Chemistry Reference
In-Depth Information
Scheme 4.8 Bromination of thiophene.
100% conversion [202]. Many more such high-T examples are available in Obermayer
et al. and Razzaq et al. [203,204].
Flow-based operation at much higher temperatures than are conventionally used in batch
operations was also established for organometallic or similar reactions, usually carried out
under cryogenic conditions [8,24]. The use of cryogenic conditions has three motivations.
Most notably, they 'freeze' the reaction in order to make it slower than the mixing;
otherwise, the very fast reactions tend to be severely mixing-masked, which results in much
lower yields. Second, unstable intermediates must be handled. Third, some of the fast
reactions are highly exothermic, so again 'freezing' is the right concept under batch
conditions with less improved heat transfer. Intensified mixing and heat and mass transfer
allow the limits mentioned to be overcome, so that the former cryoreactions can be
processed at ambient temperature without any compromises in yield - often with even
better yields - and always with much improved space-time productivity.
The bromination of thiophene (see Scheme 4.8) is conventionally performed at 0
C due
to the high reactivity. Through variation of the molar ratio of the reactants under short
reaction times in a microreactor, full control over the distribution of the products was
achieved, ranging from mono- to tetra-substituted. No change in product distribution was
observed in the range 0-60
C, keeping the molar ratio of bromine to thiophene 2.0 and
favouring the formation of 2,5-dibromothiophene. The selectivity increased to 80% [205].
Beyond many organometallic reactions investigated in microreactors, ambient-cryo
shift was also carried out for an autocatalytic and explosive nitration reaction of phenol.
The nitration was safely performed under tight temperature control at room temperature
and with high concentrations in flow. The highest combined flow-derived yield of p- and o-
nitrophenols was 77% greater than those obtained in batch [95].
4.3.4 High Pressure
4.3.4.1 Reaction Volume Effects
Apart from facilitating superheated conditions, high p affects reaction through its influence
on reaction equilibrium and rate constant and affects reaction medium through its effect on
its physical properties. In cases where the volume occupied by the product following the
reaction is lower than that given for the two separate reactants, the overall volume change is
negative and application of high p will have an acceleration effect. Thus, cycloadditions are
good candidates for intensifcation by elevated pressures.
Kappe et al. [204] performed a Diels-Alder cycloaddition of 2,3-dimethylbutadiene and
acrylonitrile under high-T (300
C) and high-p (200 bar) conditions in a stainless-steel
tubular (1000
m
m, id
1
) reactor. Upon raising the temperature from 250 to 280
C, the
reaction was accelerated from 5 to 2 minutes' residence time at a yield of 82%, total
throughput of 80.4 g/hour and space yield of 1.4 kg/m
3
/s.
1
internal diameter
Search WWH ::
Custom Search