DECOMPOSITION OF BIBEKZYL COMPOUXDS
NOVEMBER, 1964
3341
Generation of Methyl Radicals by Decomposition of Bibenzyl Compounds Containing a-Methoxy Substituents G. E. HARTZELL A K D E. S. HUYSER Edgar C . Britton Research Laboratory, The Dow Chemical Company, Midland,Michigan Received July 6, 1964 Bibenzyl compounds containing a-methoxy substituents have been found t o undergo homolytic dissociation a t temperatures in the range of 100 to 300". The resulting a-substituted a-methoxybenzyl radicals undergo rapid elimination of methyl radicals with the formation of ketones. The rates of decomposition depend upon the nature of the substitution surrounding the central carbon-carbon bond. Utilization of these compounds as high-temperature, free-radical initiators was demonstrated.
The bond strength of the normal carbon-carbon bond of 83-85 kcal./niole is sufficiently great to prohibit free-radical dissociation of ethane into methyl radicals, except at extremely high temperatures. This normal bond-dissociation energy is greatly reduced by the presence of bulky substituents surrounding the carboncarbon bond, as in the case of polyarylethanes.* I n hexaphenylethane, for example, the dissociation energy is lowered to about 10-11 kcal./mole, and this coinpound, as well as many other polyarylethanes, is dissociated to an appreciable extent in solution a t room t e m p e r a t ~ r e . ~ This ease of dissociation is attributed both to steric repulsion between phenyl groups in the undissociated molecule and to resonance stabilization of the dissociated triphenylniethyl radicals. a-Alkoxyalkyl radicals produced by hydrogen abstraction at the a-carbon atom of ethers are reported to decompose, yielding alkyl radicals and carbonyl-containing compound^.^ This observation suggests that compounds of the general structure I should undergo homolytic dissociation, producing a-substituted amethoxybenzyl radicals (11),which should then decompose further, producing a ketone and methyl radicals according to the sequence shown in eq. 1. OCHa Ar-C-
'
OCH,
A I
-Ar
R'
R
I
"
OCHI
6H3 IV
\
6H3
\
V
VI
benzyl methyl ethers using di-t-butyl peroxide as the radical s0urce.68~ All four coinpourids prepared were found to undergo the expected decomposition reaction. For example, thermal deconiposition of a,a'-dimethoxy-a,a'-diiiiethylbibenzyl (111) in diphenylniethane yielded acetophenone, methane, and 1,1,2,2-tetraphenyletharle as products, consistent with this sequence (eq. 2-5) of
1
+ 2Ar-C. + I R
I1 2CH3.
+ 2Ar-
B
-R
CH3 CHa I11
CH3
(1)
Production of methyl radicals by thermal decomposition of such compounds suggests their potential utility as initiators of free-radical reactions. lZoreover, a series of free-radical initiators could be proposed, which would produce methyl radicals at rates dependent upon the nature of the substitution surrounding the central carbon-carbon bond. Four compounds of the general structure I which were studied are a,a'-dimethoxy-a,a'-dimethylbibenzyl (111), 4,4'-di-t-butyl-a,a'-diiiiethoxy-a,a'-dimethylbibenzyl (IV), a,a'-diniethoxy-a, a'-diphenylbibenzyl (V), and a,a'-diphenyl-a-methoxybibenxvlj (VI). These compounds were prepared in low yield by free-radical coupling of the appropriate a-substituted (1) T. L Cottrell, 'The Strengths of Chemical B o n d s " Butterworth a n d Co. (Publishers). Ltd.. London 1954,p. 201. (2) M. Gomberg J A m . Chem. Soe., 22,757 (1900). (3) C. Walling, "Free Radicals in Solution " John Wiley and Sons, Inc., New York, i%. Y., 1957,p. 530. (4) L. P. K u h n a n d C. Wellrnan, J . Org. C h e n . . 22, 774 (1956). (5) Methyl 1,1,Z,Z-tetraphenylethyl ether.
(6) R . L.Huang and S. S.Si-Hoe, Proc. Chem. Soc., 354 (19571. (7) This coupling reaction is the exapt rererse of the expected decomposition reaction and invol\.es the same intermediate radical (11). The temperatures involved in the preparative reactions were considerabls lower t h a n those required for the subsequent decomposition reactions, and the coupled product was sufficiently stable t o perinit Its isolation. Formation of small amounts of formaldehyde during preparation is thought t o result from attack of the t-hutoxy radicals on the methoxy-group hydrogen atoms.
HARTZELL A N D HUYSER reactions. After 3.5 hr. at 243O, 78Oj, of the theoretical quantity of methane had been obtained, and acetophenone and 1,1,2,2-tetraphenylethane were isolated in 86 and 96% yields, respectively, based upon the methane evolved. Evolution of methane was first order in 111, with good first-order plots being obtained. Similarly, IV, V, and V I were thermally decomposed in diphenylmethane to the corresponding ketones with evolution of methane. First-order rate constants were determined both from conventional first-order plots and using the method of Guggenheim.8 Plots of log k against 1 / T gave good straight-line relationships from which activation energies were determined. Kinetic data are summarized in Table I . TABLE I DECOMPOSITION OF BIBENZYL COMPOUNDS IN DIPHENYLMETHANE ki X
104,
Compd.
Temp., "C.
sec. -1
I11
235, 5 240 5 243 245 250 255 258 236 5 242 5 255 256 130 131 5 135 140 175 5 181 5 186 5 196
0 49 0 97 1 10 1 43 2 48 4 01 5 20 0 38 0 78 2 97 3 22 1 96 2 40 3 46 7 79 0 62 1 21 2 03 5 50
IV
V
VI
Esot. kcal./mole
58
57
TABLE I1 DECOMPOSITION TEMPERATURE RANGES,"C Compd.
Of particular interest in Table I is the fact that replacement of the two methyl groups of I11 by phenyl groups (V) lowered the activation energy by 14-15 kcal./mole and lowered the decomposition temperature. The effect of replacing a methyl group by a phenyl group 011 the activation energy was therefore about 7.0-7.5 kcal. This figure is consistent with the report that the difference between the effect of a phenyl group and a methyl group on the heat of dissochtion of substituted ethanes is about 6-7 k~al./niole.~ First-order rate constants were determined by following the rate of methane evolution. Side reactions occurring during the decompositions which did not evolve methane therefore did not interfere with determination of the rates of the reactions. Side reactions were not serious in the cases of compounds 111, IV, and V I as evidenced by the high yields of methane produced. However, only 2.5-30y0 of the theoretical quantity of methane could be obtained from decomposition of V in the temperature range of 108 to 140'. Compound V I was formed from the decomposition of V in diphenylmethane, as indicated by the n.m.r. spectrum of the reaction mixture arid by the observation that methane was again evolved when the temperature was increased (8) nique York, (9)
to the decomposition temperature range for compound VI.l0 Although a sequence of reactions can be written to explain the identified products, only about half of compound V could be accounted for after decoinposition. The decomposition of V is therefore not con^ pletely understood. Utility of the bibenzyl coiiipounds as initiators of free-radical chain reactions was demonstrated in two reactions. The free-radical additiop of 2-propanol to 1-octene at 240-250' was accomplished using a,cy'-dimethoxy-a,a'-diniethylbibenzyl (111) as the initiator. The yield of 2-methyl-2-decanol was 37%, based upon 1-octene consumed. These results were almost identical with those reported for this reaction using di-t-butyl peroxide as the initiator at 135O." Cyclohexane reacted with formaldehyde a t 240-250' to produce cyclohexanemethanol in 24% yield using I11 as the initiator. This reaction has been reported using di-tbutyl peroxide as the initiator a t 1350.12 I t was demonstrated that neither of these reactions occurs a t the temperatures involved in the absence of an initiator. a,a'-Diphenyl-a-methoxybibenzyl (VI) has been reported to be an effective substitute for peroxides as synergists in the formulation of self-extinguishing polystyrene foam.13 The calculated ranges of temperatures over which the bibenzyl compounds decompose to provide methyl radicals at useful rates are shown in Table 11.
43 5
46
E. Guggenheim. Phil. Mag., 2 , 538 (19361, A. Weissberger. "Techof Organic Chemistry," Val. 8, Interscienoe Publishers. Inc.. New N. Y., 1953, pp. 195-196. J . R. C o n a n t , b. Chem. Phys.. 1, 427 (1938).
VOL. 29
V VI I11 IV
ti/,
= 20 hr.
108 156 220 224
ti/,
=
1 hr.
130 186 248 252
ExperimentalL4 Methyl a-Methylbenzyl Ether.-A modification of a previously described procedure was used for the preparation of methyl cymethylbenzyl ether.ls A pressure vessel was charged with 360 ml. (3.12 moles) of styrene, 1200 ml. of methanol, 36.0 ml. of concentrated sulfuric acid, and about 0.6 g. of hydroquinone. The vessel was sealed under nitrogen and then heated wit,h stirring a t 90" for 24 hr. After cooling to room temperature, the vessel was opened and 39.6 g. of monoethanolamine was added. The reaction mixture was filtered and added to 600 ml. of water. The mixture was extracted four times with low-boiling (b.p. 30-60") petroleum ether. The combined extracts were dried over magnesium sulfate and concentrated under vacuum on a steam bath until the pot temperature reached 40". The residue was then distilled at 30 mm. The first fraction, b.p. u p to 73", weighed 48.1 g. and was determined by refractive index to be composed of 557, st,yrene and 48% product. The methyl cymethylbenzyl ether fractions weighed 315 g., b.p. 73-78' at 30 mm., n Z 61.4910 ~ (ki5 b.p. 71-76' a t 32 mm., nZ5D 1.4911). The yield was 86%, including the product contained in the first fraction. (IO) Decomposition of V into VI also occurs during preparation of V f r o m coupling of diphenylmethyl methyl ether if temperatures or reaction periods appreciably exceed those reported in t h e experimental procedure. (11) W. H. Urry, F. C. Stacey. E. S. Huyser, and 0. 0. Juveland, J . A m . Chem. Soc., 1 6 , 450 (1954). (12) G. Fuller and F. F. R u s t . ibid., 80, 6148 (1958). (13) J. Eichhorn. J . A p p l . Polymer Sci.. in press. (14) T h e n.m.r. spectra of all compounds prepared were consistent with t h e assigned structures. T h e authors are indebted to Dr. J. P. Heeschen of the Chemical Physics Research Laboratory of T h e Dow Chemical Co. for aid in interpretation of n.m.r. spectra. (15) S. I. Miller, J. Org. Chem., 21, 247 (1956).
NOVEMBER, 1964
DECOMPOSITION OF BIBENZYL COMPOUNDS
a,a'-Dimethoxy-or,a'-dimethylbibenzyl (III).-A mixture of 335 g. (2.46 moles) of methyl a-methylbenzyl ether and 60.0 g. (0.41 mole) of di-t-butyl peroxide was heated a t 120-125" for 48 hr. During the course of the reaction, 7.0 g. of distillate, largely t-butyl alcohol, was obtained and a small amount of paraformaldehyde was deposited on the attached condenser. The reaction mixture was cooled in an ice bath for about 2 hr., during which time crystallization of the product occurred. The white crystals were obtained by filtration and then recrystallized from 550 ml. of acetone. The weight of recrystallized I11 was 11.7 g., m.p. 170.5-172' (lit.16for meso 111, m.p. 171-173'). Anal. Calcd. for C18H2202: C, 79.96; H, 8.20. Found: C, 79.40; H, 8.13. Distillation of the remaining reaction mixture afforded recovery of 231 g. (1.67 moles) of unreacted methyl a-methylbenzyl ether. The yield of I11 was 11%, based on methyl a-methylbenzyl ether consumed. 4-t-Butyl-a-methylbenzyl Methyl Ether.-Preparation was analogous to that of methyl a-methylbenzyl ether. From 345 g. (2.16 moles) of p-t-butylstyrene was obtained 213.7 g. (52%) of 4-t-butyl-a-methylbenzyl methyl ether, b.p. 99-100" a t 10 mm., n z 5 ~1.4913. 4,4 '-Di-t- butyl-a, a'-dimethoxy-a, '-dimethylbibenz yl (IV) .Preparation was analogous to that of 111. A mixture of 192 g. (1.0 mole) of 4-t-butyl-a-methylbenzyl methyl ether and 24.3 g. (0.167 mole) of di-t-butyl peroxide was allowed to react for 24 hr. a t 120-125'. Recrystallized product (5.5 g., m.p. 220-221') was obtained. Anal. Calcd. for C26N380z:C, 81.62; H, 10.01. Found: C, 81.67; H, 9.81. Distillation of the remaining reaction mixture afforded recovery of 111.3 g. (0.58 mole) of unreacted 4-t-butyl-a-methylbenzyl methyl ether. The yield of IV was 77,, based on 4-t-butyl-amethylbenzyl methyl ether consumed. Diphenylmethyl Methyl Ether.-A solution of 10 ml. of concentrated hydrochloric acid in 314 ml. of methanol was heated a t reflux temperature, while a solution of 313.9 g. (1.71 moles) of benzhydrol in 314 ml. of methanol was added dropwise over a period of 4.5 hr. The reaction mixture was then heated a t reflux temperature for an additional 12 hr., after which 540 ml. of methanol was removed by distillation. Distillation was stopped when the pot temperatdre reached 100'. The remaining reaction mixture was cooled to room temperature and shaken with 200 ml. of 10% sodium bicarbonate solution. The resulting mixture was extracted with 160 ml. of chloroform. The chloroform solution was washed with 160 ml. of water. The two aqueous layers were combined and extracted with 160 ml. of chloroform. The chloroform layers were combined, dried over magnesium sulfate, and concentrated under vacuum on a steam bath. The residue was then distilled, with diphenylmethyl methyl ether being collected a t 157-158' a t 20 mm. (lit.'? b.p. 153" at 14.5 mm.). The yield was 302.0 g. (897,). a,a'-Dimethoxy-a,a'-diphenylbibenzyl(V).-A mixture of 99.0 g. (0.50 mole) of diphenylmethyl methyl ether and 36.6 g. (0.25 mole) of di-t-butyl peroxide was stirred a t 115-120' for 6 hr. The reaction mixture was then cooled and added to 275 ml. of methanol. Upon cooling in an ice bath, the white product crystallized from the solution. The crude product was recrystallized from 350 ml. of acetone. The yield of V was 3.5 g. (3.67,). This material did not possess a sharp melting point, but decomposed to liquid products when heated above 100". The observed melting point therefore depended upon the rate of heating (lit.18m.p. 157-159"). Anal. Calcd. for CZSH2602:C, 85.24; H, 6.64. Found: C, 85.66; H , 6.56. a,a'-Diphenyl-a-methoxybibenzyl(VI) .-A solution of 39.6 g. (0.2 mole) of diphenylmethyl methyl ether, 33.6 g. (0.2 mole) of diphenylmethane and 35.0 g. (0.24 mole) of di-t-butyl peroxide in 150 ml. of chlorobenzene was heated a t 125-130" for 24 hr. The chlorobenzene was removed by distillation under reduced pressure. To the residue was added 350 ml. of methanol, causing crystallization of the crude product, which was then recrystallized from 250 ml. of acetone. The yield of VI was 9.5 g. (137,), m.p. 160.5-162'. Anal. Calcd. for C Z ~ H Z ~ C, O : 88.97; H, 6.64. Found: C, 88.98; H , 6.67. (16) D. J. C r a m and K. R. Kopecky, J. Am. Chem. Soc., 81, 2748 (1959). (17) C. M. Welch a n d H. A. Smith, ibid., 1P, 4748 (1950). (18) E. Bergman and S. Fujise, Ann., 4SS, 65 (1930).
3343
Decomposition Reactions.-Thermal decompositions were carried out using redistilled diphenylmethane (b.p. 121-122' a t 10 mm.) as the solvent. The gaa evolved was collected by displacement of water which had been saturated with methane. I n each case, the gas was identified as methane by mass spectroscopy. Volumes were corrected to standard temperature and pressure. The identity of the ketones produced was established by infrared analysis and by preparation of 2,4-dinitrophenylhydrazone derivatives which were found to be identical with authentic samples. Decomposition of a,a'-Dimethoxy-a,a'-dimethylbibenzyl(111). -A solution of 5.00 g. (0.0185 mole) of I11 in 95.0 g. (0.56 mole) of diphenylmethane was heated to 243" as rapidly as possible. As soon as this constant temperature was reached, the volume of methane evolved was followed as a function of time. A total of 647 ml. (0.029 mole) of methane (787, of theory) was evolved in 3.5 hr. The reaction mixture was then distilled. The first fraction, b.p. up to 112" a t 10 mm., (largely 80" a t 10 mm.) weighed 5.17 g. (acetophenone, b.p. 78' a t 10 mm.). Infrared analysis of this fraction showed it to contain 58% acetophenone. The yield of acetophenone, based on the infrared assay, was 3.0 g. (0.025 mole). The second distillation fraction, b.p. 112-120" a t 10 mm. (largely 119.5-120" a t 10 mm.), weighing 85.6 g., waa recovered diphenylmethane. The distillation residue was washed with several small portions of cold ethanol. The resulting white, crystalline product was recrystallized twice from ethanol. This product, weighing 3.05 g., was identified as lt1,2,2-tetraphenylethane,m .p . 21 1.5-212.5 '. Allowing for the solubility of this material in the recrystallization solvent, the actual yield was calculated to be 4.7 g. (0.014 mole). Anal. Calcd. for CZ6HZZ: C, 93.37; H,6.63. Found: C, 93.24; H,6.46. Decomposition of 4,4 '-Di-t-butyl-a ,a'-dimethoxy-a, '-dimethylbibenzyl (IV).-A solution of 0.7090 g. (0.00185 mole) of IV in 19.5 g. (0.116 mole) of diphenylmethane was heated a t 255". A total of 6.8.6 ml. of methane was evolved in 3.0 hr. (83% yield). Decomposition of a,a'-Diphenyl-a-methoxybibenzyl (VI).-A solution of 1.0160 g. (0.00278 mole) of VI in 19.5 g. (0.116 mole) of diphenylmethane was heated a t 196". A total of 48.2 ml. of methane was evolved in 85 min. (77% yield). Decomposition of a,a'-Dimethoxy-a ,a'-diphenylbibenzyl (V) .A solution of 0.7280 g. (0.00184 mole) of Vin 19.5 g. (0.116 mole) of diphenylmethane was heated a t 130'. At the end of 170 min., 22.8 ml. of methane (257, yield) had been collected and the rate of gas evolution was almost negligible. Infrared analysis of the reaction mixture a t this point showed the presence of benzophenone. The presence of VI wae indicated by the n.m.r. spectrum of the reaction mixture. The temperature of the reaction mixture was then raised to 186". Methane evolution began again a t a measurable rate (due to decomposition of V I present) with 26.6 ml. (28% yield) being evolved within 160 min. At this point methane evolution again virtually ceased. Decomposition Rate Data.-The rates of decomposition of the bibenzyl compounds were followed by determination of methane evolution as a function of time. The general procedure was to prepare about 25 ml. of an approximately 0.05 M solution of the bibenzyl compound (weighed to j~0.0001g.) in redistilled diphenylmethane (b.p. 121-122" a t 10 mm.). (Complete solution was obtained only after heating.) The solution was saturated with methane and then heated to the desired temperature as quickly as possible in a silicone oil bath. As soon as constant temperature was attained, the apparatus was connected to gasmeasuring burets. The methane was collected by displacement of water which had been saturated with methane. Temperature was controlled to 3~0.5". Data were treated in two ways. Conventional first-order plots were obtained by using the methane evolution data to calculate concentration of the bibenzyl compound as a function of time. Methane evolution data were also used directly to determine first-order rate constants by the method of Guggenheim.8 Reaction of 2-Propanol with 1-Octene .-This reaction was carried out essentially as described, using a,a'-dimethoxy-a,a'dimethylbibeneyl (111) ws the initiator in place of di-t-butyl peroxide.ll The reaction was run at 24c250' in a sealed glass tube. The yield of 2-methyl-2-decano1, b.p. 49-52" a t 0.2 mm., n Z 61.4365 ~ (lit." b.p. 50" a t 0.2 mm., n z 01.4359), ~ was 377,, based upon 1-octene consumed. Vapor phase chromatographic
SAYIGH,TILLEY, AND ULRICH
3344
analysis showed that no 2-methyl-2-decanol was formed when the reaction was carried out in the absence of 111. Reaction of Formaldehyde with Cyclohexane .-This reaction was carried out essentially as described, using a,a’-dimethoxya,a’-dimethylbibenzyl (111) as the initiator in place of di-t-butyl
VOL.29
peroxide.12 The reaction was run a t 240-250’’ in a sealed glass tube. The yield of cyclohexanecarbinol was 24%, determined by vapor phase chromatography and infrared analysis. No product was formed under identical conditions in the absence of
111.
The Dehydrochlorination of Allophanoyl Chlorides. A New Synthesis of Isocyanates A. A. R. SAYIGH,~ JAMES N. TILLEY, AND HENRIULRICH Carwin Research Laboratories, The Upjohn Company, North Haven, Connecticut
06473
Received March 12, 1964 The thermal dehydrochlorination of N,X ’-dialkylallophanoyl chlorides afforded isocyanates in high yield. While X,S’-dialkylallophanoyl chlorides are virtually unaffected by base, the cyclic allophanoyl chlorides (2-imidazolidinone-N-carbonyl chlorides) could be dehydrochlorinated t o the corresponding diisocyanatee a t room temperature using a tertiary amine. The influence of various catalysts on the thermal decomposition of allophanoyl chlorides was investigafed and the mechanism of the hydrogen chloride elimination reaction is discussed.
Isocyanates are generally synthesized from primary amines and phosgene. I n certain special cases, however, this reaction does not proceed in the desired manner. Thus ethylene, as well as propylene diisocyanate, could not be synthesized from the corresponding diamines and phosgene. Although both alkylformamides and N-alkylureas have been converted to the corresponding isocyanates by treating the former with sulfuryl chloride3 and the latter with either boron trifluoride4 or nitrous acid,6 the N,N’-dialkylureas have not been utilized in the synthesis of isocyanates. According to eq. 1, N,N’-dialkylureas could react with 1 equiv. of phosgene to afford equiv. of isocyanate and hydrogen chloride. The r action would have to proceed by Nattack. Since the I\;-attack intermediates are the chlorides of the hypothetical allophanic acid, we will refer to these compounds from here on as “allophanoyl chlorides. ”
1
R-NH-G-NH-R
11
+ COClz +2RNCO + 2HC1
(1)
0
N,N’-diphenylurea is known6 to react with phosgene a t 150°, where the urea is sufficiently dissociated into phenylisocyanate and aniline,’ to yield phenyl isocyanate ; however, the formation of alkyl isocyanates from N,N’-dialkylureas has never been observed. It has been reported8 that N,N’-dialkylureas react with phosgene to give the undesired 0-attack products, the N,N’dialkylchloroformaniidine hydrochlorides. We have demonstrated recentlyg that allophanoyl chlorides also form; however, this involves the attack of phosgene by the nitrogen of the X,N’-dialkylureas. (1) To whom inquiries should be directed. (2) W.Siefken, Ann., 662, 75 (1949). (3) E. Kiihle, German P a t e n t 1,090,197(1960): Chem. Abstr., 66, 19799 (1961). (4) F. T. Sowa, U. S. P a t e n t 3,013,045(1961); Chem. Abstr.. 66, 8632 (1962). (5) S. Ross, A. Riva, a n d B. Piantanida. Chim. Ind. ( M i l a n ) , 42, 1243 (1960): Chem. Abstr.. 66, 25850 (1961). (6) W. Hentschel, Ber., 17, 1284 (1884): see also Production of Phenyl Isocyanate, O.P.B. R e p o r t No. 68,913 F r a m e No. 1082-1087. (7) H.Eckenroth a n d R.I. Wolf, R e r . , 26, 1463 (1893). (8) H. Eilingsfeld, hl. Seefelder. and H. Weidinger, Angew. Chem., 78, 836 (1960). (9) (a) H. Ulrich, J. N. Tilley, a n d A. A. R . Sayigh, J. Oru. Chem., 29, 2401 (1964): (b) J. N. Tilley a n d A. A. R. Sayigh, ibid., 29,3347 (1964).
O R-NH-
0 I1
R-NH-&NH-R
R
e’ -N
+ HC1
\
COCl
N-attacY
+ coci,
0-attach
[
R-NH-
I’
8’
=NH-R
C1-
+ CO2
The allophanoyl chlorides are stable a t room temperature and the lower members, such as N,N’-di-nbutylallophanoyl chloride (I),could be distilled in vacuo without decomposition. Above 100’ on prolonged heating, and faster a t 150-180’, they decompose to 2 moles of isocyanate and 1 mole of hydrogen chloride. This thermal decomposition was studied extensively on I and ethyleneallophanoyl chloride (11) as model compounds. Since the straight-chain allophanoyl chlorides differ in many respects from the cyclic, both series will be discussed. I on refluxing in benzene for several hours was unchanged; however, in refluxing toluene (115’) a slow decomposition with formation of tri-n-butyl isocyanurate (111) was observed. I n chlorobenzene a t 132’ a 70% conversion to n-butyl isocyanate occurred in 3 hr. The decomposition of I in concentrations of less than 10% in o-dichlorobenzene a t 180’ gave a 91% yield of n-butyl isocyanate. 0
I1
R-N
/c-cl
-
HC1 -4
2RNCO
\
C-NH-R
I/
0
I, R VII, R VIII, R IX, R X,R
= n-butyl = methyl = isopropyl = =
n-octadecyl cyclohexyl
The thermal decomposition without a diluent led to the formation of appreciable amounts of residues with I11 being the main product. This can be explained by Scheme I outlined below and is perhaps general for the aliphatic series.
