2.2 The Need for Metrics
19
impact of both the chemicals and equipment required to purify it are fast becoming
non-innocuous issues (Jiménez-González and Constable 2011).
Energy reduction via process intensification and modification are pursued by large
industries, including the petrochemical and pharmaceutical sectors. This key point is
largely overlooked in bench chemistry. Energy metrics are usually similar to those for
mass, accounting for the energy required to produce starting materials and products,
the recycling of materials and solvents and waste treatment. Accordingly, energy
valorization should look to reduce the energy used by an entire protocol, rather
than just the energy used to heat or cool the reaction vessel (Jiménez-González
and Constable 2011). The best way to do this is to measure energy consumption
using some kind of electricity metre linked in series with whatever apparatus is
being used in a reaction’s operation. In fact, ca 80% energy reduction, relative to
heating in an oil bath, has been reported for a Suzuki coupling performed under
MW (microwave), irradiation (Gronnow et al. 2005), using this method. This kind
of energy measurement can be easily accomplished in ultrasound-assisted reactions
(Cintas 2016) and extrapolated to sequential designs where both mass and energy
consumption are evaluated (Andraos 2016).
2.3 Reactivity in Ultrasonic Fields
It is now well accepted that the chemical effects of sonication arise from cavitation, as acoustic radiation does not interact with matter at the atomic or molecular
levels. Bubble collapse leads to quasi-adiabatic heating of the vapour inside the
bubble, giving rise to local hot spots in the fluid. Overall, cavitation transforms the
low-energy-density sound field into high kinetic energy which is released in very
short bursts. The immediate consequence of the presence of pyrolyzed vapour is the
potential formation of high-energy excited species, such as hydrogen radicals (H),
and hydroxyl radicals (HO), which can then participate in reactions in the bulk liquid
at room temperature. The most innovative aspect of sonochemistry lies in reaction
switching, which is the formation of products that are not usually produced in regular
thermal reactions and/or the alteration of their distribution.
The rationalization of sonochemical reactivity, i.e. an understanding of what kind
of chemical reactions will be particularly sensitive to sonication and to what extent,
is a matter of empiricism. As formulated by Luche in the early 1990s, a tentative
classification of reaction types often means considering their mechanisms (Luche
1993). While we will not go into too much detail here, Fig. 2.1, which shows the
possible mechanisms that can take place under sonication using conventional organic
synthesis nomenclature, provides a sufficient overview.
In a convergent process, the substrates can react by means of either a radical
or a polar pathway and furnish identical products. Some organometallic reactions
(Barbier-like, for instance), may be classified in this way as radical ions stemming
from a radical mechanism will be preferentially generated by ultrasound and yet
produce the same organometallic reagent as derived from a polar route. By con-
19
impact of both the chemicals and equipment required to purify it are fast becoming
non-innocuous issues (Jiménez-González and Constable 2011).
Energy reduction via process intensification and modification are pursued by large
industries, including the petrochemical and pharmaceutical sectors. This key point is
largely overlooked in bench chemistry. Energy metrics are usually similar to those for
mass, accounting for the energy required to produce starting materials and products,
the recycling of materials and solvents and waste treatment. Accordingly, energy
valorization should look to reduce the energy used by an entire protocol, rather
than just the energy used to heat or cool the reaction vessel (Jiménez-González
and Constable 2011). The best way to do this is to measure energy consumption
using some kind of electricity metre linked in series with whatever apparatus is
being used in a reaction’s operation. In fact, ca 80% energy reduction, relative to
heating in an oil bath, has been reported for a Suzuki coupling performed under
MW (microwave), irradiation (Gronnow et al. 2005), using this method. This kind
of energy measurement can be easily accomplished in ultrasound-assisted reactions
(Cintas 2016) and extrapolated to sequential designs where both mass and energy
consumption are evaluated (Andraos 2016).
2.3 Reactivity in Ultrasonic Fields
It is now well accepted that the chemical effects of sonication arise from cavitation, as acoustic radiation does not interact with matter at the atomic or molecular
levels. Bubble collapse leads to quasi-adiabatic heating of the vapour inside the
bubble, giving rise to local hot spots in the fluid. Overall, cavitation transforms the
low-energy-density sound field into high kinetic energy which is released in very
short bursts. The immediate consequence of the presence of pyrolyzed vapour is the
potential formation of high-energy excited species, such as hydrogen radicals (H),
and hydroxyl radicals (HO), which can then participate in reactions in the bulk liquid
at room temperature. The most innovative aspect of sonochemistry lies in reaction
switching, which is the formation of products that are not usually produced in regular
thermal reactions and/or the alteration of their distribution.
The rationalization of sonochemical reactivity, i.e. an understanding of what kind
of chemical reactions will be particularly sensitive to sonication and to what extent,
is a matter of empiricism. As formulated by Luche in the early 1990s, a tentative
classification of reaction types often means considering their mechanisms (Luche
1993). While we will not go into too much detail here, Fig. 2.1, which shows the
possible mechanisms that can take place under sonication using conventional organic
synthesis nomenclature, provides a sufficient overview.
In a convergent process, the substrates can react by means of either a radical
or a polar pathway and furnish identical products. Some organometallic reactions
(Barbier-like, for instance), may be classified in this way as radical ions stemming
from a radical mechanism will be preferentially generated by ultrasound and yet
produce the same organometallic reagent as derived from a polar route. By con-