For the Ñuorinated gallium(III) oxide material, 98% of the
total Ñuorine exchanged was found in the 1,1,1-chloro-
triÑuoroethane fraction. Fluorinated indium(III) oxide utilised
whereas Ñuorinated indium(III) oxide yields CClF (0.5 mol.%)
3
and the C compounds CCl FCClF (5 mol.%) and CCl CF
2
(6 mol.%).
2
2
3
3
all the exchangeable Ñuorine to give C chloroÑuorocarbons,
2
namely a mixture of 1,1,2-trichloroÑuoroethane (29%, 2
mol.%) and the thermodynamically stable isomer 1,1,1-
trichlorotriÑuoroethane (71%,
5
mol.%).25 Similarly,
Discussion
tetrachloroethene gives a high ratio of chloroÑuorocarbons as
products. Reaction of C Cl with Ñuorinated gallium oxide,
The ability of chloroethane probe molecules to undergo intra-
molecular dehydrochlorination has been shown to be a func-
tion of the degree of chlorine substitution on the b-carbon,
such that CH CHCl \ CH ClCHCl \ CH Cl .24 Studies
have shown that this intramolecular dehydrochlorination
mechanism is consistent with the formation of a carbocation
intermediate at the catalyst surface.21,22,26 1,1,1-Trichloro-
ethane is thermodynamically unstable with respect to disso-
ciation into dichloroethene and gaseous hydrogen chloride.24
The room temperature equilibrium constant (K ) for the reac-
tion is 2 ] 10~3 mol l~1 equivalent to ca. 1.5% dissociation.
In the presence of Ñuorinated or chlorinated c-alumina, mag-
netite, and cobalt(II,III) oxide respectively, 1,1,1-trichloroethane
readily undergoes dehydrochlorination with the subsequent
formation of polyene species at the catalyst surface.4 The
ability of b-gallium oxide to induce low temperature dehydro-
chlorination, conÐrms the presence of strong Lewis acid sites
at the halogenated surface. The change in the colour of the
catalyst from white through to blackÈpurple on exposure of
gaseous 1,1,1-trichloroethane is consistent with the build up of
polyene moieties at the surface.1 Notably the formation of
polyenic species at the catalyst surface was not the case for the
indium(III) oxide, even though the extent of the distribution of
Ñuorine containing volatile materials is comparable (Tables 1
and 2). The Ñuorinated gallium(III) oxide displays an ability to
2
4
results in 66% of the exchanged Ñuorine distributed between
the CCl CF and CCl FCClF fractions, of which the b-
3
3
2
2
scission product CClF contains 33% of the exchanged Ñuo-
3
3
2
2
2
3 3
rine. The reaction of tetrachloroethane with Ñuorinated
indium(III) oxide, gives 90% of the products as C chloro-
Ñuorocarbons, with 1,2,2-trichloro-2-Ñuoroethane (5 mol.%)
and 1,2-dichloro-2,2-diÑuoroethane (2 mol.%) making up the
Ñuorine balance (Table 1). Fluorinated indium(III) oxide
exhibits b-scission capability towards the sp2 hybridised
carbon of the tetrachloroethene molecule. For the F/Ga O
2
c
2
3
reaction with C Cl , chlorohydrocarbon compounds identi-
2
4
Ðed by 1H NMR and/or gas chromatographic mass spectros-
copy (GCMS) are carbon tetrachloride (1 mol.%), 1,2,2-
trichloroethene (2 mol %), pentachloroethane (2 mol.%)
and 2-chloromethyl-1,3-dichloropropane (d \ 3.35, 1.58; 1
mol.%). Pentachloroethane can be formed by the hydro-
chlorination of tetrachloroethene in the presence of Lewis acid
H
sites.1 The production of (CH Cl) CCl however, suggests the
2
3
role of radical species at the catalyst surface. The reaction of
Ñuorinated indium(III) oxide with the tetrachloroethene results
in 45% of the exchanged Ñuorine incorporated into the
product CCl CF . Analysis of the exchanged Ñuorine content
3
3
of CCl FCClF is 29% to give a CCl CF /CCl FCClF ratio
2
2
3 3 2 2
of 1.7, the relative ratio of k
for the isomers of trichlo-
formation
rotriÑuoroethane at ambient temperatures for the catalyst
system. The Ñuorine ratio of the isomers of trichlorotriÑuoro-
ethane for the Ñuorinated gallium oxide material is 1.8. These
results are consistent with the same rates of forward and back-
ward reactions operating in the isomerisation process for
trichlorotriÑuoroethane produced from the probe molecule
C Cl . The relative ratio of Ñuorine in the CClF /
induce b-scission to C compounds for 1,1,1-trichloroethane,
whereas for the Ñuorinated indium(III) oxide only
dehydrochlorination/hydroÑuorination products are observed.
b-Scission of C compounds at the Ñuorinated gallium(III)
oxide is a measure of the Pearson hardness of the surface and
shows that the relative acidity of F/Ga O is greater than that
1
2
2
3
of F/In O .
2
4
3
2 3
CCl FCClF fractions is 2.7 and 0.3 for F/Ga O and
Increasing the thermodynamic stability of the probe mol-
ecule towards the dehydrochlorination reaction by probing
the freshly prepared catalyst Ñuorinated gallium and indium
oxide surfaces with asym-tetrachloroethane, highlights the dif-
ferences in the reactivity of these materials. The Ñuorinated
gallium oxide material gives the Cl-for-F exchange compound
1,2-dichloro-2,2-diÑuoroethane and the chloroÑuorocarbon
molecule 1,1,1-trichlorotriÑuoroethane. Conversely, Ñuori-
nated indium oxide produced only chloroÑuorocarbons,
namely, 1,1,2-trichlorotriÑuoroethane, and 1,1,1-trichloro-
Ñuoroethane. The formation of 1,2-dichloro-2,2 diÑuoroethane
is consistent with the catalyst inducing a dehydrochlorination/
hydroÑuorination mechanism in the adsorbed compound. The
presence of chloroÑuorocarbons in the product mixture evi-
dences an exchange mechanism that removes hydrogen and
inserts the halogen into the molecule. This halogen exchange
mechanism is observed by both Ñuorinated gallium and
indium oxides, respectively. For CH ClCCl the thermo-
2
2
2 3
F/In O , respectively. This result indicates the enhanced effi-
2
3
ciency of the Ñuorinated gallium oxide to induce b-scission in
both isomers of trichlorotriÑuoroethane relative to Ñuorinated
indium oxide. Using CCl FCClF and CCl CF as probe
2
2
3
3
molecules conÐrms that b-scission of the carbonÈcarbon bond
only occurs with F/Ga O to yield CClF , with isomerisation
2
3
3
of CCl FCClF to CCl CF (Tables 1 and 2). No b-scission of
2
2
3
3
the chloroÑuorocarbon occurs for F/In O .
2
3
The reactions of the C probe molecules methylene chlo-
1
ride, chloroform and carbon tetrachloride are also presented
in Tables 1 and 2. The reaction of CH Cl with Ñuorinated
2
2
gallium(III) oxide produces CH CCl F (9 mol.%), CH CClF
3
2
3
2
(15 mol.%) CH ClCClF (11 mol.%) and CCl FCClF (3
2
2
2
2
mol.%). Hydrocarbon and chlorohydrocarbon compounds
identiÐed are (CH ) CCl (d \ 1.58; 1 mol.%), (CH ) CH
3 3
H
3 3
(d \ 1.56, 0.88; 1 mol.%), (CH Cl) C (d \ 3.35; 1 mol.%)
H
2
4
H
and CHCl CCl (2 mol.%). The presence of the symmetrical
2
3
2 3
dynamic ratio of the CCl CF /CCl FCClF of 2.5 is greater
than the values of 1.7 and 1.8 observed for the tetrachloro-
ethene system, and double that of 1.2 observed for the carbon
tetrachloride system. The ratios of the isomers of CCl CF are
proportional to the rates of formation of the isomers of
trichlorotriÑuoroethane from the forward and reversed isom-
erisation reactions. Based on these results, reactivities for the
formation of C Cl F isomers from CH ClCCl are a factor
butyl complexes is consistent with the presence of radical
species during the reaction. The reaction of trichloromethane
with Ñuorinated indium(III) oxide gives trichloro-
Ñuoromethane (9 mol.%), and dichloroethene (3 mol.%),
3
3
2
2
3
3
whereas the Ñuorinated gallium(III) oxide material gives CClF
3
(2 mol.%), CH ClCClF (4 mol.%), CH ClCF (2.5 mol.%)
2
2
2
3
and CH CCl (4 mol.%). The chlorohydrocarbon compounds
2
2
identiÐed are sym-tetrachloroethane, asym-tetrachloroethane,
(CH Cl) C (1 mol.%), (CH ) CCl (1 mol.%) and pentachlo-
roethane (0.5 mol.%). The reactivities of the Ñuorinated
gallium and indium oxide surfaces are reversed for the reac-
2
3 3
2
3
of two greater than that for C Cl and Ðve times greater than
2
4
3 3
2 4
that for CCl . Given that the heats of adsorption are the same
4
for the adsorbed states of the chloroÑuorocarbon isomers,
tion of CCl , which for Ñuorinated gallium(III) oxide, gives
these results suggest that the CCl FCClF or CCl CF pre-
cursor species are formed directly from the probe molecule
4
2
2
3
3
only C products, CCl F (2 mol.%) and CCl F (14 mol.%),
1
2 2
3
J. Chem. Soc., Faraday T rans., 1998, V ol. 94 1883