Micro Process Technology and Novel Process Windows –Three Intensification Fields Svetlana Borukhova and Volker Hessel - Process Intensification for Green Chemistry - page 122

Pressure effects are also expected for reactions that undergo cyclic transition states on
their course to the product, such as Claisen and Cope rearrangements. Sato et al. [206]
achieved a conversion of 98% upon Claisen rearrangement of allyloxybenzene to 2-
allylphenol in aqueous solvent at 265 C and 50 bar. The lower concentration compared to
batch (0.27 versus 0.77 mol/kg) resulted in a longer residence time (149 versus 81 seconds)
but in a higher yield (98 versus 73%).
Apart from the effect upon the reaction volume, pressure also affects the concentration of
charge and ionization, accelerating reactions of the following types [207]:
Those that proceed via dipolar transition states (Menshutkin reactions and electrophilic
aromatic substitution reactions).
Those with steric hindrance.

4.3.4.2 Density and Viscosity Effects

Increased pressure affects both density and viscosity. The viscosity of liquids increases
approximately by a factor of two every 100MPa; thus, diffusion control of the reaction is
important [208]. Increased viscosity at high p (kbar range) has been found to accelerate
chemical processes such as Diels-Alder reactions, 1,3-dipolar cycloadditions and Claisen
rearrangements [209]. It has been demonstrated that apart from the acceleration due to the
negative reaction volume, a certain portion of the reaction-rate increase is associated with
the increase in viscosity; this is given in cases where the activation volume is more negative
than the overall reaction volume.
Transition state theory deviates for highly viscous media, where diffusion processes
play an important role. Since pressure increases viscosity coefficient exponentially,
such effects apparently must be taken into account. To illustrate this, in Z-E isomeriza-
tion of 4-(dimethylamino)-4 0 -nitroazobenzene, the rate constant initially increases with
pressure, then decreases at higher pressures when the medium is a viscous silicone
oil [210].
Packing and the corresponding interaction of the solvent molecules around the reactants
and around the transition state complex have relevance for the reaction outcome when high
pressure is applied. Electrostriction and steric effects have impact. Even the regio- or
stereoselectivity can be enhanced by using bulky groups as blocking units at one site while
enhancing the addition and substitution at the less bulky site.

4.3.4.3 Dielectric Constant Effects

Dielectric constant, , of any liquid is a weak function of temperature but strongly

dependent on pressure.

e

e p0
e p ¼

A ln ð

B þ p Þ

1 (4.8)

ð B þ 1 Þ

A and B are characteristic parameters for a liquid and are not dependent on pressure, while
B decreases with increasing temperature. Dielectric constant determines the magnitude of
electrostriction induced by the solvent in the vicinity of charged species, thus playing a
significant role in activation volume calculations. To illustrate the effect, the dielectric