Indian Journal of Chemistry Vol. 44B, June 2005, pp. 1283-1287

Hydroxylic solvent effects on the reaction rates of diazodiphenylmethane with 2-substituted cyclohex-1-enylcarboxylic and 2-substituted benzoic acids: Part II

Gordana S Ušćumlić*, Jasmina B Nikolić* & Vera V Krstić* Department of Organic Chemistry, Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, P.O. Box. 3503, YU-11000 Belgrade, Serbia & Montenegro E-mail: [email protected] Received 20 April 2004; accepted (revised) 12 October 2004

The rate constants for the reaction of diazodiphenylmethane with 2-substituted cyclohex-1-enylcarboxylic acids, determined in butan-1-ol, 2-methylpropan-1-ol and ethylene glycol, together with the rate constants determined previously in methanol, ethanol, propan-1-ol and propan-2-ol, are correlated using the total solvatochromic equation, of the form log k ∗ = A ο + sπ + aα + bβ, the two parameter model log k = Ao+ sπ* + aα and the single parameter model log k = ∗ Ao + bβ, where π , β and α represent the solvent dipolarity/polarizability, solvent hydrogen bond acceptor basicity and hydrogen bond donor acidity, respectively. The correlation of the kinetic data are carried out by means of multiple linear regression analysis and solvent effects on the reaction rates have been analysed in terms of initial state and transition state contributions. The results obtained for 2-substituted cyclohex-1-enylcarboxylic acids are compared with the results for 2-subsituted benzoic acids under the same experimental conditions.

IPC: Int.Cl.7 C 07 C 63/00

Earlier work1-4 showed that in the reaction between γ-hydrogen atoms in the ). The favourable carboxylic acids and diazodiphenylmethane (ddm) in influence of the dielectric constant was expected for the , the rate-determining step involves a proton reaction in which the route from the initial to the transfer from the to ddm to form a transition state involves charge separation, but the diphenylmethanediazonium-carboxylate ion pair. stepwise regression showed that the σ∗ term is more Subsequent fast product-governing stages have been important. This indicated the dominant role of the variously formulated5-7. The diphenylmethanediazo- solvating properties of the alcohol, i. e. Lewis bacisity nium-carboxylate ion pair seems most likely to or acidity of the solvent. collapse to give an or react with the solvent to In a recent paper9, the hydroxylic solvent effects are give an . examined on the reaction of cycloalkylcarboxylic and 1,8 In our previous work , the reactivity of 2- cycloalkenylcarboxylic acids with ddm by Linear substituted cyclohex-1-enylcarboxylic acids with DDM Solvation Energy Realtionships (lser) method. The in various alcohols was examined and the kinetic data correlation equations obtained by stepwise regression for these acids were correlated using the simple and for all the examined acids showed that the best extended Hammet equation. The results have shown approach which helps to understand hydroxylic solvent that the linear free energy realtionships are applicable effects in the reaction lies in the separate correlations of to kinetic data for the 2-substituted cyclohex-1- the kinetic data with the hydrogen bond donating (hbd) enylcarboxylic acid system. Comparisons were made and hydrogen bond accepting (hba) ability of a solvent. with corresponding 2-subsitututed benzoic acid system. For the first time, hydroxylic solvent effects on the The solvent effects on the reactivity of cyclohex-1- reaction rates of carboxylic acids and ddm have been enylcarboxylic acids are proportional to their influence analysed in terms of the initial state and the transition on benzoic acids. It was also concluded that the solvent state contributions. effect is best described through multiple regression of In the present work, we have extended our study9,10 log k vs f (ε) [the Kirkwood function of dielectric of the hydroxylic solvent effects on the reaction of constant (ε−1)/(2ε+1)], σ∗ (the Taft polar constant for carboxylic acids with ddm by means of the lser 11 the alkyl group of the alcohol) and nγH (the number of concept developed by Kamlet et al . 1284 INDIAN J. CHEM., SEC B, JUNE 2005

The second order rate constants for the reaction of The correlation analysis of the log k for all the 2-substituted cyclohex-1-enylcarboxylic acids1,8 examined acids with the solvent parametres π*, α and 12 (system 1) and 2-substituted benzoic acids (system β in protic solvents, showed that there were no 2) were correlated using total solvatochromic satisfactory results. For each examined acid the same equation of the form: problem occurred ⎯ coefficient related to hba log k = A + sπ* + aα + bβ …(1) parameter (b) had a standard error overcoming its o value and making the equation unreliable. As the where π*, α and β are solvatochromic coefficients solvent effect on the examined reaction could not be and Ao is the regression values of the solute property clearly presented when all the solvent properties are in the reference solvent, cyclohexane. taken together, the attempt was made to separate them According to strucutral analogy between the into those that stabilize the transition state and the systems 1 and 2 it seemed of interest to compare ground state. Taking into consideration the reaction obtained results for these acids with identical mechanism (Figure 1) it can be noticed that because substituents. of the charge separation in the transition state solvents of high polarity can stabilize it, making the reaction COOH COOH faster. The similar effect can be produced by the electrophylic ability of the solvent, affecting the carboxylic anion which exists in the transition state. X X On the contrary, the nucleophylic solvating ability can 1 2 be prominent in the ground state, stabilizing the X = H, CH , C H , Cl, Br, I carboxylic proton and slowing down the reaction. 3 2 5 Further examination, using two- and one- Results and Discussion parameter equations with parameters π*, α (effects supporting the transition state) and β (dominating in Values of second-order rate constants for the the ground state) showed more convincing results reaction of 2-substituted cyclohex-1-enylcarboxylic (Tables II and III), using the following forms: acids with ddm, determined in this work at 30oC in butan-1-ol, 2-methyl-propan-2-ol and ethylene glycol, log k = A o + sπ* + aα ...(2) are given in Table I. The reaction rate constants log k = A + bβ ...(3) presented in Table I together with the rate constants o determined previously12 for the reaction of 2-subsituted From the results presented in Tables II and III it cyclohex-1-enylcarboxylic acids1,8 and 2-subsituted can be concluded that the protic solvents influence the benzoic acids12 with ddm in various alcohols were carboxylic acid – ddm reaction by two opposite correlated with the solvent properties, using the total effects. The opposite signs of the electrophylic and solvatochromic equation (1). The solvent parameters nucleophylic parameters are in accordance with the are available for only seven solvents for which they are described mechanism (Figure 1). The positive signs determined by Kamlet et al.13 of the s and a parameters prove that the classical solvation and hbd effects dominate the transition Table I ⎯ Rate constants (dm3mol-1min-1) for the reaction of state and increase the reaction rate, and the negative 2-substituted cyclohex-1-enylcarboxylic acids with o sign of the b parameter points out that hba effects (β) diazodiphenylmethane at 30 C in various alcohols stabilise the initial state before the reaction starts and Substituent Butan- 2-Methyl- Ethylene are responsible for a decrease of the reaction rate. It 1-ol propan-2-ol glycol can be noticed that the equation including the proton- acceptor (β) parameter has lower values of correlation Ha 0.480 0.220 1.962 coefficient (r), from which can be concluded that it is CH 0.238 0.042 1.631 3 a less realiable of the two possible models. C2H5 0.308 0.125 1.583 From Table II it could be seen that the classical Cl 1.307 0.597 5.440 solvation and proton-donor solvent effects are more Br 1.438 0.726 4.984 pronounced for the 2-substituted benzoic acids, than I 1.650 0.853 5.480 for 2-substituted cyclohex-1-enylcarboxylic acids. aRef. 9 The explanation of the fact probably lies in the

UŠĆUMLIĆ et al.: HYDROXYLIC SOLVENT EFFECTS ON THE REACTION OF CARBOXYLIC ACIDS WITH DDM 1285

Table II ⎯ Results of correlation of log k for 2-substituted cyclohex-1-enylcarboxylic1,8 and 2-substituted benzoic acids12 with equation (2)

a a b c Acid Ao s a Rsd Cyclohex-1-enylcarboxylic -1.92 1.05±0.23 1.30±0.52 0.977 0.08

2-CH3- Cyclohex-1-enylcarboxylic -4.33 1.18±0.43 3.83±0.98 0.971 0.15

2-C2H5- Cyclohex-1-enylcarboxylic -2.39 1.44±0.30 1.41±0.60 0.980 0.08 2-Cl- Cyclohex-1-enylcarboxylic -1.48 1.28±0.29 1.15±0.57 0.976 0.08 2-Br- Cyclohex-1-enylcarboxylic -1.25 1.11±0.25 1.03±0.48 0.976 0.07 2-I- Cyclohex-1-enylcarboxylic -1.14 1.07±0.23 0.98±0.47 0.937 0.09

Benzoic -2.87 0.83±0.36 3.02±0.73 0.975 0.10

2-CH3- Benzoic -4.25 1.57±0.67 4.09±1.32 0.966 0.18

2-C2H5- Benzoic -2.59 1.66±0.33 2.21±0.65 0.985 0.09 2-Cl- Benzoic -1.85 1.93±0.38 1.96±0.76 0.982 0.11 2-Br- Benzoic -1.53 1.48±0.32 1.96±0.64 0.982 0.09 2-I- Benzoic -1.83 1.67±0.31 2.21±0.61 0.987 0.09 acalculated solvatochromic coefficients, bcorrelation coefficient, cstandard deviation of the estimate

Table III – Results of correlation of log k for 2-substituted cyclohex-1-enylcarboxylic1,8 and 2-substituted benzoic acids12 with equation (3)

a b c Acid Ao b R sd Cyclohex-1-enylcarboxylic 1.00 -1.59±0.26 0.937 0.11

2-CH3- Cyclohex-1-enylcarboxylic 1.65 -2.75±0.37 0.958 0.16

2-C2H5- Cyclohex-1-enylcarboxylic 1.04 -1.86±0.29 0.944 0.12 2-Cl- Cyclohex-1-enylcarboxylic 1.45 -1.61±0.27 0.936 0.11 2-Br- Cyclohex-1-enylcarboxylic 1.32 -1.40±0.23 0.938 0.10 2-I- Cyclohex-1-enylcarboxylic 1.34 -1.36±0.22 0.937 0.09

Benzoic 1.69 -2.07±0.29 0.954 0.12

2-CH3- Benzoic 2.51 -3.21±0.46 0.952 0.20

2-C2H5- Benzoic 2.06 -2.42±0.31 0.960 0.14 2-Cl- Benzoic 2.84 -2.53±0.38 0.948 0.16 2-Br- Benzoic 2.62 -2.16±0.29 0.957 0.13 2-I- Benzoic 2.83 -2.43±0.31 0.962 0.13 acalculated solvatochromic coefficient, bcorrelation coefficient, cstandard deviation of the estimate

Ph Ph R Ph R R δ+ δ- δ+ + C + C H +O C N2 HOCO N2 HOC N2 C PRODUCTS δ- - Ph Ph O Ph O

⇑ ⇑ nucleophylic electrophylic solvation solvation Figure 1 ⎯ The solvent effects on the mechanism of the reaction of carboxylic acids with DDM

1286 INDIAN J. CHEM., SEC B, JUNE 2005

planarity of the ring of the benzoic acid system, which carboxylic acids with diazodiphenylmethane5-7. The makes it easier for the solvent molecules to approach classical solvation and proton-donor (hbd) effects of and arrange themselves around the ionic par in the protic solvents dominate the transition state and transition state (Figure 1). The same results were increase the reaction rate, by solvating the intimate obtained for proton-acceptor solvent effects (Table ion-pair created during the rate-determining step, III). The data from Tables II and III confirm that the (shown in Figure 1). The more pronounced these interactions with the solvent are stronger with 2- accelerating solvent effects are, the reaction is methylcyclohex-1-enylcarboxylic acid than with any faster14. other 2-substituted cyclohex-1-enylcarboxylic acid. A On the basis of all the information presented, it similar influence the hydroxylic solvents have on the may be concluded that the solvatochromic treatment 2-methylbenzoic acid compared to other 2-substituted of Kamlet and Taft is applicable to kinetic data for the benzoic acids. This is in accordance with the fact that reaction of 2-substituted cyclohex-1-enylcarboxylic the methyl group on the sp2 hybridized carbon atom of and 2-substitued benzoic acids with ddm in various the ring, by the effect of hyperconjugation, extends alcohols. The satisfactory results of the correlations of conjugation between the double bond and the the kinetic data by equation (2) indicate that the carboxylic group. The degree of success of equation selected model was correct. This means that this (2) is shown in the Figure 2 by means of a plot of model gives a detailed interpretation of the solvating log k calculated versus log k observed for ddm effect of the carboxylic group in different hydroxylic reacting with 2-substituted cyclohex-1-enylcarboxylic solvents. For these reasons we consider that the acids and 2-substituted benzoic acids at 30oC. results presented in this work may be used to

As the regression analysis of log k on π*and α quantitatively estimate and separate the overall solvent parameters gave the best agreement between solvent effects into initial-state and transition-state experimental and calculated data for all acids included contributions in the reaction of diazodiphenylmethane in this research (Table II, Figure 2), it was concluded with carboxylic acids. that the effects described by it most clearly represent Experimental Section the hydroxylic solvent influence on this reaction. Using this correlation, the effects of the hydroxylic Materials. 2-subsituted cyclohex-1-enylcarboxylic solvents can be clearly shown by separating them into acids were prepared by the method of Wheeler and those influencing the ground and the transition state. Lerner15 from corresponding 2-subsituted cyclo- The results presented in this paper are in agreement hexanone as reported previously1. Diazodiphenyl- with the proposed mechanism of the reaction of methane was prepared by Smith and Howard's

2.0

1.5

1.0

0.5

exp 0.0

logk logk = 0.001 + 1.000logk exp π∗α -0.5 r = 0.992, s=0.081, n=84

-1.0

-1.5

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 logk π∗α

Figure 2 ⎯ The plot of log k observed against log k calculated from the equation (2) for diazodiphenylmethane reacting with 2-substituted cyclohex-1-enylcarboxylic and 2-substituted benzoic acids at 30oC. UŠĆUMLIĆ et al.: HYDROXYLIC SOLVENT EFFECTS ON THE REACTION OF CARBOXYLIC ACIDS WITH DDM 1287

method16 and the stock solutions were stored in a References refrigerator and dilluted for use. Solvents were 1,2 1 Ušćumlić G S, Krstić V V & Muškatirović M D, J Chem Soc purified as described in previous papers . All the Perkin Trans 2, 1993, 999. solvents used in kinetic studies were examined by 2 Ušćumlić G S, Krstić V V & Muškatirović M D, J Chem Soc GLC and no impurities were detected. Perkin Trans 2, 1994, 1799. 3 Bukley A, Chapman N B, Dack M R J, Shorter J & Wall H M, Kinetic measurements. Rate constants k2 for the J Chem Soc B, 1968, 631. reaction of examined acids with ddm were determined 4 More R A, O'Ferral R M, Kwok W K & Miller S I, J Am as reported previously by the spectroscopic method of Chem Soc, 86, 1964, 5553. Roberts and his co-workers17 using a Shimatzu 160A 5 Bowden K, Buckley A, Chapman N B & Shorter J, J Chem Soc, 1964, 3380. spectrophotometer. Optical density measurements 6 Diaz A F & Winstein S, J Am Chem Soc, 88, 1966, 1318. were performed at 525 nm with 1 cm cells at 30 ± 7 Bethell D & Howard R D, Chem Commun, 1966, 49. 0.05 oC. 8 Ušćumlić G S, Krstić V V & Muškatirović M D, J Serb Chem Soc, 58, 1993, 881. The second order rate constants for all examined 9 Nikolić J, Ušćumlić G & Krstić V, Indian J Chem, 43B, 2004, compounds were obtained dividing the pseudo-first 1995. order rate constants by the acid concentration (the 10 Ušćumlić G, Nikolić J & Krstić V, J Serb Chem Soc, 67, concentration of acid was 0.06 mol dm-3 and of ddm 2002, 77. -3 11 Kamlet M, Abboud J & Taft R W, Progress in Physical 0.006 mol dm ). Three to five rate determinations Organiic Chemistry, Vol 13, 1981, 485. were made on each acid and in every case the 12 Aslan M H, Burden A G, Chapman N B, Shorter J & Charton individual second-order rate constants agreed within M, J Chem Soc Perkin Trans 2, 1981, 500. 3% of the mean. 13 Kamlet M, Abboud J, Abraham M H & Taft R W, J Org Chem, 48, 1983, 2877. 14 Reinchart C, Solvents and Solvent Effects in Organic Acknowledgement Chemistry, 3rd edition, Wiley – VCH, 2003, 455. The authors acknowledge the financial support of 15 Wheeler O H & Lerner I, J Am Chem Soc, 78, 1956, 63. 16 Smith L I & Howard K L, Org Synth Colln, Vol III, 1955, 351. the Serbian Ministry of Science and Technologies. 17 Roberts J D, McElhill E A & Armstrong R, J Am Chem Soc, (Project 1694). 71, 1949, 2923.