Equilibria for the isomerization of (secondary-alkyl)phenols and cyclohexylphenols

O-305 J. Chem. Thermodynamics 1%9,21, 385-395

Equilibria for the isomerization (secondary-alkyl)phenols and cyclohexylphenols

of

T. N. NESTEROVA, A. A. PIMERZIN, A. M. ROZHNOV, and T. N. KARLINA Kuibyshev Polytechnical Institute, Department of Chemical Technology, Kuibyshev 443010, U.S.S.R. (Received 25 July 1988; in final form 3 January 1989) Equilibria of a series of isomerizations and tram-alkylations of alkylphenols have been investigated in the liquid phase over a wide range of temperatures. Equilibria of isomerizations connected with the displacement of a substituent on a benzene nucleus were studied for secondary-butyl-, -amyl-, -hexyl-, and cyclohexyl-phenols, and di-(secondary-butyl)phenols. Equilibria of positional isomerization connected with the displacement of an oxyphenyl radical in an alkyl chain were investigated for oxyphenyl-pentanes, -hexanes, -octanes, and -decanes. Runs-alkylation was investigated for di- and tri-(secondary-butyl)phenols. Values of A,H; and A,Si were found for all investigated reactions. An analysis was made of the thermodynamic quantities for the reactions. Enthalpies of formation of isopropylphenols (IPP) in the gaseous state were calculated. The values of A,Hk/(kJ.mol-r) were found at 298.15 K: o-IPP, -(175.3+2.4); p-IPP, -(175.3+2.4); m-IPP, -(175.3*2.4); 2,4-di-rpp, -1254.1 k2.8); 2,5-di-rpp, -(254.1+ 2.8); 2,6-di-rpp, -(254.1 f 2.8); 3,5-di-rpp, -(254.1 k2.8); 2,4,6-tri-rpp, -(333.0* 3.1).

1. Introduction Many (secondary-alkyl)phenols and cycloalkylphenols are technically important substances. That is why a knowledge of their thermodynamic properties is necessary to make different practical calculations. The evaluation of the properties of these compounds in the liquid state is of special interest. That is due to the conditions of their production and chemical processing on the one hand, and special features of alkylphenols as associated liquids on the other hand. Nevertheless, dependable thermodynamic properties, even for the lower representatives of the alkylphenol series, are practically absent nowadays. For example, literature sources give results for the enthalpies of combustion of isopropylphenols (IPP) and enthalpies of formation in the liquid and gaseous states,(lm3) yet the uncertainties in the values are as large as 13 to 17 kJ*mol-‘. This fact surely leads to serious errors in thermodynamic calculations. No less important is the fact that a discrepancy of a serious nature appears in the enthalpies of formation of isomers in groups of mono- and di-rpps. For example, the largest value belongs to the enthalpy of formation of the or&-isomer in the group of mono-rpps. For the gaseous state, AfHg(o-r~~, g) is 23 to 29 kJ. mol-’ higher than 0021-9614/89/040385+

11 $02.00/O

0 1989 Academic Press Limited

T. N. NESTEROVA

386

H-AL.

for the meta- and para-isomers. This is in good accord with the equilibrium results from isomerizations of isopropylphenols in the liquid phaset4) and calorimetric results for ethylphenols. (‘) Yet, in the group of di-mps, the standard molar enthalpy of formation of 2,6-di-rpp is 4 to 17 kJ * mol-’ lower than the values of A,Hg for other isomers; that is contradictory to observations for the present property in the group of mono-rPPs,(1~3) and does not agree with the results of research on the equilibria of isomerization of di-rpps. (4) The studies mentioned include all experimental information on the energetic properties of the compounds discussed, notwithstanding that only the lower representatives of the (secondary-alkyl)phenol series are included. The transition to higher alkylphenols will demand a knowledge of properties not only for the substitutional isomers around a nucleus but also for the positional isomers of oxyphenylalkanes (isomers of position of the oxyphenyl substituent in the aliphatic chain). At present there is no information concerning either the properties of substitutional isomers or results about the equilibria. Only results for the isomerization equilibria of phenylalkanes@’ are indirectly connected to the present problem and can be used in establishing the dependence of thermodynamic properties of alkylphenols on the compositions of their molecules if consistencies are found in the properties of alkylphenols and alkylbenzenes. We have performed research, which led to consistent thermodynamic values for (secondary-alkyl)phenols and cyclohexylphenols. The present study shows the results of the study of chemical equilibria of the transformations of the following types. Isomerization of position of alkylphenols, connected with the displacement of the oxyphenol substituent along the aliphatic chain: CH,-CH,-CH-(CH&-CH3 Ar OH CH,-(CH&-CH-(CH&

= CH,-CH-(CH,),, I ArOH

(14

1-CH3 = CH,--CH,-F-(CH,).-CH,,

Ar OH CH3-(CH,)3-CH-(CH,),-2-CH3

i-CH,,

W

ArOH = CH,-(CH&-CH-(CH&

Ar OH where n = 1, 2, 4, or 6. Isomerization connected with the displacement substituent in the aromatic nucleus:

1-CH3,

UC)

ArOH of an alkyl

I R

or cycioalkyl

EQUILIBRIA

where

R

387

OF ISOMERIZATION

= cyclohexyl or 2-butyl, -amyl, -hexyl; or

RI&R1

=

eR’

=

Rl&R1

=

Rl&Rl,

(IIb)

RI where R, = 2-butyl. Trans-alkylation of (secondary-butyl)phenols (independent reactions are shown, needed to establish the connection between groups of mono-, di-, and tri-(secondary butyl)phenols):

(III)

RI

Rl

2. Experimental The study of equilibria was made in the liquid phase. Mole percentages of 3 to 10 of AlCI, and AlBr, were chosen for isomerization and trans-alkylation of alkylphenols. Zirconium oxide was used in the isomerization of cyclohexylphenols, as halides of aluminium were not very effective in this case. The method of preparation of zirconium oxide was taken from reference 7, and the mass of the catalyst was 20 to 25 per cent of the mass of reactants. The methods of the study of chemical equilibria were the same as in our previous work.@*’ The criteria for the establishment of equilibria were the constancy of the ratio of concentrations of components of the studied reaction with wide variations in compositions of primary mixtures (and as a consequence, of the equilibrium mixtures), with changes of the mass fraction of catalyst and of the time of reaction. The conditions of the study were selected to reduce to a minimum the formation of side-products and so to obtain correct experimental information. All studied transformations (individual reactions or groups of reactions) were acceptable as the mass percentage of side-products did not exceed 7. Special runs have shown that in all cases displacement of equilibrium does not occur with selecting and processing probes as well as in their analysis. Noting that alkylphenols are associated liquids,

T. N. NESTEROVA

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ET AL.

TABLE 1. Results of equilibria of reactions in solution: z, time of the experiment; n, number of the experiments; m, number of series of experiments; (K), mean values of the equilibrium constant; s, standard deviation of an equilibrium constant. 3-(3-A@, 3-(3-amyl)phenol; 2-(3-HX), 2-(3-hexyl)phenol; etc.

TIK

n

r/h 3-@-AM)

to to to to to

13 8 6.5 4.5 1.7

m

00

3-(~-AM) 3 3 5 6 3

=

1 0.5 0.3 0.3 0.3

443 473 503 533 573

1 to 0.5 to 0.5 to 0.3 to 0.3 to

503

1to5

35

3-(3-HX) 1 to 5

= 3-(2-HX) 28 5

(4)

503

= 4-(2-HX) 32 5

(5)

503

4-(3-H@ 1 to 5

a-@-AM)

13 8 6.5 4.5 1.7

r/h

473

1 to7

(2)

443 463 483 523 573

2-(2-BU)=4-(2-Su)(19) 4 to 67 55 1to11 22 0.3 to 6 29 0.3 to 2 32 0.1 to 4 33 1 0.5 0.3 0.3 0.3

2 2 2 1 1

1.63

1

443 473 503 533 573

1.64

2

503

2-(2-nx) 1 to 5

1.62

1

533

2-(3-ocr)= 0.3 to 1.8

2-(2-OCT)(6) 21 3

1.61

1

2-CHX 0.5 to 22 0.5to16 0.2 to 10 0.2 to 16

533

3-(3-OCT)= 3-(2-OCT)(~) 0.3 to 1.8 21 3

453 483 523 573

1.66

1

533

4-(3-OCT)= 4-(2-OCT) (~) 0.3 to 1.8 21 3

443 483 523 573

4-(2-BU)= 7 to i4 1 to 6.5 0.4to2 0.1 to 0.6

473 503 533 573

2 0.5 0.5 0.7

3-(3-DEC)

473

1to7 4-(3-DEC)

473

2to7

2-(2-HX)(3) 5

= 3-(2-DEC)

21 18

2-(4-OCT)= 0.3 to 1.8

2-(3-0~~) 21 3

533

3-(4-OCT)= 0.3 to 1.8

3-(3-OCT)(12) 20 3

533

4-(4-OCT)=4-(3-OCT)(13) 0.3 to 1.8 21 3 2-(%DEC)

473

1to7

473

1to7

4-(d-DEC)

2-(5-DEC)

473

1to7 %($DEC)

473

1to7

= 2-(3-DEC)

21 21 21

1

1.22

1

1.19

1

1.20

1

1.15

1

1.00

1

0.97

1

(15)

%&AM)

503

to to to to

8 7 5 3

4-(2-HX)= 2to5

10%

3

1.00

1

7 3 4 4 5

1.72 1.65 1.59 1.43 1.29

(18)

4-(2.~~)(20) 33 3 32 4 31 4 28 4 14 2

1.80 1.65 1.55 1.47 1.33

= 4-(2-rrx) (21) 35 5 1.52 57 15 23 42

7 3 5 4

3-(2-su)(23) 14 2 22 3 21 3 11 2 = j-(&AM) 27 20 25 17

1

(22)

= ‘t-CHX

1.37 1.28 1.23 1.20

1 13 2 2

3.39 3.39 3.22 3.38

4 3 12 2

(24) 4 3 4 3

3.58 3.66 3.73 3.70

3-(2-ox) 17 3

3.82

3

3.18

2

(27) 6.59 6.01 6.09 5.27 4.29

12 27 38 13 24

2-(2-BU)+4-(2-Su)(28) 60 8 1.34 25 3 1.31 37 4 1.20

1 2 2

573

4-CHX=3-CHX 0.2 to 16 10

443 463 483 523 573

2,6-di(2-au) 4 to 77 2 to 9 0.5 to 9 0.4to2 0.1 to 4

(16) (17)

3

13 8 6.5 4 1.3


(26) 4

1

(14)

3

= 3-(‘6DEC)

21

1.53

1.17

3

= 2-(4-DEC)

3

(11)

3

= ‘t-(3-DEC)

1.64 (10)

3

533

1

(9)

3

= ‘t-(2-DEC)

1.58

2-(2-AM)=

to to to to to

m

= ‘t-(‘t-DEC)

21

2.16 2.73 2.69 2.67 2.66

2-(3-HX)=

3 4 5 6 3

n

‘t-(5-WC)

2.83 2.80 2.69 2.73 2.67

= d-(2-AM)

33 32 39 41 22

T/K

(1)

443 473 503 533 513

32 23 40 42 22

10%

2,4-&(2-BU)=

443 463 483

4to77 2toll 0.3 to 9

= 2,4-di(2-au) 26 4 6 1 24 3 15 2 23 4

EQUILIBRIA

OF

l-continued

TABLE

523 573 443 483 523 573

443

0.3 to 1.7 0.1 to 1.8 2:2(&B”’

1 to 6.5 0.4 to 1.8 0.2 to 2

15 21

2 3

;;{3-(2-BU)}4 33 29 17

2,5-di(2-su) = 2-(2-BU) 4 to 77 40

5 4 3

1.21 1.11 (30) 5.71

34

5.26 4.96 5.17

13 14 17

+ 3-(2-BU) (29) 5 2.22

389

ISOMERIZATION

8 2

3

463 483 523 573

ltoli 0.3 to 9 0.3 to 1.7 0.2to4

26 26 10 31

3 3 1 5

2,4,6-tert-(hu) = 2,4-di(2-su) + 443 4to24 16 2 483 0.3 to 9 21 3 523 0.3 to 1.7 17 2 573 0.1 to 1.3 11 2

2.16 2.08 2.03 1.87 2-(2-BU)

8.72 7.54 7.97 7.63

4 4 6 6 (31)

26 21 20 23

special attention was directed to the exclusion of possible systematic errors, the source of which could be the deviation of the system from the model “idealassociated solution”. Nevertheless, the constancy of the concentration constants of equilibrium with wide changes in the compositions of primary mixtures permitted the conclusion that ratios of activity coefficients of compounds taking part in the transformations were unity (within the limits of error) and, thus, the concentration constants of equilibria of the studied reactions could be taken as equal to the thermodynamic equilibrium constants. The analysis of the compositions of the reaction mixtures were made using g.i.c. (“Chrom-4 and -5” apparatus with a flame-ionization detector and glass-packed capillary columns). The analysis of mixtures containing isomers of one group was made with trimethylsilyl esters (TMSE) using a capillary column 50 m in length and 2.5 x lop4 m in diameter with the immobile phase OV-101. The column was prepared by a static method at a high pressure and had an efficiency of 3000 theoretical plates for a length of 1 m (for pentadecane at 383 K). The separation of the reaction mixtures containing mono-, di-, and tri-alkylphenols was done in packed (Chromaton NAW-DMCS with 5 mass per cent of SE-30 and 0.25 mass per cent of NPGS) and capillary (OV-101) columns with the combined chromatograms processed later. The packed column was used to analyse phenols; the capillary column to analyse their silyl esters. The transformation into TMSE was made according to the method stated in reference 9. The error caused by incomplete silylation did not exceed 0.4 per cent. The coefficients for composition in the phenol series and their TMSES, found experimentally in the interval of studied mole percentages from 0.2 to 99.9 were (1.00+0.02) with regard to phenol or its TMSE. Compounds necessary for study and identification were synthesized by the alkylation of phenol with the appropriate olefins, cycloolefins, or their halogen derivatives, and separated by multiple distillation in vacuum. The mass percentage of the basic isomer in the primary mixtures was not less than 95 in all cases (according to g.1.c.). In the reaction mixtures there were no unidentified components, and all compounds in the study were completely separated chromatographically.

390

T. N. NESTEROVA

ETAL.

3. Results The results of the investigation of chemical equilibria are shown in table 1, where for each studied reaction are given: 7’, the temperature of the investigation; r, the duration of the experiment; n, the number of experiments; and m, the number of series of experiments. A group of experiments different from others by the composition of the primary mixture, quantity, or type of catalyst is taken as a series. Table 1 shows also values of (K), the mean value of the equilibrium constant, and its standard deviation s. Most of the reactions were studied over a wide range of temperatures. These transformations are basic in establishing the dependence of thermodynamic properties of compounds on their molecular compositions. Some reactions were studied at one temperature and fulfil the function of test transformations for the evaluation of the relationships in terms of other representatives of the series. Accordingly, treatment of the experimental results with approximate equations was made both for individual reactions and groups of transformations of the same type. With the aim of finding the type of dependence A,GL = f(T), experimental results were treated with the approximation AC,,, = f(T). It was found that a linear approximation is satisfactory for all cases. The results are shown in table 2, in which the changes of enthalpy and of entropy of reaction are also given as well as the errors represented at the 95 per cent confidence level. The evaluation of the confidence level included both the errors of the experiment and the deviations of In& from the approximate dependence. More detailed methods of processing experimental results are given in references 10 and 11. 4. Discussion The compounds in this study show all the structures of secondary alkylphenols; thermodynamic quantities for their transformations give important information about the dependence of the properties of the compounds on the composition of the molecules. However, in the analysis of the experimental equilibrium results, many problems need to be taken into account. It can be stated a priori that the answers to some of them can be found, both at the present time and in the foreseeable future, only on a qualitative level. Nevertheless, this does not diminish the value of the analysis and permits conclusions of a principal nature to be made. The interpretation of the results of the study of chemical equilibria in the liquid state involves certain difficult problems in general. This difficulty is connected with the fact that the study has included associated liquids with a different level of screening of the oxygroup and consequently with a different level of association. The equilibrium results deal with different types of transformations, including isomerization on the aromatic nucleus and aliphatic chain. The analysis of thermodynamic parameters shown in table 2 has required the inclusion of (p,T) information and the enthalpies of evaporation. In view of the limited number of experimental results, we have tried different calculation methods for the compounds of interest. We have used dependable experimental results for phenol,‘15) cresols, xylenols,” ‘* ’ 6, ethylphenols,‘5) ortho-isopropyl- and amyl-phenols,(l 3, and or+-(secondary butyl)

III

= M-IPPH)

= p-IPP’4’ IPP(4*e)

I;:; 2,4-di-rpp

(29)

(28)

= o-IPP

+ WIPP

2,4-di-w = 2,5-di-rr@) 2,5-di-mp = 3,5-di-rpp(4)

p-IPP

;i;

(22) (27)

o-IPP

(20) (22)

(I), (2)AMBc (3), (4), (5) HXB ’ (9), (lo) DECB’ (3) to (lo) HXB, HP& DECB (11) to (15) HP& DECB’ (16) to (18) DECB (19)

(2)

(1)

Reaction

’

488 507 513 459 498 494 507 513 459 459 472 473 469 511 495 459

495 499 372 381 362 370

K

(T) 1.3kl.O 0.7kO.3 0.4 + 0.2 0.6kO.3 0.3 kO.1 0.5kO.2 0.1+0.1 O.lkO.2 4.6 f 0.8 4.6kO.5 2.4kO.4 8.0 f 0.2 3.5 f 1.9 5.3 k 1.8 0.1 kO.9 -0.9kO.4 1.4kO.3 1.3kO.l 9.2 + 0.6 3.2+ 1.9 2.6& 1.1 1.3k2.1 1.75 1.5 0.7+ 1.0

kJ.mol-’

-ArfM<~)) 5.8 + 2.0 6.9 f 0.5 7.6kO.6 3.OkO.7 3.2kO.3 3.OkO.6 1.3 +0.4 -0.2kO.3 -5.9k1.7 -5.5kl.O -2.7kO.9 - 14.6 kO.4 -4.8k3.9 3.7k3.8 9.8 + 1.7 12.5 +0.8 5.9 kO.6 0.8 + 0.2 - 13.5+ 1.4 -4.8+4.0 0.8k2.3 11.2k4.1 13.7k3.1 0.9 f 2.2

4%((T))

J.K-‘.mol-’

48 4(13,f)

81.1 b,d 80.9 bsd 50.9”2’

63.0 b*d 81.4 b,d

57.9 b

kJ.molF’

&a,JG((VLrt

51,8”4.f’

81.4 b,d 81.1 b*d 56.1 b

63.8 b,d 82.2 b*d

58.5 b

kJ ‘mol-’

&,,HXV),,,

-3.4

-0.3 -0.2 -5.2 -5.6

-0.8 -0.8

-0.6

kJ.mol-’

Wv&GY

b Calculated by Lee-Kesler. ’ Joint processing of values for the mentioned transformations of alkylphenols and alkylbenzenes from reference 6. d Values for paw-alkylphenols. e Joint processing of values for the mentioned transformations and isopropylphenols from reference 4. ’ Values for isopropylphenols.

aWLi,,&J= =va,HX~)Lr, -W,,H;(U-)L,.

IIb

IIa

IC

lb

la

X

TABLE 2. Thermodynamic quantities for reactions of transformation type X

1.2 1.0 1.6 1.7 1.3 1.4 1.5 0.8 0.9 0.7 1.5 1.4 1.3 1.1 0.8 2.0 1.5

1.3 1.4 6.0 3.4 3.6 2.9 1.5 2.0 1.3 2.1 5.4 8.0 1.3

1.7 1.6

1.2 1.0 1.6 1.7

1.7 1.6

1.4 1.4

Ki”t at 473 K

2.8 2.7

K

Zl Xl Xl 1.2 1.3 1.0 0.8 1.0 2.2 1.1

1.5 1.5f 1.6b

1.5 1.6

1.05b’d l.Obsd

1.1 b,d 1.1 bed

l.lb

z Ki

ii g

8 ;3

F F

g p

392

T. N. NESTEROVA

ETAL.

and para-(tertiary butyl)phenols.“2) In addition, the (p, T) dependence of p-isopropylphenol was determined experimentally as literature sources do not yield dependable values for compounds with this structure. It has been found that, out of many correlations recommended for normal liquids, some of them can successfully be used for the prediction of the properties of alkylphenols. For example, the correlation of Lee-Kesler: A,,,,H,/R

= AZ(T,/K)(6.09648

- 1.28862T,+ 1.016T7 +~(15.6875-13.4721T,+2.615T,~)).

(1)

makes it possible to evaluate enthalpies of evaporation of alkylphenols having no ortho-alkyl substituent, with an error not exceeding 2 kJ.mol-’ within the range 0.5 < T, < 0.75. The enthalpies of evaporation of alkylphenols can be calculated with the same precision, i.e. those which contain one or two ortho-alkyl substituents in a molecule using the Pitzer correlation: AvapH,/R

= (T,/K)(7.08(1-

T,)“.354+ 10.950(1-

T,)o.456),

(4

with T, 2 0.5. The pressure of vapours of any composition can be predicted with satisfactory precision (at T, > 0.6) on the basis of the Pitzer equation: lnp, = fO(T,)+wf ‘(T,)

(3)

with functions f ’ and f 1 given by Lee-Kesler. As our experiment was made at higher temperatures, the limitations on the above temperature are insignificant. Calculated values of enthalpies of vaporization of reactants and their differences referred to the mean temperature of the equilibrium are given in table 2. Table 2 also shows the relationship of pressures of the vapours at 473 K. In some instances, the calculated results are shown only by the relationship of the pressures of vapours which indicates the absence of valid literature values for the normal boiling temperatures of at least one of the reagents. Naturally the relation (pstart/pend) should be considered only approximate. Table 2 shows also thermodynamic quantities for mutual transformations of isopropylphenols and (secondary-alkyl)benzenes determined earlier in our laboratory. (4*6) Moreover, table 2 shows the results of the joint treatment of results for transformations of one type. The comparison of the enthalpies of reaction with differences of the enthalpies of vaporization of the reagents brings us to the conclusion that for the majority of cases A,H~((T)) w A{A,,,H~((T))), which is probably correct for other temperatures also. Thus, the source of differences in the enthalpies of formation of isomers of (secondary-alkyl)phenols or cycloalkylphenols in the liquid state is intermolecular interactions. This fact helps us to work out an approach to the calculation of enthalpies of formation of compounds of the types discussed here. Firstly, it can be stated for sure that in calculating enthalpies of formation of (secondary-alkyl)- and cycloalkyl-phenols in the gaseous state, correlations paying no attention to intramolecular interactions can be used successfully. This does not contradict the dependence of enthalpies of formation of ethylphenols in the gaseous state on the molecular composition”’ and was supported by the results of calorimetric studies of some alkyl-substituted 1,2-dihydroxybenzenes.‘17’ It was shown that the experimental enthalpies of formation of 3-isopropyl- and

EQUILIBRIA

393

OF ISOMERIZATION

3-methyl-6-isopropyl-12-dihydroxybenzenes and those calculated by the Cox and Pilcher correlation are in good agreement. There is no need to include any additional correlations in order to take into account the interaction of substituents. Thus, enthalpies of formation of (secondary-alkyl)phenols in the gaseous state do not depend on the positions of substituents on the nucleus. The exceptions are probably represented only by compounds with two or more alkyl substituents at the neighbouring carbon atoms of an aromatic nucleus. These compounds were not included by us because of their low thermodynamic stabilities. On the basis of the foregoing discussion, enthalpies of formation of isopropylphenols have been calculated. The calculation used the enthalpies of formation of benzene, isopropylbenzene, and phenol taken from reference 20. The values obtained are shown in table 3, where they are compared with the results of Bertolon et LI~.‘~,~) which have also been included in compilations. (la, lg) The results shown in table 3 support our opinion about the misleading results. (‘3 3, 18, lg) We have performed a detailed analysis of the Bertolon et al. experimental results”-3’ and think that they should be excluded from compilations which are widely used. The calculation of enthalpies of formation of alkylphenols in the liquid state needs the solution of the problem of dependence of enthalpies of evaporation of compounds on their molecular compositions and temperature, up to 298.15 K. This work has been almost finished by us and its results will be reported elsewhere. Another problem that has to be solved in investigating chemical equilibrium is the analysis and evaluation a priori of the change of entropy in the process of reaction, and the definition of its contribution to A,Gk(T) and the equilibrium constant. This problem appears more complicated than the analysis of A,Hi(,(T), especially in cases when the processes involve the participation of condensed phases. The experiment shows that in such cases the best test for a hypothesis is the equilibrium constant. This can be explained by the fact that it is a highly sensitive thermodynamic quantity and besides is found experimentally and does not include approximations. TABLE 3. Standard molar enthaipies of formation of isopropylphenols in the gaseous state Compound 0-IPPO p-IPP m-IPP 2,4-di-rpp 2.5di-rw L(6-di-IPP 3,5-di-1~~

2,4,6-tert-IPP n IPP,

isopropylphenol.

Our calcutation

Literature

A,Hk/(kJ.mol-t) -161.1’2,3’ -182.2f 12.7”8~‘e’ -175.3k2.4 -184.1’2*3’ -209.4+ 12.8”8*‘9’ - 175.3 52.4 - 190.4’233) - 196.Ok 12.8”8,‘9’ -254.lk2.8 - 228.0r2s3, -254.l~k2.8 -240.6’2.3’ -2cj&+2~3’ -254.1 z2.8 -254.1 k2.8 -236.4r2~” -333.Ok3.1 -290.8r2’ -175.3Ir2.4

Calculation minus literature value - 14.2 + 6.9 +8.8 -I-34.1 t 15.1 -I-20.7 -26.1 -13.5 -9.3 - 11.1 -42.2

394

T. N. NESTEROVA

ETAL.

The problems appearing in the analysis of transformations of alkylphenols are stipulated by their specificity, which has been mentioned before in the general form. The solution of this problem is somewhat simplified as it has been established experimentally that intramolecular interactions of substituents in compounds of the type studied are almost negligible. Thus, the values of the substantial equilibrium constants Kint must depend only on the relations of the pressures of reagent vapours and their molar volumes. The latter contribution to the equilibrium constant for the transformations studied is not large and may be roughly approximated as (pstart/pend). Such comparison can be made with the results in table 2. Thus, the problem has been reduced to the calculation of the values of the substantial constants Kinl on the basis of the experimental equilibrium constants. Accounting for the number of optical isomers and the symmetry numbers of the molecules is not difficult. Yet, for correct calculations, values are needed of the conformer composition of compounds and the distribution of oligomeric associated forms of alkylphenols and phenol under the conditions of the investigation. At present, the exact solution of the latter two problems is hardly possible; that is why we have studied the two most probable versions by testing them with the method mentioned above. To do this, we have taken the following basic prerequisites on the basis of the literature values of alkylphenols, and alkyl- and cycloalkyl-benzenes. The oxy-group lies in the plane of an aromatic nucleus (we do not cite the literature sources here as this concept is now generally accepted). Preferential conformations stipulated by the restricted rotation of an alkyl substituent are such that the hydrogen atom at the a-carbon atom of the substituent overshades the benzene ring. (21,22) Compounds not containing orthoalkyl substituents are represented by the mixture of all stable conformers in equal molalities for instance, four conformers in rneta-IPP, two conformers in para-IPP, and four conformers in 3,5-di-IPP. (23*24) For compounds having one or&o-alkyl substituent, the trans-conformation is more stable as stipulated by the mutual position of the oxy-group and the alkyl substituent. The basis for this is the results of spectroscopic investigations of isopropylphenols’25’ as well as the results on the influence of temperature and polarity of a solvent on the position of equilibrium of cis-to-trans-isomerization of ortho-alkyl-phenols.‘26’ Because of the absence of information leading to a clear conclusion about the concentrations of conformers stipulated by the rotation of an ortho-alkyl substituent, we have studied two independent equally probable variants: only eclipsed (e) conformers are present in an equilibrium mixture; eclipsed and staggered (s) conformers possess equal stability. The variants have been studied in a similar way for compounds having two orthoalkyl substituents: the stable conformer is ee, and in an equilibrium mixture four conformers are present (ee, es, se, ss). In accordance with reference 25, the number of conformers due to the rotation of the cis-ortho-alkyl substituent has been taken to be 7. The smaller concentration belongs to the conformation in which the oxy-group is blocked by two alkyl groups (in the case of 2,6-d’-1 IPP or 2,4,6-tri-Ipp, two methyl groups). The order of axes of symmetry has been defined for each stable conformer and introduced into the calculation with the weight stipulated by the conformer concentration.

EQUILIBRIA

OF ISOMERIZATION

395

In accordance with the results of some studies, (26327)it has been accepted that compounds studied by us do not have meaningful steric obstacles for the formation of hydrogen bonds between the components of studied systems, i.e. the oligomeric composition of associated forms has been accepted as the same. Comparison of the results obtained has shown that the calculation agrees best with experiment for a model in which compounds having one or two o&o-alkyl substituents are represented by one e or ee conformer. All other conditions have been kept unchanged. The results of that calculation are shown in table 2. Such an analysis can be made only for reactions in which alkyl substituents do not undergo any structural isomerization, and the aliphatic chain does not have any displacement of oxy-phenyl substituent. Some disagreement in the substantial equilibrium constants Kin, calculated only with account of the quantity of optical isomers and symmetry numbers in the reactions of type 1 is stipulated by different conformer composition of isomers of position in an aliphatic chain. Good agreement of the equilibrium results for the single-type transformations of alkylphenols and alkylbenzenes supports that assumption.@) REFERENCES

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