Hawkins, James Alexander

English

Edinburgh Research Archive

THE INFLUENCE OF SOLVENTS ON THE VELOCITYOF FORMATION OF QUATERNARY AMMONIUM SALTS.*>yJAMES A. HAWKINS, B.A. (Amherst)Thesis for the Degree of Ph.D. • ^^THE INFLUEHOE OF SOLVENTS ON THE VELOCITYOF FORMATION OF QUATERNARY AMMONIUM SALTS.N. Menschutkin (Zeit. phy3ikal Chem., 1890, 5,589) and in several later papers shows that theinfluence exercised by the solvent on the velocity ofcombination of tertiary bases and alkyl halides isvery great.The experiments which follow were made in orderto test the validity of certain suggestions thathave been advanced to explain the nature of the actionof the solvent.Patterson and Montgomerie (T., 1912, 101, 26)show that change of solvent frequently affectsdifferent reactions in the same order. From this,Cox (T., 1921, 119, 142) assumes that reactions takeplace by the formation of intermediate compounds orsolvent-solute complexes, the velocity of reactionbeing dependent on the constitution of the complex,is also dependent on the constitution of the solventand solute.To test this assumption, freezing point curvesof mixture of solvent and tertiary base and ofsolvent and alkyl hallde were determined. Maas andMacintosh/2.Macintosh (J.A.C.S., 34, 1278) and J. Kendall(J.A.C.S., 36, 1222) used this method for the examina¬tion of organic addition compounds. The molecularconcentration is plotted against the temperature ason the addition of one pure substance to another puresubstance, the melting points of both are lowered andtwo curves are obtained intersecting at their eutecticpoint. If a compound is formed it being a puresubstance also, the further addition of either of thetwo former substances will lower its melting pointand more than one eutectic point will be obtained.Marcelin and Rice treating the problem of re¬action velocity from the point of view of statisticalmechanics correlate the velocity constant with thetemperature. Rice (Brit. Ass. Rep., 1915, 397) showsthat the relation between the velocity constant kand the absolute temperature is —— - = ——— .dt RTXThis equation of Marcelin and Rice is identical withArrhenius1 equation log kt, = A (1 - 1 ) whichT( T^agrees well with experimental data.Lewis and Lamble (T, 1914, 105, 2330) deducefrom Arrhenius' equation that "when a positive catalysjtis present in large quantity a diminution of thetemperature-coefficient should take place "and byapplying/3.applying the quantum theory to the Marcelin-Riceequation Lewis (T., 1916, 109, 796) draws the conclus¬ion that "any agency which increases the reactionvelocity diminishes the temperature-coefficient ofthe constant."Lewis assumes that the critical energy increment,E, the quantity of energy necessary to make an averagegram molecule react, is supplied by the absorption ofinfra red radiation of a given frequency which thesubstance is capable of absorbing. The radiation ispresent in the system by virtue of its temperature.Temperature-coefficients and the value of E forthe velocity of combination of pyridine and allylbromide# were obtained in order to investigate whetheror not the influence exercised by the solvent is dueto infra red radiation.EXPERIMENTAL.FREEZING POINT CURVES.Freezing point curves were used to determinewhether or not addition compounds were formed.Temperatures from 10° to -15° were obtained byusing ice and salt, -15° to -70° by solid carbondioxide and acetone, temperatures lower than -70°0.fey/4.by the use of liquid air. The apparatus used re¬sembled the usual Beckman apparatus. The outer tube*was kept at a temperature slightly lower than thefreezing point of the mixture in the inner-tube, whichwas constantly stirred. The inner tube with themixture to be examined was contained in a slightlylarger, carefully dried glass tube. This was in¬serted in the bath of solid carbon dioxide and acetoneor liquid air which was kept in an unsilvered Dewarflask. When the mixture had solidified the innertube was transferred to a dry outer tube and placed inan empty Dewar flask to warm up slowly so thatfrostiness was avoided. The temperature was recordedwhen the first crystals appeared on lowering thetemperature and again when the last crystals dis¬appeared on raising the temperature.mol. ) pyridinecono. ) acetophenonemelting point1.0.- 42°.9.1. o- 49.8.20- 36oistoto••031mol. ) pyridinecono.) benzenemelting point1.0.- 42°.9.1- 49°.8.2- 56°.7.3- 46°mol. ) allyl bromide)conc.) acetophenonemeltingvpoint1.0.-106°.975.025- 109°.9'.1- 94°.8.2- 66°5.6 .5 .4 .3 .2 .1 0..4o12.50- 3.6o+ 3.7o4- 8• 8+ 12°.9+ 16°1.+ 20°.6 .5 .4 .3 .2 .1 0..436°.5- 28°.6- 18°.7-12°• 8- 5°.94 1°1.4 4°.7 .6 .5 .4 .3 .2 .1.325°.4- 12°.50- 3.6+ 3°.74 8°.8+ 12°.9+ 16°mol. ) allyl bromideconc.) benzenemelting pointmol. ) piperidinecono.) acetophenonemelting point6.7• 3noc.6.4- 92.5.5.4.6- 37 - 17.3 .2.8.1.901.- 12° - 4° 4 2° + 4°.7.3- 18C.6.4- 8C.5.5- 3°.4.64 1°.3 .2 .1 0.7 .8 .9 1.44.5° 411° +15° 4 20° 7.When these numbers are plotted curves are obtain¬ed which in all cases have only one eutectic point.(Pig. I.) Consequently no addition compounds weredetected either in the case of mixtures of the tertiarjybase and the solvent or in mixtures of the alkylhalide and solvent. The two solvents chosen were asdiverse as possible, the reaction proceeding veryrapidly in acetophenone and very slowly in benzene.The freezing point curve of piperidine and aoeto-phenone was also determined as although piperidine isa secondary base its reaction with allyl bromideproceeds much more rapidly than the reaction ofpyridine and allyl bromide.Prom this, it would appear that the reaction doesnot take place by the formation of intermediate com¬pounds or solvent-solute complexes, consequently thevelocity of reaction cannot be determined by thesesolvent-solute complexes.8REACTION VELOCITIES.Solutions of pyridine and allyl bromide indifferent solvents were prepared and sealed in glasstubes, the resulting solution being .37 N with re¬spect to both reactants. The quaternary ammoniumsalt precipitated out as a heavy red oil in allsolvents except alcohol. The weight of the mixturein each tube was determined and the tubes immersed inoa thermostat, temperatures used being 28.3 C.,38.3°C., and 56.5°C. A tube was withdrawn at inter¬vals and broken into a white poroelain dish containingcold distilled water and was titrated with silvernitrate, using potassium chromate as an indioator.Time was taken in minutes and reckoned from the with¬drawal of the first tube from the thermostat, a fewminutes after immersion. The velocity of reactionwas calculated by the formulaAll the solvents and reactants were carefullypurified and dried as efficiently as possible.The alcohol was placed over calcium oxide and allowedto stand, finally refluxed over metallic calcium anddistilled. Alcohol 99.8^ absolute was obtained.9.Solvent toluene Temperature 28.5°C.time C£ % k1146 9.16 .0002551185 9.35 .0002351273 10.30 .000243benzenetime 0^ ^ k1115 11.20 .0003051183 12.12 .0003121589 16.27 .000316alcoholtime 0.j. % k65 6.74 .0029995 8.55 .00254193 15.81 .00253acetonetime C.J. % k121 19.36 .00503161 23.78 .00502197 27.68 .00508p nitrotoluenetime G^, fo k172 30.46 .00659192 32.61 .00653245 38.37 .0065810acetophenonetime Ct fo k178 41.05 .01015227 46.00 .01014253 50.90 .01061nitrobenzenetime ct ^ ^107 34.41 .0127140 41.48 .0131202 46.84 .0114toluene temperature 38.5°0.time Ct fo k243 4.22 .000489270 4.70 .000475280 5.11 .000518benzenetime Ct i° k188 4.05 .000640207 4.92 .000647275 6.63 .000655285 6.74 .000656353 8.13 .000650alcoholtime k120 21.17 .00557210 30.59 .00551245 35.09 .0056811acetonetime C.J. % k44 14.04 .0095954 16.75 .0096665 16.89 .00964180 39.82 .00950p. nitrotoluene Temperature 37°C.time O k51 15.70 .00944109 33.12 .01150127 36.73 .01185acetophenone Temperature 3S.3°0.time °t * k152 55.19 .0209190 59.46 .0200202 60.85 .0199nitrobenzene Temperature 37°0.time k53 31.90 .022971 39.38 .023684 43.42 .0237*Temperature 56.5°0.toluene reactants .79 N.time Gt . k time Gt 1" k87 5.88 .00186 162 19.28 .00186117 8.12 .00195 190 22.59 .00193■331 77.78 .0017712benzene reactants .63 N.time Ct i k time Ct f k74 7.19 .00240 31 4.28 .00229109 8.12 .00220 59 7.73 .00225109 13.53 .00227alcohol acetone reactants .79 IT.time °t 1' k time °t i' k25 16.32 .0201 30 43.49 .032855 31.54 .0202 61 60.35 .035866 33.75 .0208 95 71.65 .0337p. nitrotoluene acetophenone reactants .56 Ntime Ct * k time Ct f k15 17.85 .03759 11 23.95 .051936 34.41 .03769 17 32.81 .051428 45.27 .0532acetophenonetime °t k20 28.87 .0526solvent nitrobenzenetime °t * k86 73.71 .084596 75.84 .0847125 80.49 .0854These results are tabulated below.Solvent/13.kSolvent. temperature28.3°G. 38.3°C. 56.5°Ctoluene .000231 .000517 .00186benzene .000305 .000642 .00223alcohol .00251 ' .00557 .0202acetone .00605 .0102 .0338p. nitrotoluene .00657 .0126 .0376acetophenone .0104 .0203 .0526nitrobenzene .0129 .0251 .0848There is a very marked difference in thevelocity of reaction in these solvents, being overforty times faster in nitrobenzene than in toluene.MIXED SOLVENTS.Allyl bromide and pyridine were added in thesame concentration to mixtures of two solvents andthe reaction velocities determined.Molecular concentration. temperature 56.5°G.Solvents acetone toluene.0338reactants .79 Ntime ot i k.27 32.50 .022643 49.56 .0289159 75.09 .023914.Molecular concentration.Solventsacetone toluenetime * k0.75 0.25 48 40.57 .0180.112 60.87 .0176152 67.16 .0171time Ct 1° k0.59 0.41 152 60.80 .0129190 66.00 .0129223 08.59 .0129time °t * k0.4 0.0 80 37.31 .00936100 41.84 .00918time °t % k0.23 0.77 30 13.01 .0070964 23.06 .006670 1. .00193>1ventsacetophenone benzene reactants .63 N. k1 0 .0526time 0t 1° k0.68 0.32 10 17.38 .036015 25.43 .033337 44.09 .0338time Ct k0.43 0.57 29 26.26 .0195639 33.22 .020273 48.69 .0206 15.Molecular concentration.Solventsacetophenone benzenetime ct 1' k0.19 0.81 10 5.4-3 .0091129 14.31 .0091476 27.89 .00806k0 1. .00227When the molecular concentration of the twosolvents is plotted against the reaction velocity(Fig. II) it is seen that the curve does not falloff in a straight line from the highest to thelowest velocity, but falls slowly until a concentra¬tion is reached where the solvent in which thereaction takes place more slowly predominates to avery great extent and then drops rapidly.From this, it appears that the solvent in whichthe reaction proceeds more rapidly has a greatereffect on the reaction velocity than correspondsto its concentration.16./TEMPERATURE-COEFFICIENTS.The usual form of expressing temperature-k+ + 10coefficients is by the ratio —£ . The values*tof this ratio over the range 28.3°C. to 56.5°C. aregiven in the following table.Solvent k 38.3° k 56.5° k 56.5° k+.+lO for k 56.5°k 28.3° k 38.30 k 28.3° kt k 28.3°8.05 1.977.31 1.908.04 1.986.70 1.82p. nitro-toluene 1.92 2.98 5.72 1.63aceto-phenone 1.95 2.59 5.06 1.42nitro¬benzene 1.94 3.37 6.57 1.85From these results it is seen that the valueper degree of temperature for the ratio isslightly higher than for the ratio 56.5° -which agrees38.5°with Lewis* theory that "any agency which increasesthe reaction velocity diminishes the temperature-coefficient of the constant."The results are shown to agree with the inte¬grated formula of Arrhenius or of the radiationtheory by the graph (Fig. Ill) in which the logarithmof/toluene 2.25 3.59benzene 2.11 3.47alcohol 2.21 3.62acetone 2.02 3.31ni£roacs,ben*>e^e,ber\one,top>,tro,U,laer\£.^.aceWettyA a'olcoVio',ben*to'A»^eneer\e-17of the velocity constant is plotted against thereciprocal of the absolute temperature within thelimit of experimental error. The slope of the lineIndicates the temperature-coefficient.The effect of the temperature on the velocity isbetter expressed by the term E, denoting the differencebetween the critical energy and the mean internalenergy reckoned per gram molecule, assuming that amolecule reacts when its internal energy has beenraised to a certain critical value, which is generallylarge compared to the average internal energy of themolecule. E represents the amount of energy peraverage gram molecule necessary in order that thereaction may proceed.log - log kt| = | (I - IjR = 1.985 calories.The following are the mean values of E for thisreaction.-Solvent E k 28.3°0toluene 15,100 .000231benzene 14,400 .000305alcohol 15,100 .00251acetone 13,600 .00505p. nitrotoluene 12,600 .00657acetophenone 12,100 .0104nitrobenzene 13,300 .012918.E in these results is in the inverse order tothe velocity of the reaction except in the case ofalcohol and nitrobenzene. All the other results arein agreement with the conclusion of Lewis and Lamble,"that any agency which increases the velocity of thereaction diminishes the temperature-coefficient of thevelocity constant". The temperature-coefficient isnot inversely proportional to the reaction velocity indifferent solvents, so other influences besides thatof radiation alone must control the velocity of the+•»reaction.Attempts were made to find a connection betweenreaction velocity and the viscosity of the medium,but these were unsuccessful as in this reaction, thereaction velocities are very different in media ofnearly the same viscosity. There is a rough agree¬ment between the dielectric constants and the reactionvelocities, the reaction velocities generally in¬creasing in the same order as the dielectric constantsof the solvents.Solvent k 28.3^0 viscosity 26 0 dielectric constanttoluene .000231 .0053 2.31benzene .000305 .0060 - 2.26alcohol .00251 .0109 25.8acetone .00505 .003 21.5aceto-phenone .0104 .019 18.1nitro¬benzene .0129 .018 36.519.The values of the viscosity and dielectric con¬stants of the solvents were taken from the LandoltBornstein Tables except in the case of the viscosityof acetophenone which was determined with referenceto benzene.In the case of alcohol and nitrobenzene wheretthe temperature-coefficients of the velocity constantsare abnormally high* the dielectric constants are alsoabnormally high.SUMMARY.Freezing point curves of mixtures of solventand reactants have been constructed. No additioncompounds were detected. The influence of the solventis therefore not due to addition compounds or solvent-solute complexes.The temperature-coefficients of the reaction havebeen determined in a number of different solvents.The value of the temperature-coefficient per degree of38 • 3®temperature for the ratio gg?"gOC. is slightly higherthan for the ratio. which agrees with Lev/is*theory.The mean value of E in different solvents hasbeen determined. E is in the inverse order to thevelocity/20.velocity of the reaction, except in the case ofalcohol and nitrobenzene, whicli also agrees withLewis' theory "that any agency which increases thevelocity of the reaction diminishes the temperature-coefficient of the velocity constant".The temperature-coefficient is not inverselyproportional to the reaction velocity in differentsolvents, so other influences besides that of radiatiopalone must control the reaction velocity.There is a rough agreement between the dielectricconstants and the reaction velocities.ft:■

Influence of solvents on the velocity of formation of quaternary ammonium salts

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Influence of solvents on the velocity of formation of quaternary ammonium salts

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