Side reactions and products

PTT shares several similar thermo-oxidation degradation mechanisms with PET [32], Some of the more important ones are as follows  

During polymerization, PDO dimerizes into dipropylene ether glycol (DPG) which incorporates into the PTT chains as a copolymer. DPG formation is more severe in the acidic TPA process. The incorporated DPG lowers the polymer s melting point and affects fiber dye uptake [34], 

The above side reactions can be suppressed to various extents by adding antioxidants and phosphites, using higher purity PDO and controlling the polymerization conditions [35], 

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During this process, the metal catalyst transforms the slow self-accelerated oxidation into the fast accelerated chain process. One can lower the temperature of oxidation and decrease undesirable side reactions and products. 

We studied the hydrogenation of acetophenone, where a great deal of side-reactions and production of intermediaries take place in a three-phase reactor, with liquid batch and gas continuously fed, at constant pressure and temperature. The catalyst is Rhodium (3%) over activated carbon. The solvent is Cyclohexane. Samples are taken at different instants and analysed by gas chromatography. The species for which measures are available are acetophenone, AC, phenyl-ethanol, PE, methyl-cyclohexyl-ketone, ethyl-benzene, EB, ethyl-cyclohexane, cyclohexenyl-ethanol, CNE, methyl-cyclohexenyl-ketone and cyclo-hexyl-ethanol, CE. 

The key step in the reduction of oxygen at a catalytic surfece is the breaking of the 0—0 bond that requires four coupled proton and electron transfers, opening up the possibility of many side reactions and products (see Figure 2.4) [6]. The complexity of the ORR and its numerous potential side products means that it is still relatively poorly understood, although the consensus is that it proceeds either via a direct four-electron reduction pathway or via a peroxide intermediate in a 2 + 2 serial four-electron pathway [16-18]. 

Competing side reactions and product degradation reactions are identified. 

In principal, synthesis route prediction can be done from scratch based on molecular calculations. However, this is a very difficult task since there are so many possible side reactions and no automated method for predicting all possible products for a given set of reactants. With a large amount of work by an experienced chemist, this can be done but the difficulty involved makes it seldom justified over more traditional noncomputational methods. Ideally, known reactions should be used before attempting to develop unknown reactions. Also, the ability to suggest reasonable protective groups will make the reaction scheme more feasible. 

Liquid Fuels via Methanol Synthesis and Conversion. Methanol is produced catalyticaHy from synthesis gas. By-products such as ethers, formates, and higher hydrocarbons are formed in side reactions and are found in the cmde methanol product. Whereas for many years methanol was produced from coal, after World War II low cost natural gas and light petroleum fractions replaced coal as the feedstock. 

The chemistry of side reactions and by-products may also offer opportunities for increasing the inherent safety of a process. For example, a process involving a caustic hydrolysis step uses ethylene dichloride (EDC 1,2-dichloroethane) as a solvent. Under the reaction conditions a side reaction between sodium hydroxide and EDC produces small but hazardous quantities of vinyl chloride ... 

The product composition from these reactions is influenced by the location of the functional group in the substrate. Olefin formation is the most common side reaction and in certain cases, especially with reductions of tosyl-hydrazones (section IV-B), it may become dominant so that the reaction can be used for the preparation of mono-labeled olefins. 

The results presented in Tables 3 and 4 deserve some comments. First, a variety of enzymes, including whole-cell preparations, proved suitable for the resolution of different hydroxyalkanephosphorus compounds, giving both unreacted substrates and the products of the enzymatic transformation in good yields and, in some cases, even with full stereoselectivity. Application of both methodologies, acylation of hydroxy substrates rac-41 and rac-43 or the reverse (hydrolysis of the acylated substrates rac-42 and rac-44), enables one to obtain each desired enantiomer of the product. This turned out to be particularly important in those cases when a chemical transformation OH OAc or reverse was difficult to perform. As an example, our work is shown in Scheme 3. In this case, chemical hydrolysis of the acetyl derivative 46 proved difficult due to some side reactions and therefore an enzymatic hydrolysis, using the same enzyme as that in the acylation reaction, was applied. Not only did this provide access to the desired hydroxy derivative 45 but it also allowed to improve its enantiomeric excess. In this way. 

The simplest routes to azoniaspiroalkanes involve condensation of ammonia with dihalogenoalkanes. Owing to the competing side reactions and subsequent low yields of the desired product, no new examples of these types of reactions have been reported <1996CHEC-II(8)1109>. 

Maleic acid is a linear four carbon molecule with carboxylate groups on both ends and a double bond between the central carbon atoms. The anhydride of maleic acid is a cyclic molecule containing five atoms. Although the reactivity of maleic anhydride is similar to other cyclic anhydrides, the products of maleylation are much more unstable toward hydrolysis, and the site of unsaturation lends itself to additional side reactions. Acylation products of amino groups with maleic anhydride are stable at neutral pH and above, but they readily hydrolyze at acid pH values around pH 3.5 (Butler et al., 1967). Maleylation of sulfhydryls and the phe-nolate of tyrosine are even more sensitive to hydrolysis. Thus, maleic anhydride is an excellent reversible blocker of amino groups to temporarily mask them from reactivity while another... 

If a solvent is used as part of the catalyst solution, then it also must be separated from the product. Finally, the buildup of various byproducts from ligand degradation, from raw material side reactions and from subsequent reaction of the desired product must be addressed so that the catalyst solution remains fully functional to achieve an economic catalyst life. 

This preparation and oleoyl chloride (p. 66) illustrate the use of the general form of a laboratory-sized continuous reactor.6 This device has many advantages over the commonly used flasks (batch procedure). In particular, the short time of exposure to heat results in a better quality of product, as shown by less color, fewer side reactions, and better melting point, often unchanged by recrystallization. Furthermore, the unlimited capacity, very short reaction time, and use of concentrated solutions permit a larger output with no increase in size of apparatus and less delay required for removal of solvents. 

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