Many synthetic reactions are second or higher kinetic order. Once initiated in a particular reactor at a particular concentration (solvent volume), they proceed most rapidly in the initial stage and then slow down as the starting materials are consumed and their concentrations decline. As a consequence, the major portion of reaction time is spent waiting for the last small part of the reacting to finish because the concentrations of agents in these multi-order kinetics have become relatively low.
From these same considerations when a reaction is exothermic, the larger part of the exotherm occurs in the early stage when concentrations are highest. It is for this reason that process chemists religiously avoid mixing the full stoichiometric quantities of all the reactants together first and then initiating the reaction (say by heating). The reason: this is a recipe for a disastrous runaway reaction. Instead, in the preferred approach, one essential reactant is added gradually to a mixture of the other chemicals at the reaction temperature. Operating this way, any unwanted exotherm above what can be balanced by cooling, can be choked off by stopping the addition.
The question considered here is whether, after the faster part of the reaction has passed, anything can be done to accelerate the later slower stage of the reaction so that the overall reaction time can be reduced? If the reaction is being conducted at the reflux temperature of a single pure solvent, the reaction can in principle be accelerated, without changing the steady reaction temperature, by distilling away part of this reaction solvent. In this situation the reaction temperature is the boiling temperature of the solvent and such distillation removes solvent and increases starting material concentrations without changing the reaction temperature. Because removing solvent increases the concentrations of all the solutes including all the starting materials, the rate of their consumption will increase and the point of effective disappearance of starting materials will arrive quicker. For example, if the volume for a bimolecular reaction is reduced in half, the concentrations are doubled and the rate of reaction will be increased by a factor of four.
Of course there is a limit to how low the volume can be taken in a standard reactor. The volume cannot practically be reduced below where the reactor contents can be effectively stirred (the minimum stirrable volume). Also the volume must not be reduced below the level at which the reacting materials begin to precipitate because the reaction’s kinetics are almost certainly dependent upon a homogeneous solution.
Another advantage for the process of concentrating the reaction mixture is that the volume at the point of maximum volume is likely to be lowered. This will result in a higher product throughput; that is, more kilograms can be synthesized in fewer batch repeats. If the volume at the point of maximum volume can be reduced in half (for the sake of simplicity of example) you would only need half as many repeats of that process step to transform the same amount of starting materials.
A potential difficulty with such a concentrating procedure as I am describing can arise if some important element of the process co-distils with the solvent and is so removed. Again for example a volatile catalyst co-distilled when the solvent was being reduced this would slow down or stop the desired reaction despite the increased concentrating of the co-reactants. Although some reaction ingredients may not be blown out of a reaction mixture when distilled in the lab, distilling in the plant can have substantially different characteristics and one needs to be aware of the possible loss of even quite non-volatile materials via an aerosol. There are physical traps (called impingers) that can capture aerosol droplets and return them to the reactor to overcome this.
Resort to this concentration strategy described above is only needed when an unacceptably long time is required to get complete reaction at an acceptably low temperature. Of course it can only be practiced if a solvent is found that facilitates the desired reaction at the solvent’s boiling point.
Alternately the pressure in the reactor can be controlled so that the solvent that is most desirable for the reaction boils at the desired temperature.
Reactions that are bimolecular but exhibit pseudo-first order kinetics because one reactant is present in large excess can also be accelerated by this strategy.
This strategy could also be applied to a reaction conducted at the azeotropic boiling point of a binary solvent mixture.
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