solvent effects on dimerization

7/28/2019 Solvent Effects on Dimerization

http://slidepdf.com/reader/full/solvent-effects-on-dimerization 1/6

1070-3632/05/7511-1712 + 2005 Pleiades Publishing, Inc.

Russian Journal of General Chemistry, Vol. 75, No. 11, 2005, pp. 1712 ! 1716. Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 11, 2005, pp. 1794 ! 1798.Original Russian Text Copyright + 2005 by Karpyak, Makitra, Marshalok, Pal’chikova, Yatchishin.

Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í

Solvent Effect on the Rate of Dimerization of CyclopentadieneN. M. Karpyak*, R. G. Makitra**, G. A. Marshalok*,

O. Ya. Pal’chikova**, and I. I. Yatchishin*

* L’vovskaya Politekhnika National University, L’viv, Ukraine** Institute of Geology and Geochemistry of Fossil Fuels, National Academy of Sciences of Ukraine, L’viv, Ukraine

Received August 27, 2004

Abstract - The rate of dimerization of cyclopentadiene and some other cycloaddition reactions is deter-mined by electrophilic solvation of the substrate with some contribution of other solvation factors, primarilynonspecific solvation. The corresponding dependence is described by multiparameter equations.

Spontaneous dimerization of cyclopentadiene slowsdown upon dilution with various solvents; the

maximal deceleration (by a factor of 3.5) was ob-served in strongly polar acetonitrile [1]. However,C 1oster and Pfeil [2] studied the rate of cyclopentadi-ene dimerization in 16 solvents and found no clearrelation betweem the secondorder rate constant k andsuch solvent parameters as dielectric constant e ,dipole moment m , Hildebrand solubility parameter d ,and Reichardt electrophilicity parameter E T [3]. Acertain relation (with a number of exceptions) wasrevealed between log k values and enthalpies of vaporization of the solvents: reduction of the latterparameter by a factor of 2 in going from 1-butanolto diethyl ether leads to decrease in k to a comparable

extent. The authors concluded that, in the general case,solvent effect on a dissolved substance cannot bedescribed using a single parameter owing to specificinteractions.

On the other hand, it is known that interactions of a solute with the solvent can be generalized on thebasis of the linear Gibbs energy relationship principleby summation of effects produced by various solva-tion processes via multiparameter equations [4].Among the latter, one of the most effective is the ex-tended Koppel’ 3 Pal’m equation which takes intoaccount not only specific and nonspecific solvation

but also structural features of the medium [5]. In thepresent work we made an attempt to generalize thedata given in [2] on the rate constants for dimerizationof cyclopentadiene at 40 o C in 0.6 M solutions usingEq. (1):

log k = a0 + a 1 7 7 7 + a2 7 7 7

n23 1

n2 + 2e 3 12 e + 1

+ a3 B + a4 E T + a5 d 2 + a6V M . (1)

Here, n and e are, respectively, the refractive indexand dielectric constant which determine solvent

polarizability and polarity (i.e., factors responsible fornonspecific solvation); B is the basicity according toPal’m [4], and E T is the Reichardt electrophilicityparameter [3] (these quantities characterize solventability for acid 3 base interactions, i.e., specific solva-tion); d is the Hildebrand solubility parameter (itssquared value is proportional to the cohesion energydensity); and V M is the molar volume which takes intoaccount possible effect of structural factors. Thesolvent parameters were taken from [4] and reviews[6, 7], and the calculation procedure conformed to theIUPAC recommendations concerning the applicationof correlation analysis in chemistry [8]. The corres-

ponding data are collected in Table 1.However, processing of the data for 17 solvents

according to Eq. (1) resulted in a too low [8] multiplecorrelation coefficient ( R = 0.925). However, to reachan acceptable correlation quality, it was sufficient toexclude the most deviating data for only one solvent,tert -butyl alcohol. We thus obtained Eq. (2) with afairly high correlation coefficient.

log ( k 0 105) = 3 2.52 + (2.87 + 0.46) f (n 2) 3 (0.67 + 0.29) f (e )

3 (0.05 + 1.22) 0 10 ! 4 B + (2.97 + 0.74) 0 10 ! 2 E T

3 (0.06 + 0.28) 0 10 ! 3d

2 + (4.66 + 1.00) 0 10 ! 3V M

, (2)

R 0.951, S + 0.04.

The very poor pair correlation coefficients ( r i =0.1 3 0.3) between log k and particular terms of Eq. 2do not allow us to estimate their significance, but themean-square deviations of the coefficients at B andd

2 exceed their absolute values, indicating probableinsignificance of these terms. Therefore, in keeping



RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 11 2005

SOLVENT EFFECT ON THE RATE OF DIMERIZATION OF CYCLOPENTADIENE 1713

Table 1. Experimental [2] and calculated [by Eq. (3)] rate constants log k for dimerization of cyclopentadiene (40 o C)Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Â Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Â Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä

Solvent ³ 3 log( k exp 0 105) ³ 3 log( k calc 0 105) ³ D log( k 0 105)Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Å Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Å Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä

1-Butanol ³ 0.2676 ³ 0.2653 ³ 0.0023Chlorobenzene ³ 0.3188 ³ 0.3183 ³ 0.00051-Propanol ³ 0.3279 ³ 0.3594 ³ 3 0.0315Toluene ³ 0.3979 ³ 0.3518 ³ 0.0461

Benzene ³ 0.4089 ³ 0.4208 ³ 3 0.0119Carbon tetrachloride ³ 0.4202 ³ 0.4786 ³ 3 0.0584Hexane ³ 0.4685 ³ 0.4651 ³ 0.0034Chloroform ³ 0.5086 ³ 0.4906 ³ 0.0180Methanol ³ 0.5086 ³ 0.5070 ³ 0.00161,4-Dioxane ³ 0.5376 ³ 0.5075 ³ 0.0301Methylene chloride ³ 0.5850 ³ 0.5952 ³ 3 0.0102Tetrahydrofuran ³ 0.6021 ³ 0.6138 ³ 3 0.0117Diethyl ether ³ 0.6021 ³ 0.6223 ³ 3 0.0203Acetone ³ 0.6778 ³ 0.6359 ³ 0.0418Ethanol a

³ 0.3372 ³ 0.4506 ³ 3 0.1134tert -Butyl alcohol a

³ 0.2924 ³ 0.4395 ³ 3 0.1470Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Á Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Á Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä

a The data were not included in the calculation by Eq. (3).

with the recommendations given in [8], the signifi-cance of particular terms was determined by succes-sively excluding them from Eq. (2) and calculatingeach time multiple correlation coefficients R for theresulting equations containing a lesser number of terms. In order to raise the statistical reliability of these equations, we excluded the data for one moresolvent (ethanol) which showed an appreciable devia-tion. Equation (3) thus obtained was characterized bya higher R value:

log ( k 0 105) = 3 2.49 + (2.90 + 0.34) f (n 2) 3 (0.57 + 0.21) f (e )

3 (0.02 + 0.09) 0 10 ! 3 B + (2.66 + 0.56) 0 10 ! 2 E T

3 (0.003 + 0.21) 0 10 ! 3d

2 + (4.93 + 0.75) 0 10 ! 3V M , (3)

R 0.973, S + 0.03.

After exclusion of low-significant terms d 2 and B,

we arrived at four-parameter Eq. (4).

log ( k 0 105) = 3 2.50 + (2.93 + 0.29) f (n 2) 3 (0.57 + 0.14) f (e )

+ (2.66 + 0.21) 0 10 ! 2 E T + (4.93 + 0.55) 0 10 ! 3V M , (4)

R 0.973, S + 0.03.

However, further elimination of any term stronglyimpairs or even destroys the correlation: Exclusion of f (e ) leads to R = 0.936, and of f (n2) and V M , to 0.73and 0.78, respectively. The parameter E T is clearlydetermining; the multiple correlation coefficient forthe equation containing no E T term is as poor as R =0.56. We can conclude that the rate of dimerization of

cyclopentadiene is determined mainly by two factors,electrophilic solvation of the transition state (whichfavors the reaction, obviously due to some deficit of p -electrons in the ring as a result of charge transfer tothe solvent) and nonspecific solvation of the initialhydrocarbon (which slows down the reaction and isrelated to the solvent polarity). On the other hand, thepolarizability factor favors the process. An analogouseffect is produced by increase of the molar volume of the solvent. This is consistent with the assumption [2]that the reaction rate increases in going to solventswith high boiling points and enthalpies of vaporiza-tion; the latter properties are related to the molecularweight and molar volume of liquids. However, therole of this factor is difficult to rationalize.

The absence of effect of self-association of themedium on the reaction rate should be noted; thisindirectly disproves the concept [2] implying theeffect of [ mutual attraction of molecules ] on log k .Some authors presumed the determining and favorableeffect of d

2 on the cycloaddition. For example, Wongand Eckert [9] drew such a conclusion while consideri-ng the data on dimerization of cyclopentadiene andreaction of isoprene with maleic anhydridom; however,the log k 3 d

2 dependences given therein showednumerous deviations from linearity.

Table 1 contains log k values calculated by Eq. (3)and the corresponding deviations from the experi-mental values D log k = log k calc 3 log k exp . It is seenthat D log k values (except for the value for excluded2-methyl-2-propanol) generally do not exceed the




1714 KARPYAK et al.

Table 2. Experimental [12] and calculated [by Eq. (6)] rate constants log k for cycloaddition of 2,3-dimethyl-1,3-buta-diene and 1,4-naphthoquinone (25 o C)Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Â Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Â Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä

Solvent ³ 3 log( k exp 0 104) ³ 3 log( k calc 0 104) ³ D log( k 0 104)Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Å Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Å Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä

Cyclohexane ³ 3 0.0757 ³ 3 0.1342 ³ 3 0.0585Hexane ³ 3 0.0655 ³ 3 0.0428 ³ 0.0227Ethyl acetate ³ 0.1461 ³ 0.3195 ³ 0.1734

Benzene ³ 0.1875 ³ 0.1343 ³ 3 0.0532Diethyl ether ³ 0.2355 ³ 0.3991 ³ 0.1636Acetone ³ 0.4200 ³ 0.4957 ³ 0.0757Diethylene glycol diemthyl ether ³ 0.5328 ³ 0.5210 ³ 3 0.01181,4-Dioxane ³ 0.6010 ³ 0.6009 ³ 3 0.0001Acetonitrile ³ 0.6170 ³ 0.5824 ³ 3 0.0346Dimethylformamide ³ 0.6493 ³ 0.6365 ³ 3 0.0128Dimethyl sulfoxide ³ 0.9191 ³ 0.8539 ³ 3 0.0651tert -Butyl alcohol ³ 1.0043 ³ 0.7040 ³ 3 0.30032-Propanol ³ 1.0828 ³ 0.9973 ³ 3 0.0854Ethanol ³ 1.0969 ³ 1.1943 ³ 0.0974Methanol ³ 1.1644 ³ 1.2985 ³ 0.1342Acetic acid ³ 1.4150 ³ 1.3699 ³ 3 0.0450Chloroform a ³ 0.6946 ³ 0.1709 ³ 3 0.5237

Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Á Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Á Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä Ä

a The data were not included in the calculation by Eq. (6).

error S = + 0.04 or exceed it only slightly. Pytela [10]has already made an attempt to generalize the data of [2] using a four-parameter Koppel’ 3 Pal’m equation,i.e., Eq. (1) without d

2 and V M terms. However, thecorrelation obtained for 14 solvents was characterizedby unsatisfactorily low coefficient R = 0.724.

Data on the solvent effect on the rate of dimeriza-tion of cyclopentadiene were also given in [1, 9, 11],but the low number of solvents ( n = 6) makes it im-possible to correlate them using a six-parameterequation. Only the data from reference book [11] maybe generalized with some accuracy through a four-parameter equation having no basicity and cohesionenergy density terms. The corresponding values of log k at 25 o C are equal to 3 5.94 (benzene), 3 5.85(carbon tetrachloride), 3 6.20 (carbon disulfide), 3 5.64(ethanol), 3 5.80 (acetic acid), and 3 5.60 (nitroben-zene). The equation is characterized by an R value of 0.999, and exclusion of the E T term almost does notimpair the correlation quality ( R = 0.998). The coef-ficients at f (e ) and V M are positive, while that at E T isnegative, which is consistent with the data of [2].

Shuikin and Naryshkina [1] reported the rate con-stants k 0 104 (l mol ! 1 h ! 1) for dimerization of cyclo-pentadiene at 20 o C in its 1 : 1 mixtures with the follow-ing solvents: acetonitrile (5.9), diethyl ether (8.2),benzene (10.5), carbon tetrachloride (13.2), chloro-benzene (17.3), and dichloroethane (17.6). Dilutionof cyclopentadiene with a solvent reduces the rate of

its dimerization. However, the above values cannotbe used in correlations (2) 3 (4) for the data of [2],presumably due to difference in the solvation effectsat different temperatures. For example, recalculationof the rate constant k in carbon tetrachloride at 20 o C(13.2 + 10 ! 4 l mol ! 1 h ! 1 or 0.0367 + 10 ! 4 l mol ! 1 s ! 1)[1] using the energy of activation E a = 74.79 kJ mol ! 1

to 40 o C gives k = 0.24 0 10 ! 5 l mol ! 1 s ! 1, whereas thevalue given in [2] is higher by a factor of 1.5 ( k =0.38 0 10 ! 4 l mol ! 1 s ! 1).

In order to check the approach in use for the ap-plicability to other Diels 3 Alder reactions, we analyzedthe data given in [12] on the solvent effect on the rateof addition of 2,3-dimethyl-1,3-butadiene to 1,4-naphthoquinone. The reactions were carried out insealed ampules at 80 o C, the quinone concentrationwas 0.0003 3 0.005 M, and 2,3-dimethyl-1,3-butadienewas taken in excess. The second-order rate constantswere determined by spectrophotometry (Table 2).

Corsico Coda et al . [12] observed acceleration of

the process as the solvent polarity rises: by a factor of ~30 in going from alkanes to acetic acid; however,they failed to correlate log k with the Kirkwoodfunction ( e 3 1)/(2 e + 1), Reichardt electrophilicityparameter E T , and other solvent parameters. A certainrelation was observed only with the Gutmann acceptornumbers ( AN ), which was considered to be [ a goodhyperbolic correlation. ] However, a number of pointsappreciably deviated from the general curve. The




SOLVENT EFFECT ON THE RATE OF DIMERIZATION OF CYCLOPENTADIENE 1715

authors presumed a donor 3 acceptor mechanism of thereaction whose rate depends on electrophilic coordina-tion of the quinone to the solvent. In the general case,the process involves shift of excess p -electrons on thedouble bonds in the diene to the electrondeficient di-enophile up to formation of an intermediate charge-transfer complex (in the limiting case). Here, solva-tion of electron-acceptor dienophile molecule by elec-

tron-donor solvent competes with the reaction withdiene; therefore, the effective concentration of dieno-phile is reduced, and the apparent reaction rate de-creases. On the other hand, the hyperbolic characterof the log k ?AN dependence indicates possible effectof other factors.

Generalization of the data in Table 2 using Eq. (1)gives a correlation with a fairly high coefficient ( R= 0.961). To estimate the effect of particular solvationeffects on the reaction rate more reliably, we excludedthe most deviating data for chloroform and obtainedsix-parameter Eq. (5) for 16 solvents:

log ( k 0 104) = 3 3.25 + (2.84 + 1.25) f (n 2) 3 (2.09 + 0.54) f (e )

+ (1.63 + 0.41) 0 10 ! 3 B + (9.48 + 0.92) 0 10 ! 2 E T

3 (0.87 + 0.32) 0 10 ! 3d

2 + (1.29 + 1.65) 0 10 ! 3V M , (5)

R 0.977, S + 0.10.

The signs at the particular terms in Eq. (5) [exceptfor the polarity term f (e )] are the same as in the di-merization of cyclopentadiene, though in the lattercase the same molecule acts as both diene and dieno-phile. Unlike Eq. (2), Eq. (5) clearly shows the de-termining effect of electrophilic solvation: the cor-

responding pair correlation coefficient is fairly highr (0 3 E T ) = 0.859. Taking into account that both AN and E T characterize the electronacceptor power of asolvent, these findings are consistent with the conclu-sions drawn in [12].

Verification of particular terms in Eq. (5) for theirsignificance via successive exclusion (according to[8]) showed that the parameters V M and f (n2) arealmost insignificant; elimination of V M and f (n2) leadsto only a light decrease in R. The rate constant isdescribed with a sufficient accuracy by four-parameterEq. (6):

log (k 0 104) = 3 2.09 3 (2.40 + 0.62) f (e )

+ (1.94 + 0.45) 0 10 ! 3 B + (8.58 + 0.95) 0 10 ! 2 E T

3 (0.73 + 0.33) 0 10 ! 3d

2, (6)

R 0.968, S + 0.116.

As in the dimerization of cyclopentadiene, the rateof addition of 2,3-dimethyl-1,3-butadiene to 1,4-

naphthoquinone is determined mainly (and favored)by electrophilic solvation. The effect of the cohesionenergy density is relatively weak [Eq. (7)]:

log (k 0 104) = 3 1.82 3 (2.38 + 0.71) f (e )

+ (1.77 + 0.51) 0 10 ! 3 B + (7.19 + 0.81) 0 10 ! 2 E T , (7)

R 0.957, S + 0.133.

Thus the important role of solvent coordination tothe quinone and 2,3-dimethyl-1,3-butadiene moleculesin the solvent effect on the reaction rate [12] is con-firmed mathematically.

Table 2 contains log k values for the addition of 2,3-dimethyl-1,3-butadiene to 1,4-naphthoquinone,calculated by Eq. (6), and the corresponding devia-tions D log k from the experimental values. As in thedimerization of cyclopentadiene, most points fit theerror S = + 0.116 or insignificantly exceed its range(ethyl acetate, diethyl ether, tert -butyl alcohol); an

exception is chloroform which was not included intothe correlation.

We can conclude that the factor determining theeffect of the medium on dimerization of cyclopenta-diene and cycloaddition 2,3-dimethyl-1,3-butadieneto 1,4-naphthoquinone is the ability of solvents forelectrophilic solvation; nevertheless, reliable quantita-tive generalization also requires that other solventparameters be taken into account using multiparameterequations. This conclusion is indirectly supported bythe data of Piatek et al. [13] who studied diastereo-selectivity in the addition of cyclopentadiene tofumaric acid derivatives; in this case, the determiningfactor is also electrophilic solvation ( E T term);however, correct description of the process may beachieved only with the use of multiparameterequations.

REFERENCES

1. Shuikin, N.I. and Naryshkina, T.I., Zh. Fiz. Khim.,1957, vol. 31, no. 2, p. 493.

2. C 1oster, G. and Pfeil, E., Chem. Ber., 1968, vol. 101,no. 12, p. 4248.

3. Reichardt, Ch., Solvents and Solvent Effects in Or-ganic Chemistry, Weinheim: Wiley, 2003, pp. 178,284, 294.

4. Koppel, I.A. and Palm, V.A., Advances in Linear Free Energy Relationships, Chapman, N.B. andShorter, J., Eds., London: Plenum, 1972, p. 203.

5. Makitra, R.G. and Pristanskii, R.E., Zh. Obshch. Khim.,2004, vol. 74, no. 12, p. 1948.

6. Makitra, R.G., Pirig, Ya.N., and Kivelyuk, R.B.,




1716 KARPYAK et al.

Available from VINITI, Moscow, 1986, no. 628-V86.

7. Abboud, J.L.M. and Notario, R., Pure Appl. Chem.,1999, vol. 71, no. 4, p. 645.

8. Recommendations for Reporting the Results of Cor-relation Analysis in Chemistry using Regression Analysis, Quant. Struct. Act. Relat., 1985, vol. 4,no. 1, p. 29.

9. Wong, K.F. and Eckert, C.A., Ind. Eng. Chem.Process Des. Devel., 1963, vol. 8, no. 4, p. 568.

10. Pytela, O., Collect. Czech. Chem. Commun., 1988,vol. 53, no. 7, p. 1333.

11. Denisov, E.T., Konstanty skorosti gomoliticheskikh zhidkofaznykh reaktsii (Rate Constants of HomolyticLiquid-Phase Reactions), Moscow: Nauka, 1971.

12. Corsico Coda, A., Desimoni, G., Ferrari, E., Righet-ti, P.P., and Tacconi, G., Tetrahedron, 1984, vol. 40,no. 9, p. 1611.

13. Piatek, A., Chapuis, Ch., and Jurczak, J., J. Phys. Org.Chem., 2003, vol. 16, no. 10, p. 700.

solvent effects on dimerization

Documents