H. Bozorgzadeh et al. / Journal of Fluorine Chemistry 107 (2001) 45±52
51
4
. Discussion
isomerization reaction is not facile kinetically [37,40] and it
is therefore unlikely that Eq. (4) is an important pathway to
CCl FCF in this situation.
The most noteworthy ®nding from this study is the
relatively large quantities of CCl FCF that are formed from
2
3
The ostensible ¯uorination steps that convert C Cl F to
2 3 3
2
3
CCl FCClF , in addition to the expected formation of the
2
CCl FCF are described most obviously by dismutation
2 3
2
isomer, CCl CF (113a).
3
reactions. These could involve CCl CF3 (Eq. (2)) or
3
3
Isomerization of CCl FCClF in the presence of alumi-
2
CCl FCClF (Eq. (5)).
2
2
2
nium(III) chloride under moderate conditions, was ®rst
observed many years ago [36]. A re-investigation of the
reaction at room temperature, undertaken as a precursor to
the present study, has shown that the isomerization, which is
intramolecular, occurs not at an aluminium(III) chloride site
but at Al(III) in a disordered chloro¯uoride environment.
The latter site is generated via chlorination reactions
between CCl FCClF , or more highly chlorinated CFCs,
2
CCl2FCClF2 ! CClF2CClF2 CCl3CClF2
(5)
If the latter reaction were dominant, significant quantities of
CClF CClF would be expected in product mixtures, since,
as indicated above, the isomerization to the asymmetric
isomer is not facile. The former reaction (Eq. (2)) however,
has a severe disadvantage from a kinetic standpoint that
2
2
2
2
catalytic activation of the CF group is required. There is
3
and aluminium(III) chloride [37]. At room temperature
however, isomerization of CClF CClF was not observed.
abundant evidence from synthetic work involving these and
similar compounds, that the chemical reactivity of both C±
Cl and C±F bonds decreases as the fluorine content of the
2
2
The intramolecular nature of the isomerization of
CCl FCClF suggests that this species is adsorbed in a
2
2
molecule increases [41]. Activation of a CF group, which is
3
dissociative fashion at an isomerization site in contrast to
its adsorption at sites where halogen exchange (dismutation)
can occur. This implies that the acidity of an isomerization
site is greater than that of a Lewis acid site at which halogen
exchange occurs but there is, at present, no direct evidence
for this proposal. Aluminium chloro¯uoride is recognised as
a highly Lewis acidic material and on electronegativity
grounds, replacement of Cl by F might be expected to
required by Eq. (2), is a disadvantage for this pathway, even
though the reaction is probably thermodynamically favour-
able. Similar considerations apply to the formation of the
minor product, CClF CF .
2
3
Analyses of product distributions in themselves do not
permit an unambiguous differentiation to be made between
the possible routes to CCl FCF from CCl FCClF . On
2
3
2
2
�
balance however, the pathway described by Eq. (1) then
Eq. (2), is preferred. What has been de®nitely established,
however, is that these aluminium-based catalysts are capable
of achieving the conversion of unwanted CCl FCClF to
increase Lewis acidity. Calculation of F ion af®nities
for isolated molecular AlCl3 Fn species suggests that order
� n
is AlF3 < AlClF2 < AlCl2F < AlCl3 [38] or AlCl3 <
2
2
AlClF2 < AlCl2F, AlF [39]. The differences among the
3
CCl FCF which can then undergo hydrodechlorination to
2
3
data from the two studies are, however, very small and the
important conclusion in each case is that signi®cant Lewis
acidity is to be expected at a surface Al(III) site in an
halogenated environment.
give the useful compound, CH FCF .
2
3
Acknowledgements
Any rationalisation proposed for the reactions studied
here must account for the formation of CCl CF and
CCl FCF as major products and must recognise that ¯uor-
3
3
We thank the European Union for support of this work
under contract ENV4-CT97-0601.
2
3
ination of a C±Cl species by Al±F is thermodynamically
unfavourable [3,37]. The isomerization of CCl FCClF to
the thermodynamically preferred isomer, CCl CF (Eq. (1))
2
2
3
3
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ditions, irrespective of whether CCl FCClF or CCl CF are
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