T H E J O U R N A L O F
PHYSICAL CHEMISTRY Registered i n U.S. Patent Office
0 Copyright, 1979, by the American Chemical Society
VOLUME 83, NUMBER 10
MAY 17, 1979
Evidence for Walden Inversion in High Energy Chlorine-for-Chlorine Substitution Reactionst A. P. Wolf,” P. Schueler, Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973
R. P. Pettljohn, Kar-Chun To, and E. P. Rack” Depaflment of Chemistry, ,University of Nebraska, Lincoln, Nebraska 68568 (Received October 23, 1978; Revised Manuscript Received February 12, 1979) Publication costs assisted by Brookhaven National Laboratory
The stereochemistry of high energy chlorine-for-chlorinesubstitution was studied in gaseous and condensed phase 2(S)-chloropropionyl chloride and 2(R)-chloropropionyl chloride. Greater than 80% inversion of configuration was observed at the chiral center for both high energy 38Cland 34mClsubstitution. Net retention is observed in gaseous 2(S)-chloro-4-methylvalerylchloride where steric hindrance to backside attack is enhanced chloride. Condensed state data suggest caged radical recombination reactions. relative to 2(S)-chloro~propionyl
Introduction The fundamental question of whether “high-energy” chlorine atoms replace chlorine by retention or inversion of configuration a t asymmetric carbon atoms X*i + R,Xj
Xi for Xj
RX*i + Xj
in the gas phase is of importance since it can provide insight into the dynamics of hot-atom reactions. R~wlarndl-~ and Stocklin4i5have investigated the stereochemical course of halogen-for-halogen substitution reactions in halocarbon molecules with multiple asymmetric centers. These studies have been restricted to diastereomeric compounds such as ruc- and meso-(CHFCl), and rac- and meso-2,3-dichlorobutane because of experimental difficulties in resolving chiral molecules with a single chiral center. In their study of the hot-atom stereochemistry of rac- and meso-2,3-dichlorobutane, Rowland and cow o r k e r ~found ~ ~ ~greater than 93% retention of configuResearch carried out at Brookhaven National Laboratory under contract with the U.S. Dlepartment of Energy and supported by its Office of Basic ]Energy Sciences.
ration at the asymmetric centers in gas phase reactions while racemization occurred in the liquid and solid phase systems. Their results suggested cage radical reactions in the condensed state. In an important study, Stocklin et aL4 found the substitution of chlorine in liquid ruc- and meso-2,3-dichlorobutane by recoil 38Cl can lead to 110th retention and inversion depending on conformational change due to solvent interactions. In a subsequent study of 38C1-for-C1substitution in meso- and rac-(CHFC1)P Machulla and StBcklin5 postulated that hot substitution can proceed via two channels: (1)a direct replacement with retention of configuration in accordance with an impact m0de1,~-~ (2) a backside attack leading to the formation of a highly excited complex with lifetimes long enough to allow inversion of configuration to occur. However, recent studies by Ache, Rack et al.lOJ1observed no conformational effects in the stereochemical course of 38C1-for-C1substitution in various dichloroalkanes and (CHFC1)2. The stereochemical course of the substitution process is controlled by the properties of the solvent molecules, most likely intermolecular interaction between reactants and solvents. Their results1’ also suggest that the excited complex is formed by both direct and backside
0022-365~4/79/2083-1237$01.00/0 0 1979 American Chemical Society
1238
The Journal of Physical Chemistry, Vol. 83,No. 10, 1979
attack in 38C1-for-C1substitution reactions in (CHFCI),. In this paper we report for the first time the studies of high energy C1-for41 substitution in the gaseous and condensed phases at the asymmetric carbon atom of the enantiomers 2(S)-(+)-and 2(R)-(-)-chloropropionyl chloride in which steric effects are significantly minimized and on the sterically hindered enantiomer, 2(S)-chloro4-methylvaleryl chloride. In addition, attack in the gas phase on this molecule with a single asymmetric center minimizes or obviates the influence of conformational effects on the substitution reaction (vide infra).
Experimental Section Sample Preparation. 2(S)-(-)- and 2(R)-(+)-chloropropionic acids were prepared from optically pure R- and S-alanine according to the method of Fu et a1.12 2(S)-(+)and 2(R)-(-)-chloropropionyl chlorides were prepared by the method described by Lucas and co-workers.13 Polarimetric analysis of the R and S acid chlorides showed specific rotations [(wIz5D of -4.38 f 0.01 and [ o ~ ] ~+4.38 ~D f 0.01, respectively. (S)-(+)-valine methyl ester was prepared by passing anhydrous hydrogen chloride gas into a slurry of @)-valine in anhydrous methanol at 0 "C for 1.5 h. After recrystallization the optical purity of the ester was found to be greater than 98%. From 50 to 100 wL of the acid chlorides was used for the study. An aliquot of either S or R acid chloride was transferred to a quartz irradiation vessel. To minimize the possibility of hydrolysis of the acid chlorides the storage container and the sample vessel were purged with dry nitrogen before use. After transfer the samples were outgassed on a vacuum line by freeze-thaw cycles until gas evolution was no longer visible. The cyclotron vessels were closed off by turning the teflon stopcock while the reactor vessels were sealed off quickly with a gas-oxygen flame while the sample was frozen in the far end of the sample vessel. Sample Irradiation. The 34mClstudies were carried out a t the Brookhaven National Laboratory 60-in. cyclotron. A 20-MeV proton beam was used to produce 34mClfrom 35Clwhich has a natural abundance of 75.53%. By using 20-MeV protons for the in-situ production of 34mCl,llC isotope production can be suppressed to acceptable levels without seriously decreasing 34mClproduction. Nitrogen-13 yields at EOB are comparable to 34mClyield. In order to irradiate an adequate gas phase concentration, sample holders were heated during irradiation by an aluminum target holder. The irradiation vessel was inserted into the holder and allowed to come to equilibrium ( 30 min) before irradiation was begun. A temperature of 110 "C was used. The samples received an average irradiation of 50 bA s. The 10-mil quartz window of the irradiation vessel degraded the 20-MeV proton beam from 20 to 18.5 MeV. The beam was further degraded to 18.15 MeV in the target gas. Thus, the energy deposited during a proton irradiation of 2-chloropropionyl chloride at a pressure of 380 torr was 0.37 eV per molecule at 50 PA s. The 38Cl samples were irradiated at the epithermal patient facility at the Brookhaven Research Reactor at a thermal neutron flux of 3 X 1Olo n cm-2 s-l. The in-situ production of 38Cl was affected by the 37Cl(n,y)3sC1reaction. The gas phase samples were heated in an aluminum block which was surrounded by quartz thermal insulation. Heating was controlled by a heating tape. Temperature was measured on a calibrated ironconstantan thermocouple. Low temperature studies were carried out by irradiating the sample in an unsilvered quartz dewar containing either liquid nitrogen or dry ice. Dose rates to the sample were of the order of 60 rd/min. N
Wolf et al.
The neutron dose to gaseous sample was approximately lo-' eV/molecule for a 1-min irradiation. The samples were usually irradiated for periods of 5-15 min. Chromatographic Separations of Enantiomers. The stereochemistry of the replacement of chlorine atoms in alkyl halides which contain a single asymmetric center has not been studied, partly because of the experimental difficulties in separating the enantiomers within the period of time allowed by the half-life of the nuclide in question. Halpern and Westley14J5 have shown that the diastereomers of N-chloroacylamino acid methyl esters can be rapidly separated by gas chromatography. This technique was employed to resolve the enantiomers. A systematic search for the optimum resolving agent for 2-chloropropionyl chloride indicated that the @)-valine methyl ester gave the optimum resolution in the shortest time. Two columns were used for the separation of the diastereomers. Column I was 1 / 8 in. X 12 ft packed with 10% Carbowax 20M on Chromosorb-W 100/200 HMDS and run at 175 "C with a He flow rate of 37 mL/min. Column I1 was 1/4 in. X 20 f t of glass construction filled with 17% Carbowax 20M on Anakrom 80/90 HMDS with a He flow rate of 100 mL/min. For column I the SR and RR diastereomers eluted at 14.9 and 16.6 min, respectively, after injection; while column I1 gave a retention time of 55.5 and 61 rnin for the SR and RR diastereomers, respectively. Resolution using column I was superior to the resolution using column I1 but owing to the greater capacity of the latter (hence allowing larger quantities of sample to be injected) column I1 was used for all analytical determinations. The gas chromatograph used in this work was a Perkin-Elmer 880-FID. The column, injector end, and detector were all heated to 180 "C. The exit port of the chromatograph was fitted with a 1/8 in. swagelock to a 1/4-in.Ultra Torr fitting and was heated to approximately 200 "C. Fractions eluting from the chromatograph were trapped in glass tubes packed with 40/60 mesh activated charcoal with cotton plugs at both ends. The samples trapped in activated charcoal were then placed in vials for static counting in a NaI well counter. Deriuatization of Diastereomers. Post-irradiation derivatizations of diastereomers was accomplished by adding a solution of 0.05 g of (S)-(+)-valine methyl ester in 0.5 mL of chloroform to the irradiated sample. Triethylamine (0.2 mL) was added slowly and reaction was allowed to proceed for 5 min at 0 "C. At the end of 5 min 0.1 mL of absolute methanol was added to convert the unreacted acid chloride to methyl ester. The organic solution was then washed with distilled water, extracted, and dried with CaS04. An aliquot of the organic solution was injected into the gas chromatograph. The relative percent retention was determined from the activities found in the diastereomer peaks. The results listed in Table I have been corrected for optical impurities present in the resolving agent and in the acid chloride. Absolute Yields. Absolute yields for the C1-for-C1 substitution were determined by adding 0.5 mL of absolute methanol to the activated sample. The rate of formation of the methyl ester was very fast and was found to proceed to greater than 99% completion within 1 rnin at 25 "C. Due to the formation of the radionuclide 13N (tip = 10 min) during sample activation the samples were counted for about 100 min after the end of the irradiation in order to minimize the 13Ncontribution, There was no detectable 13N activity in the assayed peaks at EOB + 100 min. The specific activity of the irradiated sample was determined and an aliquot of the sample was injected into
The Journal of Physical Chemistry, Vol. 83, No. 10, 1979
Walden Inversion in CI-fa41 Substitutions
1239
TABLE I: Yields of "C1 and MmCl-for-ClSubstitution at the Asymmetric Carbon of 2(S)-(t ) and 2(R)-(-)-Chloropropionyl Chloride in the Gas (at t 110 "C) and Condensed Phases enantiomer -- nuclide
phase
MmC1 9mc1 3
4
~
1
"C1 3
~
1
j4mC1 MmC1
wmcl "C1
"c1
"C1
gar; (400 torr) ;ai; (400 torrj gas; (590 torr) gar! (590 torr) gas (590 torr) gw (800 torr) liquid ( t25 "C) liquid (+ 25 "C) liquid ( t25 "C) glass (-78 "C) solid crystalline (- 196 "C)
the gas chromatograph. The amount injected was determined gravimetrically. The methyl ester eluted after 13 min a t 130 "C with a He flow rate of 65 mL/min and was trapped on charcoal and counted (vide in€ra). The absolute yield after corrections for decay and for split of the effluent to the FXD detector was the activity found in the trapped ester peak divided by the total activity injected onto the column. Control Experiments. Derivatization of the diastereomers of N-2-chloropropionyl-(S)-valinemethyl ester has been described in the previous section. The derivatization takes place in chloroform in the presence of a considerable quantity of chloride ion and triethylamine. Isotopic exchange reactions are therefore possible, though unlikely, during the derivatization. If exchange reactions occur during the derivatization steps, the experimental results will not represent the true percent retention in C1-for-C1 substitution. A series of control experiments was carried out in which the derivatization was run in the presence of externally generated 34mClin admixture with the substrate. H34mCl was generated by irradiating a gaseous sample of methylene chloride with 20-MeV protons a t a dose similar to that used for 2-chloropropionyl chloride irradiations. The irradiated methylene chloride samples were washed with methylene chloride and H34mClwas isolated by gas chromatographic separation, using a 6% Halocarbon grease on Fluropak E10 column. The H34mCleluted rapidly (- 1-2 min) at room temperature and was trapped in a solution of (S)-(+)-valine methyl ester and chloroform. Derivatizatiori was performed under conditions identical with those described previously. After gas chromatographic separation, no detectable activity was found in the trapped diastereomers. To check the feasibility of 34m61-for-C1 exchange reaction in a more reactive mbstrate under these specific conditions, isopropyl tosylate was treated with H34mClundeir the same conditions. Labeled isopropyl chloride was readily isolated. We, therefore, conclude that post-irradiation 34mCl-for-C1exchange reactions are not significant urtder the conditions employed for the derivatization of the diastereomers. Effect o f Free Radical Scavengers in t h e Gas Phase. Wai and Rowland3 have used 1,3-butadiene as a free radical scavenger for the 38Cl reactions with 2,3-dichlorobutane and have found that the substitution of 3sC1-for-C1 proceeds almost entirely with retention of configuration in the presence of the free radical scavenger. This led to their suggestion that a free radical recombination mechanism is involved in tlhe formation of the inverted product. We have investigated the effect of 1,3-butadiene on the 38C1-for-C1substitution in 2-chloropropionyl chloride. In a study of the effect of concentration of 1,3-butadiene on the percent retention in the gas phase, a slight increase
absolute yield, % 1.3 f 1.3 f 1.0 f 1.0 2 1.0 f 2.4 f 4.3 2 4.5 f 4.1 t 6.6 f 7.8 f
0.1 0.1 0.1 0.1 0.1 0.9 0.2 0.2 0.3 0.1 0.1
ratio of retention to inversion
retention, %
18.8 21.3 t 17.6 f 17.7 f 20.0 t 37.5 f 53.0 f 53.4 f 52.7 2 47.5 * 49.2 t
2.0 2.0 0.9 8.0 7.3 7.1 2.0 2.0 4.7 4.1 0.4
-
0.23 f 0.03 0.21 c 0.03 0.21 f 0.01 0.21 f 0.13 0.25 f 0.12 0.60 i. 0.13 1.1.2 f 0.10 1.15 f 0.10 1.11 f 0.24 0.90 f. 0.17 0.97 i: 0.01
in retention (-8%) is observed when large amounts of the additives are present. The retentions are 15.2 and 23.5% with 2.89 and 23.5 mol % of 1,3-butadiene present, respectively. This may be due to the high efficiency of 1,a-butadiene as a chlorine moderator. I t is therefore concluded that the free radical combination mechanism described by Wai and Rowland is not operative in this system.
Results and Discussion Charge S t a t e o f Energetic Chlorine. High energy species produced by nuclear transformations are usually ionic. It can be assumed that species with very high recoil energies (in the kiloelectronvolt range) will undergo multiple charge-transfer collisions with the bath molecules before reaching the chemically reactive region and react as energetic atoms. For low energy species ( lo2 eV) this may not be the case. In their study of the reactions of energetic chlorine with gaseous alkyl halides, Wai and Rowland3showed that there was no difference in the chemical reaction pattern using (E,,, recoil = 530 eV) chlorine produced by 37Cl(n,y)38C1 and 40Ar(y,p)39C1(E,,, recoil lo3 eV) as one would expect from theoretical consideration. Additional kinetic moderation studies using argon and xenon as moderators indicated that the reacting chlorine species are not ionic. This is expected since a chlorine ion in a chlorocarbon environment of I P = 10.5 f 0.5 eV will be neutralized by exothermic charge transfer reactions. Therefore, it is assumed that the energetic 34mCl(Ernmrecoil = 0.3 MeV) and 38Cl,whose reactions are reported in this study, react with 2-chloropropionyl chloride as neutral atoms. Evidence for Inversion of Configuration. Presented in Table I are the absolute yields, percent retention, and ratio of retention to inversion of optical configuration for the gas to condensed phase C1-for-C1 substitution at the asymmetric carbon atoms of 2(S)-(+)- and 2(R)-(-)chloropropionyl chloride. In the gas phase where mechanistically complicating cage effects are probably not present, our results show a surprisingly small degree of retention of configuration at the asymmetric carbon atoms for both %C1and 34mClsubstitutions. The level of inversion has been determined independently for both the R and S isomers of 2-chloropropionyl chloride and they are identical within experimental error. This is the first report where net inversion has been observed in any gas phase halogen hot atom reaction. While it may be argued that 50% of the apparent inversion is actually due to racemization (the complete retention of configuration in other gas phase studies makes this an implausible consideration) there is the remaining 30% of the substitution reaction that must result from a Walden type inversion. Fukushima16 has studied the rotational conformations of 2-chloropropionyl chloride in the gas and liquid state.
-
-
The Journal of Physical Chemistry, Vol. 83, No. IO, 1979
1240
TABLE 11: Stereochemistry of 38Cland 34mC1-for-Cl Substitution at the Asymmetric Carbon of 2(S)-Chloro-4-methylvalerylChloride enan-
tiomer nuclide
s S
=c1 %“C1
tepp, C
phase
25 130
liquid gas
retention, %
* 1.3 59.3 ? 0.8
48.6
It was found that in the liquid state the molecules exist in three rotational conformations: trans, gauche, and gauche prime forms. In nonpolar solvents and in the gaseous state the gauche prime conformation becomes significantly more abundant than in the liquid phase while only the trans form persists in the solid state. The gauche prime conformation provides a relatively unhindered approach to attack of the asymmetric carbon from the backside (with respect to the 2-chlorine atom). In addition, trajectories for chlorine atom attack leading to Walden inversion involve a wide range of angles. The backside approach to 2-chloropropionyl chloride is less hindered than for any rotational conformation of 1,2-dichloro1,Zdifluoroethane or 2,3-dichlorobutane, both of which are relatively hindered in all conformations. If it is the open conformation which determines gas phase C1-for-C1 substitution with such a high degree of inversion, then blocking the backside with large substituent groups should hinder the inversion mode. In addition the increased inertia of the alkyl group should prohibit extensive motion during a reactive collision. The data in Table I1 show the percent retention of 38Cl and 34mC1-for-C1 substitution at the asymmetric carbon of 2(S)-chloro-4-methylvalerylchloride. The percent retention of 37mC1-for-C1 at the asymmetric carbon was found to be 59.3 f 0.8%. This large increase in retention compared to 2-chloropropionyl chloride is consistent with the hypothesis that the degree of retention (or inversion) is strongly affected by steric hindrance. There are several important implications to high energy chemistry. According to the impact m0de1,~-~ hot atom displacement reactions are fast (10-13-10-14s) and direct, independent of bond strengths, but dependent primarily on the point and direction of impact (steric factors). The inertial model, an outgrowth of the impact model, places severe restrictions on the hot atom substitution process: Hot atom reactions requiring nuclear motions which are slow relative to the time of collision tend to be forbidden. Specifically, substitution is affected by the presence of substituents other than those being displaced; substitution yields are related to the ability of the target molecule to rotate rapidly; and inertial effects are of greater magnitude than steric effects. Experimentally we have found C1-for41 substitution leading to preferential inversion of configuration in the gas phase for 2(S)-chloropropionyl chloride and 2(R)chloropropionyl chloride. However, C1-for-C1substitution in gaseous 2(S)-chloro-4-methylvaleryl chloride leads to predominant retention of configuration supportive of a steric factor being operative. It would appear that, in backside attack, either involving a multicentered collision complex or an inelastic reactive collision, little excitation energy is involved. However, the collision time must be long enough to ensure the inversion process (movement of substituent groups during the reactive collision). In propionyl chloride, substitution is not as predicted by the impact model, i.e., direct with retention of configuration, nor consistent with an inertial model. The general obvious implication of our experimental results is that both the impact and inertial models in their present forms do not have general validity in halogen hot atom chemistry.
Wolf et al.
Based on our results we can suggest the predominant factor controlling the substitution event is not inertial restriction but steric restriction. Not all hot atom substitution reactions are fast and direct involving a front-side attack, but can be from the backside of the molecule, colinear with the bond being broken, involving Walden inversion, perhaps by an inelastic reactive collision. If there is steric obstruction to Walden inversion then reactions can occur via other pathways. It is interesting to note that Bunker and PattengilP in their trajectory studies of hot atom reactions of tritium with methane suggested that some substitutions in methane may proceed with Walden inversion. T h e Gas to Condensed Phase E f f e c t . As can be seen in Table I, the absolute yields for the 38Cland 34mC1-for-C1 substitution display a density dependence with an increase in yield being observed as one proceeds from low to higher pressure to the condensed phase. This effect is similar to those previously reported by recoil fluorine: ~ h l o r i n e , ~ bromine,18 and i ~ d i n e . l With ~ - ~ ~the increase in absolute yield, there is an increase in the retention to inversion ratio, becoming one and remaining constant, within experimental error for the liquid, gas, and solid phases. This increase in stereospecificity in the gas to condensed phase transition is in contrast to the previous s t u d i e P on the stereochemistry of substitution to high energy halogen atoms at asymmetric carbon atoms in diastereomers. If the C1-for-C1substitution in the gas phase were mainly by direct substitution with retention of configuration and produced a racemic mixture in the liquid and solid phase, no conclusions could be made as to whether the substitution event were the result of ( a ) complex formation in excited states with lifetimes long enough to allow configurational changes5 (and stabilized by the cage effect to a greater extent in the condensed phase) or (b) caged radical reactions, as both modes are possible. If the C1-for-C1substitution in the gas phase were mainly by “inversion” of configuration and in the liquid and solid phases produced racemic mixtures, we could rule out significant contributions from excited complex formation with subsequent configurational changes, as one might expect their yield contributions would increase (increasing “inversion” product yield) with increasing density. Therefore, caged radical effects may be the explanation of the condensed phase experimental results. Interpretation of condensed phase stereochemical experiments using diastereomers is difficult due to the difference in the thermodynamic stabilities of such compounds as the meso and racemic modification of 2,3-dichlorobutanea If cage effects are present in these studies then the effect of the solvent on the conformation of a chloro-sec-butyl radical must be considered. Furthermore, the difference in energies between the apparent retained product and the apparent inverted product during recombination may give a distorted view of the stereochemical course of the replacement reactions taking place. Enantiomeric molecules provide a means of studying replacement reactions in which both the initial and final energy states of the reaction are thermodynamically identical. The experimental values of the retention of optical configuration in the condensed phases are 50% (within experimental error) as shown in Table I. These results could be due to racemization and/or equal rates of retention and inversion reaction channels. In order to test these possibilities a condensed phase study was carried out with 2(S)-chloro-4-methylvaleryl chloride. The properties of this molecule have been
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 CH3CIICOC1 and (CH3)2CH2CHCOC1radicals 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 a t 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 a t 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
