An NMR Kinetics Experiment Don Kaufman, Carl Sterner, Brian Masek, Ralph Svenningsen, a n d Greg S a m u e l s o n Kearney State College, Kearney. NE 66847
A kinetics e x ~ e r i m e n its found in virtuallv all undermaduate physical chemistry laboratory texts and indeed iseven common in manv organic laboratory texts. Furthermore. increasing applica&sof various ~ p e c t r o ~ h o t o m e t rmethods, ic particularlv nuclear magnetic resonance. are heina made a part of the laboratory experience in both of these courses. Thus, a kinetics experiment utilizing N M H specrrosco~yto obtain rate data should he particula;ly attractive t o instiuctors in these laboratories. Advantages of the Experiment The experiment presently discussed also offers the following advantages: Experimental Detalls
I . The reactant, p-methux)~phenylncerylene,is easily synthesized fnrm
readily nvailahlestarting mnterials.'
Procedure Used to Obtain the Kinetic Data The solvent system used for the initial hydration resetion was 5 mL of acetic acid-ds 0.5 mL of DzO,and 10 pL of cone. HzSO4. Compound I11 was readily soluble in this system and the deuterated solvents prevent the appearance of large obscuring hydroxyl proton peaks in the NMR s m t r u m of thereaction mixture. The hvdration solution was prepared by add~ngu.4 mLul the nhow solvent -+Rrmnto :XI mg ol I l l irugive a 0 5 M sdutiun) in asmallvml.The rmrtimtime was begun at this point and the solution was transferred with a syringe to an NMR tube for the duration of the experiment. At selected intervals the sample tube was'placed in the spectrometerfar integration of the soectrum to determine the relative size of the methoxvl sinelet of I11 a i 3.8 6 and the methoxvl , sinelet . ~" ~ of IV a t 3.9 6. (see .~ thk aceompanying illuitration to note the rhangc in these signal4 d u r m ~the prog~grcssofthe reaction.) It should he n c ~ that d k.au..of d~utcrium exchange the integration of the methyl protons of IV could not be used. It was also found more appropriate to integrate the 3 me'thoxyl protons of I11 rather than the single acetylenie proton in order to reduce the relative error inherent in the integrations. In all runs, intemations were nerformed at intervals raneine 10'70 .. . from aooroximatelv .. rwaction through at least 2 hall-lives. Runs were also eonduetea at JO and 40 degrees with the aid of a thermostatted water hath tu maintain the desired temperature. In these runs, the reaction tubes were immersed in an ice bath upon removal from the water bath to effectively quench the reaction during the short time intewd required for intemation. The reaction time was "stoooed" when the tub& were olaced in the ice bath and started againas the tubes were replacedin the heated water hath after the integration h d b e e n conducted.
2. As little as 30 mg of wmpound is required per student or pair 3.
of students. Other than the NMR spectrometer, the only required apparatus includes an analytical balance, syringe, NMR tube in which the entire reaction is conducted, and a temperature controlled water hnth (ootionall. . ,~~ The reat tion involwd, the hydration of an nlkyne, is simple and ahuuld he known r u d l students who would he cwdurting the experiment. The experiment gives reproducible data from which a rate exoression is easily extracted which is consistent with the accented mechanism of the reaction. 'l'hr reaction i* ronvenicntly run at elevntpd temperatures of on& uhcming and therehy prrmitri thr calculation of the energy of activation for the reaction. ~
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The Chemical Reactions Involved in the Experiment P-n~ethox)phenylacetylene(lII ) can be prepared according t o a number of schemes reourtrd in the literature (1-121. We first prepared 111 via the f&wing route in which N~N&was used a s the base in hexamethvl~hosohoramidea s shown
We have recently developed a procedure for t h e synthesis of Ill which a p p e a k superi;)r to any reported in the-litrruture. A crown ether assisted reaction is utilized to dehydrohalugenate 11 with ootassiurn tertiai-v butoxide to eive Ill under mild conditions and in excellent yield. T h e acid catalvzed hvdration of I11 leads t o the exoected - - ~ acetophene, IV, via a mechanism universally accepted LO be t h r ft,llowine in which the first s t w is rate determininn and is thus c o n s k e n t with the observed pseudo-first-ord&kinetics. ~
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This paper was presented in part at the 16th Midwest Regional Meeting, American Chemical Society, Lincoln. Nebraska, November 7, 1980. prnemoxystyrene is available from Pfaltz and Bauer. lnc. WM09620).
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The NMR spectra. in the range of 3.5-4.06. of the reaction mixture during progression of the reaction to show the change in size of lhe &xyi pesks ot Ill and IV.
Volume 59
Number 10
October 1982
885
Table 1. Rates of Hydration 01 Ill at Varlous Temperatures Absolute Rate Constants at Temoerature
Table 2.
kX
lo5
Rate of Hydration of Ill at 40° Calculated
Time in Minutes
%
1. 67
Reaction
k X lo5
14.1 30.0 42.4 59.3 67.6 72.3 76
2. 161 3. 241 4. 381 5. 479 6. 545 7. 604
3.78 3.69 3.81 3.93 3.92 3.93 3.87
Average = Table 3.
Rate of Hydratlon of V at 30° Calculated
Time in Minutes 1. 62 2. 125 3. 163 4. 310 5. 429 6. 494 7. 631
%
Reaction 14.3 28.0 36.7 55.1 68.8 71.8 78.9
k X lo5 4.15 4.38 4.68 4.31 4.51 4.26 4.11 Average = 4.34
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reported rates represent the average of two separate runs each consisting of at least seven measurements. To illustrate the consistency of the calculated rates over the duration of the reaction, representative data is presented in Table 2 for a run at 40'. The Reaction Rate Under More Acidic Conditions Rralizing that an accelerated reaction rate wuuld be necessary or nr lrast prrierable in some lahuratory sirualions, the hydration was repented n s d ~ s r r i b e d a t wexcept t that thcconrentratim~,fH2SOI was increased threefold. As expected from the mechanism of the re: action, the reaction rate was approximately 3 times faster in this more acidic solvent system. For example, at 30" the new rate constant was 2.90 times greater than when run in the original solvent system. An Attempt To Hydrate A Commercially Available Alkyne Although 111has proven to be easily synthesized, it was deemed worthwhile to determine whether the commercially available, but less reactive phenylacetylene(V) could be hydrated a t a satisfactory rate to permit its use in place of 111. When V was reacted in the original solvent system, no detectable reaction had occurred even after 48 hr. When the concentration of H2SO4 was increased tenfold, V had reacted only to the extent of approximately 15%in 24 hr. When the concentration of HzSOa was finally increased fiftyfold, V reacted at a rate acceptable for the present experiment. It should also he noted that the use of an increased concentration of Hz801 did not produce an obscuring hydroxyl proton peak in the NMR spectrum. However, it is noteworthy to realize that with V one must integrate the single acetylenic proton to obtain rate data in contrast to the 3 methoxyl protons integrated in the reaction of 111.Thus, the relative error due to inteeration is sienifieantlv with V. ,greater T ~ h l t:I. indudpi mlrulnted rarer for a representative run of V at 30' using the muit agidlc solwnr sysfrm descrilled ahwe. ~
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Energy of Activation Using the data of Tahle 1,a plot of -Ink versus 1/T yielded a straight line and a calculated e n e-. m of activation of 19.7 kcal (82.3 GJ) which was in close agreement with a n energy of activation of 19.8 kcal(82.8 k J ) ohtained from a least souares treatment. Our valueis i n close accord with t h a t reported for t h e hydration of similar alkynes (12). ~
Synthesis of p-methoxyphenylacetylene(llf~ In a typical preparation of III,6.4 gofp-methoxystyrene was dissolved in 50 mL of chloroform. A 1.1molar excess of bromine (8.51 g) dissolved in 25 mL of chloroform was added to this solution, with stirring, at a slow dropwise rate. Upon completion of the addition, the excess hromine was destroyed with aqueous sodium thiaaulfate. Aftel. drying over maghesium sulfate, the chloroform solution was stripped of solvent on the rotary evaporator to yield the crude dibromide(I1) in 98% yield. A 2.1 molar excess (12.52 g) of potassium tertiary hutoxide and 400 mg of 18-crown-62in 50 mL of toluene was refluxed for 30 min after which 13.05 g of 11, used as obtained above without further purification, in 25 mL of toluene was added with stirring over a period of 30 min. The reaction mixture, which developed a dark brown color, was refluxed for an additional hour. After cooling, the reaction mixture was filtered to remove KBr and subsequently passed over a short silica gel column to remove the crown ether. The resulting solution was stripped of toluene to give I11 in 92% yield. An NMR spectrum of the crude product in CDC13 showed no extraneous peaks; only the following expected peaks: a singlet at 3.256(1H),a singlet at 3.86(3H), and a complex multiplet a t 6.8 to 7.56(4H). Final purification of I11 was achieved via vacuum distillation, h.p. 829/10 torr. [lit 86'110 tom ( Z ) ] . Upon cooling, the distillate crystallized to give a colorless solid, m.p. 30' [lit 29' (911.
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Conclusions T h e rate of hydration of p-methoxyphenylacetylene(II1) was followed by N M R spectrometry provides t h e basis for a kinetics experiment in which the objective of t h e experiment is n o t shrouded in comulicated lahoratorv urocedures. T h e rate of reaction can hi readily adjusted'to meet t h e time framework of a uarticular lahoratorv hv altering t h e concentration of ~ 2 used~or t h e 0 reaction ~ temperahre. Finally, phenslacetslene can b e used i n d a c e of 111urovided a suffi. ciently acidic reaction mixture is used. It is, however, our recommendation t h a t 111he utilized because of t h e greater accuracy possible i n integrating its N M R spectrum. Literature Cited (11 King, A. 0 , Negishi, E., Vallani, Jr., F. J.,and Silvairs,Jr.. A,, J. Org. Chm..43.358 11978!.
Rate Data Obtained Rates o f Reaction in Least Acidic Solvent System Tahle 1 includes representative data obtained in our laboratory using the solvent system described in the Experimental Details. The Appropriate care should be exercised in using the toxic cmwn emer. Contact with skin should be avoided.
886
Journal of Chemical Education
(6) Schlasser, M.and Ladenbeqer, V., Cham. Bor., l W , 3901 (1967). (7) B0tt.R. W.,Esborn,C.,snd Ws1tan.D. R. M., J Chem. S~.,384(19651. (8) A1ien.A. D. endCmk,C.D.,Con. J.Chem.,41.1084 (1963). (9) Co0k.C. D.snd Uany1uk.S.S.. Trfrahedron. 19,17711963). (10) Smissman, E. E.,Jonspn,R H.,Cc!m~so,A. W., and A Y Y ~ , B.F., J.AAA. Chhm. SS.,
78.3395 (1956). 111) MeBee, E. T,Raberts, C. W., and C. G.Ha".J.Amer Cham. Soc., 78,3393 (1956). (12) Noyrr, D. S., Matesich, M. A , and Petemn, P. E., J. Amer C h m . Soc., 89,6227 (19671.
