COMMUNICATION
Carbocation chemistry at room temperature on solid acids
Abdelkarim Sani Souna Sido, Jeremy Barbiche and Jean Sommer*
Received (in Cambridge, UK) 8th January 2010, Accepted 3rd March 2010
First published as an Advance Article on the web 17th March 2010
DOI: 10.1039/c000513d
The existence of carbocationic intermediates in a zeolite
environment even at room temperature is supported by hydride
(and deuteride) transfers between isoalkanes as monitored
ex situ by 13C NMR.
grease-free batch recirculation reactor) was evacuated at
10ꢀ2 Torr and purged with N2 in order to evacuate
the adsorbed residual 2MP. Monodeuterated isobutane
(2-methylpropane-2d; 22.4 ml; 1 ꢁ 10ꢀ3 mol) was then introduced
in the recirculation loop and the gas phase was trapped at the
outlet of the reactor in a precooled U tube (ethanol–liquid N2),
then transferred to the NMR tube using CF2Cl–CCl2F as
Acid-catalysed transformations of hydrocarbons are of prime
importance to the petrochemical industry.1,2 The chemical
inertness of alkanes, the main components of oil, and the
low acidity of the catalysts (silicoaluminates or zeolites) are
overcome by operating at high temperature and pressure. Only
liquid superacids3 are known to activate saturated hydro-
carbons at room temperature and well below in relation with
the s-basicity of C–H and C–C bonds.4 If carbocations are
generally accepted as reaction intermediates in acid-catalysed
hydrocarbon reactions, their mode of formation as well as
their true nature is still a matter of controversy.5,6 Whereas the
number of Brønsted acid sites of a catalyst can be measured
with a good precision by various techniques,7,8 the heterogeneity
of their environment and strength have been demonstrated by
experiment and theory.9 After the alkane activation step,10
carbocation intermediates are generally present at very low
concentration11 and thus difficult to detect and observe.
Moreover, with the exception of the gas phase, carbocations
are never present as free ions but solvated by interaction with
the lone pairs of the solvent. In the case of silicoaluminates or
zeolites this solvation generates a more stable form: the alkoxy
species which is in fact a mesomeric (or resonance) form of an
alkoxonium ion (Scheme 1).
1
2
solvent and CDCl3 as reference. The H, H and 13C NMR
spectra of the resulting solution showed, next to an excess of
monodeuterated isobutane and residual 2MP, a very small
amount of non deuterated isobutane and an equivalent
amount of 2-methylpentane-2d deuterated in the tertiary
position. The logical explanation of this result is the deuteride
transfer to the 2-methyl-2-pentylium ion generating 2-methyl-
pentane-2d and the adsorbed t-butyl cation or t-butoxy species
(Scheme 2).
Hydride transfer between alkanes via carbocation inter-
mediates is a classical reaction in hydrocarbon chemistry in
the large scale isomerisation reaction used to obtain high
octane gasoline:
RH + R0 # R+ + R0H
+
In our experiments, the alkoxy species generated on the solid
acid are tertiary substrates for which, due to steric hindrance,
SN2 reactions are not allowed.13 We must admit for this reason
that a small fraction liberates the carbenium ion intermediate
in order to facilitate the hydride transfer. With the purpose of
checking the reversibility of this reaction we carried out the
following experiment: 2-methylpentane (100 ml, 0.75ꢁ10ꢀ3 mol)
and 2-methylpropane-2d (18 ml gas, 0.75ꢁ10ꢀ3 mol) were
recirculated over previously activated HUSY (250 mg) during
24 h under the same conditions as above, after which the
hydrocarbon mixture was condensed and analysed by NMR.
The 13C NMR spectra show that the tertiary deuteriums and
protons were equally redistributed over both isoalkanes
resulting in the presence of the four compounds 2MP, 2MP-2d,
isobutane and isobutane-d. Fig. 1 shows the 13C spectrum of
the mixture. Due to the isotope effect on 13C chemical shifts
and 13C–D coupling the various lines are easy to assign. In
comparison with the non deuterated alkanes the tertiary
carbons connected to deuterium experience an upfield shift
Oxonium ions or salts12 are widely used as reagents in
organic synthesis due to their ability to generate carbenium
ions. In this paper we describe our results which, we believe,
for the first time strongly support the existence of carbenium
ion intermediates on zeolites even at a temperature as low as
room temperature.
In a first experiment, in order to generate the alkoxy species
on the catalyst, we recirculated, during 24 h at room
temperature, 2-methylpentane (2MP; 220 ml), 5% in nitrogen
at a rate of 20 ml minꢀ1 over 200 mg of HUSY (titrated as
4 ꢁ 10ꢀ3 acid sites gꢀ1)w. The recirculation loop (an all glass
Scheme 1 Equilibrium between alkoxy species and carbenium ions.
´
´
Institut de chimie associe au CNRS (UMR-7177), Universite de
Strasbourg, 1 rue Blaise Pascal, 67008 Strasbourg Cedex, France.
E-mail: sommer@chimie.u-strasbg.fr
Scheme 2 Deuteride transfer to the adsorbed 2-methyl-2-pentylium
ion.
ꢂc
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 2913–2914 | 2913
Sani Souna Sido, Abdelkarim
