FLUORINE AS A HALOGEN Reaction with a highly deactivated aromatic nucleus Lucius A. Bigelow, Reade Y . Tompson', and Paul Tarrant' DUKE UYIVERSITY, DURH.A\l. 3 . C.
THE fundamental reasons are giben for the gre'at \igor of organic fluorinations with the free element, as coniparecl with corresponding chlorinations. W a j s and means are discussed by I+ hich these reactions h a l e been controlled and caused to proceed in orderl) fashion. The chlorination of t h e aromatic nucleus under conditions fa,oring a 1 1 atomic chain mechanism is compared with the Taporphase fluorination of benzene. Both are shown to proceed, first by addition, then substitution. followed, in the case of t h e more reactile fluorine, by fragmentation and polymerization. 1 description is included of the direct vapor-phase fluorination o\er a metal catalkst of the highl? deactilated aromatic nucleus of i-chloro-1,3-bis(trifluoromethy1)benzene wider niild operating conditions. The di- and tetrafluorides were formed bj addition, the heptafluoride and perfluoro-1,3-dimethjlc) clohevane b j substitution, and perfluoromethy I c y clohevanc and perfluorocjclohewane bj fragmentation. These results and interpretations are presented in support of the general proposition t h a t organic fluorinations proceeding b? atomic chain mechanisms are fundamentally similar to the corresponcling chlorinations, and may readily be ac-rounted for O I I the basis of modern theoretical con*ideratioii-.
ficult. Consequently, in organic fluorinations the fast chain rcactions n-hirh actually take place might well be expected. Uockeniueiler ( 1 ) directed attention t o another fundamental cause of t h e violence of these reactions-namely, the very high heat of formation of hydrogen fluoride and of carbon-to-fluorine honds:
,
--C--H
many years, especially in the field of organic chemistry,
F O the R element fluorine was believed to be a thing ai?art, following no generalizations, obeying no rules, and c~liaracterized by disruptive and explosive reactions. S o w , rapidly developing knowledge makes clear t h a t 'the reactions of fluorine \vitli orguiiic compounds fit in very well x i t h existing theoretical considerations, and are nearly as predictable as those of t h e other halogens. As Figure 1 shows, any halogen, X,, may react by either of two mechanisms: -4, ionic, carrier-catalyzed, leads to t h e active S ion; B , atomic, often light- and heat-catalyzed, leads to tlir active X. atom. Since fluorine, the most electronegative of a11 elements, has a high positive nuclear charge, it should react atomically rather than ionically, because t h e complete remora1 of a n electron from it, t o form a positive F ' ion, would be very dif-
+
-~ substitution , I -it €IS+ --C--S i
heat of dissociation of C-C
bond
=
+ .lI
kg.-cal. (1)
71 kg.-cal. (approx.)
Reactions 1 and 2 represent substitution and addition, respectively. When X is fluorine, the heats of reaction, J..and S , exceed t h e heat of dissociation of a carbon-to-carbon bond by a subrtantial margin, even n-ithout t h e heat of activation; n-hen S iz chlorine, corresponding values do not do this. Consequently fluorine n-ould be expected t o disrupt' a n organic molecule easily, whereas chlorine should not. T h e powerful reactions actually observed can, in general, be moderated arid caused to take place in a n orderly fashion, first, by breaking u p the reaction chains, and second, by dissipating the lieat. T h e more common means for moderating direct fluorinations are iis fo1lon.s:
1. Diluting the fluorine by a n inert gas. 2 . 'Conducting the reaction in a n inert solvent. 3. Fluorinating by means of compounds which easily release fluorine atoms or molecules, such as CsHJFp, SbFj, PbF,, I.InFJ, ion in the vapor phase, n-ithin the iiieshes of a metal packing, n-ith or without other catalyst. $. Reacting the fluorine Tvith a relatively inert compound. 6. 1-arious combinations of the preceding.
Of these, dilution of the fluorine with an inert gas, which surrounds the halogen n-it11 unreactive molecule., has not heen m c cessfulalone; conducting the reaction in an inert solvent, m-hich similarly envelops t h e reactant, has often failed becau-e nearly all organic liquid, + F ~ X Q (Ionic) react with fluorine, and fluorocarhoii< themselves are not good solvents for organic compounds. By combining items 1 and 2, Bockemueller ( 1 ) fluorinated some aliphatic compounds successfully in carbon tetrachloride solution at .* 0-15' C.; but Jve have found in the aromatic x (Atomlc) series t h a t these conditions often result in the for.* mation of C l F rvhich then cauSes deep seated chlorination of the organic moierules. Liquid IiyFigure 1. Activation of Halogen Molecules drogen fluoride has, however, heen used successfully as a solvent by Simons (6) and others. T h e reagents which easily relraqe fluorine atomi or molecules have Present address, General Chemical Company, Laurel Hill, K. Y. t h e common advantage that it portion of the heat of fluorina1 Present address, University of Florida, Gainesville, Fla.
..
: x+
..
:
f
360
361
INDUSTRIAL AND ENGINEERING CHEMISTRY
March 1947
A. subs t i t ! A t i o n
ion
B.
C H
Hexachlorobenzene
light, heat.
radical Figure 2.
A 2 d lt io n
Hexachlorlae
Ionic and Atomic 3Iechanisms for the Attack of Chlorine o n .iromatic Nucleus
tion is consumed in dissociating the reagents, and the overall reaction is thus moderated to a certain extent. Their disadvantage is t h a t they have to be regenerated unless the reactant is sufficiently inert to fluorine so that they can be used as carrier catalysts, n-hich is usually not the case. Of those mentioned, the aryl iodofluoride has not been used to any extent,, and antimony pentafluoride is a powerful fluorinating agent only
recently available commercially. The metal fluorides have all been used n i t h considerable success, but only the cobalt fluoride has been employed for a considerable time o n a large commercial scale in a regulation t x o . cycle process. Vapor-phase fluorination within the meshe3 of a metal packing (sometimes silver-plated) has proved successful in this laboratory ( 3 ) and elsewhere, since the large mass of metal rapidly conducts heat away from the re-
c1 c 1
ci
ci
S t a b l e CGCl1,
Stable C6ClI2
g Cl Cl
C
Cl c1
C
c1
/3
H
c 1 c1
isomer ( s a t u r a t e d )
S t a b l e CeKCll
CN C l z addition
; J f Hc
and s u b s t i t u t i o n
0C
,B
Y isoners
Figure 3.
Reactions i n Liquid Chlorine Solution under Pressure i n Sunlight ( 5 )
e1 c1
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 39, No. 3
'
action zone, and t h e relatively great metal halide surface exposed tends t o break u p t h e reaction chains. At least one important example of t h e use of a deactivated reactant will be discussed in detail later in this article. CHLORINATIOX COJIPARED WITH FLUORINATION
Turning now t o chlorine, a n d confining ourselves t o t h e unsubstituted aromatic nucleus, Figure 2 s h o w t h a t a h e n this element acts ionically, as in A , t h e result is orthodox substitution, leading finally t o t h e formation of hexaclilorobenzene. As already pointed out, fluorine does not behave in this manner. On the other hand, when i t act's atomically, as in B , addition occurs instead, Kith the ultimate formation of benzene liesachloride. Van der Linden (5) studied t h e intensive chlorination of aromatic rings under conditionsfavoring atomic reactions-namely, in 0 CYo IO liquid chlorine solution under pressure, and simultaneously exposed t o bright sunlight. A few represenF i W r e 4. .iPParatue for Vapor-Phase Fluorinn tion of Volatile tative results from this n-ork are illustrated by Figure 3. The aroOrganic Liquids ( 4 ) matic, b u t still unsaturated, hesnchlorobenzene by addition yielded t h e highly stahle compounds CaC1,o and c,Cln, depcndilig 1i1)on addition colnp(,untls. Ti~rouglithe courtesy of the x a v a l Reconditions. On the ot'her hand, since the 8-stereoieonier of henscarc.11 Idixljor:~t~~ry, rl, substance .,%-ell suited for thi.3 purpose zene hexachloride was completely saturated under these coilmade avuilat,le. Til;, \vas 4-chloro-1,~-t~ij(triiluorometliyl) ditions, the stable C6HC111 was ultimately formed, presumakdy benzene, ~ r h i c hn-as prepared and purified in this laboratory achy aliphatic substitution involving an orthodox chain mcchacording to tile follotving scheme: nism. Finally, benzonitrile reacted both by addition arid subCH, stitution as shown. S o evidence of ring cleavage by direct chlorination was reported. I n contrast, benzene was fluorinated in t h e vapor pliase over a metal catalyst in this laboratory ( 4 ) by Fukuhara, using the ape1 paratus shown in Figure 4. T h e sample in the small flask, A , under compressed air pressure entered t h e heated vertical reCF3 actor, C, through calibrated capillary D; the vapor mingled in HF t h e meshes of t h e packing, n-ith the fluorine entering a t H and rising from below in countercurrent f l o i ~ . T h e products left the reactor at J , passed over sodium fluoride in L ttnd L' to remove c1 c1 hydrogen fluoride, were condensed in liquid air t r a p M , and were This proccdure involved nuclear and side-chain substitution, then rectified. follon-ed by side-chain replncement under pressure; the details Table I summarizes the results, Tvhich include CaHFll a2d will be described elsen-here by P. 11. Gross, C. K. Bradsher, and csF12,formed by addition and substitution; they should be co-aorlcers. pure, iiicrt compouI1d, boiling a t 148-150' c.9 pared with the corresponding CBHCllland C6Cl12just mentioned. is stable t o alcohclic alkali a t reflus temperature. It \vas highly Then come CF,, CeF6, C3F8, C,Flo, and CsFlo, produced by fragdeactivated by withdrawal of electrons from the ring tiy t h e mentation, and finally ClzF,, Tj.\-hichresulted from dimerization. strongly electronegative substituents, and was fluorinated ( 7 ) aromatic products could be isolated. The fUndalllerltal in the v w J r phase over a silver-plated copper wire packing in similarity of tile primary attack of atomic cf,lorine and atomic
u
fluorine upon the aromatic nucleus by addition and then substitution is thus obvious. The more reactive fluorine atom also produced extensive fragmentation, often follo\ved by polymerization. FLUORINATION O F A H I G H L Y DEACTIVATED NUCLEUS
I n the fluorination of benzene, no products intermediate between the hydrocarbon and CeHFl, could be isolated; therefore, i t seemed desirable t o fluorinate, under mild conditions, a compound with a highly deactivated nucleus, in the hope of moderating the reactions still further and isolating such intermediate
TAIJLI:I. Compound
CFI CrFa C ~ F ~ CiFio CIF,o Carla CsHFii
c,,F~~
P LYRE COLIPOPiDS I 5 9 L A T E D BESLESI:(4) B.P., C . Literature Found
- 128 - 78 - 36
-5to-3 23 51 I
.
.
...
-128 - 78 - 36
-
2
22 50 6% 90
hfol. Wt. C d c d . Found 88 138 188 238 250 300 282 562
88 138 188 238 250 300 282 563
FROM
FLCORIX.ATED
Yo Fluorine Calcd.
.. . .., ...
79.8 76.0 76.0 74.1 74.4
Found
... ... . . I
80 0 26.1,78,." (6.1,75.8 74.2, 74.1 74.1
March 1947
INDUSTRIAL AND ENGINEERING CHEMISTRY
A
0
Figure 5 .
(j
Apparatus for Direct Fluorination
Fa
+
C&13FaC1(CFS)a
Fa
CF3
C1 I
I1
/J
