Thermolysis of αꢀpinene
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 6, June, 2006
991
purpose of possible optimization of the conditions of liꢀ
monene preparation, it was necessary to elucidate whether
limonene underwent any transformations in the superꢀ
critical medium or not. It was known beforehand that for
the gasꢀphase transformation of limonene its conversion
rate below 415 °C is low but already at 450 °C limonene is
consumed completely within the same reaction duration.26
It can be assumed that at temperatures below 400 °C
limonene, which is formed from αꢀpinene in the superꢀ
critical medium, does not react further. This assumption
required additional studies, especially after a mixture of
products similar to the pyrolysis products of αꢀpinene has
been reported to form from limonene at high temperaꢀ
tures.27
the reaction is insensitive to the presence of inhibitors of
radical species.12
In the conclusion, it has been found that the therꢀ
molysis of αꢀpinene in supercritical alcohols results
in the same products as gasꢀphase and liquidꢀphase
thermal isomerizations. At the same time, the superꢀ
critical reaction medium makes it possible to enhance
the conversion rate by several orders of magnitude with
retention of the reaction selectivity for limonene and
isomeric allooocimenes, whose maximum yield is 52
and 38%, respectively. The efficient parameters controlꢀ
ling the reaction rate are the temperature and pressure.
A comparative study of the influence of the solvent naꢀ
ture on the isomerization shows that the conversion of
αꢀpinene at the same parameters increases in the series of
alcohols C1—C3. In the range of chosen temperatures
and pressures, limonene is thermally more stable than
αꢀpinene.
To estimate the conversion of limonene, we carꢀ
ried out the thermolysis of its ethanolic solution (see
Table 2).
At 330 °C the conversion of limonene is not higher
than 3% and the main product of its transformation
is αꢀpinene, which can be formed from limonene as
a result of ene reaction.15 When the temperature inꢀ
creases to 430 °C, the conversion increases insignifiꢀ
cantly (~8%) and αꢀ and βꢀpyronenes appear along
with αꢀpinene; however, alloocimenes are entirely absent
in the reaction mixture. According to the proposed
mechanism (see Scheme 1), pyronenes are formed from
alloocimenes due to intramolecular cyclization.14,21,28
The absence of alloocimenes in the reaction mixture
is related, most likely, to the fact that at 430 °C they
are rapidly transformed into αꢀ and βꢀpyronenes and
other pyrolysis products. This is indirectly indicated by
the earlier observed decrease in the maximum concenꢀ
tration of alloocimenes in experiments on αꢀpinene
thermolysis at elevated temperatures and pressures (see
above).
Thermolysis of αꢀpinene in the presence of phenol.
To study additional possibilities of the process in superꢀ
critical media, we carried out the thermolysis of αꢀpinene
(the initial concentration in EtOH is 0.1 mol L–1) in
the presence of phenol (concentration 0.3 mol L–1) at
Tconst = 380 °C, three different pressure values (100,
150, and 200 atm), and a residence time of 140 s.
However, a phenol additive exerted no noticeable efꢀ
fect on the reaction course. Pinene is consumed comꢀ
pletely (residual concentration <0.5%), the concentraꢀ
tion of limonene that formed remains close to the maxiꢀ
mum value (47—49%), and the content of phenol does
not virtually change with the pressure increase. No other
compounds, except for the starting phenol and isomerizaꢀ
tion products of αꢀpinene, were found in the reaction
mixture.
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The presence of phenol exerts no effect on the radical
character of thermal isomerization of αꢀpinene (see
Scheme 1). This fact confirms available information that