By-products, undesirable

Foremost we hope - and believe - that chemoinformatics will become of increasing importance in the teaching of chemistry. The instruments and methods that are used in chemistry will continue to swamp us with data and we have to manage these data to increase our chemical knowledge. We have to understand more deeply, and exploit, the results of our experiments. Concomitantly, demands on the properties of the compounds that are produced by the chemical and pharmaceutical industries will continue to rise. We will need materials that are better we need them to be more selective, have fewer undesirable properties, able to be broken down easily in the environment without producing toxic by-products, and so on. This asks for more insight into the relationships between chemical structures and their properties. Furthermore, we have to plan and perform fewer and more efficient experiments. 

Many of the reactions listed at the beginning of this section are acid catalyzed, although a number of basic catalysts are also employed. Esterifications are equilibrium reactions, and the reactions are often carried out at elevated temperatures for favorable rate and equilibrium constants and to shift the equilibrium in favor of the polymer by volatilization of the by-product molecules. An undesired feature of higher polymerization temperatures is the increased probability of side reactions such as the dehydration of the diol or the pyrolysis of the ester. Basic catalysts produce less of the undesirable side reactions. 

Uses. Boron triduoride is an excellent Lewis acid catalyst for numerous types of organic reactions. Its advantages are ease of handling as a gas and the absence of undesirable tarry by-products. As an electrophilic molecule, it is an excellent catalyst for Friedel-Crafts and many other types of reactions (63-65) (see Friedel-craftsreactions). 

Ultimately, as the stabilization reactions continue, the metallic salts or soaps are depleted and the by-product metal chlorides result. These metal chlorides are potential Lewis acid catalysts and can greatiy accelerate the undesired dehydrochlorination of PVC. Both zinc chloride and cadmium chloride are particularly strong Lewis acids compared to the weakly acidic organotin chlorides and lead chlorides. This significant complication is effectively dealt with in commercial practice by the co-addition of alkaline-earth soaps or salts, such as calcium stearate or barium stearate, ie, by the use of mixed metal stabilizers. 

This process has been widely studied and led to the constmction of new and original industrial units. Interest in the reaction stems from the simplicity of the process as well as the absence of undesirable by-products. However, in order to be economically rehable, such a process has to give high yield of dihydroxybenzenes (based on hydrogen peroxide as well as phenol) and a great flexibiUty for the isomeric ratio of hydroquinone to catechol. This last point generated more research and led to original and commercial processes. 

An excess of phosgene is used during the initial reaction of amine and phosgene to retard the formation of substituted ureas. Ureas are undesirable because they serve as a source for secondary product formation which adversely affects isocyanate stabiUty and performance. By-products, such as biurets (23) and triurets (24), are formed via the reaction of the labile hydrogens of the urea with excess isocyanate. Isocyanurates (25, R = phenyl, toluyl) may subsequendy be formed from the urea oligomers via ring closure. 

Manufacture and Processing. Mononitrotoluenes are produced by the nitration of toluene in a manner similar to that described for nitrobenzene. The presence of the methyl group on the aromatic ring faciUtates the nitration of toluene, as compared to that of benzene, and increases the ease of oxidation which results in undesirable by-products. Thus the nitration of toluene generally is carried out at lower temperatures than the nitration of benzene to minimize oxidative side reactions. Because toluene nitrates at a faster rate than benzene, the milder conditions also reduce the formation of dinitrotoluenes. Toluene is less soluble than benzene in the acid phase, thus vigorous agitation of the reaction mixture is necessary to maximize the interfacial area of the two phases and the mass transfer of the reactants. The rate of a typical industrial nitration can be modeled in terms of a fast reaction taking place in a zone in the aqueous phase adjacent to the interface where the reaction is diffusion controlled. 

Unsymmetrical dialkyl peroxides are obtained by the reaction of alkyl hydroperoxides with a substrate, ie, R H, from which a hydrogen can be abstracted readily in the presence of certain cobalt, copper, or manganese salts (eq. 30). However, this process is not efficient since two moles of the hydroperoxide are consumed per mole of dialkyl peroxide produced. In addition, side reactions involving free radicals produce undesired by-products (44,66). 

Water formed in the reaction as well as some undesirable by-products must be removed from the acetic acid solvent. Therefore, mother Hquor from the filter is purified in a residue still to remove heavies, and in a dehydration tower to remove water. The purified acetic acid from the bottom of the dehydration tower is recycled to the reactor. The water overhead is sent to waste treatment, and the residue still bottoms can be processed for catalyst recovery. Alternatively, some mother Hquor from the filter can be recycled directiy to the reactor. 

Side reactions in the primary reformer are those which form carbon, an undesirable by-product. 

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