DALLAS L. RABENSTEIN
1848
Table XVI: Calculated us. Observed Changes in Ln ke for Anisoyl Chloride at 35.00' in Ethanol-Benzene AXEtOH
4 A+H/R T
~AZ~RT
0.5-0.1 0.5-0.2 0.5-0.3 0.5-0.4 0.5-0.6 0.5-0.7 0.5-0.8
-8.96 -4.460 -2.48 -1.04
-2.72 -1.18 -0.58 -0.22 0.15 0.22 0.24
0.80
1.48 2.04
A In ks
4(AdSEe)/R
A(ASE)/R
(celcd)
A la ks (obsd)
-4.12 -1.40 -0.47 -0.12 0.048 -0.048 -0.024
-1.70 -0.74 -0.37 -0.16 0.12 0.20 0.28
-5.86 -3.50 -2.22 -0.98 0.78 1.548 2.204
-6.87 -3.82 -2.14 -0.86 0.71 1.31 1.85
of mixing data are those of RIrazek and Van N e d g and Brown, Fock, and Smith.20 The treatment of the reaction of anisoyl chloride in requires that four species Solvate the Carbonyl oxygen andl possibly, the chlorine. As its self-association constant indicates, ethanol is acidic than isopropyl alcohol. The enthalpy of mixing data are those of Van Xess and coworkers.21
The free energies of mixing were those of Brown, Fock, and Smith; the entropies were calculated from these and the Van Xess enthalpy data. (19) R. V. Mraeek and H. C. Van Ness, A.I.Ch.E. J., 7, 190 (1960). (20) I. Brown, W. Fock, and F. Smith, Aust. J. Chem., 9, 364 (1956).
(21) C. S. Savini, D. R. Winterhalter, L.H. Kovach, and H. C. Van Ness, J . Chem. Eng. Data, 11, 40 (1966), and personal communication of Professor Van Ness.
Nuclear Magnetic Resonance Study of the Reaction of Methoxide Ion with Methyl Formate in Methanol Solution by Dallas L. Rabenstein' Chevron Research Company, Richmond, California 94808
(Received October 83,1969)
Collapse of the long-range spin-spin coupling in I e proton nmr spectrum 0. methyl formate in methanolsodium methoxide solution has been investigated. At high sodium methoxide concentrations, the collapsed methyl resonance of methyl formate merges with the averaged methyl resonance of the solvent which estr blishes that the spin-spin coupling is collapsed by the symmetrical transesterification reaction HCOOCHs -OC*H8 e HCOOC*Ha -OCHs. The rate constant for this reaction was determined to be 76 =t 5 M-I sec-1 at 31 '1 from the dependence of the collapse of the spin-spin coupling on the sod um methoxide concentration. Experimental methyl formate lifetimes were obtained by comparison of partik lly collapsed spectra with theoretical spectra calculated as a function of methyl formate lifetime.
+
+
Introduction Long-range spin-spin coupling is observed in the high-resolution proton nuclear magnetic resonance (nmr) spectrum of methyl formate. Fraenkel attributed the long-range coupling to a 25% partial doublebond character in the carbonyl carbon-ether oxygen bonda2 The coupling has been observed in the spectrum of neat methyl formate, in the spectrum of methyl formate dissolved in several polar and nonpolar solvents, and in the spectrum of methyl formate in strongly acidic solutions. The long-range coupling is also observed in the nmr spectrum of methyl formate disThe Journal of Phyaical Chemistry
solved in methanol; however, the coupling collapses upon the addition of sodium methoxide to the methanol solution, the amount of collapse depending on the sodium methoxide concentration. I n the present paper, the reaction of methoxide ion with methyl formate which causes the collapse of the spin-spin coupling is established, and the kinetics of this reaction are determined from partially collapsed multiplet patterns. (1) Address inquiries to this author at Department of Chemistry, The University of Alberta, Edmonton, Alberta, Canada. (2) G . Fraenkel, J . Chem. Phys., 34, 1466 (1961).
1849
REACTION OF METHOXIDE IONWITH METHYLFORMATE IN METHANOL SOLUTION Results and Treatment of Data The proton nmr spectrum of methyl formate in methanol solution consists of a doublet at 3.70 ppm due to the methyl protons and a quartet at 8.04 ppm due to the formyl proton. The coupling constant for the interaction between the methyl and formyl protons is 0.85 Hz. Addition of sodium methoxide to a methanol solution of methyl formate causes the multiplet patterns to collapse, the amount of collapse depending on the sodium methoxide concentration as shown in Figure 1. The methyl and formyl proton resonances are shown (not a t the same instrument sensitivity) a t each concentration of NaOCH3 except 1.0 M . At NaOCH3 concentrations greater than 0.8 M , the methyl resonance of methyl formate is merged with the methyl resonance of the methanol-sodium methoxide solvent. It is well known that multiplet patterns due to spinspin coupling begin to collapse when chemical exchange processes involving one of the partners in the coupling interaction are occurring a t rates of the order of the spin-spin coupling constant (measured in cycles per second) . 3 There are two different exchange reactions involving methyl formate in methanol-sodium methoxide solution which would result in the collapse of the spin-spin coupling if their rates were fast enough, namely the symmetrical transesterification reaction (or methoxide exchange reaction) represented by eq 1, k
HCOCH3
I/
+ -06H3 )JH C 0 6 H 3 + -OCH3
(1)
/I
0
0
and the methoxide-catalyzed exchange of the formyl proton of methyl formate with the hydroxyl proton of the solvent, represented by eq 2. GCOCH,
II 0
+ HOCHs
-0CHa
HCOCH3
I1
+ 6OCH3
(2)
0
The reaction causing the collapse can be established from the spectra shown in Figure 1. I n reaction 1, the formyl proton experiences only one magnetic environment because it remains bonded to the carbonyl carbon, while in reaction 2, it is exchanging between methyl formate and methanol. Thus, if reaction 1 is causing the collapse of the multiplet patterns, a single narrow resonance will be observed a t 8.04 ppm for the formyl proton a t high rates of exchange while reaction 2 will cause the formyl proton resonance to merge with the hydroxyl resonance of the solvent at high rates of exchange. For the same reason, if reaction 1 causes collapse of the multiplet patterns, the methyl resonance of methyl formate will merge with the methyl resonance of the solvent at high rates of exchange while reaction 2 will give a single narrow resonance a t 3.70 ppm for the methyl protons of methyl formate at high rates of exchange. Experimentally, the formyl proton doublet collapses to a
Figure 1. Nmr spectra of 2.0 M methyl formate in methanol-sodium methoxide solution a t the following sodium methoxide concentrations: (A) 0.0 M , (B) 0.025 M , ( C ) 0.035 M , (D) 0.07 M , (E) 0.14 M , (F) 1.0 M . For 0.0 M NaOCH3, the quartet is a t 8.04 ppm and the doublet is at 3.70 ppm relative to TMS. The temperature is 31 f 1”.
single narrow resonance at 8.04 ppm a t high sodium methoxide concentrations while the methyl resonance of methyl formate merges with the solvent methyl resonance. Thus, the spin-spin coupling is collapsed by the methoxide exchange reaction. The rate of decrease in the concentration of methyl formate, -d [HCOOCH3]/dt, by reaction 1 is given by eq 3. Dividing eq 3 by the methyl formate concen-d[HCOOCH,]/dt = k[HCOOCH3] [-OCH3]
(3)
tration gives eq 4, which relates the average lifetime of methyl formate between events which cause exchange of methoxide ion, THCOOCH,~,to the methoxide ion con-
~-1
- k[-OCH3]
THCOOCHs
(4)
centration. T H C O O C H ~is the kinetic parameter experimentally measured from the nmr spectra, and exchange rate constant k was obtained from plots of l/THCOOCHa US. [-OCH,]. Dividing eq 3 by the methoxide ion concentration, [-OCH,], gives an equation similar to eq 4 for the average lifetime of methoxide ion between events which cause it to exchange with methyl formate, r O c H s . I n methanol solution, rapid alcoholic proton exchange between methanol and methoxide ion results in one averaged resonance for the methyl protons of methanol (3) J. W. Emsley, J. Feeney, and L. H. Sutcliffe, “High Resolution Nuclear Magnetic Resonance,” Pergamon Press, Inc., Kew York, N. Y., 1965, p 488.
Volume 7.4?Number 9 April 30,1970
1850
DALLAS L. RABENSTEIN
and methoxide ion. However, it was not possible to independently determine rocH3from the averaged methanol-methoxide methyl resonance because the concentration of methanol is much greater than the concentration of methyl formate and thus exchange of methoxide ion with methyl formate causes negligible broadening of the averaged methanol-methoxide methyl resonance. T H C O O C H ~was determined from the shapes of the partially collapsed methyl formate resonances. For NaOCH, concentrations less than 0.07 M , the multiplet patterns are not collapsed completely; and, in this concentration region, T H C O O C H ~was determined by comparison of the experimental resonance line shapes with computer-calculated line shapes. Partially collapsed quartet spectra were calculated for different THcoOCH~ values using the equation presented by Grunwald, et aL4 The most convenient parameters to measure from the calculated and observed spectra were the valley-to-peak ratios. For the calculated quartet spectra, the ratio of the central valley 0.0 0. 04 0. 08 0. 12 0.16 to the central pealis and the ratio of the outermost [NaOCH3], 8 valleys to the outermost peaks were measured for the different degrees of collapse. These ratios were Figure 2. Experimental data for methoxide exchange with methyl formate. The temperature is 31 f 1". plotted vs. T H C O O C H ~and experimental THCOOCH~)Swere determined from the experimental valley-to-peak ratios using these plots. height of the exchange-broadened resonance and W1l2 Exchange-collapsed doublet spectra were calculated is the width at half-height in the absence of exchange.' using the equation for two-site exchange originally The W1,%values used in the calculation of T H C O O C H ~by derived by Gutowsky and Holmus Application of this eq 5 were measured from doublet spectra calculated equation to the calculation of partially collapsed douusing life times predicted for the appropriate concentrablet patterns has previously been discussedS6 Doublet tions of methoxide ion from the lower concentration spectra were calculated for a range of T ~ C O O C H ~ ' Sand , the results. The uncertainty jn W1/, limits the accuracy valley-to-peak ratios of the calculated spectra were with which rate constant k can be determined to 7%. plotted vs. THCOOCH~. Experimental T H C O O C H ~ ) Swere After the quartet resonance collapses, it becomes determined from the experimental valley-to-peak ratios more narrow as the NaOCH3 concentration is increased using this plot. until a limiting width a t resonance half-height of 0.25 When the KaOCHa concentration is greater than Hz is reached at 1.0 M NaOCH3. No broadening of 0.07 M , the width of the collapsed methyl resonance of this resonance occurs because the formyl proton is not methyl formate at resonance half-height increases as exchanging between two environments of different the NaOCH, concentration increases. This is because chemical shift, Evidence for this is that the chemical the OCH, group of methyl formate is exchanging beshift of the center of the formyl proton resonance pattween two environments of different chemical shift tern is independent of the NaOCH3 concentration. (methyl formate and exchange-averaged methoxide Sample kinetic data are plotted according to eq 4 in ion-methanol) , and exchange broadening is observed Figure 2. The slope of the straight line gives a value of when the exchange rate is on the order of the separation 76 J4-l sec-l for the methoxide exchange rate constant in cycles per second between the two environments. a t 31 i 1". The nonzero concentration axis intercept At XaOCH3 concentrations greater than 0.8 M , the methyl resonance of methyl formate merges with the (4) E. Grunwald, A. Loewenstein, and S. Meiboom, J . Chem. Phys., averaged methanol-methoxide ion methyl resonance to 27, 630 (1957); Z. Luz, D. Gill, and S. Meiboom, ibid., 30, 1540 give one exchange-averaged methyl resonance. I n the (1959). Equation A6 of the first reference is applicable t o the exchange system studied in the present paper; however, several concentration range where a separate exchangesigns are in error in eq A6. The correct equation is given in footbroadened methyl resonance is observed for methyl note 10 of the second reference. formate, T H C O O C H ~was calculated from the width of this (5) H. S. Gutowsky and C. H. Holm, ibid., 25, 1228 (1956). resonance using eq 5 , where W f l l ais the width a t half(6) Reference 3, p 489. (7) J. A. Pople, W. G. Schneider, and H. J. Bernstein, "High Resolu~-1 - 7r(WfI,*- W1,J. (9 tion Nuclear Magnetic Resonance," McGraw-Hill Publications, New THCOOCHa
The Journal of Physical Chemistry
York, N. Y., 1959, p 221.
REACTION OF METHOXIDE IONWITH METHYLFORMATE IN METHANOL SOLUTION in Figure 2 is due to the presence of a small amount of water in the methanol solvent used. The water reacts with methoxide ion forming methanol and hydroxide ion, thus decreasing the concentration of methoxide ion and shifting the concentration axis intercept. A plot having the same slope was obtained a t a methyl formate concentration of 4.0 M .
Discussion There are no previously reported studies of the symmetrical methyl formate transesterification reaction involving methoxide ion in methanol or in other solvents. This is probably because the reaction is inaccessible t o most kinetic methods. The reaction is so rapid it is outside the kinetic range observable by conventional techniques. Also, the products are identical with the reactants unless radioisotope labeling or spin labeling, as in the nmr spectrum, is employed. Thus, no rate constants are available for direct comparison with the one reported in the present paper; however, a rate constant can be predicted from literature data for other reactions of esters of carboxylic acids. The rate constant for the reaction of hydroxide ion with methyl formate in aqueous solution is 50.9 M-’ sec-’ at 29.97°.8 This reaction is formally similar to the reaction of methoxide ion with methyl formate, and experimental evidence has been presented indicating that the similarity can be extended t o the reaction mechani~ms.~*’OAssuming that this similarity does exist, a value of 82 M-’ sec-l is calculated for the reaction of methoxide ion with methyl formate in aqueous solu-
1851
tion from the hydroxide rate constant and the relative nucleophilicity of hydroxide and methoxide a t carbonyl carbon.8v11 It has been reported that the rate of reaction of alkoxide ions with several carboxylic acid esters in alcohol-water solvents decreases as the alcohol content increases.1° Thus, the rate constant for the reaction of methoxide ion with methyl formate in methanol is predicted to be less than 82 M-’sec-l but greater than 50.9 M-’ sec -I, in good agreement with the rate constant determined in this investigation.
Experimental Section Absolute reagent grade methanol (Baker) and sodium methoxide (Matheson Coleman and Bell) were used as received. The methyl formate (Baker) was dried by shaking with Drierite before using. Nmr measurements were made on a standard Varian A-60 nmr spectrometer operated a t an ambient probe temperature of 31 A 1”. Chemical shifts are reported relative to TMS. For the exchange rate measurements, spectra were recorded five times, and the average valley-to-peak ratios or the average widths a t resonance half-height were used in the lifetime determinations. (8) H. M. Humphreys and L. P. Hammett, J. Amer. Chem. Soc., 78, 521 (1956). (9) R. W. Taft, Jr., M. S. Newman, and F. H. Verhoek, {bid., 72, 4511 (1950). (10) M. C. Bender and W. A. Glasson, ibid., 81, 1590 (1959). (11) J. March, “Advanced Organic Chemistry: Reactions, Mec-
hanism and Structure,” McGraw-Hill Publications, New York, N. Y., 1968, p 298.
Volume 74,Number 9 April $0, 1970
