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Chemistry Letters Vol.32, No.3 (2003)
Alkylation and Acetal Formation Using Supercritical Alcohol without Catalyst
Yoshiteru Horikawa, Yuki Uchino, and Takeshi Sakoꢀ
Department of Materials Science, Shizuoka University, 3-5-1 Hamamatsu, Shizuoka 432-8561
(Received October 17, 2002; CL-020885)
The aromatic ring alkylation of phenols, N-alkylation of
(critical temperature = 240.7 ꢁC, critical pressure = 6.1 MPa)6 as
alkylating reagents was investigated to evaluate the possibility of
alternatives to hazardous compounds such as dimethylsulfate or
methyl iodide. First, the alkylation of the aromatic ring of phenols
was examined. The experimental results are given in Table 1. The
Friedel-Crafts alkylation using a Lewis acid is well known.
Compared with the traditional Friedel-Crafts reaction, alkylation
using the supercritical alcohol did not require any catalyst.
Furthermore it is remarkable that only a monoalkylated com-
pound was synthesized in the reaction of hydroquinone with
supercritical alcohol, while the Friedel-Crafts reaction gives the
complex product mixture of mono- and polyalkylated com-
pounds. A maximum yield of 15% was obtained for hydroqui-
none. The order of the reactivity of the aromatic compounds was
hydroquinone > p-cresol > phenol, which agreed with the order
of the activation of the aromatic ring by hydroxy and methyl-
activating groups. On the other hand, the yield of the reaction of
hydroquinone with supercritical ethanol was only 6% under the
same conditions. The reactivity of supercritical ethanol was
smaller than that of supercritical methanol.
aniline, O-alkylation of phenols and acetal formation from
acetaldehyde or acetone were examined using supercritical (SC)
alcohol without any catalyst. Highly selective syntheses of
monoalkylated compounds were achieved for the aromatic ring
alkylation and N-alkylation. The O-alkylation proceeded more
preferentially than the aromatic ring alkylation for phenols which
have a deactivating group. The acetal formation went on in more
than 96% selectivity.
The alkylation of an aromatic ring or a functional group is an
important reaction in the chemical industry. At the present time,
toxic alkylating reagents, such as alkyl halides and dialkylsul-
fates, are often used along with a catalyst but have to change to
less hazardous substances in the near future. Recently, super-
critical alcohols have attracted much attention due to their high
reactivity and relatively small impact on the environment. There
are some literatures on the reactions of supercritical methanol or
ethanol: the esterification of fatty acids,1 the transesterification of
polyethylene terephthalate2 and the depolymerization of phenol
resin3 without any catalyst. Supercritical alcohol can significantly
accelerate the reaction, keeping a high selectivity. The analysis of
the reaction mechanism begins on a molecular level. Several
reports are available on the microscopic properties of super-
critical alcohols which have been carried out mainly by spectro-
scopic techniques.4;5 For example, about 70% of the hydrogen
bonding network among methanol molecules are broken at the
critical point to produce dimers and monomers. As a result, the
reactivity of the alcohol molecules increases significantly and the
catalyst is not needed because the molecules can move fast and
collide strongly with other molecules. Here, we report our
preliminary results of the cleaner, uncatalyzed and selective
alkylation and acetal formation with supercritical alcohol, where
supercritical alcohol was used not only for an alkylating or acetal
formation reagent but also for a reaction solvent.
Table 1. Alkylation of aromatic ring of phenols with super-
critical alcohola
The reactions with supercritical alcohol were conducted
using a batch-type reactor made of 316 stainless steel and with an
inner volume of 20 cm3. The experimental procedure was as
follows: The test compound of 0.1 g and methanol of about 5.5 g
were loaded into the reactor. The air in the reactor was replaced
with argon gas. The reactor was sealed and immersed into a sand
bath, which was already heated to the reaction temperature. A
small amount of alcohol was added to the reactor to get the desired
reaction pressure for a specified reaction temperature. After the
desired reaction time, the reactor was removed from the sand bath
and cooled quickly in water to stop the reaction. The reaction time
was defined as the duration for which the reactor was kept in the
sand bath. Subsequently, the solution sample was collected with
alcohol and analyzed by GC, GC-MS, and LC.
Secondly, N-alkylation was investigated. The N-alkyation of
aniline proceeded using supercritical alcohol without any
catalyst.
The performance of supercritical methanol (critical tempera-
ture = 239.4 ꢁC, critical pressure = 8.1 MPa)6 and ethanol
Copyright Ó 2003 The Chemical Society of Japan