T H E ACTION OF FLUORINE ON CERTAIN AROMATIC COMPOUKDS AND A THEORY OF RING SUBSTITUTIOK* BY STEPHEN F R A S C I S WHEARTY, J R .
The Theories of Substitution and Orientation in the Benzene Ring When halogens act on benzene derivatives containing side chains there will be substitution in the nucleus or side chain depending upon conditions. In the case of bromine and toluene the facts may be summarized as follows:--’ “ I . Ring and chain substitution in the dark, temperature effect. 2 . Chain substitution in the sunlight. 3 . Chain substitution in the dark with HzO or dilute NaOH. 1. Chain substitution with PC15. j. Chain substitution with sulphur as a carrier. 6. Some chain substitution in the dark with HzO or dilute NaOH. j . Ring substitution with FeCl,, IC1, SbC13, etc. 8. Ring substitution by electrolysis. 9. Some ring substitution in the dark with H20 or dilute KaOH.” Various explanations have been offered to account for these phenomena, that of Bruner? perhaps being the first of real significance. He maintains that, nuclear substitution is due to a dissociation of the halogen molecule into atoms or ions and assumed that chain substitution is due to molecular halogen only. This hypothesis is favored by reaction velocity measurements since both side chain and nuclear substitution processes apparently take place according to the requirements of the equation for a unimolecular change. Bancroft3 enlarges upon Bruner’s theory by assuming that nuclear substitution is caused by negative halogen ions and that side chain substitution is caused by positive ions. His five assumptions are as follows:”(a) There is a elight reversible dissociation of bromine into positive and negative particles or gaseous ions. Brz = Br‘ Br’ (b) There is a slight reversible reaction between bromine and the positive gaseous ion. Brz . Br = Br2 Br’
+
+
(c) Substitution takes place in the ring when the negative gaseous ions are present in excess; otherwise it takes place in the side chain. (d) The addition products of benzene are to be considered as analogous to the chain substitution products of toluene. * This work was done under the programme now being carried out a t Cornell University
and supported in part by a grant from the Heckscher Foundation for the Advancement of Research established by August Heckscher a t Cornell University. Bancroft: J. Phys. Chem., 12, 417 (1908). * Z.physik. Chem., 41, 513(1902);Cracovie: Bull. Akad. Sei., 1907,691;1910,516,560. Bancroft: J. Phys. Chem., 12,417 (1908).
3122
STEPHEN FRASCIS WHEARTY, JR.
(e) There is a reversible dissociation of the halogen carriers with formation of so-called gaseous halogen ions. Whether these ions are positive or negative depends on the nature of the carrier.” “Of these assumptions the first and fourth are not new; the first half of the fifth has been made implicitly by everybody who has tried to account for halogen carriers without postulating the formation of an intermediate compound. It was explicitly made by Bruner. The second half of the fifth assumption is new but it is a necessary consequence of the first part of the assumption.” “The second assumption is new, so far as I know, and is made because I cannot get along without it. The third assumption is also new but it is a necessary consequence of the first two assumptions.” “I will now show how these assumptions apply to the cases we have to account for. At low temperatures and in the dark, there will be a formation of Brl.Br’ and consequently an excess of Br’. The relative excess will be greater, the greater the concentration of the bromine. We should therefore expect ring substitution in concentrated solutions with an increasing percentage of substitution in the side chain as we start with a more and more dilute bromine solution. This is exactly what was found by Bruner rising temperature we get increasing dissociation of Br2. Br’, the ra to Br’ increases, and consequently we get an increasing percentage of benzyl bromide among the reaction products. This was also found experimentally Sunlight will increase the dissociation of Br2 and Br2. Br’ and the ratio of Br to Br‘ up very close to unity and consequently under the third assumption we shall get side chain substitution only, provided the intensity of the light is sufficient. In diffused light we shall of course get a state of things intermediate between the results for darkness and bright light. Both of these predictions have been confirmed experimentally.” “With a carrier such as ferric chloride our fifth assumption calls for a reversible dissociation into gaseous halogen ions and ferrous chloride. With iron we can, of course, only have negative gaseous halogen ions and the equilibrium will be represented by the equation FeC18 = FeClz zC1’ Cnder these conditions there will be enough negative gaseous ions to overbalance any positive ions which might come from the chlorine gas and we should expect to get ring substitution even in boiling toluene and even when the system is exposed to bright sunlight. This is exactly what happens.” “With the chlorides of iron, antimony, molybdenum and aluminum, there can be no question but that the chlorine is the negative radical. With iodine monochloride, however, the matter is more open to doubt. Since chlorine is a more powerful oxidizing agent than iodine, we might reasonably expect that iodine would be the positive radical and chlorine the negative radical. The margin of safety is not large however and the experimental results are just what we should expect under these circumstances. Iodine monochloride as carrier does substitute in the ring; but the dissociation is not sufficient to
+
ACTION OF FLUORINE ON AROMATIC COMPOUNDS
3123
prevent substitution in the side chain in boiling toluene, especially if the concentration of iodine monochloride or monobromide is low. Of course, if one were to decrease sufficiently the concentration of ferric chloride as carrier there would come a point a t which side chain substitution would also occur in boiling toluene in spite of the iron; but no such point has been determined experimentally.” “While iodine monochloride is on the line, phosphorus pentachloride is apparently over it and either forms positive gaseous chlorine ions only or an equal quantity of positive and negative gaseous chlorine ions. It makes no difference which reaction we Dostulate
+ +
PClS = PC13 c 1 C1’ or Either one will account for the formation of benzyl chloride by the action of chlorine and phosphorus pentachloride on toluene. Since phosphorus pentachloride accelerates the rate, we must look upon it as a halogen carrier. This differs from the case of iodine monochloride because side chain substitution takes place with phosphorus pentachloride and chlorine in sunshine. Willgerodt has shown that phosphorus pentachloride, chlorine and benzene form benzene hexachloride, so that its behavior with benzene corresponds to the behavior with toluene. . . .” “It would strengthen the argument if it were possible to show why negative chlorine substitutes in the ring and positive chlorine in the side chain.” Another explanation to account for the mechanism of substitution is that of Holleman.’ He accounts for side chain substitution by assuming that the replacement is due to molecular halogen. Nuclear substitution on the other hand he postulates as being the result of the action of a perhalide. Fry2 criticizes these theories and takes the opposite view to that of Bancroft by assuming that nuclear substitution is due to positive halogen and side-chain substitution to negative halogen. Using the electronic conception of positive and negative valence to the atoms composing the benzene nucleus, Fry maintains that the following model represents the formula for benzene in which the hydrogen atoms in the 1 , 3 , 5 positions function negatively and those in the 2,4,6 positions function p~sitively.~ H-
,+c,p+, H-
C-
H-
I
H+ Rec. Trav. chim., 27, 435 (1908). “The Electronic Conception of Valence and the Constitution of Benzene” (1921) ”. physik. Chern., 76, 385, 398, 591 f1911).
,3124
STEPHEN FRANCIS WIIEARTY, JR.
Evidence of the existence of positive chlorine has been furnished by Soyes‘ and StieglitzL while the positivity of chlorine in the 2,4,6 positions has been pointed out by Chattaway and O r t ~ n . Also ~ there is proof of the alternate negativity and positivity of the nuclear hydrogen atoms.‘ On the basis of evidence such as this, Fry has formulated his theory to account for the mechanism of side chain and nuclear substitution and the processes of orientation following the Brown and Gibson ru1e.j “Now the application of the electronic conception of positive and negative valences to the constituent atoms of the benzene molecule and to the principles of the Brown and Gibson rule, not only rendered possible an interpretation of the rule, but also indicated a mechanism according to which substitution must take place in one way rather than in another way. This was a consequence of the evidence, both theoretical and experimental, that in benzene the hydrogen atoms in positions I ,3 , and 5 are negative, while the hydrogen atoms in positions 2,4, and 6 (relatively speaking) are positive. Accordingly, when substituents are of the same sign or polarity they occupy positions which are meta to each other, but if two substituents are of opposite sign or polarity they will occupy positions either ortho or para to each other. The extension of these principles, and the electronic conception of valence to the phenomena and conditions of nucleus and side chain substitution may afford not only an explanation of the phenomena, but also indicate a possible and a probable mechanism of the process.” Because of the fact that the presence of moisture, low temperature and the absence of sunlight favors nucleus substitution and also because these factors cause the stabilization of hyprobromous and hypochlorous acids, Fry concludes that these substances are intermediates in nuclear Substitution. The halogen therefore comes from the hypoacid and is positive. On the other hand the conditions which favor side chain substitutions Le., high temperature, sunlight and absence of water also cause hypobromous and hypochlorous acids to act as oxidizing agents and Fry points out that side chain substitution is essentially an oxidation-reduction reaction. Accordingly the mechanism of nuclear substitution is as follows :H+H/\HT 1 1
- +
+ HO C1 =
-H\/HH+
- +
+“i‘;+
+HOH
vHH+
J. Am. Chem. SOC., 23, 460 (1901);35, 76 (1913);42, 991 (1920).
* J. Am. Chem. 5
SOC.,23, 797 (1901).
J. Chem. SOC.,75, 1046 (1899); Ber., 32, 3572 (1899). J. Am. Chem. SOC.,38, 1324 (1916). “Electronic Conception of Valence and the Constitution of Benzene,” 133 (1921).
ACTION OF FLCORINE O N AROMATIC COMPOUNDS
3125
H-
+ I -HvH-
- +
+ HO H
+HONOz=
~
H+
IT+
H-
H-
- +
+HOH H+
Clf CH3=
CH3-
H+
H+
CH3-
+HAH+ - -
1
i
+HOH -H\/HC'lf It can be seen from this that the sign of the group already in the ring determines the position of the entering group. The KO2 groups directs into meta position and the CH3 group directs into ortho-para positions, this is in accord with the Gibson and Brown rule. The mechanism of the side chain substitution may be summarized as an intra-molecular oxidation-reduction process.
/\E~
-H
h
+ Cl+.Cl-+
d
i'icl + v -
/):- - C1+ + H+. C1IG" +:\/ - '
i
c1-
H
'\/
Fry correlates this change from posit'ive halogen to negative halogen in the side chain when halogenation occurs in the sunlight, by the fact that heat or sunlight will also convert HO- C1- to H+ e l - with an evolution of oxygen. I n other words the halogen is reduced in both cases.
3126
STEPHEN FRANCIS WHEARTY, JR.
He explains the action of halogen carriers as follows:--‘ “R represents any halogen carrier such as water, pyridine, iodine chloride, phosphorus-, antimony-, and molybdenum-trihalides, or any other compound containing an atom which in uniting with chlorine or bromine, XZ,increases 2 ) as follows:its valence from (n) to (n
+ +R + X-X+X---R-X
+-
,+-
CHa+ X--R--X
f
-
+
or Hf
Hf
“It should be observed that the halogen acid eliminated during the course
+ -
- +
of the substitution is of the type H - - X, and not H - -X, the latter electromer never having been identified. If neither oxidation nor reduction
+ -
has occurred during nucleus substitution then the elimination of H - - X is conclusive evidence that the substituted halogen atom is positive, the reaction having preceded as indicated above. . . .” Fry’s ideas have been criticized from time to time. Holleman2 states that “on studying this hypothesis more closely it seems to me that t’here are so many objections against it, that it cannot be accepted.” Brune13 raises objections to the electronic conceptions of valence put forward in Fry’s work. “The chemical evidence advanced in support of this hypothesis in as far as it deals with simple phenomena is quite unconvincing. Any application of the theory involves the constant use of assumptions that render it too elastic to be proved or disproved by these applications.” Another one of the earlier theories developed to explain substitution in carbon chains, orientation in the henzene ring etc., is that of Flurscheim.’ He goes back to the accepted ideas of the physicist as regards the structure “Electronic Conception of Valence and the Constitution of Benzene,” 138 (1921). Chem. SOC.,36,2495 (1914). J. Am. Chem. SOC.,37, 722 (1915). Chern. Ind. Review, 3, 246 (1925).
* J. Am. 4
ACTIOS OF FLUORINE O X AROMATIC COMPOUNDS
3127
of the atom i.e., the Rutherford-Bohr-Sommerfeld conception of the rotation of electrons around a positive nucleus in orbits governed by the quantum theory and explains the behavior of a substance under certain conditions on the basis of “reactivity.” “Accordingly, in addition to external factors, such as temperature, medium, concentration, reactivity is governed by three internal factors :the amount of affinity (quantitative factor “q”), the kind of affinity (polar factor “p”) available at each reacting atom and the magnitude of any steric hindrance (steric factor “s”) affecting each reacting atom” . . . . The nature of the affinity of all atoms in a molecule is modified, by a substituting atom, in the direction of the nature of the affinity of the latter, and it has been found that the extent, to which atoms in direct combination modify the kind of each other’s affinity (“p”) is a direct function of the affinity content of the bond between them (“q”).” Using this idea that the reactivity is governed by the three internal factors “p,” “q,” and ‘ I s , ’ ’ Flurscheim interprets and predicts orientation in the benzene ring, the degree of hydrolysis of salts of tautomeric acids and bases, the relative order of electrolytic dissociation constants of acids and bases, the thermionic dissociation of quaternary salts and bases and the mobility of substituents. He explains benzene substitution and orientation as follows,’ “The equilibrium principle when applied to benzene, leads to formula A, in which we have a symmetrical distribution of free and bound affinity. The amount of residual affinity at each carbon atom is less than at ethylene carbon, but more than at methane carbon. Consequently, the hydrogen atoms are more saturated even than in methane, and substitution is preceded by molecular addition exclusively at one of the six equivalent carbons (see formula B). Substitution of one of the hydrogens creates a new equilibrium with an asymmetric distribution of affinity. Thus the unsaturated, bivalent oxygen of methoxyl in anisol makes a bigger affinity demand on nuclear carbon than did the hydrogen of which it has taken the place, and this results in a shifting of bound and free affinity as shown in formula C, wherein the strong lines denote bonds with an affinity content greater than in benzene, dotted lines bonds with a lesser affinity content, and long and short arrows, respectively, amounts of free (residual) affinity greater and smaller than in
1
Chem. Ind. Review, 3, 248
(192j)
3128
STEPHEN FRASCIS WHEARTT, JR.
benzene. It is seen that residual affinity is reduced at the meta carbons and increased at the ortho and para carbons, to which latter a substituting reagent must be attracted. When, on the other hand, nitrobenzene, with its highly saturated pentavalent nitrogen, is considered, the distribution is reversed, as shown in formula D, leading to meta substitution.” Other workers in this field of benzene substitution have been principally concerned with the mechanism of orientation. Torlander’ put forth the assumption that the atoms of a radical may exhibit an alternate polarity which may be indicated by means of the signs and - attached to the radical. He writes the formulas for nitrobenzene and aniline as follows:-
+
0- = N+ = 0I
H+
- N-
- Ht
I
+I
\ - /
\/
and points out that the carbon-nitrogen bonds in these two cases are not of the same type. He assumes each hydrogen to be positive, which means that in the case of nitrobenzene the positive NO2 group replaces the positive hydrogen and the general character of the benzene system is retained. However, in the case of aniline the carbon-nitrogen union is different and Vorlander assumes a strain to exist in the aniline molecule. He indicates the difference between the two by using long and short lines in his structural formula:0- = N+ = 0-
-I
1-I /-
.~
:--+. 1 i I+ H
C
HO
NO2
--t
~
HO--f;JO* 6 - 6 +
HOtNO2
- +
“. . . . . . When the atom to be replaced is not hydrogen (which is normally stable as its positive ion), but is an atom or group such as chlorine which is most stable as its negative ion, a group of positive- centre- seeking, or, to use Bronsted’s term, “basic” reagents (HXRR’, OR, SR, - etc.) became especially effective in leading to substitution. .” The above mechanism illustrating the introduction of the NO2 group is similar to that of Fry. The symbol “6” represents the electron repulsion or attraction necessary to start the reaction. I n the case of halogenation Ingold, Smith and Vass‘ state “In explanation of the process of halogenation, Fry, Cofman, Francis and others have supposed that the halogen molecule ionizes, the positive ion being the active agent. Whilst, however, it seems unlikely that such a decomposition involving the exclusive appropriation of electrons by one atom would take place without external excit’ation, the electron repulsion of a negative centre (e.g., in an aromatic nucleus) might supply the necessary stimulus by tending to divest a halogen atom of its shared electrons during combination, leaving the other halogen atom to escape as negat’ive ion. Thus, the activity of a molecular chlorinating agent, XC1, should increase with the electron affinity of X, and similarly for bromination and iodination, which agrees with the fact that bromine chlorides is a powerful brominating agent, but does not chlorinate, that iodine monochloride iodinates. . .” The fact that iodine trichloride is a chlorinating agent is explained on the basis of the theory of “singlet” linkings. Prideaux* writes the formula for iodine trichloride thus :-
.
.
J. Chem. S O ~ 130, . , 1 2 4 j (1927). Chem. Ind., 42, 672 (1923).
s,
ACTION O F FLUORISE ON AROMATIC COMPOUXDS
(C1 ,!‘ . ..I. Clz :)
3 I33
or
in which the binding electrons attached to the negative chlorine atoms are furnished by the iodine atom. This leaves the iodine atom positive and the chlorine atoms negative. Ingold and Ingold’ explain halogenation by compounds of this nature as follows :“It has also to be remembered that the dissociation of a reagent into neutral atoms, or radicles, may lead to substitution in positions determined by the consideration that those neutral atoms (e.g., C1) which have to gain electrons to form their stable ions will seek out negative centres, and in this respect simulate a positive group, and vice versa. This may well be the mechanism of chlorination by phosphorus pentachloride, iodine trichloride and aryliodide dichlorides each of the two loosely held chlorine atoms in these compounds being regarded as bound by a single electron and a s in a state of incipient atomic dissociation of the following kind:-
..
R : I..:
---t
*.
c1 j (f-)
R:I:
+ ,ci. ...
(4
(n)
“Thus it happens that chlorine although negative (0.5 unit charge) in the original group, tends to separate as a neutral atom which so dt. :es an electron that it simulates a positive group. Thus the apparent anomoly of negative chlorine leaving positive iodine or phosphorus to become attached to negative carbon is explained.” Studying the halogenation of phenol from the reaction velocity viewpoint Soper and Smith2 show that the reaction between phenol and hypochlorous acid involves the phenoxide ion and the un-ionized hypochlorous acid and that in the case of iodination the main reactions are those between the hypoiodous acid and ionized and unionized phenol. Experiments on the electrolytic oxidation of aromatic compounds? show that a negat,ive hydroxyl group is introduced into the nucleus in the same positions as those into which a positive nitro group would enter. Oxidation 1
J. Chem. Soc., 129, 1310 (1926). J. Chem. Soc., 129, 1582 (1926); 2757 (1927). Fichter and Alder: Helv. chim. acta, 8, 74 (1925).
3134
STEPHEN FRANCIS WHEARTY, JR.
at the anode gives a neutral hydroxyl radical which contains one electron less than the number required to form the stable group and will therefore seek out the negative centers of the aromatic nucleus. This is similar to the behavior of a positive group. From reaction velocity measurements it has recently’ been shown that the general electron drift influence is usually the predominating factor in determining aromatic reactivity. Moreover, Bradfield and Jones2 indicate from reaction velocity measurements that groups keep their relative reacting powers under widely differing reaction conditions. Another aid to the study of substitution processes is the investigation of the local electrical fields in the neighborhood of the various atoms. “Such an investigation can be carried out in certain cases by measurement of the electrostatic dipole moment of the molecule and though the electrical polarization of the normal ‘resting form’ of the molecule which is that measurable by purely physical methods, may be radically different from that of the ‘activated form’ of the molecule at the moment of reaction there is remarkable agreement between the theoretical deductions drawn from such measurements and the known facts of aromatic sub~titution.”~ The mass of evidence for and against each of the theories presented, is enormous. There seems t o be agreement on several points but in general the problem is still a very complex one. The following ideas on the subject seem to have been accepted by most everyone :(I) The chlorine molecule when reacting with benzene dissociates into positive and negative ions; the positive ion enters the nucleus and the negative ion unites with the positive hydrogen to form HC1. I n this process the benzene is activated in some manner. The modern electronic views of atomic structure, together with the (2) modern conceptions of electro-valence, co-valence and coordinated valence are applicable to aromatic substitution and orientation. (3) Because of the facts of ortho, para and meta substitution the substituent causes an alternate induced polarity along the nuclear carbons. This involves a key atom in the case of groups such as CHB, XO2, etc. However, although these several points seem to be generally accepted they have not met with universal approval. Because of this, new terms and new theories are constantly being introduced in attempts to solve the problem. Indeed it is the opinion of some people that introducing these various conceptions of co-valence, semi-polar linkages, induced alternate polarities, etc., leads only to confusion. For instance, Cranston4 points out that there has been no uniformity in the choice of symbols to represent the various types of linkages used in the development of the electronic theory of valence. As a ~~
Shoesmith and Slator: J. Chem. SOC.129, 216 (1926); Olivier: Rec. Trav. chim., 41, 646 (1922); 42, 516 (1923); Berger and Olivier: 46, 517 (1927); Berger: 46, 545 (1927). * J. Chem. SOC.,1928, IOIO. Waters: Science Progress, 23, 649 (1929). J. SOC.Chem. Ind., 47, 208 (1928).
ACTION OF FLUORINE ON AROMATIC COMPOUKDS
3133’
result of this it very often happens that two people, supporters of the same theory, are unable to comprehend each other. Also these new conceptions are probably making the problems far more complicated than they actually are. Fry’ in showing why chemists are inclined to impose upon structural formulas an octet system of valence notation says: “Most chemists, I believe, are more partial to the Lewis-Langmuir conception of electronic shells than to the Bohr conception of electronic orbits. A possible reason for this attitude may be the fact that the cubical octet conception is primarily and fundamentally an outgrowth and a pictorial elaboration, in terms of atomic structure, of the original conception of Abegg and Bodlander, who, in 1899, stated that atoms display different kinds of valency termed “normal” and “counter” valency of opposite polarity according as they are united with electropositive and electronegative atoms. Furthermore, it is a question in the minds of many chemists whether or not the Lewis-Langmuir valence conceptions and notations possess any more, or perhaps as much, truly chemical significance as that which characterizes the Abegg and Bodlander system.” “Now when we come to consider the actual chemical properties of atoms and molecules, no matter what attempts may be made to explain valency by an electronic pictorial notation, all that the chemist knows pragmatically about the valence of an atom may be embodied in the simple fact that if (n) be the empirically determined valence of a given atom, that atom may function in (n I ) different ways. This has been fully illustrated in the numerous publications of the interpretations of a great variety of chemical reactions. This is also a direct and simplified modification of the conceptions of Abegg and Bodlander and it does not require the amplification and entailed ambiguity necessarily encountered when the cubical octet or other systems of electronic valence notation are imposed upon structural formulas. I t enables us to correlate in a simple fashion the several ways in which atoms and radicals react, positively, negatively, and amphoterically, in strict conformity with the actual chemical behavior of the molecule as amply illustrated by the well established chemical reactions actually being dealt with such as ionization and electrolysis, hydrolysis, and oxidation-reduction processes. In fact, practically all chemical phenomena may be classified under these types.” There seems to be therefore a great demand for some simple workable theory that will explain the facts sufficiently well without going back to the fundaniental conceptions of the structure of the atom. It is the purpose of this paper to call attention to the theory of substitution proposed by Bancroft in 1908, substantiate it and to show that slightly modified it will explain the facts of nuclear and side chain substitution in a simple manner. One of the big objections to Bancroft’s theory has been his postulation that the entering halogen is negative in ring substitution. Most all of the other theories have it as positive and this view point has been generally accepted as has been pointed out above. However there are still arguments in favor of the negative halogen. Flurscheim,* to cite one instance, when speak-
+
Chem. Reviews, 5, 559 (1928). chim., 48, 819 (1929).
* Rec. Trav.
3136
STEPHES FRANCIS WIIEARTY, JR.
ing of the recent theory of induced alternating polarities states that, “Since the new theory depends . . . . . on the assumption that the entering group has a positive charge this assumption may be refuted.’, Also the assumptions it is necessary to make when contradictions to the positive theory appear complicate the problem. Illustrations of this, already mentioned are the single linkages of Prideaux to explain chlorination by iodine trichloride and the “tautomeric effect” of Robinson, Kermack, Ingold, et. al., to explain why the chlorine atom in benzene monochloride directs into ortho and para positions when it should direct into meta position if positive. In addition reaction velocity measurements have shown that very often in benzene substitution the reaction is not a function of the halogen at all but that the unionized reactant is the predominating factor.
Purpose of This Investigation It is the purpose of the first part of this paper to offer an explanation for the mechanism of the process of ring substitution and to uphold Bancroft’s theory by showing that the entering halogen is negative. I n the second part the theory will be extended and applied to cover cases where the substitution is not a function of the halogen alone.
Introduction to Experimental Part When ferric chloride or chlorine and ferric chloride reacts with benzene or toluene to form chlorobenzene, the chlorine is certainly negative; but we do not know whether the ring hydrogen is positive or negative until we know whether the first step in the reaction is the replacement of hydrogen by chlorine or the reaction of hydrogen with chlorine. Q7e can picture two quite different mechanisms :(I) CBHG-I- Clz (2)
=
C&fsC1
+ [H f
C1 = HCl];
CsH6 f C11 = HC1 f [CsHs -I- C1 = CsHsCl].
I n the first reaction we have negative chlorine replacing a negative hydrogen and the replaced hydrogen reacting with the residual chlorine. This was the case postulated implicitly by Bancroft’ in his theory of halogen substitution. In the second reaction negative chlorine reacts with positive hydrogen and the resulting phenyl radical, CBHs,combines with the residual chlorine. When chlorine acts on benzene in the sunlight there is formed benzene hexachloride, which breaks down under suitable treatment into trichlorobenzene and hydrochloric acid. CsHsC16 -+ C6H3Cls
+ 3HC1.
In this case it seems certain that the primary reaction is the splitting off of hydrogen chloride and the residual chlorines combine with the C6H3 group to give trichlorobenzene. This is the more certain because the reaction takes J. Phys. Chem.,
12, 41; (1908).
ACTION O F FLUORINE ON AROMATIC COMPOUNDS
3'37
place on boiling with alkali, which would of course tend to split off hydrochloric acid. I t is extremely probable that the six chlorines add on in pairs and not simultaneously. On this assumption the reaction would take place in three stages theoretically though not practically: C6H6
+ 3Clz = CsH6C1z + Zclz = C6H6Cla + clz = C6H~C116.
If we had a substance which accelerated the splitting off of hydrogen chloride at the end of the first stage, the reaction would be written CsHs
+ Clz = CsH6Clz
CcH,CI
+ HC1
If ferric chloride were such a substance, the chlorination of benzene in presence of light and of ferric chloride would involve the same first stage and we could say that Equation z was right. Unfortunately it turns out experimentally that ferric chloride is not that kind of a catalyst. Some benzene hexachloride was heated in an electric furnace to over zooo with and without ferric chloride. In both cases there was a small amount of hydrogen chloride evolved; but there was no catalysis by the ferric chloride. This indicates that Equation z does not hold for the chlorination of benzene in presence of ferric chloride. If Equation I is the only other possibility, and it seems to be, then Equation I must hold for the chlorination of benzene in the presence of ferric chloride, stannic chloride, aluminum chloride, etc., and the ring hydrogen in benzene and toluene must be negative relatively. On the other hand a negative proof is not very satisfactory and it was therefore hoped that the action of fluorine on benzene might be helpful if it were found possible to make the reactions take place slowly.
Preparation of Fluorine The method of preparing fluorine was essentially the same as that worked out by Argo, Mathers, Humiston and Anderson,' later modified by Meyer and Sandow2 and by S i m ~ n s . ~ The apparatus used during the first part of the work was that set up by Jones' in this laboratory but later a cathode pot of larger capacity kindly loaned us by Professor Frank C. Mathers of Indiana University was employed. Several other changes were made also; these will be pointed out later. The cell (Fig. I ) consists of a magnesium pot which serves as cathode, heated electrically by a nichrome element, and a magnesium diaphragm or bell through the top of which is suspended the anode, a carbon rod. The diaphragm serves to prevent the union of hydrogen and fluorine generated a t their respective electrodes. A thin magnesium plate is placed over the lower part of the diaphragm in order to deflect any hydrogen rising from the bottom of the pot. Several holes are drilled in the diaphragm to allow for Trans. Am. Electrochem. SOC.,35, 335 (1919).
* Ber., 54, 759 (1921).
J. Am. Chem. SOC.,46, 2 1 7 5 (1924). J. Phys. Chem., 33 801 (1929).
3138
STEPHEN FRANCIS WHEARTY, JR.
FIQ.I
circulation of the electrolyte. The anode is connected to the electrolyzing circuit by a heavy copper wire driven into a hole in the end of the carbon. In order to insulate the copper lead from the diaphragm the space around it is filled with pasty Portland cement and then baked a t zoo°C. This seal holds the anode rigid and will withstand the action of fluorine for several months before breaking down. The temperature of the bath is determined by a thermometer incased in a magnesium well immersed in the electrolyte. The cathode pot is 4y8 inches internal diameter and 8% inches high; the diaphragm is 7 inches high by z inches internal diameter. Both pieces are
ACTION O F FLUORIh'E Oh' AROMATIC COMPOUNDS
3139
1/4 inch in wall thickness. The anode is a one-inch graphite rod 6% inches long. The thermometer well was made by drilling a 5/16 inch hole almost through a I/Z inch magnesium rod 9 inches long. A 1/4inch copper tube leads off from the top of the anode compartment and connects to a purification train by means of a copper flange union made gas tight by a gasket of thin copper. Because of the ease with which fluorine attacked the several soldered joints on the purification U tubes used by Jones, it was decided to employ a straight copper tube which would involve two threaded joints only. The threads corrode quickly so the apparatus remains gas-tight indefinitely. To save space this straight purification tube can be bent into a U. Sintered lumps of sodium fluoride are used in the purification train to take out hydrofluoric acid given off by the molten bath.
'I NO
I
vo/h
Rc'
'91
vol i n c h ZL€CTROLYZ/NC
C/RCUIT
FIG. 2 In the past, workers in this field have used for the electrolyte either sodium or potassium bifluorides. In making a choice the following factors were taken into consideration:( I) Sodium bifluoride is cheaper than potassium bifluoride. (2) Sodium bifluoride has more fluorine per gram. (3) Potassium bifluoride is hygroscopic; sodium bifluoride is not. (4) Potassium bifluoride tends to creep over the side of the pot and thus destroy the heat-insulating material and to short-circuit the heating unit. (5) Sodium bifluoride decomposes before the melting point is reached; potassium bifluoride does not. (6) A mixture of the two would give a lower melting point. This would save time in heating the bath and also save electrical energy. It was decided to use a mixture of the two salts containing 3 jyosodium bifluoride. This mixture has a melting point of 1 7 0 O which is joo lower than potassium bifluoride alone and about the lowest of any combination of the two salts. There is little or no creeping by the electrolyte and the hygroscopic nature of the potassium bifluoride is diminished to such an extent that there is hardly any anode polarization by oxygen resulting from the electrolysis of water. However more hydrofluoric acid is given off than from a bath of potassium bifluoride alone so a larger purification train is necessary.
3 140
STEPHEN FRANCIS WHEARTY, JR.
The bath is kept heated by an element connected through an ammeter and rheostat to the I I O volt A.C. line, (Fig. 2 ) . Six amperes are used until the charge is molten and the electrolysis started. This is then reduced to about three amperes depending on the amount of fluorine left in the bath. When the electrolysis is started, the anode covers itself with a film of oxygen because the water in the bath electrolyzes out before any fluorine will be generated. This film is pierced by I I O volts when the current is reversed. The electrolysis is carried out at three amperes, the cell delivering about 900 cc. fluorine per hour. A charge of 3400 grams replenished from time to time to compensate for volume decrease was still in operation after over IOO hours of continuous usage. The fusion temperature starting a t 170' for a fresh bath rises gradually as the fluorine is used up.
FIG.3
Apparatus and Manipulation The apparatus (with the exception of the generator) is shown in Fig. 3. The copper purification tube (J) is joined to the rest of the apparatus by a glass-on-copper seal a t (H). This union may also be made by using glass tubing of slightly larger diameter than that of the copper, slipping it over the copper and sealing with paraffin wax. Fluorine from the generator is collected over mercury in a bulb of approximately 600 cc. capacity (A), and then run into the reaction chamber (B) by raising the levelling bulb (C). The rate of flow is regulated by the stop cock (D) and is estimated by the number of bubbles forming a t the delivery tube (E) which dips into carbon tetrachloride. This arrangement is also used to indicate the rate of flow of fluorine into the reservoir from the generator. Stop cock (F) is opened and if the liquid levels inside and outside a t the delivery tube are the same, fluorine is flowing into
ACTION O F FLUORINE ON AROMATIC COMPOCNDS
3141
the reservoir as fast as it is being generated. This flow is regulated by stop cock (D). I n this manner loss of fluorine] or contamination by air in case there are leaks in the apparatus is prevented. Because of the fact that leaks develop around the stop-cocks it is necessary to replace most of the apparatus about once a week. “Lubriseal” stop cock grease is used and although it is attacked by fluorine the action is not fast enough to set fire to the material and break the stop cocks as was the case with several other greases tried. Two different types of reaction vessels are used depending on whether the material to be treated with fluorine is a liquid or a solid. In the case of solids the arrangement (B) is employed. Fluorine passes up through the material, the reaction product (if liquid) drips down and is drawn off through the stop cock (G). A fresh supply of reactant can be added from time to time through the top. K i t h solutions a wash-bottle arrangement (I) proves to be satisfactory. The Action of Fluorine on Benzene When fluorine was passed into dry benzene at room temperature] small explosions occurred, CF, and HF were evolved and carbon deposited. However the action was made to take place quietly by diluting the fluorine with dry nitrogen in the proportion I : I and cooling the benzene to about 7’. Under these conditions the solution became murky and a thick material] with an odor somewhat like that of diphenyl] formed in the delivery tube. After prolonged fluorination the solution became very dark and some of the tarry material settled t o the bottom. This was filtered off and the filtrate, shaken with dilute alkali until free from HF. KO attempt was made to separate the fluorobenzene suspected of being present from the excess benzene by distillation, because of the fact that the two boilingpointsareveryclosetogether. However an attempt wasmade to establish the presence of fluorobenzene by nitration with I : I nitric-sulphuric acid mixture. This converts the benzene into the dinitro derivative,’ m.p. 9o01b.p. z97’, and fluorobenzene into a 9:1 mixture of para and ortho nitrofluorbenzenes, m.p. of the mixture 18.55’.? Sodium methylate converts the latter compound quantitatively into the corresponding anisole m.p. jaol with a separation of sodium f l ~ o r i d e . No ~ fluorobenzene could be detected by this method. The solid material remaining was washed as well as possible with dilute alkali and then tested qualitatively for fluorine by a potassium fusion and precipitation of CaF2 from faintly alkaline solution by Ca(X03)2. Fluorine was found to be present by this method. KOfurther analysis was attempted] but from the tarry nature of the material and the similarity of its odor to diphenyl it was assumed to be a fluor-substituted condensation product. Karnrn: “Qualitative Organic Analysis,” 165 (1923). Hollernan: Rec. Trav. chirn., 24, 140 (1905). Hollernan: Rec. Trav. chirn., 23, 225 (1904).
3142
STEPHEN FRANCIS WHEARTY, JR.
The Action of Fluorine on Toluene Toluene behaved similarly to benzene when diluted fluorine was bubbled through. No fluor-substituted toluenes could be detected and a tarry residue which gave a qualitative test for fluorine was formed as in the case of benzene.
The Action of Fluorine on Anisole Anisole was chosen because it is supposed to have an active hydrogen atom Xevertheless fluorine without dilution could be bubbled fairly rapidly through this material without a violent reaction taking place. S o tarry material was formed in this case but the solution changed from colorless to dark red. By fractional distillation after refluxing to get rid of dissolved gases, the excess anisole was removed and the dark red liquid remaining gave a qualitative test for fluorine.
The Action of Fluorine on 1,3,5-Trinitrobenzene Fluorine set fire rather readily to 1,3,5-trinitrobenzene but by dilution and careful regulation a reaction could be made to take place quietly. After many hours of endeavor the reaction was abandoned because although the mixture became sticky and pasty it seemed to burn away slowly. The Action of Fluorine on Hexachlorobenzene When fluorine was passed slowly over hexachlorobenzene a reaction took place which was evidenced by a heating of the reaction vessel and an evolution of chlorine. If however, the rate of flow was increased, small explosions occurred and carbon was formed. If the reaction chamber was cooled there seemed to be little or no reaction. It was necessary therefore to regulate carefully the flow of fluorine, reducing the rate when small explosions began yet having it flow fast enough to keep the reaction mixture warm. After many hours of fluorination a colorless sticky liquid of peculiar odor started to form. This dripped away from the sphere of action and was withdrawn for purification and analysis. It was suspected that the mixture resulting from the fluorination consisted of some new compound or compounds, unattacked hexachlorobenzene, carbon tetrafluoride, fluorine and chlorine all in solution and carbon in suspension. This mixture was subjected to a fractional distillation, first however refluxing until there was no longer tests with starch iodide or blue litmus paper. The fractionation was not sharp but most of the material came over within two ranges 14oo-15o0and z30°-z400.
The Action of Fluorine on 1,3,5-Tnchlorobenzene When fluorine was passed over 1,3,5-trichlorobenzene in a manner similar to that used for hexachlorobenzene a sticky, oily liquid not dissimilar to that obtained above was formed and chlorine evolved. However to speed up the reaction a concentrated solution of 50 grams of I ,3,5-trichlorobenzene in
ACTION O F FLUORINE ON AROMATIC COMPOUSDS
3143
carbon tetrachloride was made up and fluorine bubbled through this slowly a t first and then rapidly for about 53 hours. The mixture was subjected to a fractional distillation under reduced pressure, first however refluxing to get rid of the dissolved gases. At 2 . 5 cm. Hg. and 75’ a pale yellow liquid came over; this had a boiling point of about 150’ under atmospheric pressure but was not stable. The Action of Fluorine on 1,2,4 Trichlorobenzene Attempts to introduce fluorine into 1,2,4or unsymmetrical trichlorobenzene failed, although the reaction was not studied under all conditions. Under ordinary conditions when fluorine was bubbled through the liquid, small explosions occurred as in the case of benzene and toluene and an odor was noticed similar to that formed when these two substances were treated with fluorine. Method of Analysis Because of lack of information in the literature concerning the properties, derivatives, etc., of the compounds suspected of being present, it was decided to run ultimate analyses for chlorine and fluorine. Of the several methods in use for halogens in organic compounds, that of Piccard and Buffat’ for fluorine and that of Drogin and Rosanoff2 for chlorine were judged to be the best. The first method calls for a potassium fusion a t an elevated temperature to get the fluorine as potassium fluoride. The second method, excellent for chlorine, bromine, and iodine, consists in treating the material in alcoholic solution with sodium. I t was decided not to risk the loss of the small amount of material available for analysis by attempting the bomb reaction necessary in the Piccard and Buffat method for fluorine but rather to analyse for chlorine and fluorine simultaneously by a modification of the Drogin and Rosanoff method. Their scheme is followed exactly except that 40 W grams of potassium instead of 21.5 W grams of sodium and 135 W grams instead of 160W grams of alcohol is used. W representing the Weight of sample. An outline of the procedure is as follows:0.2 to 0.3 grams of unknown is dissolved in 135 W grams of alcohol (dried over sodium) and treated with 40 W grams of potassium. The reaction takes place in a Kjeldahl flask fitted with a reflux condenser. The potassium is added slowly thru the condenser and the flask gently heated to help dissolve any metal remaining. After cooling, the reaction mixture is diluted with zoo cc. of water and adjusted to neutrality. The separation of chlorine and fluorine is made on the basis of the solubilities of their calcium salts in slightly alkaline solution.a Chlorine is determined by precipitating it as AgCl with Helv. chim. acta, 6, 1047 (1923). J. Am. Chem. SOC.,38, 711 (1916). Treadwell-Hall : “Quantitative analysis,” 11, 406 (1928).
3'44
STEPHEN FRANCIS WHEARTY, JR.
a known amount of N/I j and then titrating the excess AgN03 with N / I ~NHICNS. Fluorine is determined gravimetrically as CaF2.' This method when tried with fluorobenzene gave results which were low. However it was thought to be satisfactory for the complex type of compound analyzed in this problem in view of the fact that complex compounds break up easily. Results of Analyses The results obtained from the analyses of the compounds produced by fluorination of hexachlorobenzene and 1,3,j-trichlorobenzene are as follows:-
A. From hexachlorobenzene I. Fraction boiling a t 230'-240' 7% c1 I 53 7 2 53'7
11. Fraction boiling a t
%F I.
2.
I ~ O O - I ~ O ~
46 4 45 3
I . 2.
B. From 1,3,j-trichlorobenzene I. Fraction boiling at 7 j"
c;c1
16.4 14.7
(2. j
I.
20
9
2 .
20
6
cm. Hg)
%F __
1.
35.5
I.
28.j
2.
Aj.1
2.
28.7
The theoretical proportions of chlorine and fluorine in the compounds that fit the closest to these are as follows:-
"'; F
__ 56 3 45 2
I j I
35 3
2s.3
24.2
I t hos thwefore been concluded that these new compounds have been isolated. Unfortunntrly there was not enough material for further investigation and identification but recently Kraay? and De Crauw3 have prepared thru the diazo reaction ~,~-dichlorofluorobenzene b.p. 17;'1 2 , j-dichlorofluorobenzene b.p. 168' and 2,4,5-trichlorofluorobenzenem.p. 62O, and these compounds have properties similar to those of the compounds prepared above. 2
Treadwell-Hall: "Qualitative analysis," 11, 406 (1928). Rec. Trav. chim., 48, 1 0 g j (1929). Rec. Trav. chim., 48, 1061 (1929).
ACTION OF n U O R I N E ON AROMATIC COMPOUNDS
3145
In the reaction between fluorine and hexachlorobenzene, the fluorine has of course displaced the chlorine, because there is no question ol the formation of a stable chlorine fluoride. I n the reaction between fluorine and symmetrical trichlorobenzene, one chlorine and two hydrogens are substituted by fluorine. From the fact that the hydrogens and chlorines behave much alike, it seems probable that we are dealing with a direct replacement in both cases.
Activation by Charcoal Regardless of how we account for it, we know that sunlight activates chlorine and that this activated chlorine reacts with toluene to form benzyl chloride, benzyldichloride, and benzyltrichloride, all the substitutions occurring in the side chain. This is a commercial process and precautions must be taken to prevent the presence of iron. We know also that phosgene can be made from carbon monoxide and chlorine either under the influence of light or in the presence of charcoal. V e know from the work of Jones* that charcoal does not activate carbon monoxide at all and that consequently the charcoal activates the chlorine. In the reactions between hydrogen and chlorine, it is the chlorine and not the hydrogen which is activated. The validity of this conclusion is supported by the fact that sunlight has no appreciable effect on the combination of hydrogen and oxygen to form either? water or hydrogen peroxide while it is known that monatonic hydrogen reacts with oxygen3 to form hydrogen peroxide. I t was therefore expected that the chlorination of toluene in the presence of activated charcoal would give substitution in the side chain-nothing of the sort happened. When chlorine was passed into benzene and into toluene in the presence of activated charcoal (“Nuchar” No. ooo from the Industrial Sales Corporation) ring-substituted products were formed exclusively. The obvious explanation is presence of iron; but that seems not to be the right answer. In order to be sure that these resultswere not due to small traces of iron, the charcoal was heated almost to the melting-point of combustion tubing, and chlorine was passed through it for about four hours to drive off the iron as ferric chloride. Fhile it might not be possible to make charcoal behave like sunlight, we ought to be able to make sunlight behave like charcoal. Apparently the only way to account for the results, if we consider that iron has been ruled out, is to postulate that the charcoal activates the toluene, loosening one of the ring hydrogens more than the side-chain hydrogens. If this is the explanation, light which is absorbed by toluene should cause substitution in the ring to some extent. Curtis4has reported that ring substitution takes place in benzene and toluene if the reaction is carried out in a quartz flask placed a few centimeters from an iron arc; light from a carbon arc gives negative results. This J. Phys. Chem., 33, 1415 (1929). Mellor: “Modern Inorganic Chemistry,” 3Traube: Ber., 15, 2434 (1882). J. Franklin Inst., 184, 875 (19x7).
‘
102
(1922).
3 146
STEPHEN FRANCIS WHEARTY, JR.
seems to prove that light of very short wave-length, such as is found in the spectrum of the iron arc activates the benzene and toluene, causing ring substitution. The results of Curtis have been confirmed, using the following procedure:Chlorination of Benzene About 300 cc. of thiophene-free dry benzene was placed in a quartz flask fitted with a reflux condenser. The flask was then placed a few centimeters from an arc formed between two iron rods carrying I j amperes. After about three hours, or until the delivery tube clogged, the reaction was stopped and the flask allowed to cool. The benzene hexachloride was filtered off and the filtrate distilled to get rid of the excess benzene. The ring-substituted products were detected by their unmistakable odor and by the fact that hexachlorobenzene was formed in the vapor and caught in the condenser tube. This was recognized by its melting point. Chlorination of Toluene Dry redistilled toluene was treated in a manner similar to that used for benzene. The chlorotoluenes were partially separated from the excess toluene and benzyl chloride by fractional distillation under reduced pressure. After converting the mixture to the corresponding acids by oxidation with KMnOa and separating the acids by fractional crystallization from ligroin' the presence of chlorotoluenes was confirmed by the melting point of the product. It was not to be expected that ultra-violet light would give only ring substitution, as the charcoal does, because chlorine absorbs also in the ultraviolet, and is consequently activated also. I n the visible spectrum the chlorine is activated and the benzene and toluene are not. Consequently we get zero substitution of the ring hydrogen in the visible spectrum and partial substitution in the ultra-violet. These results with ultra-violet light do not prove that our chlorine-treated charcoal was free from iron. The assumption was a good working hypothesis because it permitted us first to predict and then to confirm the experiments of Curtis which seem to have been ignored by organic chemists because they did not fit in. The general conclusions of this paper are as follows:I . Chlorine set free a t the anode reacts primarily with the ring hydrogen. 2. The activation of chlorine by ferric chloride must give an essentially negative chlorine and this chlorine reacts primarily with the ring hydrogen. 3. Fluorine reacts with benzene hexachloride replacing chlorine. This must involve a displacement of chlorine. 4. The compounds C6C14F2 and CsClaFa have been prepared by the fluorination of hexachlorobenzene. 1
Bornwater and Holleman: Rec. Trav. chim., 32, 231 (1913).
ACTIOK OF FLUORISE ON AROMATIC COMPOUNDS
3 147
5 . The compound C6HF8C12has been prepared by the action of fluorine on symmetrical trichlorobenzene. There is no reason as yet to postulate any difference between the substitution of chlorine and the substitution of hydrogen in this compound. On the other hand there seems to be no proof that there is no difference. 6 . Ferric chloride is not a catalyst converting benzene hexachloride into trichlorobenzene and hydrochloric acid. Consequently, the chlorination of benzene in presence of ferric chloride does not involve a preliminary addition of two chlorines. 7 . Since the conversion of the addition product, benzene hexachloride, into trichlorobenzene undoubtedly involves the reaction of half the added chlorine with the ring hydrogen, it is probable that the chlorination in presence of ferric chloride does not involve this reaction. 8 . Activated charcoal, which has been treated red-hot with chlorine to remove iron, causes ring substitution with chlorine and toluene. This is probably due to activation of toluene. 9. Ultra-violet light activates toluene and benzene and causes some ring substitution. I t of course activates the chlorine and causes side-chain substitution thereby. I O . When attached to an amino group, a phenyl group is known to be more negative than a methyl group. This is an argument in favor of considering the ring hydrogens as more negative than the side-chain hydrogens. The matter cannot be said to be proved definitely.
Acknowledgments This problem was suggested by Professor W. D. Bancroft and carried out under his direction. The author welcomes this opportunity to express his appreciation for the many helpful suggestions and kindly criticisms offered by Professor Bancroft during the course of the investigation. The author also wishes to thank Dr. S. C. Jones for his invaluable assistance with the fluorine cell. Cotnell Uniuersil y .
