[CONTRIBUTION FROM
THE G-EORQE
HERBERT JONESCHEMICAL LABORATORY OF T"
U N I V E R S I T Y OF CHICAGO]
EFFECT OF ORGANIC PEROXIDES I N CHLORINATION REACTIONS M. S. KHARASCH
AND
MICHAEL G . BERKMAN
Received June 6 , 1941 SATURATED EIYDROCARBONS AND MONOCHLORIDES
A considerable accumulation of experimental evidence strongly indicates that, a t least in many cases, the chlorination of saturated hydrocarbons proceeds by means of a chain mechanism involving chlorine atoms and free radicals (1). In general such chlorinations are expedited by agents which promote chain initiation in one of the two following ways: (a) By the direct production of chlorine atoms. Heat, light, and traces of olefins (in "induced" substitutions on alkanes and alkyl chlorides) act in this way (2). (b) By the formation of free radicals which then react with molecular chlorine to liberate chlorine atoms. In chlorinations, the catalytic effects of hexaphenylethane in the liquid phase, of azomethane in the vapor phase, and of tetraethyllead in both phases (3) are probably examples of this mechanism, although there is also the possibility of direct production of chlorine atoms. In brominations and in hydrogen bromide additions to olefins, both oxygen and organic peroxides are well known to be effective catalysts, presumably because they initiate reaction chains by producing bromine atoms. Although liquid-phase addition of chlorine to olefins is but little affected by the presence of oxygen, the inhibitory effect of oxygen in vapor-phase chlorine substitution, has been well established (1, 3, 4). It is significant, however, that substitutive chlorination of olefins is facilitated by low (ea. 0.5010) oxygen concentrations at temperatures near 270" ( 5 ) . Under proper experimental conditions, oxygen may be capable of initiating both bromine-atom and chlorine-atom chains. The inhibitory effect of high oxygen concentrations is attributed to the chain-breaking reaction
c1. + 0 2 + ClO2. So far as is known, the analogous bromine-oxygen reaction does not take place. The effects of organic peroxides on the initiation of chlorine-atom chains remain t o be investigated, and the present study is a preliminary effort towards that end. 810
ORGANIC PEROXIDES IN CHLORINATIONS
811
General procedure. I n an air-free system a t mm. a measured volume of chlorine was condensed (by the aid of liquid nitrogen) into a bomb tube containing the hydrocarbon to be chlorinated. The reaction tube was then sealed off and maintained a t 0" in the dark for the desired length of time. Usually 0.05 mole (ca. 5.0 cc.) of hydrocarbon was treated with 0.005 mole of chlorine, although preliminary experiments showed that considerable variations in relative chlorine concentration do not significantly affect the results. At the completion of the experiment, analyses were made for free chlorine and for hydrogen chloride liberated; by these it was possible to account for all the chlorine originally introduced. CHLORINATION OF ALIPHATIC AND ALICYCLIC HYDROCARBONS AND HALIDES
Cyclohexane. Air-free cyclohexane absorbed by substitution 25% of the chlorine to which it was exposed a t 0" in the dark for 20 hours. The accelerative effect of light was demonstrated by control experiments; in systems illuminated by a 500 watt Mazda lamp at 20 cm., 100% of the chlorine was absorbed in one minute or less. The inhibitory effect of oxygen (2%) in the dark was shown by experiments in which no appreciable substitution took place a t 0" in 120 hours. Ascaridole, unlike oxygen, markedly accelerated substitution in the dark in air-free systems. For example, the presence of as little as 1% of this perioxide brought about complete reaction in 15-20 minutes a t 0". Within the range of concentrations studied, the inhibitory effect of oxygen appears to outweigh the accelerative effect of ascayidole, for, in 20 hours a t 0" in the dark, there was about 13% substitution in the presence of both reagents (oxygen 2-3%, ascaridol 2%) as compared to 25% substitution when both reagents were absent. In supplement to the foregoing brief digest, the data on the chlorination of cyclohexane are summarized in Table I. The studies on n-heptane, n-butyl chloride, and cyclohexyl chloride thereafter reported in Table I1 follow essentially the same scheme. AROMATIC HYDROCARBONS
It is well known that substitutive nuclear chlorination of aromatic hydrocarbons is facilitated by so-called halogen-carriers, among which are iodine, aluminum chloride, ferric chloride, iron, stannic chloride, activated charcoal, and aluminum oxide. On the other hand, side-chain chlorination is favored by light, by heat, and by the presence of peroxides, Le., in general, by the factors which facilitate the chlorination of saturated hydrocarbons. Nuclear and side-chain substitution may be simultaneous, as when chlorine is passed into toluene a t room temperature. It has also been shown that under suitable conditions chlorine reacts by addition as well as by substitution with some aromatic nuclei, notably those of benzene and toluene (6).
812
M. S. KHARASCH AND M. G. BERKMAN
So far as the present authors are aware, the addition of chlorine to t-butylbenzene or to m-xylene has not hitherto been reported. In order to test the hypothesis that additive chlorination should be favored by low temperature and by relatively high initial chlorine concentrations, studies were mttde on benzene, chlorobenzene, toluene, t-butyl-
-DURATION OF REACTION
1-2 min. 20 hours 40 hours 60 hours 1 min. 20 hours 120 hours 20 hours 5 hours 15 min. 10 min. 15 min. 20 hours 20 hours 0
TABLE I CHLORINATION OF CYCLOHEXANE" (In Vacuo in the Dark)
MOLE % OF ABCARIDOLE
7 0 REACTION
____-
-
1 25 40 60 100
-
2 2 1 0.5 0.1 0.5 2
N U M B E R OF EXPERIMENTS
0 0
-
100 100 100 98 85 12 13
6 10 4 4 6 5 5 6 3 2 1 1 1 1
REMARKS
Reaction illuminated 2 mole % O2 used 2 mole % O2used
2 mole % O2used 3 mole % 02 used
No individual experiment deviated from the average by more than 2%. TABLE I1 CHLORINATION OF ALIPHATIC COMPOUNDS IN
THE
ABSENCEOF AIR AT 0"
10% in 20 holm N o reaction in 100% in 20 min. 100% in 1 min. 20 hours 7% in 20 hours No reaction in 85% in 20 hours 100% in 1 min. Butyl chloride 96 hours Cyclohexyl chlo- 3% in 20 hours N o reaction in 52% in 20 hours 100% in 1 min. 96 hours ride
Heptane
benzene, and m-xylene at 0'. Exploratory experiments were conducted with toluene; hence, the chlorination of this hydrocarbon is described in detail. Table I11 gives a complete summary of the results obtained with toluene. Toluene. Five cubic centimeters (0.032 mole) of toluene and 0.005 mole of chlorine were allowed to react in the dark at ' 0 in the absence of air.
813
ORGANIC PEROXIDES I N CHLORINATIONS
At the end of 20 hours 99% of the chlorine had reacted, about 5470 by substitution in the side chain, and about 45% by addition to the nucleus. Illumination accelerated the chlorine consumption markedly, the time for complete reaction being 30 seconds under illumination, as against 20-24 houirs in the dark, but light did not appreciably alter the ratio of additive to substitutive chlorination (cf. expts. 186,210). The illuminated reaction is not much affected by ascaridole, and is only moderately retarded by oxygen (expts. 208, 209, 210). The only factor investigated which materially affects the ratio of substitution to addition is the rate of chlorine introduction. I n expt. 212, about 25 g. of chlorine was added in successive 5-g. portions to 150 cc. of toluene. Each chlorine addition was made at the temperature of a carbon dioxide-acetone mixture; the system was then
1
NtE& 221 182 186 203 2084 21.0 209 21.3b8 21.2bs c 23.1b a
GAO ADDED
None None Xone Sone None None 1% 0 2 Air Air None
1
TABLE I11 CHLORINATION OF TOLUENE TIME
2 hrs. 20 hrs. 24 hrs. 36 hrs. 30 secs. 30 secs. 1; hrs.
1
LIGHT O R DARg
70 REACTION ~
Subst.
50 54 55 56 58 55 55 7585 40-45 40-45
1
~
Add'n
25 45 45 44 42 45 45 15-20 50-55 50-55
NO. OF EXPTS.
1 6 4 2 2 2 2 1 1 1
T w o mole per cent peroxide (ascaridole) used. One hundred fifty cubic centimeters of hydrocarbon and about 25 g. of chlorine. Ilifference in rate of introduction of chlorine (see text).
allowed to warm to 0", and, when reaction ceased, the system was recooled and the process was repeated. In expt. 213, the presence a t any time of an excess of chlorine was avoided by bubbling the chlorine slowly into the tolu'ene held a t 0". The results indicate that addition is favored a t the expense of substitution by relatively high chlorine concentrations. t-Butylbenzene. About 5 cc. (0.032 mole) of hydrocarbon was treated with1 0.005 mole of chlorine in the dark a t 0" in the absence of air. After twenty hours, 87% of the chlorine had reacted, 59y0 by substitution, and 28%) by addition. In a similar 2-hour run, 77% of the chlorine reacted, 51% by substitution, and 26% by addition. I n 20-minute runs with 2Oj, ascaridole, conducted in the dark a t 0" in the absence of air, 84% of the chlorine reacted, 55% by substitution, and 29% by addition. Thus, or-
814
M. S. KRARASCH AND M. Q. BERKMAN
ganic peroxides accelerate both addition and substitution, but not so effectively as they accelerate the chlorination of saturated hydrocarbons. Benzene. Benzene (150 cc., 1.67 moles) was cooled (as in expt. 212 with toluene) and 27 g. (0.38 mole) of chlorine was introduced in 4- to 5-g. portions. When the excess of benzene had been removed by distillation, the residue readily crystallized. The melting point (157") indicated that the product was hexachlorocyclohexane, a conclusion further substantiated by analysis (Table IV, Experimental Part). At least 95% of the chlorine consumed was thus accounted for. Chlorobenzene. Experiments similar to those with benzene were conducted with chlorobenzene. As with benzene, there was little substitution under the experimental conditions; about 85% of the chlorine consumed was accounted for in the form of heptachlorocyclohexane. m-Xylene. In one experiment with this hydrocarbon, 25-26 g. (0.36 mole) of chlorine was passed rapidly into 150 cc. (1.2 moles) of m-xylene maintained a t 0" in diffut;ed light. Reaction was rapid, and there was profuse evolution of hydrogen chloride. Distillation of the resultant mixture under reduced pressure yielded, aside from m-xylene, a fraction which distilled over a range of about 20" and contained 27% of chlorine (evidently a mixture of monochloro substitution products), and a higherboiling fraction which contained about 55% of chlorine. In an otherwise similar experiment the xylene was maintained a t about -55", and the chlorine was added in 5-g. portions. When the addition was complete there was no perceptible chlorine coloration, and there had been practically no evolution of hydrogen chloride. Vigorous evolution of hydrogen chloride took place, however, when the mixture was warmed to room temperature. The final distillation fraction contained 52% of chlorine, and was shown bi? subsequent fractionation and analysis to consist of 4,6-dichloro-l ,3-dimethylbenzene and a tetrachloro substitution (?) product of m-xylene. In view of the observations recorded it seems probable that although there is undoubtedly some side-chain substitution, the initial reaction is principally addition. I n all probability the unstable tetrachloro addition product, when warmed, loses hydrogen chloride to produce the 4,6-dichloro-m-xylene isolated. A similar process would also account satisfactorily for the tetrachloroxylene, which was shown to contain two atoms of nuclear chlorine. The authors wish to express their appreciation to Dr. Frank R. Mayo for help in connection with this investigation. EXPERIMENTAL PART
Materials. Chlorine (Ohio Chemical and Manufacturing Co.) was found t o be 99% pure. In some experiments the chlorine from the tank was dried and distilled
ORGANIC PEROXIDES I N CHLORINATIONS
815
before use, but the results in these instances did not differ from those in which the chlorine had not been so treated. Therefore, in subsequent experiments, the chlorine was used directly from the tank, precautions being taken t o prevent the introduction of moisture. Reagent grade cyclohexane (Eastman) was washed first with concentrated sulfuric acid and then with fuming sulfuric acid. The cyclohexane thus treated, after being washed with water and dried over calcium chloride, was distilled through a column and stored over sodium wire in a dark bottle: b.p. 80" at 750 mm.; n: 1.4258. n-Heptane (Eastman) was treated with concentrated sulfuric acid. The hydrocarbon was washed first with m-ater, then with sodium carbonate solution, and final1.y with several portions of water. The n-heptane after being dried over calcium chloride, was distilled through a 35-cm. column. The fraction distilling a t 97.297.4" a t 746 mm. was collected and stored in a dark bottle over sodium wire; n: 1.3870. n-Butyl chloride (Eastman) was distilled through a column; the fraction boiling a t 76-77" at 750 mm. was collected and stored in a dark bottle; n: 1.4021. Cyclohexyl chloride was prepared by the peroxide-catalyzed chlorination of cyclohexane with sulfuryl chloride (7). The product was distilled through a 30-cm. column. The fraction boiling a t 141-143" a t 750 mm. was collected and stored in a dark bottle; n: 1.4610. Toluene (reagent grade) was refluxed over sodium and then distilled through an 8-ball Schneider column. The fraction boiling a t 109.5-110" at 750 mm. was stored over sodium wire; n," 1.4949. t-Butylbenzene was prepared by the Friedel-Crafts synthesis. The refractive index (n: 1.4927) and the lack of reactivity of the product towards bromine established the purity of the fraction collected a t 166.5-166.8" at 739 mm. Benzene (best grade, thiophene-free) was distilled before use; b.p. 79.5-80" at 750 nim.; n: 1.5005. Chlorobenzene (best grade, Eastman) was distilled before use; b.p. 131-132' at 747 nim.; n: 1.5241. Bromobenzene (reagent grade, Merck) was distilled before use; b.p. 153-155" at 747 mm.: TL; 1.5579. m-Xylene was distilled through a 30-cm. column. A fraction boiling at 138-139" a t 750 nim. (n: 1.4951) was used. Apparatus. -4vacuum line was used in the experiments conducted under airfree conditions. I n most cases about 5 cc. of the hydrocarbon (or chloride) was by pipette into a 10-cc. bomb tube which was then cooled and sealed to the line. The sample was then thrice degassed. Chlorine was introduced from the tank into a chlorine chamber of known volume attached to the line, and was brought t o atmospheric pressure by means of a sulfuric acid trap. The reaction tube was then cooled with liquid nitrogen, and by suitable manipulation of the stopcocks, the chlorine was distilled into this tube. Oxygen (when used) was introduced from a 2-1. chamber attached to the line, and brought to the required pressure (measured by a manometer). I n the vacuum-line experiments the extent of reaction was estimated by determining !,he unused chlorine and the hydrogen chloride evolved. I n all experiments with saturated compounds the sum of the two in moles was found t o equal the number of moles of chlorine originally introduced. Uncombined chlorine was determined by absorption in potassium iodide solution and titration of the liberated iodine. Sodium iodate was next added to the titrated portion; additional iodine, representing the hydrogen chloride formed in the substitution reaction, was thus liberated. It was titrated with standard sodium thiosulfate (8). For the quantitative determination of total halogen, in such substances as the
816
M. S. KHARASCH AND M. G. BERKMAN
toluene-chlorine product, the sodium and liquid ammonia method was used (9). About 0.3 g. of the substance t o be analyzed was dissolved in ether. This solution was added t o about 100 cc. of liquid ammonia in a 200-300-cc. round-bottomed flask. Small pieces of sodium were added until the blue color no longer faded. When this color had persisted for about one-half hour, i t was discharged by the addition of ammonium nitrate. The ammonia was then allowed t o evaporate. Ten cubic centimeters of ethyl alcohol was added to the residue, and the solution was evaporated t o dryness. Ten cubic centimeters of water was then idded, and the solution was boiled. The halide was titrated by the Mohr method. Halogens in the side chains of aromatic compounds or in aliphatic or alicyclic hydrocarbons were determined by the use of alcoholic alkali (10). About 100 mg. of the substance to be analyzed was dissolved in 10 cc. of absolute ethyl alcohol. To this solution, 5 cc. of 1 N sodium methoxide in methyl alcohol was added. The resulting mixture was refluxed gently for one hour, and the halide was titrated by the Mohr method. The results of the analyses appear in Table IV. TABLE IV
ANALYSESOF PRODUCTS SUBSTANCE TR E A IED
TOTAL CHLOBINE PRODUCTS
1 I
CHLORINE BEMOVED BY ALCOHOLIC NaOH
Found, %
Cal'd, %
Found, %
Cal'd, %
Toluene
Benzyl Chloride Chloromet h ylhexachloroc yclohexane
27.0 72.8
27.0 73.2
27.0 31.5
27.0 31.6
Benzene
Hexachlorocyclohexane
73 .O
73.2
36.5
36.6
I Heptachlorocyclohexane I
76.0
76.5
33.0
32.8
Chlorobenzene m-Xylene
4,6-dichloro-l, 3-dimethylbenzene Tetrachloro-m-xylene (?)
40.6
58.0
1 I
Sone Xone M.p. 68" M.p. 68" 29.0 29.0
SUMMARY
The chlorination of cyclohexane, n-heptane, n-butyl chloride, and cyclohexyl chloride proceeds slowly a t 0" in the absence of light and catalysts. The presence of organic peroxides markedly accelerated substitution in the dark. The reactions are also tremendously accelerated by light. Oxygen completely inhibits chlorination. Aromatic hydrocarbons are more readily chlorinated than aliphatic or alicyclic hydrocarbons. Chlorine absorption is relatively rapid in the dark at 0"; hence the effects of light and of peroxides are less marked. Oxygen inhibits reaction only slightly. Under the conditions described, aromatic chlorination takes place both by addition to the aromatic nucleus and by substitution. The percentage of addition product formed in the studies
ORGANIC PEROXIDES I N CHLORINATIONS
817
here described is: for benzene, 100%; for chlorobenzene, over 90%; for toluene, 45%; for t-butylbenzene, 35%. At -50" chlorine adds to mxylene, but a t 0" the addition product loses hydrogen chloride to give substituted m-xylenes with two atoms of chlorine in the aromatic nucleus. CHICAGO, ILL. REFERENCES (1) PEASEAND WALZ,J . Am. Chem. SOC.,63, 3728 (1931); KHARASCH AND BROWN, J . Am. Chem. SOC.,61, 2142 (1939); KHARASCH, BROWN, AND CHAO, J . Am. Chem. SOC.,62, 3435 (1940). (2) DIEANESLEY, J. Am. Chem. SOC.,66, 2501 (1934). (3) VAUGHAN AND RUST,J . Org. Chem., 6, 449 (1940). (4) BUNSENAND ROSCOE,Pogg. Ann., 96, 373 (1855); HASS,MCBEE,AND WEBER, Ind. Eng. Chem., 28, 333 (1936). (5) RUSTAND VAUGHAN, J . Org. Chem., 6, 472 (1940). (6) SIMITH, NOYES,AND HART,J. Am, Chem. soc., 66, 4444 (1935); HARDIE,u. s. Patent 2,218,148 (1940); LEEDSAND EVERHART, J. Am. Chem. SOC.,2, 206 (1880); VAN DER LINDEN,Rec. trav. chim., 67, 1075 (1938). (7) KHARASCHAND BROWN,J . Am. Chem. SOC.,61, 2142 (1939). (8) GROLL,HEARSE,RUST,AND VAUGHAN, Ind. Eng. Chem., 31, 1239 (1939). (9) MEYER,A n d . u. Konstit. org. Verb., 4te Auflage 263 (1922). (10) v . 4 ~DER LINDEN,Rec. trav. chim., 67, 1075 (1938).
