It refers to the treatment of drugsdoseIt happened undermedicineMost of the inherent functions are minor and predictable.Due to low selectivity, pharmacological effects involve multiple organs. When a certain effect is used for therapeutic purposes, other effects become side effects (usually called side effects).Side effects refer to the reactions that usually occur. Side effects often refer to the side of drugs that is unfavorable to the treatment of diseases. Most of them do not occur.
The side reaction is that the reactants are carrying out two or more different reactions at the same time under certain conditions, not only generating one substance we want, but also generating substances we don't want. It is usually called a by-product. The reaction that generates substances we don't want is called a side reaction, such as toluenenitrificationIn the reaction, the nitro group can be substituted in the ortho, para, and meta positions to become nitrotoluene. In this way, we only want one of them, and the reaction that produces the other two products is a side reaction.
For example, inmanganese dioxideIn the experiment of catalytic decomposition of potassium chlorate to produce oxygen, the equation is
But this reaction is not completed in one step. There are three side reactions, namely
In fact, it is not difficult to find that the addition of these three side reactions is exactly the original reaction, andmanganese dioxideIf the left and right sides of the equation are omitted, it will not participate in the reaction at the macro level, but it will still be written in the equation as a catalyst.
Control of Side Reactions in Xylene Isomerization Industry
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stayXylene isomerization catalystThe occurrence of side reaction is closely related to the acidic function of catalyst and operating conditions.Most of the side reactions are mainly caused by the conversion process of ethylbenzene. The key to reduce the side reactions is to optimize the matching of reaction temperature and pressure and control the appropriate conversion degree of ethylbenzene.[1]
Main reaction and side reaction
NonequilibriumMixed xyleneOne of the main reactions is the conversion of ethylbenzene to xylene mixture with composition close to thermodynamic equilibrium, and the other is the conversion of ethylbenzene to xylene.In terms of acid function and metal function of xylene isomerization catalyst, the expected isomerization conversion and ethylbenzene conversion can be achieved through the main reaction.
The main reaction is accompanied by a series of side reactions, such as the disproportionation and transalkylation of ethylbenzene itself, ethylbenzene and xylene, and xylene itself, the dealkylation of ethylbenzene and xylene, the hydrogenation saturation and ring opening cracking of ethylbenzene.The main side reactions are as follows: disproportionation reaction, alkylation transfer reaction, dealkylation reaction, hydrogenation ring opening cracking reaction.[1]
Influence of acid sites on side reactions
The acid center of molecular sieve is not only xyleneIsomerization reactionIt is also the active site of disproportionation, alkyl transfer, hydrocracking and other side reactions.It can be seen from the above side reactions that ethylbenzene participates in the majority of side reactions, so improving the selectivity of ethylbenzene conversion process is the key to reduce the degree of side reactions.Xylene is mainly involved in disproportionation and alkylation;Under suitable process conditions, the possibility of dealkylation and hydroring opening of xylene is small in the process of catalytic isomerization of xylene.Disproportionation and alkylation belong toBimolecular reaction, a large intermediate transition is required（Diphenylmethane）Its steric hindrance is large, and this kind of reaction can be completed only under the action of strong B acid of molecular sieve in a large pore channel;The dealkylation reaction needs a high energy barrier to break the C-C bond, and also needs the catalysis of strong B acid center;Hydrogenation ring opening reaction also needs the catalysis of strong B acid center to complete.Therefore, although strong B acid sites are important active sites for xylene isomerization, the more strong B acid sites in the catalyst, the more side reactions will be caused.[1]
Influence of process conditions on side reaction
The operating conditions of the catalyst will also affect the degree of side reaction.For the catalyst with pre designed activity, the change of process operating conditions (temperature, pressure, heavy time space velocity, hydrogen hydrocarbon molar ratio) is an important means to adjust the catalyst activity.The change of process conditions will have a greater impact on the reaction results: with the increase of reaction temperature, the reaction activity increases and the selectivity decreases;With the increase of reaction pressure, the reactivity increased and the selectivity decreased;The reaction activity decreased and the selectivity increased with the increase of the weight space velocity;With the increase of the molar ratio of hydrogen to hydrocarbon, the activity decreased slightly and the selectivity increased slightly.[1]
Teaching of Side Reaction Coefficient in Analytical Chemistry
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This paper discusses the teaching method of side reaction coefficient, gives a clear definition of side reaction coefficient, and summarizes the application of side reaction coefficient in analytical chemistry.By analogyCumulative stability constant, willAcid effect coefficientAnd coordination effect coefficient become easier to understand and remember.[2]
Proposing the definition of side reaction coefficient
While the main reaction is going on, many side reactions are also going on, and the side reactions will change the reaction degree of the main reaction.The degree of side reaction is expressed by side reaction coefficient α.Both reactants and products in the main reaction may have side reactions. Both reactants and products have two forms of existence, one is the expected form of existence in the main reaction, and the other is the various forms of existence in which side reactions occur.Therefore, the side reaction coefficient is defined as the ratio of the total concentration of all existing forms of reactants (or products) in the main reaction to the expected concentration of existing forms of reactants (or products) in the main reaction, that is
α=(total concentration of all existing forms of reactants (or products) in the main reaction) ÷ (expected concentration of existing forms of reactants (or products) in the main reaction)
The more serious the side reaction is, the more the side reaction products will be. If the total concentration of all existing forms of reactants (or products) in the main reaction is far greater than the expected existing form concentration of reactants (or products) in the main reaction, the greater the side reaction coefficient α will be.
If there is no side reaction, there is only one form of reactant (or product) in the main reaction, that is, the expected form of reactant (or product) in the main reaction, then α=1.[2]
Application of side reaction coefficient
1. Side reaction coefficient in coordination titration
In addition to the coordination reaction between metal ion M and ligand EDTA, there are side reactions such as coordination effect, acid effect and coexistence ion effect in the coordination titration process.To accurately express the degree of titration reaction, it is necessary to calculate the conditional stability constantKΘ′f, while calculatingKΘ′fIt is necessary to know the side reaction coefficient.
The side reaction of reactants M and Y is not conducive to the main reaction, while the side reaction of product MY is conducive to the main reaction.Here, α M represents the side reaction coefficient of metal ions, α Y represents the side reaction coefficient of EDTA, and α MY represents the side reaction coefficient of MY.According to the reaction formula, for reactant M, the expected form of the main reaction is free M ion, while the reaction with ligand OH－The existing forms of reactant M and free M ions after side reaction of, L are all existing forms of reactant M.
2. Side reaction coefficient in redox titration
When calculating the conditional electrode potential in redox titration, the side reaction coefficients of oxidation state and reduction state should be used.For example, according to the electric pair HthreeAsOfour/HthreeAsOthreeStandard electrode potential E ofΘ（HthreeAsOfour/HthreeAsOthree）Calculate the conditional potential E of the electric pair at pH=8Θ′（HthreeAsOfour/HthreeAsOthree）。
In the precipitation gravimetric method, it is used to calculate the solubility of insoluble electrolyteConditional solubility productKspΘ′, while K is calculatedspΘ′Side reaction coefficient is also required.For example, calculate AgI in NHthreeSolubility in, where NHthreeThe acid effect of is negligible.[2]
