Amlno-Amide Proton Exchange in Methylamine
The Journal of Physical Chemistry, Vol. 83, No.
14, 1979
1029
Isotopic Exchange Reactions of Amines. 2. Amino-Amide Proton Exchange in the Potassium Methylamide-Methylamine System James D. Halllday" and Patrick E. Blndner Physical Chemistry Branch, Chalk River Nuclear Laboratories, Atomic Energy of Canada Limited, Chalk River, Ontario, KOJ IJO, Canada (Received November 3, 1978; Revised Manuscript Received March 5, 1979) Publication costs assisted by Atomic Energy of Canada Llmited
The kinetics of the methylamine-potassium methylamide amino proton exchange reaction were studied by lH NMR as a function of temperature and catalyst concentration. Rates were determined by analysis of the line shapes of both the amino and methyl peaks. The exchange kinetics cannot be described by a single bimolecular rate constant based on solvent and stoichiometric amide concentration. By using published data for the dissociation constant of potassium methylamide in methylamine, we calculated bimolecular rate constants for two catalytically active species in solution: free methylamide ions and potassium methylamide ion pairs. These are at 25 "C as follows: k2(free ion) = 1.42 X lo9 L-mol-ld, It2(ionpair) = 1.04 X lo8 L.mol-'.s-', with activation parameters = 12.8 and 10.3 kJ-mol-l and AS*= -27 and -57 J.mol-l.K-', respectively. The NMR spectrum of anhydrous methylamine was recorded as a function of temperature over the range -82 to +75 "C. Values of 46.7 and 7.04Hz, respectively, were found for the I4N-H and H-N-C-H spin coupling constants in methylamine. In addition the spin lifetimes of the methylamine 14Nyielded an activation energy for quadrupole relaxation, a parameter characterizing rotational diffusion in the liquid phase, of 8.7 kJ-mol-l.
Introduction Spin coupling between amino or hydroxyl protons and the group's parent molecule has been observed for most amines and alcohols subjected to stringent p~rification.l-~ However, liquid methylamine (MA) has proven anomalous in this r e ~ p e c t ; ~H-N-C-H -~ coupling has not been recorded. Usually this coupling is removed by a fast intermolecular exchange reaction between the amino or hydroxyl protons, catalyzed by minute traces of an acidic or basic This paper describes the preparation of pure anhydrous methylamine based upon an application of Swift et al.'s technique6 for purifying and drying ammonia. Intermolecular exchange in the pure solvent was reduced sufficiently to enable a proton NMR study of the effect of temperature and potassium methylamide (PMA) concentration on the kinetics of reaction 1. CH3NHz+ CH3-NHK+ + CH3-NHKf
+ CH,NH,
(1)
Experimental Section The chemical reactivity and high vapor pressure of PMA-MA solutions required that special handling techniques be used.' Preparation of Pure Anhydrous Methylamine. Anhydrous methylamine was purified in the apparatus shown in Figure 1, which was also used to prepare the low concentrations of PMA in MA for the exchange reaction 1study. This apparatus incorporates the principal features of the design by Swift et ala6and suitable optical cell attachments for in situ concentration measurements similar to the conductivity cell designed by Bhattacharyza et a1.* The thoroughly washed and dried (vacuum pumped and flamed) system was charged, through valve A and a vacuum adaptor, with -50 mL of a 0.1 mol-L-l PMA in MA solution prepared by allowing 0.4 g of K metal to react with 100 mL of purified MA9 over a platinum black catalyst. The apparatus was pickled with this concentrated PMA solution for a minimum of 48 h. During this time it was frequently rotated to ensure all internal surfaces were treated. With the solution transferred to bulb C the
methylamine was distilled into bulb B, the apparatus rinsed, and the solution poured back into C. This was repeated until all of the PMA residues were washed out of the apparatus from the glass coil (path length 5 ft) forward. A glass frit (70-100 pm) between bulb C and the glass coil (not shown in Figure 1)gave additional protection in preventing carryover of PMA during the distillations, After many rinses (usually 10) the amine was tested for amide contamination by taking off an NMR sample in one of the eight attached Wilmad 528,5-mm NMR tubes and observing the amino triplet (H-14N coupling) resolution at room temperature. The apparatus was rinsed again several more times and another sample removed. This process was continued until there was no further change in the NMR line width of the amino NMR signal. Dilute P M A Samples. Absorbance measurements of PMA solutions were done by using a valved glass apparatus with three optical cells (0.1, 1.0, and 10.0 cm) and a calibrated reservoir to allow dilution of the PMA solution. UV/visible spectra of PMA/MA solutions yield only a broad absorption shoulder for the PMA before the cutoff due to the solvent absorbance. Plots of optical density at 350 nm vs. concentration of PMA (low2to lo4 mo1.L-l) showed that Beer's law was obeyed. For preparation of the dilute PMA exchange measurement samples, the same apparatus as used to prepare anhydrous CH3NHzwas employed (Figure 1). The procedure for sample preparation was to ascertain initially that the MA was dry and free of amide or other contaminants by removing a sample for comparison with the NMR. spectrum of a known sample. Then a small quantity of PMA was allowed into the system from bulb C and the absorption measured at 350 nm (shoulder) in one of the optical cells E (0.1, 1, and 10 cm; 0.1 not shown in Figure 1). The PMA concentration was adjusted to the required value by distillation of the methylamine between flask C and reservoir B. NMR samples were taken by pouring the solution into an NMR tube (D) and accurately measuring the height (-4 cm) at room temperature, -23 "C. The solution was frozen in the NMR tube and in bulb B ensuring that there was no large change introduced from
0022-365417912083-1829$01.0010 0 1979 American Chemical Society
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1030
The JwMl of physical ChmWty, Vd. 83. No. 14. 1979
J. D. HalMay and P. E. Bindmn
3. Temperahre e n d s (A. 75 1 OC. and E. -56 5 ' C ) on me
amho poton swcbun OfmSmykJmhe Thecalculated WEdJa (boxes) show bom me uncoupled spechum and me enects 01 spin coupling wim the mmyl protons (J = 7 0, Hr) The !alter are onset to Um right for
clarMcatlon
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Flp.s 1. Apparatus used fapreparatbn of M e scktkas 01 ~otasYm methylamae in anhydrous methylamine
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