A + B Gives C + D
Most chemical reactions are either unimolecular or bimolecular in their kinetics. They are either addition/condensations where there is only one product, ie C and no D, or in other instances, some transformation also provides a coproduct, D, created along with C.
Reactions proceed when the energy supplied to a system is appropriate for the free energy of the reaction. More energy being supplied to the system can cause the reaction to proceed faster and/or it can promote alternative transformations that compete with the desired one. Applying excess energy for longer than necessary can degrade any of A, B, C, or D.
If any of the reactions
C + (A or B) gives (E or H)
or
A + A gives F
or
B+ B gives G
can occur, then having the correct stoichiometry present throughout the time when the system experiences the required activation energy becomes important. If the stoichiometry is wrong then starting reactant polymerizations or overreaction modes can compete to the detriment of the desired outcome. A procedure that keeps the reactants in the correct stoichiometry throughout that period when adequate activation energy is supplied is likely to give a cleaner reaction.
Standard Chemical Processing
Now in the standard mixing procedure, a solution either of reactant A or B is added slowly to a solution of the complete charge of B or A already stirring in the reactor. If the reaction proceeds so quickly that it overheats, the addition is slowed or stopped allowing the rate of reaction to slow down as the reactant being added gets consumed and no more is being added. That is, the reaction is prevented from running out of control by letting the stoichiometry inside the reactor change radically. To control an exotherm the operators in such instances are forced to let the concentration of the reactant being added into the vessel fall towards zero.
For greater clarity and to emphasize the necessary safety issue let us perform the thought experiment of mixing all of the charge of these same A and B in methylene chloride and starting to heat the reactor. When the reaction between A and B sets in, the methylene chloride will start to reflux but as the temperature rises towards the boiling point of the methylene chloride the reaction between them goes faster and faster. The methylene chloride now boils faster than it can be condensed and gaseous methylene chloride blows liquid methylene chloride and all the rest of the reactor’s content out of the reactor and probably onto the roof of the plant building!
Identifying Reactions that Could be Affected by Stoichiometry
The standard mixing procedure is unlikely to be adversely affected by the ‘in situ’ stoichiometry if the reaction rates of all steps occurring in the reactor are slow compared to the addition time. This is easily determined. When one-half of the addition of the controlling reactant has been added, an in-process check should be done. If either the product or any reaction intermediates are significant compared with unreacted starting materials then the transformation is likely to be sensitive to the reaction’s stoichiometry. If nothing significant besides unreacted starting materials (or substances produced during the quenching of the reaction before analysis) are present, then it is very unlikely that the yield will be affected by stoichiometry. When hardly any change can be detected in this test it indicates that almost the reacting occurs mainly after the reactant being added is completely in the reactor and the stoichiometry is therefore essentially fixed throughout the actual reacting period.
Importance of the Enthalpy of Reaction Calculation
If you are going to perform experiments in which all the reactants are mixed together at once, you must take appropriate precautions.
You must do a calculation of the enthalpy of reaction based on standard bond energies; however, the enthalpy of reaction (∆H) is only a crude approximation to the free energy of reaction (∆F) since ∆F = ∆H - T∆S. The change in entropy of the reaction (∆S) also contributes to the reaction’s driving force. For example, if the reaction leads to more moles of products than the moles of reactants, the entropy change is most likely going to be positive and the overall free energy decidedly more negative because of (-T∆S). The reaction will be faster than the approximation. It will be necessary to absorb the net free energy of reaction through the agency of first- the temperature rise of the reactor contents ( mostly solvent), then the energy absorbed in vaporizing some part of the reaction solvent, and finally by the energy drained off by the refluxing of a part of the solvent.
Absolute Necessity for Small Scale Trials in a Laboratory Fume Hood
Theory is just theory. If you are going to perform a reaction in which all the reactants are to be mixed together and then reaction initiated (either by heat or catalyst), you must try out the method at a scale where even if you have miscalculated there will be no more than a mess to clean up, not a laboratory disaster! The reaction which you have examined might not even be the reaction that occurs. Something much more energetic could surprise you.
Get Emergency Control Means Ready
Of course, your reaction vessel must be equipped with a means to directly in real-time read the internal temperature. Otherwise, you won’t be alerted soon enough that you have a problem.
If despite your calculations the temperature of the reactor accelerates in an unexpected way you want to be prepared to intervene to slow things down. Having a cold -bath ready into which the reactor flask can be plunged often can tame a reaction that is misbehaving. The advantage of this form of moderation is that it can be removed when the danger passes and the experiment continued.
Some reactions need to be quenched by injection. In many cases that quench material can be as simple as water. Of course, such an experiment is wasted but a runaway condition is avoided.
When a runaway reaction is the least bit possible, the experimentalist should both wear appropriate personal protective equipment and have a fire extinguisher handy. It is important to prepare by imagining what might happen. For example, are there other chemicals in the hood you will be using that could exacerbate your runaway?
Well then—can we actually mix reactants A and B together safely so that the correct stoichiometric is maintained at all times and a runaway exotherm is prevented?
Two Simultaneous but Separate Additions
Simple in concept but mechanically difficult would describe the simultaneous addition to one reactor of two separate solutions, one carrying the A reactant and the other carrying the B reactant. The two addition rates of each need to be controlled so that the same required molar amount of each reactant is delivered throughout the addition time. If unwanted exothermicity occurs in the reactor both reactant streams need to be slowed the same amount or stopped simultaneously. Technically this is a big ask and practically it will only be approximately successful but even if it is only achieved approximately it can provide a closer approximation to constant ideal stoichiometry. This method of reactant addition, even if performed somewhat imperfectly, can give the experimentalist an indication of whether getting better stoichiometry using a different but more practical method is likely to deliver a useful benefit.
Storage Solvent and Reaction Solvent Method
Both reactants can be dissolved together completely in a volatile solvent (storage solvent) at a temperature and concentration at which reaction will not proceed and that solution can be added in a controlled fashion to a higher boiling solvent (reaction solvent) preheated to below its own bp. but above the bp. of the addition solvent so that the storage solvent will flash distill as it is added and the reaction between A and B will occur at the set stoichiometry in the reaction solvent solution.
Flow System Method
A continuous flow system may be possible where the reactants are bled separately but in the proper stoichiometry into a heated reaction zone and the product is continuously removed from the reaction zone. The problem with the flow technique is that for many reactions operating at the lowest normal reaction temperature the heated zone would be quite long. Such specially configured equipment is not typically available. This can be solved by operating at a higher temperature in a higher boiling solvent treated in the reaction zone near the higher boiling point of this alternative solvent.
For example, suppose it is known that A will react with B in refluxing dichloromethane and the reaction is essentially complete in three hours. The same reaction however might proceed equally cleanly in hot propylene carbonate in one minute. Then mixing two solutions one of A and the other of B; each dissolved in hot propylene carbonate so that a correct stoichiometric ratio and delivering it into the hot reaction zone in a flow reactor so that the transition of material through that zone was one minute could produce C more cleanly than heating much longer in dichloromethane because the correct stoichiometry has been more faithfully observed and the reaction period is very abbreviated. If the reaction time is lessened, the reaction zone length can be shortened and some standard equipment may suffice. Now, whether creating the advantage of having the correct stoichiometry throughout the reaction period outweighs the disadvantage of applying more energy to the system than is actually required to get reaction can only be answered by experiment.
Reactants- All-In Method
Another means to achieve the same result would be to mix the correct stoichiometric quantities of A and B in the bottom of a reactor and then add a high-boiling solvent to it; in our example above this would be the propylene carbonate. The reactor is then heated to start the reaction. Normally mixing all of the reactants together and then heating to start their reaction is never done at scale because, as already pointed out, the exothermicity of the reaction is likely to cause a runaway wherein the entire contents of the reactor including any solvent would overheat and potentially even explode; however, using a sufficient quantity of a high-boiling solvent with a significant heat capacity and large enough heat of vaporization will allow any exothermicity to be absorbed first by the heating of the high-boiling solvent and subsequently by the heat of vaporization (refluxing) of a small portion of this solvent.
Using High-Boiling Solvents to Control Exothermic Reactions
The advantage to using a high-boiling solvent for such a reaction is not primarily its ability to dissolve the reactants and bring them together homogeneously. Good low-boiling solvents can do that. The high-boiling solvent can use its higher heat-of-vaporization, as well as its heat capacity, as a heat sink to soak up the exothermicity of the reaction.
In fact, the high-boiling liquid in which the reaction is being conducted does not need to be able to dissolve the reactants for it to be effective as a heat sink. So long as the reactants can blend together without solvation help the liquid medium does not have to dissolve them. Reactions ‘on-water’ for example are of this type. If at least one of the reactants is itself liquid there will be a reduced need for a dissolving type solvent. Another purpose for using a high-boiling liquid medium in which the reactants are insoluble is to provide sufficient bulk volume to satisfy the minimum stirrable volume requirement for the reactor.