/mol lower in energy than the equilibrium geometry of 2,4- cyclohexadien-1-one. Consequently, the fraction of 2,4-cyclohexadien- 1-one in phenol at room temperature is extremely small. Thus, the RHF calculations verify that phenol is more stable in its enol than in its keto form.

V. Phloroglucinol

For 1,3,5-~yclohexanetrione, the carbon-hydrogen distance was set to 2.04 ao, and the angle between two hydrogen atoms bonded to the same carbon atoms was fixed at 109.0'. The carbon-carbon-oxygen and carbon-carbon-carbon angles were all fixed at 120.0'. However, we have optimized the carbon-carbon as well as the carbon-oxygen distances. In all but one calculation, the 1,3,5-cyclohexanetrione was assumed to be planar. However, 1,3,5-cyclohexanetrione is presumably nonaromatic and the molecule need not be planar. To investigate the effect of nonplanarity, we performed a calculation on 1,3,5-cyclohexantrion in a nonplanar geometry. The nonplanar geometry was constructed by fixing all bond angles in the methylene groups to 109.5", the tetrahedral angle. The nonplanar 1,3,5-cyclohexanetrione is shown in Figure 4. This revealed that the molecule is more stable in the planar than in the nonplanar geometry. The ground state of 1,3,5-benzenetriol was calculated under the restriction that the molecule is planar with D3h symmetry. For 1,3,5-benzenetriol,

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164 MANDIX ET AL.

H

H

0

H ( b)

Figure 3. The geometry of phenol: (a) in its enol form, phenol; (b) in its keto form, 2,4,-cyclohexadien-l -one.

the carbon-carbon and carbon-hydrogen distances were fixed at 2.64 and 2.04 ao, respectively. The carbon-carbon-carbon and carbon-carbon-hydrogen angles were all fixed at 120.0'. The carbon-oxygen distance and carbon-carbon-oxygen angles were optimized.

In the case of 1,3,5-cyclohexanetrione and 1,3,5-benzenetriol, the total energies are reported in Table 111. In the equilibrium geometry, the 1,3,5-cyclohexanetrione has a carbon-carbon bond length of 2.86 a0 and a carbon-oxygen bond length of 2.32 ao.

For 1,3,5-benzenetriol, the most stable molecular geometry was found for a carbon-carbon bond length of 2.64 ao, which is identical to that of benzene.

Page 7: Ab initio investigation of phloroglucinol

AB INITIO INVESTIGATION OF PHLOROGLUCINOL 165

TABLE 11. Total energy of the RHF ground state of phenol and its keto form, 2,4-cyclohexadien-l-one.

Distance Total energy c-0 Angle Dihedral angle

Molecule (ao) 0-C-C H-0-C-C 0-C-C-H (au)

2,4-Cyclohexa- dien-I-on 2.58

2.32 2.21

Phenol 2.58 2.58 2.58 2.58 2.58 2.58 2.58

120.0" 120.0" 120.0" 120.0" 122.2" 124.4" 120.0" 120.0" 120.0" 120.0"

0.0" 0.0" 0.0" 90.0" 45.0" 45.0" 90.0"

54.5" 54.5" 54.5" 0.0" 0.0" 0.0" 0.0" 0.0" 54.5" 54.5"

-305.420500 - 305.4368 13 -305,429092 -305.512412 -305.5 12830 -305.512465 - 305.5091 53 - 305.5 10584 - 305.457577 -305.452867

The rest of the geometries were chosen as the following: 2,4-Cyclohexadien-l-one: R(C-H) =

2.04 ao, (CCC = 120.0", (HCH = 109.0". Phenol: R(C-H) = 2.04 ao, (CCC = (CCH = 120.0"; R(0-H) = 1.80 ao, (HOC =109.0". a0 = 52.92 pm.

The carbon-oxygen bond length was determined to be 2.58 a0, and the carbon- carbon-oxygen angles, as 122.2, identical to the angles in phenol.

From the total energies in Table 111, it is noted that the most stable of the two configurations is 1,3,5-benzenetriol. The energy of 1,3,5-benzenetriol is 0.055567 au or 146 kJ/mol lower than that of 1,3,5-cyclohexanetrione. This energy difference is so large that phloroglucinol should exist almost entirely as 1,3,5-benzenetriol at room temperature.

H 0 \\

0 Figure 4. The geometry of phloroglucinol in its nonplanar keto form.

Page 8: Ab initio investigation of phloroglucinol

166 MANDIX ET AL.

TABLE 111. Total energy of the RHF ground state of phloroglucinol, 1,3,5-benzenetriol, and its keto form, 1,3,5-cyclohexanetrione.

~~~~~

Distance Distance Total c-c c-0 Angle energy

Molecule (ao) (a01 c-c-0 ( a 4

1,3,5- Cyclohexantrion 2.64 2.58 120.0" -455.032452

2.64 2.32 120.0" - 455.102662 2.64 2.21 120.0" -455.087350 2.80 2.32 120.0" -455.153165 2.86 3.32 120.0' -455.156110 2.91 2.32 120.0" -455.153356 2.64* 2.32* 120.0- - 455.060096 *

1,3,5- Benzenetriol 2.64 2.50 120.0° -455.204658

2.64 2.50 120.0' -455.204658 2.64 2.65 120.0" -455.210540 2.64 2.58 120.0' -455.210470 2.64 2.58 122.2" -455.211677 2.64 2.58 124.4" -455.210682

*Nonplanar geometry (see Fig. 4). The rest of the geometries were chosen as the following: 1,3,5-Cyclohexanetrione: R(C-H) =

2.04 ao, (CCO = (CCC = 120.0", (HCH = 109.0'. 1,3,5-Benzenetriol: R(C-H) = 2.04 "0, (CCC = (CCH = 120.0'; R(0-H) = 1.80 00, (HOC = 109.0". a0 = 52.92 pm.

To obtain experimental confirmation of our findings, we recorded NMR spectra of 70 mg phloroglucinol both in 600 pL pure (CD3)zSO and with 100 pL 0.1M sodium deuterooxide (NaOD) in D20 added. The two spectra are shown in Figures 5 and 6. The phloroglucinol used was commercial phloroglucinol from Riedel-De Haen AG, Hannover, which contains two equivalents of water (H20).

In the spectrum in Figure 5 , two peaks at 6 = 5.7 and 9 have identical integrals. These peaks are identified as aromatic H and the H in the hydroxy groups. A third peak at S = 3.7, with 413 of the integeal of the peaks previously mentioned, is originating from the hydrogens in water. There is no trace of peaks originating from aliphatic hydrogen atoms in the spectra. Thus, the phloroglucinol exists purely in its enol form.

The spectrum in Figure 6 was taken after the sample used for the previous spectrum was treated with NaOD in D20: One mole of NaOD per mole phloroglucinol. This spectrum displays only two major peaks: The larger of these, at S = 4.5, can be assigned to hydrogens in water. The smaller at S = 5.6 originates from aromatic hydrogens. The integrals of the peaks is significantly different from that expected if only the hydroxy protons had been exchanged with deuterium. Thus, a significant number of the aromatic protons have been exchanged with deuterium. This means that the phloroglucinol molecules are changing between the enol and keto forms. However, there is no trace of aliphatic protons in the spectrum, and, consequently, the fraction of the phloroglucinol molecules in the keto form is very small.

To verify the nonexistence of phloroglucinol in its keto form, we also recorded an IR spectrum of phloroglucinol, shown in Figure 7. There is no peak in the range

Page 9: Ab initio investigation of phloroglucinol

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AB INITIO INVESTIGATION OF PHLOROGLUCINOL 169

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Page 12: Ab initio investigation of phloroglucinol

170 MANDIX ET AL.

1670-1780 cm-', where carbonyl groups generally absorb. Thus, the IR spectrum confirms that phloroglucinol exists almost entirely in its enol form.

VI. Conclusions

Our investigation of acetaldehyde and phenol, correctly predicted the relative stability of the keto and enol forms of these molecules. The transition energy between the two forms is smaller for the acetaldehyde-vinyl alcohol system than for the phenol, 2,4-cyclohexadien-l-one system. This is due to the stabilizing effect due to the aromaticity of phenol. This finding is also in accordance with data on related compounds. For the keto-enol equilibrium for other aliphatic molecules, e.g., cyclohexanone and acetone, the ratio of the keto form to the enol form is approximately 1:1@ and l : lOh , respectively [9]. Phenol is generally known not to participate in chemical reactions in its keto form. Thus, we are confident that the results of our investigation are qualitatively correct.

The results of our calculations reveal that phloroglucinol is to be found in its enol form, 1,3,5-benzenetriol. The number of 1,3,5-cyclohexanetrione molecules in phloroglucinol is almost vanishing. This finding has been confirmed by NMR spectra taken on phloroglucinol, both in neutral as well as in basic solutions.

However, the enol-keto form transition energy for the phloroglucinol system is only 3/4 of the enol-keto form transition energy for the phenol system. This difference in transition energy means that even though the fraction of phloroglucinol molecules found in the form of 1,3,5-cyclohexanetrione is very small. It is actually a billion times larger than the fraction of phenol molecules found in the 2,4-cyclohexadien-1- one form. The difference in transition energy explains the ability of phloroglucinol to participate in chemical reactions in its keto form, an ability that is not present for the phenol system.

Bibliography

[ 11 J. Packer and J. Vaughan, A Modern Approach to Organic Chemistry, (Clarendon Press, Oxford, 1958). 121 C.C.J. Roothaan, Rev. Mod. Phys. 32, 179 (1960). [3] J. Almloef, MOLECULEflNTEGRAL program. [4] P. S. Bagus, M. Yoshimine, A. D. McLean, and B. Liu, ALCHEMY SCF program. [5] S. Huzinaga, J. Chem. Phys. 54, 2284 (1971). [6] S. Huzinaga, J. Chem. Phys. 42, 1293 (1965). 171 CRC Handbook of Chemistry and Physics 1983-84, Robert C. Weast, Melvin J. Astle, and William

181 J . Phys. Chem. Ref. Data, 8, 619 (1979); Referenced in Peter Politzer and Nagamani Sukumar,

[9] J. McMurry, Organic Chemistry, (Brooks/Cole, Pacific Grove, California, 1988).

H. Beyer, eds., (CRC Press, Boca Raton, FL, 1984).

Theochemistry 48, 439 (1988).

Received June 1, 1992 Accepted for publication October 20, 1992