The Use of Semimicro Technic in Elementary Organic Chemistry-IV Semimicro Chlorination o j Organic Compounds1 NICHOLAS D. CHERONIS Chicago City Colleges, Chicago, Illinois
C
HLORINATION of organic compounds is seldom used m the teaching of the laboratory part of orp n i c chemistry. Examination of several laboratory manuals disclosed only one experiment describ'mg the preparation of a chloro compound (1). The time required for this experiment is given as 6 to 12 hours. "Organic Syntheses" (2) lists only the chlorination of o-chlorobenzaldehyde, which requires 15 hours for completion of the chlorination of one mole of the aldehyde. In the opinion of the author the danger involved in handling liquid chlorine in tanks, and the long time required for chlorination, as usually performed, are res~onsiblefor the relative absence of chlorinations from the organic laboratory. By the application of semimicro chlorination it is possible to remove most of the difficulties mentioned and to make available a large variety of experiments which can be used in the teaching of elementary organic chemistry. The basis of semimicro chlorination is not the diminution of the apparatus from a 500-ml. reaction flask to an &inch test tube. It is based upon the principle that an appreciable reaction will take place between two reactants if a large surface of contact exists between them; this implies that if one reactant is a gas and the other a liquid, a large surface of the gas must come in contact with the liquid. This condition is not realized by most laboratory chlorinations described in the literature, because chlorine is bubbled into the liquid to be chlorinated by means of ord'mary glass tubes. Since reaction takes place only a t the interface of the gas and liquid i t is clear that only a very small part of the bubble will react as it ascends and escapes from the liquid. The small amount of chlorine which dissolves in the liquid has a better chance to react. Solvents used in chlorinations owe such efficiency as they have to their solvent action for chlorine. ~ & c only-the e surface of a bubble reacts, it is logical to expect that if a large bubble is subdivided into 100 small ones the surface of the gas that will come in contact with the liquid will be tremendously increased. The essential feature of semimicro chlorination is that it disperses chlorine throuph a microporous tube which h& very fine openings;arying from 5 to 50 microns. This principle has been successfully applied to other reactions between gases and liauids as, for examule, hydrogenation and oxidation.
-
-
Another feature of semimicro chlorination is that the small quantities which are employed permit the use of apparatus which enables better photochemical activation if light is used for the reaction. Finally, the problem of handling liquid chlorine in cylinders is solved by generating the required amount of chlorine by the action of hydrochloric acid on pyrolusite. The apparatus is described in the experimental part. The amount of chlorine required by most semimicro chlorinations does not exceed 2.5 to 5 liters. CHLORINATION OF BENZENE
From the beginning of systematic organic chemistry, the study of benzene has been an honored subject, tempting both teachers and students to indulge in speculatious as to its abnormal behavior. Here is a hydrocarbon which should exhibit all the reactions of unsaturated compounds, yet i t does not. Benzene is shown to possess three double bonds which for some mysterious reason do not exhibit the characteristic addition properties of the orthodox type. The existence of double bonds is inferred from such experimental data as the formation of an ozonide and the catalytic pressure hydrogenation to cyclohexane. With reference to addition of chlorine, most textbooks state that in the presence of sunlight and absence of catalysts, chlorine reacts slowly to form the addition compound, benzene hexachloride, or 1,2,3,4,5,6-hexachlorocyclohexane, C6H6C16. Thus, normally (except in sunlight), benzene is thought to yield substitution chloro compounds.
h
l
o
No reaction B_ zene her achloride (hElslchl
C1, in presence of dilute alkali (14) CI,i. 01 E U I ~ W ~add C (15)
eye~ohcrane) Benzene hexachloride Benzene herachloiide Benzene hexachloride Benzene heraehloride chieay chlorobenzene;
CIS in presence of: FelPeCla).
~hlorobenzeneand benzene her*. cuocme Chlorobenzene and polychlorobcn-
Cb with heat and suolight (10.11) CI" in~.~~ oresence of r r a v ~~. .r (12) . . I
in P
C of p
t
l 3
.. ..
di-
zene AI(AICIa), S, I,, SbCIs,SbCla. (16-21) Chlorobenzene, dichlorobenzene, nnd Electrolytic chlorination (22) benzene herschloride
Presented before the Division of Chemical Education of the American Chemical Society, 103rd meeting, Memphis, Tennessee, April 21, 1942, and 105th meeting, Detroit, Michigan, April 12, 1943.
61 1
Benlene heaachloride Chlarobenzene Chlorobenzene Benzene herachloride
A survey of the literature, summarized in Table 1,
shows that benzene hexachloride is formed: (a) by the action of chlorine which has been activated photochemically; (b) by the action of chlorine on benzene in presence of dilute alkali; (c) by electrolytic chlorination. The standard method for preparing benzene hexachloride is chlorination in presence of dilute alkali. The action of chlorine on benzene in presence of various metal catalysts yields chlorobenzene and polychlorobenzenes. In the course of the aresent work. suitable ex~eriments were developed by which it is possible to show that benzeue reacts with chlorine with the same ease by addition as by substitution. The formation of benzene hexachloride takes place not only by photochemical activation but also by catalytic activation in the dark. The addition of chlorine to benzeue is induced by light, alkali, x-rays, a-particles, and electrolytic chlorine, and therefore can be assumed to take place rapidly whenever molecular chlorine is activated and forms chlorine atoms. The mechanism for the reaction of chlorine atoms with benzene has been shown to be a chain reaction (13, 29) represented as follows: ~
~
---
Condirionr; Coralrri In thedark; heated at intervals lll~minationt n~vmination In the dark: benzoyl peroxide, 0.2 g. Illuminated: iron. 0.5 g. Illuminated; iron, 0.2 g.; sulfur,0.1 g. IlluminatedS; benzoyl peroxide; 0.2 g.
Readion Timc (Min.)
---Chtorinnlsd ClRaCl8
Yidd
M.P.
(G)
lo C.)
60
Ot
90 60 60
2.5 3.4
90
0
60
0
60
8.4
lot
...
129-131 152 148
ProdudC&CI CIRICI, Yidd Yield IG) (GI 0 0
0 0
o
o
0
0
. ..
7.8
8.2
...
5.5
4.5
146
0
0
-
*
In each run 13 g. (15 ml.) of pure benzene was used. Flow of chlorine 60 to 80 rnL per minute. t Ten gram. of unchanged benzene reeevaed. Residue (0.4 g.) oily. resembled odor of C*HsCls,but did not eryrtallize. t pistance of 6a-wattbulb from reaetion tube wns 2 un. Flow of d o "ne in this run was higher thanin others. I Commercial benzene was u e d without any purific~tion.
to benzene can be explained tentatively by the free radical mechanisms proposed by previous investigators (28, 29).
Substitution
+ Cln 2C1&Ha + HCI Cl + &HI H + Clz HCI + C1 C& + CIS G H S l + C1 A
Addition (ravid)
It was noted by Luther and Goldberg (10) that although oxygen inhibits the photochemical chlorination of benzene, traces of oxygen apparently activated chlorine. Recently Warasch and Brown (30) used sulfuryl chloride in presence of organic peroxides as chlorinating agent and explain their procedure as due to the formation of chlorine atoms. Kharascb and Berkman (28) showed that ascaridole markedly accelerates the reaction between cyclohexane and &lorine a t 0' in the dark in air-free systems, thus establishing that organic peroxides may initiate chlorine atom formation from molecular chlorine. It appeared, therefore, that the addition of a small amount of an organic peroxide to benzene in presence of chlorine would have the same effect as the action of photochemically activated chlorine. It was found that addition of 0.5 to 1per cent of benzoyl peroxide to benzene, while chlorine is being passed through (in the dark), causes a rapid addition of chlorine to take place, with the formation of benzene hexachloride. To the knowledge of the author it has not been previously reported that benzene hexachloride can be produced in appreciable quantities by addition of chlorine in the dark. Table 2 lists a number of experiments in semimicro scale which can be used to illustrate the reaction of benzene and chlorine by addition and substitution. The mechanism for the catalytic addition of chlorine
From the results of the experiments summarized in Table 2 it appears tbat when the microporous disperser is used, the addition of chlorine is rapid, if a peroxide catalyst is employed either in the light or in the dark. Further comparison of runs 1, 5, and 6 shows that about the same amount of chlorine reacts by addition as by substitution, in about the same period of time. Although these experiments are not quantitative, they can be used as evidence for the assumption that under appropriate conditions the addition of chlorine to benzene can be made as rapid as reactions with chlorine, which involve substitution. Also it is of interest to note that commercial benzene which contains water, thiophene, and other impurities undergoes addition of chlorine with the same ease as purified benzene. The benzene hexachloride obtained under the conditions described is a mixture of several isomers, with the or-isomer predominating. Of the eight possible hexachlorocyclohexanes, four have been reported in the literature (2, 31, 32). The melting point of the orisomer is given as 157' and 158", the &isomer as 297' and 310°, and the y- and 8-forms as 112°-1130 and 129°-1320 respectively. Linden, who isolated the last two isomers, claims that a eutectic mixture of 79.7 per cent of the a-isomer and 20.3 per cent of the 8form melts a t 155.5O, which is only 1.5' lower than the melting point often reported for the pure a-form. The product obtained in this work by the action of chlorine in the presence of light gave an initial point of 13O0132'; after two crystallizations from a mixture of benzene and ethanol the product gave a constant melting point of 155°-1560. On the other hand the product
obtained by chlorination of benzene in the presence of benzoyl peroxide in the dark had an initial melting point of 14S0, and after one crystallization, 155'-156'. On the basis of these limited observations it seems that photochemical activation leads to the formation of a mixture of several isomers, whiie peroxide-catalyzed addition gives almost exclusively the a-form. CHLORINATION OF TOLUENE, NAPHTHALENE, AND
CYCLO-
HEXANE
The results of semimicro chlorinations of toluene, naphthalene, and cyclohexane are summarized in Tables 3, 4, and 5. They indicate that it is possible to devise appropriate experiments for students to illustrate side chain and nuclear substitution in toluene. In the case of naphthalene the chlorinated products are mixtures of a number of isomers which are not easily separated. For this reason the chlorination of naphthalene is not recommended for student experiments. The crystalline product formed by addition is a mixture of: naphthalene tetrachloride (tetrachloronaphthalene-1,2,3,4tetrahydride) ; or-chloronaphthalene tetrachloride (1,1,2,3,4 - pentachloronaphthalene - 1,2,3,4 - tetrahydride); and 1,4-dichloronapbthalene tetrachloride (1,2,3,4,5,8 - hexachloronaphthalene - 1,2,3,4 - tetrahydride). The chlorination of cyclohexane takes place with great ease. It will be noted that the dichloro derivatives are the chief products after 30 minutes of chlorination, particularly if peroxide is used as a catalyst. Therefore, chlorination for 15 to 20 minutes is considered sufficient for student experiments.
-.
Iron. 0.5 g. Iron,0.1 8.; sulfur. 0.1 8.
...
90 90
7.5 8.5
...
1.5 1.0
-
*In each run 13 g. (15 ml.) of pure toluene was used. Flow of chlorine 60 to 100 m1. per minute.
Illumination Illumination Illumination Illumination Sulfur, 0.1 6.; iron, 0.2 g. (dark) Benzoyi peroxid% 0.2 p. (dark) Benzoyl peroxide, 0.2 g. (dark)
80 to 90 120 to 150 80 to 90 120 to 150 80 to 00
90 60 30 15 60
80 to 90
60
80 to 90
30
0 0 3.0 3.8 10
0 0 5.1 4.6 0.5
14 15 3.6 3.2 0
0
0
13.0
1.8
5.0
10.0
-
* Cyclohexsne, 11.7 g. (15 ml.), was placed in a 25 X 250-mm. pyrer tube, and chlotioe passed through disperser. Heated to boiling only in the beginning t In each, 11.7 p. (15 ml.) of cydohexane was med in a 25 X 250-mm. Pyrex tube and chlotine passed through the mieroporous disperser. Except in run NO.5 the hydrocarbon wa. heated and then chlorine war passed. Run No. 6 war heated at intervals. f The fraction collected boils at 140°to 145'. & T w ofraction3 were collected, one boiling at 185' to 195" and the other at 202' to 210'. The boiling points of dichlorohexanen are given as 187' t o 189'. 193' to 194'. and 201'to 202'. EWERIMgNTALPART
Apfiaratusfor Semimicro Chlorination. Figure 1represents the apparatus used for semimicro chlorination. The flask (50 to 100 ml.) is charged with 50 g. of technical manganese dioxide, and about 50 ml. of commercial hydrochloric acid is added through a separatory funnel or thistle tnbe. The flask is heated by a small flame intermittently until a constant stream of chlorine is obtained. The flow of chlorine is regulated to such a rate that little chlorine escapes from the reaction tube. The wash bottle contains 250 to 300 ml. of concentrated sulfuric acid. The height of sulfuric acid in the safety valve tube should not exceed 35 cm. If the acid column shows a pressure of 40 cm., it is an indication that the disperser is clogged. In such case, the rubber tube which connects the disperser with the generating train is disconnected and the generator cooled. The disperser is cleaned by wanning in a test tnbe with 5 ml. of aqua regia, then by drawing through it by means of a pump, first water to which a few drops of 6 N sodium hydroxide have been added, then 5 ml. of
TABLE 4
C l a o n ~ ~ m r oolar N ~ ~ ~ r a a ~ AT n w70s *l o 76'
Roocliol Time Condilionr; Cnlniyrl Illumination: no solvent CCL. 10 ml.: illuminati& CCh, 10 mi. (CIH.COO).. 0.2 g. Dark; CUA CCI,. 10 ml.; S. 0.2p.; Fe,0.3 g. Benzene, 10 ml.; Is, 0.2 g.
-
(Mi".) 60
Chlo"iro1rd ProdudSubli13ion a-ClaHCI-oil 8-CxoHEI, 55' lo 56' C~dIaCls (lo iromcn)
Addition ClaRaC4.182: C ~ H C I EI3I . C I O H ~ C I172" I.
1.5
174t
60
3.2
127t
60
3.0
151t
60
2.4
l6Ot
60
... ...
.... ....
..
...... ...... ...... ...... 7.2% 4.5 3.5
'Fifteen grams of naphthalene was used in each run. t After three eryotallizationr the melting point changed to 148' to The product melts with evolution of HCI. f Mostly polychloronaphthaleoes. Could not be separated. $ Mixtures of manochloro and diehloro (mostly 2.6-diehloro).
FIGURZ1.-APPARATUS FOR SEMIMICRO CHLORINATION
alcohol; and finally ether. The chlorine wash bottle is connected to a small wide-mouth bottle which contains calcium chloride and a layer of glass wool. The reaction vessel consists of a regular &inch pyrex tube. It is closed by a three-hole rubber stopper in which are inserted a microporous disperser, a miaocondenser, and an outlet tube for gases. The disperser has been described in the previous article of this series (33). The microcondenser is inserted about 80 to 90 mm. in the 8-inch reaction tube. Materials. Commercial benzene was purified by washing with sulfuric acid, followed by water, drying with calcium chloride, then distilling over sodium. Commercial toluene was fractionated, dried with sodium, and redistilled. The impure material gave about the same amounts of chlorination products. Naphthalene was purified by crystallization from alcohol. Cyclohexane was obtained from the Eastman Kodak Co.
A second crystallization may be necessary to obtain crystals melting a t 156". Chlorobenzene. Fifteen milliliters of benzene are chlorinated in the presence of 0.2 g. of iron powder or 0.2 g. of iodine. Illumination by means of a 60-watt electric light (at a distance of 2 to 3 cm.) is found advantageous. Chlorine is admitted for 1to 1.5 hours. The tube is removed and fitted with a semimicro fractionating column (34) and fractionated. The fraction boiling a t 125" to 140' is mostly chlorohenzene, while the fraction boiling a t 170' to 190' consists of dichlorobenzenes. The yield of monochlorobenzene is usually 5 to 7 g. and depends on the amount of chlorine admitted. Benzyl Chloride. Fifteen milliliters of toluene and 0.2 g. of benzoyl peroxide are placed in the reaction tube and the mixture chlorinated with or without illumination for 1 to 1.5 hours. The mixture is heated to the boiling point of toluene in the beginning. After chlorination the tube is removed and the mixture fractionated. The fraction boiling a t 175" to 180" is collected. The yield of benzyl chloride is 6 to 7 g. Chlorotoluenes. Fifteen milliliters of toluene and 0.5 g. of iron (or 0.1 g. of iron and 0.1 g. of sulfur) are placed in the reaction tube and chlorinated for 1 to 1.5 hours, with or without illumination. Heat is applied only in the beginning. On fractionation of the chlorinated mixture the fraction which boils a t 155" to 160' is collected. o-Chlorotoluene boils a t 159" and pchlorotoluene a t 162'. Chlorocyclohexane. Fifteen milliliters of cyclohexane and 0.2 g. of benzoyl peroxide are placed in the reaction tube and chlorinated for 15 to 30 minutes. The mixture is fractionated (34). The fraction which boils a t 140" to 145' is monochlorocyclohexane, while the fraction boiling at 180' to 205' consists of dichlorocyclohexanes. ACKNOWLEDGMENT
Benzene Hexachloride. Commercial benzene (15 ml.) is placed in the reaction tube and 0.2 g. of benzoyl per,oxide added. The tube is heated until the benzene just bepins to boil: then heatinp is discontinued. Chlorine is passed for about one hour. The benzoyl peroxide may be omitted and a 60-watt electric light bulb (frosted) is placed 2 or 3 cm. from the reaction tube. Light and peroxide used together give better results. After about one hour the reaction tube is removed, corked, and cooled in running water for about one hour. The contents of the tube crystallize into a solid mass. The mixture is filtered by suction and washed with three 5-ml. portions of alcohol. The hexachloride consists mainly of a-isomer, with variable amounts of the other three isomers, and melts below 150". The yield varies from 3 to 8 g., depending on the rate a t which chlorine is admitted into the reaction tube. A further amount of crude hexachloride is obtained by distilling most of the benzene from the filtrates and adding alcohol. The hexachloride is purified by dissolving it in the minimum amount of hot benzene and adding 3 volumes of alcohol.
The author wishes to acknowledge the help of those of his students who in parts of this work: charles ~ ~ t~~~~k t ~johnson, , and ~ d~ ~ ~ ~ ~ LITERATURE CITED
(1) . . COHEN,"Practicsl Organic Chemistry," Macmillan and Company, Ltd., London, 1924, p. 106. ( 2 ) G-ur AND BLATT,"Organ~cSyntheses," John Wiley and Sons. New York. 1941. Collective Volume I.D. 155.
d i
THE USE OF SEMIMICRO TECHNIC IN ELEMENTARY ORGANIC CHEMISTRY-IV (Continued from page 614) COHENAND DAKIN,3.Chem. Soc., 79,1118 (1901). WILLGERODT, 3. prakt. Chcm. (2) 34,264 (1886). JUENEHOMMA, I . chim. phys., 32, 173 (1935). BENDER,U. S. Pat. 2,010,%1 (1935). SCHMIDT, Ber.. 11, 1173 (1878). DUBOIS,Bull. acad. m y . Belg., 42, 126 (1876). COLEMAN AND NOVES,3.Am. Ckem. Soc., 43, 2216 (1921). HENTSCHEL, Ber., 30, 1436 (1897).
AND BERKMAN, I . OIg. Ckem., 6,810 (1941). (28) KHARASCH (29) NOYESAND SMITH,I. A m . Chem. Soc., 54, 161 (1932). (30) KHARASCH AND BROWN, {bid.. 51,2142 (1939). (31) LINDEN,Ber.. 45, 231 (1912). AND HANSON, 3.A m . Chem. Soc., 53 (1931). (32) STEWART . 488 (1943). (33) J. CHEM.E ~ u c .20, (34) CHERONIS."Semimicro and Macro Organic Chemistry," Thomas Y . Crowell Company. New York. 1942, p. 74.
