Decomposition of Chemically Activated Methylamine
previously described. These properties should cause a large increase in the percent retention (:>50%) if a direct hot C1-for41 replacement mechanism is involved. The experiment was carried out at 25 “C and resulted in 48.6 f 1.3% retention as shown in Table 11. This strongly suggests that racemization is occurring through radicalradical recombination in the liquid, glass, and crystalline solid. In addition, the presence of both labeled isomers in equal amounts is evidence that planarity of the CH3CIICOC1and (CH3)2CH2CHCOC1 radicals is established before recombination. A similar observation on “racemization” in thLe condensed phiase has recently been reported.21 Machulla and Stocklin5 have suggested that the inversion reaction chainnel is made possible by formation of an excited reaction complex with lifetime long enough to allow configurational change, including Walden inversion. They suggest that such a complex would be stabilized in condensed phases leading to the increase in the percent inversion they observed in going from a low pressure gas to a liquid. The results reported in this paper clearly show considerably greater inversion in the gas phase in a molecule with a single asymmetric center inconsistent with the general mechanism they propose on the basis of their more complex chiral system.
Conclusions The experimental results of 38Cland 34mC1-for-C1 suband 2stitution at the asymmetric carbon of 2(S)-(+)(R)-(-)-chloropropionyl chloride in the gaseous and condensed phase indicate predominant inversion of configuration in the gaseous phase and decrease to a “retention” to “inversion” ratio of about one in the liquid and solid state. These results dong with the lack of conformational effects in the condensed phase gives additional support for a caged radical combination mechanism leading to racemization. The almost identical substitution yields for both
The Journal of Physical Chemistry, Vol. 83, No. 10, 1979
1241
38Cland 34mClindicate that the initial kinetic energies are not a factor. The gas phase results indicate that not all hot-atom substitution reactions are fast and direct (as predicted by the impact modePg) involving a front-side approach, but can indeed occur from the backside of the molecule resulting in Walden inversion. From the results of the 2(S)-chloro-4-methylvalerylchloride experiments we conclude that the predominant factor controlling the substitution event is steric in character.
References and Notes (1) C. M. Wai, C. T. Ting, and F. S.Rowland, J . Am. Chem. Soc., 86, 2525 (1964). (2) F. S.Rowland, C. M. Wai, C . T. Ting, and G. Miller, “Chemical Effects of Nuclear Transformations”, Vol. 2, International Atomic Energy Agency, Vienna, 1965,p 333. (3) C. M. Wai and F. S. Rowland, J. Phys. Chem., 74, 434 (‘1970). (4) L. Vasaras, H.-J. Machulla, and G. Stocklin, J . Phys. Chem., 76, 501 (1972). (5) H.-J. Machulla and G. Stocklln, J. Phys. Chem., 78, 658 (1974). (6) R. Wolfgang, Prog. React. Kinet., 3, 97 (1965). (7) D. Urch and R. Wolfgang, J. Am. Chem. Soc., 83, 2982 (1961). (8) R. A. Odum and R. Wolfgang, J. Am. Chem. SOC.,85, 1050 (1963). (9) A. E. Richardson and R. Wolfgang, J . Am. Chem. Soc., 92, 3480 (1970). (10)J. Wu and H. J. Ache, J. Am. Chem. SOC.,99, 6021 (f977). (11) T. R. Acciani, Y.-Y. Su, H. J. Ache, and E. P. Rack, J. Phys. Chem., 82 975 (1978). *(12)S.-C. J. bu, S: M. Birnbaum, and J. P. Greenstein, J. Am. Chem. Soc., 76, 6054 (1954). (13) W. Fickett, H. K. Garner, and H. J. Lucas, J. Am. Chem. Soc., 73, 5063 (1951). (14) B. Halpern, J. W. Westley, and B. Weinstein, Nafure (London),210, 337 (1966). (15) B. Halpern and J. W. Westley, Chem. Common., 12, 246 (1965). (16) K. Fukushima, Nippon KagakuZasshi,80, 1828 (1959);Chem. Abstr., 53, 2116h(1959). (17) D. L. Bunker and M. D. Pattengill, J. Chem. Phys., 53, 3041 (1970). (18) M. E. Berg, W. M. Grauer, R. W. Helton, and E. P.Rack, J . Phys. Chem., 79, 1327 (1975). (19) K-C. To, M. E. Berg, W. M. Grauer, and E. P. Rack, J. Phys. Chem., 80, 1411 (1976). (20) M. E. Berg, A. Loventhal, D. J. Adelman, W. M. Grauer, and E. P. Rack, J. Phys. Chem., 81,837 (1977). (21) J.-L. Wu, T. E. Boothe, and H. J. Ache, J. Chem. Phys., 68, 5285 (1978).
Decompo!jition Rate of Chemically Activated Methylamine K. J. Chao,“ C.
L. Lin, M. Hsu, and S. Y. Ho
Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan (Received August 28, 1978; Revised Manuscript Received February 1, 1979)
A study of the decomposition of the activated methylamine formed by methylene radical reacting with ammonia is presented. Singlet methylene radicals were produced by diazomethane photolysis at 4358 and 3660 8, in the presence of added oxygen. The measured decomposition rates are 1.7 X 1O1O and 2.3 x 1O1O s-l in the 4358and 3660-8, systems, respectively. The result is consistent with RRKM calculations.
Introductioii Reactions of the methylene radical with hydrocarbons and the unimolecular decomposition of chemically activated alkanes and olefins have been widely in~estigated.l-~ In this paper, the insertion of a methylene radical into the N-H bond and the decomposition reaction of the excited amine are studied. Methylamine produced from the photolysis of a diazomethane-ammonia mixture decomposes at lower pressure^.^ In the presence of oxygen the triplet methylene can be quench~ed.~ The insertion of singlet methylene into
the N-H bond of ammonia produces vibrationally excited methylamine, which can be stabilized by collision or can undergo the dissociation:
- +
‘CH2 + NH, CH3NH2*
CH3NH2*
dM) ka
CH3NH2
CH3
NH2
The decomposition rate of chemically activated methylamine was determined by an internal compariison
0022-3654/79/2083-1241$01.00/00 1979 American Chemical Society
The Journal of Physical Chemistry, Vol. 83,No. 10, 1979
Chao et al.
method and calculated from Rice-Ramsperger-KasselMarcus (RRKM) theory.
In the photolysis of diazomethane with 2,2-dimethylpropane at 3660 A, Hase, Johnson, and Simon# found that most of the energetically hot 22DMB, initially formed at pressures higher than 0.4 torr, was stabilized. Under the conditions of the present experiments the excited 22DMB was stabilized by collisional deactivation and its decomposition was negligible. The amount of 22DMB produced in the photolysis is a measure of the total amount of CH3NH2*initially formed. In connection with the yield of stabilized CH3NH2,S/D ratios may be determined.' For S >> D R = (D + S)/(22DMB) N S/(22DMB)
1242
Experimental Section Diazomethane was freshly prepared5 by reaction of nitrosomethylurea with 40% aqueous KOH, and was distilled under vacuum from 196 to 77 K. Ammonia gas (Matheson Co.) was purified by trap-to-trap distillation. 2,2-Dimethylpropane (22DMP, 99.87%) obtained from Matheson was used as internal standard without further treatment. Commercial oxygen gas was passed through two traps at 77 K to remove any condensable impurity. Methylamine gas from Matheson was used for the calibration mixtures. The light source was a HBO 200-W high-pressure mercury lamp. The light was focused on the entrance slit of a Bausch and Lomb Model 33-88-07 monochromator with a linear dispersion of 16 A/mm. Both entrance and exit slits were 6 mm in width. Several Pyrex reaction vessels of 8, 137, 1181, and 5215 mL volumes, fitted with Teflon-Viton O-ring stopcocks, were used for the experiments. For the highest pressure runs the reactants were sealed into a small diameter quartz tube. All reaction vessels were covered with aluminum foil so as to reflect the transmitted light back into the reaction vessel and reduce the stray light. The typical composition of the reaction mixtures was NH3:CH2N2:22DMP:02= 1Ol:l:l. According to the pressure change the irradiation time varied from 12 to 36 h. Products were analyzed with a Varian 600 D gas chromatograph equipped with a flame ionization detector. The analyses were made on a 40-ft column of 30% Carbowax 4000 on 60/80 mesh Chromsorb P which was treated with a solution of 5% NaOH in methanol and a 8-ft column of 70/230 mesh silica gel. Calibration checks were made with standard mixtures of authentic compounds. Results a n d Discussion Some of the processes in the photolysis of diazomethane and ammonia are CH2N2 + hv 'CH,
-
+ NH3
CH3NH2"
4M) +
+ N2
(1)
CH3NH2*
(2)
'CH2
CH3NH2
-
22DMB*
w(M)
(4)
ka'
22DMB
decomposition products
(CH3NHz)/(22DMB),D/S, w, and k, are functions of total pressure as shown in Table I. The higher rate constants for lower pressure experiments are likely due to stepwise collisional deactivation.
where AHdCH3NH2)= -6.7 kcal mol-', AHANH,) = -11.04 kcal m ~ l - and ~ , ~( E t h ) E 0.96 kcal mol-l. According to
(D)
22DMB*
where w is the collisional stabilization rate constant proportional to pressure (see Appendix). A least-squares fitting of 0.102 [(22DMB)/(CH3NH2)],where 0.102 is the correction for the relative FID sensitivity of products, vs. 1 / w gives R = 27, k , = 2.3 X 1O1O s-' through the 3360-A data above 300 torr, and gives R = 26, k , = 1.7 X lolo s-l for 4350-A data. The average high pressure value 26.5 for R gives a rate constant ratio of k2/k6 = 2.7. This corresponds to a N-H vs. C-H insertion ratio of 11 per bond. The rate constant k, of decomposition of CH3NH2*can also be expressed by k , = w(D/S)
(8)
(3)
-
+ C(CH3)4
(22DMB)/(CH3"'2) = [(k6/k2) + ( h / h ) ( k a / w ) l X (C,Hiz)/("3) = ~Z/JZ~[("J/(CEBI~)I
(7)
(S)
The energetically hot methylamine, CH3NH2*,formed in the reaction 2 may transfer energy to other molecules (M) and become stabilized (S),or may decompose into CH3 and NH2 radicals (D). Oxygen was added in order to scavenge triplet methylene3 and also to suppress radical-radical recombination reactions such as CH, + NH2 CH3NH2* (5) Neopentane was adopted as an internal standard in order to eliminate the need for directly measuring the decomposition products. Singlet methylene reacted with neopentane to form excited 2,2-dimethylbutane (22DMB): lCHz
A steady-state approximation for processes 2,3,4,6,and 7 gives
Calculation The RRKM formulation was used to calculate the decomposition rate of excited methylamine. The reaction mechanism of the present system is considered to be C-N bond rupture. In the activated complex, the C-N bond distance of CH3NH2increases from 1.46 to 3.00 A, and either the CH3 group has a free radical geometry of sp2 planar that may not be affected by NH2group, or the geometrical structure of CH3remains the same. The corresponding ratios of moment of inertia of the activated complex and molecule I+/I are 3.51 and 3.49, respectively. Two different activated complex models were employed. In both cases, the C-N stretch was the reaction coordination; N-H twist, N-H scission, and C-H rock of the molecule were reduced to 50% and the C-H, N-H stretches were slightly varied. In model I, the torsion was replaced by an internal rotation. In model 11, the torsion frequency was reduced from 268 to 50 cm-'. Both models gave substantially the same results and only the latter is presented in Table 11. The excitation energy E* is given by - ( E * ) = A"f(CH3NH2) - AHf(NH3)- AHf(lCH2)E*('CH2) - ( E t h )
k.8
-CH3+NH2
and at lower pressures D/S = [R(22DMB)/S] - 1 = [R(22DMB)/(CH,NH,)] - 1
(6)
The Journal of Physical Chemistry, Vol. 83, No. 10, 1979
Decomposition of Chemically Activated Methylamine TABLE I: Experimental k , for the NH, t CH,N, System (2913 K) 0.102[(CH,press., torr
(22-)I DMB)la
w , s-'
TABLE 111: Calculation Results from RRKM Theory
;(go
2"
D/S
A. CH,N, Photolysis a t 4358 A 5.29 X 10'' 1.99 0.305
s-
'
4941 3435 3158 2087 1479 1233 754 366 169 59 30
3.66 x l o l o 3.33 X 10" 2.21 X 10" 1.58 X 10" 1.31 x l o i o 8.04 x l o 9 3.86 x l o 9 1.79 x lo9 6 . 2 0 ~lo8 3.2 x lo8
0.354 0.566 0.962 1.37 1.08 2.24 4.95 10.4 35.0 53.2
1.62 1.30 1.89 2.13 2.16 1.42 1.80 1.91 1.85 2.18 1.70
4300 3446 3380 2934 2521 2171 1568 1025 438 337 279 211 186 99 38
B. CH,N, Photolysis a t 3660 A 4.52 x 10'' 1.85 0.461 1.67 0.617 3.65 x 1 O 1 O 1.71 0.579 3.63 x 1O'O 3.13 X 10'" 1.58 0.709 1.40 0.927 2.67 X 10" 2.33 X 10" 1.09 1.48 1.68 x 1 O ' O 0.90 2.00 1.10 x lo1' 0.75 2.60 4.65 X l o 9 0.32 7.39 3.58 X l o 9 0.39 5.98 2.95 x 1g9 0.26 9.51 0.18 14.1 2.24 X l o 9 1.95 X lo9 0.25 10.0 1.05 x l o 9 0.055 48.1 4 . 0 ~l o 8 0.054 48.6
2.09 2.25 2.10 2.22 2.47 3.45 3.36 2.86 3.43 2.14 2.80 3.16 1.95 5.05 2.00
1.92 1.66 1.33 1.10 1.25 0.80 0.44 0.23 0.072 0.048
a The FID sensitivity of (CH,NH,) is relative to that of 22DMB. TABLE 11: Modela for the CH3NH, Molecule and Activated Complex
--descript mode N-H C-H C-H C-H N-H N-H N-H
stretch stretch deform rock wag scission twist C-N stretch torsion moment of inertia, g/cm2
I+/I
moleculea
complex
3394 ( 2 ) 2992 (3) 1463 (3) 1160 ( 2 ) 780 (1) 1520 (1) 1520 (1) 1044 (1) 268 (1) 3.84 x 3.65 x 10-39 0.78 x 10-39
3220 ( 2 ) 3040 ( 3 ) 1463 ( 3 ) 580 ( 2 ) 780 (1) 780 (1) 780 (1) reaction coordinate 50 (1) 1.32 x 1.30 x 10-38 0.78 x 1 0 - 3 9 3.50 1.19
F+' Reference 14.
TaylorlO et al. that AHf(lCH2)+ E*(lCH2) = 113.0 kcal mol-l a t 4358 A and IWf('CH2) E*(lCH2) = 116.8 kcal mol-l a t 3660 8, the total internal energy would then be 109.6 and 113.4 kcal mol-l for 4358- and 3660-A photolyses systems, respectively. By observing chemically activated
+
1243
CH,N, NH3-0, (4358 A )
CH,N, NH,-0, (3660 A 1
E,,, kea1 mol-'
15.0 78.0
15.0 78.0
rare constant, s-I model A model B
1.4 x 10" 1.8 X 10"
2.4 x 10'' 3.1 X 10'"
log A, s-'
Experimental Studies 1.7 x 1 O ' O rate constant, s-I
2.3 x
lolo
methylcyclobutane obtained from diazomethane and cyclobutane,ll Simons reported that the energy distribution of the singlet methylene from the photolysis of diazomethane was reasonably broad. In our work, both T(ay1or's (model A) and Simons' energy distribution pattern (model B) for E*(lCH2)were adopted. A CDC 7600 computer was used for the calculations. The average decomposition rate constant k., and Arrhenius constant A are listed in Table 111. The agreement between experiment and RRKM calculation result is satisfactory. The theoretical A factor obtained by the standard ART expression is 1015,slightly smaller than the value estimated from the radical recombination.12 Acknowledgment. This work was supported financially by the Chemistry Research Center of National Science Council, Republic of China.
Appendix The hard sphere collision number for each mixture w was calculated, taking the diameters of NH3, C(CH3)4, CH3NH2, and O2 to be 3.15, 5.15, 4.06, and 3.60 A, re~pectively.~J~ References and Notes (1) B. S.Rabinovitch and M. C. Flowers, Q. Rev. Cbem. SOC., 12:! (1964). (2) R. J. McCluskey and R. W. Carr, Jr., J. Pbys. Cbem.,80, 1393 (1976). (3) R. L. Johnson, W. L. Hase, and J. W. Simons, J. Cbem. Pbys., 52, 3911 (1970). (4) S.N. Tong and S. Y. Ho, J. Chinese Cbem. SOC.,19, 189 (1972). (5) A. H. Blatt, "Organic Synthesis",Collect. Vol. 1, Wiley-Interscience, New York, 1943, p 165. (6) W. L. Hase, R. L. Johnson, and J. W. Simons, Int. J . Cbem. Kinet., 4, 1 (1972). (7) J. F. Meagher, K. J. Chao, J. R. Baker, and B. S. Rabinovitch, J . Phys. Chem., 78, 2535 (1974). (8) P. J. Robinson and K. A. Holbrook, "Unimolecular Reactions", Wiley-Interscience,New York, 1972, p 272. (9) R. C. West, Ed., "Handbook of Chemistry and Physics", 56th ed, CRC Press, Cleveland, Ohio, 1976. (10) G. W. Taylor and J. W. Simons, Int. J. Cbem. Kinet., 3, 25 (1971). (1 1) T. H. Richardson and J. W. Simons, Cbem. Pbys. Lett., 40, 168 (1976). (12) W. C. Richardson, M.S. Thesis, Kansas State University, Manhattan, Kansas, 1969. (13) P. W. Payne, J. Cbem. Phys., 65, 1920 (1976). (14) T. Shimanouchi, Natl. Stand. Ref. Data Ser., Natl. Bur. Stand., No. 1, 39 (1972).
