Kandlikar, (Smt) Sushama

Rao, T. Navaneeth

Sethuram, B.

English

NOPR

Indian Journal of ChemistryVol. ISA, December 1977, pp, 1076-1078Solvent Effects on Rates of Base-catalysed Condensation of0-, m- & p-Hydroxybenzaldehydes with Acetone(Smt) SUSHA11A KANDLIKAR, B. SETHURA:\I & T. NAVANEETH RAODepartment of Chemistry, Osmania University, Hyderabad 500007Received 6 December 1976; revised 11 July 1977; accepted 3 August 1977The effect of changing dielectric constant (D) of the rnedlum on the rates of base-catalysedcondensation of 0-, rn - and p-hydroxybenzaldehydes with acetone has been studied by usingdifferent aquo-alcoholic mixtures of methanol, ethanol and isopropanol, at different tempera-tures. The rates of condensation decrease with decreasing D up to 60% vjv of ethanol and iso-propanol and later remained either constant or slightly increase. In the case of methanol, therates however decreased throughout. Plots of log kW vs IjD are linear with negative slope upto 60% vjv of ethanol and isopropanol and up to 90% vjv of methanol. Thermodynamicparameters and activation energies in isodielectric mixtures were calculated.ITwas reported earlier that the base-catalysedcondensation of aromatic aldehydes withketones takes place via the formation ofintermediate secondary alcohols (ketols) and involvesseveral equi libriav+, Also from substituent effectstudies, it was concluded that the ketone anionreaction with aldehyde molecule is the rate deter-mining step. To confirm this, solvent effect studieshave been carried out on the formation of differentchalkones in various aquo-organic solvent mixtures(aquomethanol, ethanol and isopropanol) in thepresent work.Materials and MethodsOrganic solvents were purified by standardmethods". The dielectric constants of the aquo-alcoholic mixtures were obtained from literature6•The metho.ls employed for following the rates areas described in our previous work",Results and DiscussionIn the condensation 4-hydroxy benzaldehydewith acetone the rate decreased with decreasingdielectric constant (D) in the case of water-methanolmixtures. However, in the cases where water-ethanoland isopropanol mixtures were used the rates de-creased up to 60% (vIv) of alcohol and later remainedeither constant, or increased slightly with increasingpercentage of alcohol, depending on the nature ofaldehyde used. The plots of log k" vs liD wherek" is bimolecular rate constant, were linear witha negative slope up to 60% (v/v) of all alcoholsstudied (Fig. 1). The slopes of these plots decreasedfrom methanol to isopropanol at constant tempera-ture. Above 60% (v/v) of ethanol or isopropanol,the plot was almost parallel to Xvaxis in the caseof 3-0H and 4-0H benzaldehydes, while it waslinear with a positive slope in the case of 2-0H-benz-aldehyde.The negative slope of the plot of log k" vs liDcould be explained on the basis of Amis equation",for an ion-dipole type of reaction. The linear plot1076~ 1';1 <,fl;0.~,,,4"2 "<,, ·0 ~'~ -.O'B0"-..,0061II0-4 rI,'5 2·0',0 2·0te> -~--~------~-------o--I~c'2·5(A.B)- 'g'H) (C)_~DFig. 1 - Plots of log k" versus ljD {Effect of (A) [methylalcohol]; (B) [ethyl alcohol]; and (C) [isopropyl alcohol]}with negative slope indicates that the reaction couldbe between a negative ion and a dipole. This is infavour of the slow step suggested in the mechanismfor the condensation of an aldehyde with a ketoneanion.The significant change in the trend of decreasingrate constants with decreasing D beyond 60% (v/\')of ethyl or isopropyl alcohol could be explained onthe basis of reaction (1) which may be taking placein the reaction mixture.OH-+ROH~H20+0R- ... (1)According to reaction (1) at higher [alcohol] (above60% v!v), the OR- produced, whose basic strength(8)1'1"4KANDLIKAR et al.: BASE-CATALYSED CONDENSATION OF ALDEHYDES WITH ACETONEdiffers from alcohol to alcohol, might be affectingthe rate of condensation. Since methanol and waterhave the same pKa values", OMe- produced maybe expected to nave almost same basic strength inthese two media. Hence, even if OMe- acts as anew catalyst, it is not going to affect the rate of thereaction. So, at higher percentage of methanol, itis probably dielectric constant effect that is pre-dominating and hence the decrease in the rate.However, in ethanol and isopropanol mediumabove 60% (v/v) of alcohol, OEt- or OCH(CHa);formed being stronger bases than either OMe- orOH-, might act as better catalysts than OH- thusincreasing the rate of condensation. In solutionsof ethyl or isopropyl alcohol, therefore, there aretwo opposing trends. On one hand the increasein percentage of alcohol (decrease in D) decreasesthe rate, while OIl the other, the OR- formationat higher percentages of alcohol increases the rate.In the case of 3-0H and 4-0H-benzaldehydes thesetwo effects appear to cancel each other, such thatthe rate, above 60% (v/v) of ethyl or isopropylalcohol, remains practically constant. If OR- effectis higher, it might more than compensate for thedielectric constant effect, resulting in an increasein rate as in the case of the reaction of 2-0H-benz-aldehyde with ketones. The increase in rate wasmuch more prominent when isopropanol was usedprobably because the OCH (CHa);; is better basethan OEt- or O:\le-.The activation enerries for the overall reactionincrease with increase in percentage of alcohol for allalcohols. In the case of ethyl and isopropyl alcoholsthe llE~ values increased up to 60% v [v of alcoholand latter remained constant. The increase inllEt may be attributed to greater solvation of thereactants as compared to the activated complex.The activation energy values along with otherthermodynamic parameters obtained for all solventmixtures are presented in Table 1.The thermodynamic parameters for all the re-actions studied in different solvent mixtures maybe explained on the basis of isokinet ic relationshipbetween llH~ and llSt values obtained for differentaquo-alcoholi- mixtures. For example, for thecondensation reaction of 4-0H-benzaldehyde withacetone the plots of llHt vs AS~ were linear for allthe solvents used (up to 60% of ethyl or isopropylalcohol and up to 90% of methyl alcohol), inaccordance with the isokinet ic equation".AHt = AH~+~llS~ ... (2)The isokinetic temperature (~) values for methanol,ethanol and isopropanol are 350°, 346° and 348°Krespectively and the corresponding experimentalrange of temperature used is 303-318°K. It isevident from these results that the ~-value obtainedis well above the experimental temperature rangeindicating that it is an enthalpy controlled reaction.This view gets further support from the data inTable 1 where activation energy for the fastest re-action is lowest and vice-versa.The experimental activation energies involve ahighly mixed dependence of the rate constant ontemperature and dielectric constant of the medium.TABLE 1 - VALUES OF ACTIVATIONPARAMETERSATDIFFERENT [ALCOHOL]{[Aldehyde] = [acetone] = 0·025M; [OH-] = 0'2M;temp. = 40°}stu /lG~ -/lS~kcal kcal e.u.mole"? mole"!Alcohol%(v/v)/lEtkcalmole-lMeOH20 8·5 7·5 22·3 48·030 10·5 9·3 22·4 42·040 12·1 10·9 22'5 37·350 14'5 13·1 22'6 30'360 16·2 15·0 22·8 25·270 18·3 17·1 23·0 19·080 20·2 19·0 23·2 13·490 22·3 21·1 23·4 7·5EtOH20 11'5 10·3 22·0 37·630 13·2 12·0 22·1 32'340 15'3 14·1 22·2 25·850 17·4 15·2 22·2 22'560 19·1 17·9 22·3 14·070 19·1 17·9 22·3 14·080 19·1 17·9 22·3 14·090 19·1 17·9 22·3 14·0ISOPROPANOL20 14·8 13·6 21-8 26·330 16·5 15·3 21·9 21·040 18·3 17·1 21·9 15·450 20·1 18·9 21·9 9·860 22·0 20'8 22·0 3·970 22·0 20·8 22·0 3·980 22·0 20'8 22·0 3·990 22·0 20·8 22·0 3·9Error limits for /lE~ and /lS~ are ±0'85 kcal mole"!and ±2·5 e.u. respectively.TABLE 2 - Ed VALUES FOR THE REACTIONBETWEEN4-0H-BENZALDEHYDE AND ACETONEIN METHYL, ETHYLAND ISOPROPYLALCOHOLSAlcohol Ed [kcal mol") at different D values(%, v/v)60 4817·718·921·1Methyl alcohol 11'3Ethyl alcohol 13·2Isopropyl alcohol 15·05613·315'317-15215'517·419·2In order to obtain a more precise treatment of theresults, the temperature dependence of the Dshould be eliminated. This was achieved by cal-culating the activation energies for isodielectricmixtures of the various solvents used, from thecorresponding rate constants in isodielectric solutions.The latter was obtained by interpolation, at chosenvalues of D, from plots of log kif against lID. Theactivation energy values, Ed, were then computedfrom the Arrhenius plots. The contribution dueto solvation phenomenon could be considered moreor less constant in these values. The values of Edfor alkaline condensation of 4-0H-bepzaldehydewith acetone below 60% (v/v) of all alcohols are given1077INDIAN J. CHEM., VOL. 15A, DECEMBER 1977in Table 2, from which it is clear that the Ea valuesincrease with decreasing dielectric constant irres-pective of the nature of the alcohol.References1. NOYCE, D. S. & REED, W. L., J. Am. chem. Soc., 81 (1959),624.2. (Mrs) KANDLIKAR, S., SETHURAM,B. & NAVANEETHRAo,T., Z. physik. Chem. (Neue Folge), 95 (1975), 587.10783. STILES, M., WOLF, D. & HUDSON, G. V., J. Am. chem-Soc., 81 (1959), 628.4. (Mrs) KANDLIKAR, S., Ph.D. thesis, Osmania University.1976.5. VOGEL, A. L, Text book of practical organic chemistry(Longmans, London), p. 167.6. AKERLOF,G. & SHORT,O. A., J. Am. chem, Soc., 54 (1932),4125.7. AMIS, E. A., Solvent effects on reaction rates and mechanism(Academic Press, New York), 1966.8. McEwEN, W. K., J. Am. chem, Soc., 58 (1936). 1124.9. LEFFLER, J. E., J. org. Chem., (1955), 1202.

Solvent Effects on Rates of Base-catalysed Condensation of o-, m- & p- Hydroxybenzaldehydes with Acetone

http://nopr.niscair.res.in/bitstream/123456789/53348/1/IJCA%2015A%2812%29%201076-1078.pdf

Solvent Effects on Rates of Base-catalysed Condensation of o-, m- & p- Hydroxybenzaldehydes with Acetone

Abstract

Similar works

Full text

Available Versions

NOPR