Stoichiometry side reactions

Perhaps the most discouraging type of deviation from linearity is random scatter of the data points. Such results indicate that something is seriously wrong with the experiment. The method of analysis may be at fault or the reaction may not be following the expected stoichiometry. Side reactions may be interfering with the analytical procedures used to follow the progress of the reaction, or they may render the mathematical analysis employed invalid. When such plots are obtained, it is wise to reevaluate the entire experimental procedure and the method used to evaluate the data before carrying out additional experiments in the laboratory. 

MPD-1 fibers may be obtained by the polymeriza tion of isophthaloyl chloride and y -phenylenediamine in dimethyl acetamide with 5% lithium chloride. The reactants must be very carefully dried since the presence of water would upset the stoichiometry and lead to low molecular weight products. Temperatures in the range of 0 to —40° C are desirable to avoid such side reactions as transamidation by the amide solvent and acylation of y -phenylenediamine by the amide solvent. Both reactions would lead to an imbalance in the stoichiometry and result in forming low molecular weight polymer. Fibers are dry spun direcdy from solution. 

Polyetherification is similar to a polycondensation process formation of high molecular weight polymer requires precise adjustment of composition to approximately 1 1 ratio of bisphenol to dihalosulfone. Trace amounts of water gready reduce the molecular weight attainable owing to side reactions that unbalance the stoichiometry (76). The reactivity of the halosulfone is in the order expected for two-step nucleophilic aromatic displacement reactions ... 

If it can be accepted that the stoichiometry is known and that no side reactions take place, the concentration of only one of the products or of the reactants needs to be measured. 

Step-growth polymerization processes must be carefully designed in order to avoid reaction conditions that promote deleterious side reactions that may result in the loss of monomer functionality or the volatilization of monomers. For example, initial transesterification between DMT and EG is conducted in the presence of Lewis acid catalysts at temperatures (200°C) that do not result in the premature volatilization of EG (neat EG boiling point 197°C). In addition, polyurethane formation requires the absence of protic impurities such as water to avoid the premature formation of carbamic acids followed by decarboxylation and formation of the reactive amine.50 Thus, reaction conditions must be carefully chosen to avoid undesirable consumption of the functional groups, and 1 1 stoichiometry must be maintained throughout the polymerization process. 

Are Side Reactions Important What is the Stoichiometry of the Reaction When a mixture of various species is present in a reaction vessel, one often has to worry about the possibility that several reactions, and not just a single reaction, may occur. If one is trying to study one particular reaction, side reactions complicate chemical analysis of the reaction mixture and mathematical analysis of the raw data. The stoichiometry of the reaction involved and the relative importance of the side reactions must be determined by qualitative and quantitative anal-lysis of the products of the reaction at various times. If one is to observe the growth and decay of intermediate products in series reactions, measurements must be made on the reaction system before the reaction goes to completion. 

It is always wise to calibrate physical methods of analysis using mixtures of known composition under conditions that approximate as closely as practicable those prevailing in the reaction system. This procedure is recommended because side reactions can introduce large errors and because some unforeseen complication may invalidate the results obtained with the technique. For example, in spectrophotometric studies of reaction kinetics, the absorbance that one measures can be grossly distorted by the presence of small amounts of highly colored absorbing impurities or by-products. For this reason, when one uses indirect physical methods in kinetic studies, it is essential to verify the stoichiometry of the reaction to ensure that the products of the reaction and their relative mole numbers are known with certainty. For the same reason it is recommended that more than one physical method of analysis be used in detailed kinetic studies. 

The reaction must be one of known stoichiometry with no side reactions. 

More complicated mechanisms of the same category are encountered in SrnI reactions (Section 2.5.6) where the electrocatalytic reaction, which corresponds to a zero-electron stoichiometry, is opposed to two-electron consuming side reactions (termination step in the chain process). 

Strategy. We will need to decide first (i) the identity and stoichiometry of the second electrode reaction. Then, we will work out (ii) how much of the charge was required to form the hydrogen by this route. Therefore, knowing the overall charge and the amount consumed in the side reaction, (iii) we can work out the faradaic fraction utilized to form copper metal. 

Hydroperoxides undergo reduction with aqueous Fe(II), which turns to aqueous Fe(III). The reaction can be followed at 305 nm (e = 2095 M cm ) ° . Although the stoichiometry of this process is straightforward, with two Fe(II) ions being consumed per molecule of hydroperoxide, the mechanism involves an alkoxide free radical, RO", that may undergo -elimination, H abstraction from R—H, or a 1,2-H-shift and reaction with other components in the system. A case in point is the determination of f-butyl hydroperoxide which consumes under 1 mol of Fe(II) per mol of analyte under inert gas cover, while in the presence of O2 four mols are consumed, pointing to extensive side reactions of the RO" free radical, both without and with O2 in the system. ... 

The extension of DKR to polymer chemistry is not trivial in practice since side reactions that are relatively unimportant in DKR (dehydrogenation, hydrolysis) have a major impact on the rate of polymerization and attainable chain lengths because the stoichiometry of the reactants is an important issue. As a result, the reaction conditions and catalyst combinations used in a typical DKR process will not a priori lead to chiral polymers from racemic or achiral monomers with good molecular weight (>10kDa) and high ee (>95%). 

Each system considered in this section has a space of overall reactions whose dimension exceeds one. In many industrial reactions involving organic substances a major product is formed, but a side reaction contributes to loss in selectivity or yield of the desired product. Such cases may be said to exhibit a multiple overall reaction, unless the ratio of desired product to by-product remains constant over a range of operating conditions, so that a simple chemical equation might be employed to express the stoichiometry. 

There are several minimum requirements for a successful titration (a) the reaction taking place must proceed quantitatively according to a particular stoichiometric equation (b) the reaction must be sufficiently rapid (c) there must be a satisfactory way of locating the equivalence point and (d) it must be possible to prepare (and maintain) a standard solution of the titrant of precisely known concentration, although this restriction is overcome in other ways in the practice of cou-lometric titrations. Clearly condition (a) implies that the titrant must not enter into any side reactions and that no extraneous material is present that could alter the stoichiometry of the desired titration reaction. 

In the stoichiometry examples worked out in the preceding section, we made the unstated assumption that all reactions "go to completion." That is, we assumed that all reactant molecules are converted to products. In fact, few reactions behave so nicely. Most of the time, a large majority of molecules react in the specified way, but other processes, called side reactions, also occur. Thus, the amount of product actually formed—the reaction s yield—is usually less than the amount that calculations predict. 

Syntheses of Bisphenol-A Carbonate Oligomers in Homogeneous Pyridine. Oligomers made by the pyridine method were prepared in the same apparatus except that drying tubes were added where necessary. It was also necessary to use carefully dried solvents since the undesirable side reaction with water converts chlorocarbonic acid to hydrochloric acid and carbon dioxide, thus upsetting the reaction stoichiometry. 

The chemical structure of UP oligomers is more complex than might be expected from the chemistry of the reactions, Eqs. (2.13) and (2.14). The addition of hydroxyl groups to the activated double bonds is one of the most important side reactions - Eqs. (2.16) and (2.17) (Table 2.4) - called Ordelt reactions. It leads to the formation of side chains and to a modification of the COOH/OH stoichiometry due to diol consumption. 

In his kinetic plots, Hair only considered a monomolecular reaction (L), and a bimolecular reaction (O). He never mentioned the secondary reaction (M) or the side reaction (N). Although his general conclusions on the stoichiometry of the reaction may be correct, it is not excluded that other reactions than the two he mentioned are involved. 

One would expect that a reaction with TCS at 623 K also would cause exclusively primary species, originating from either the main reaction (L) or the side reaction (N). Inspection of the NMR spectrum in figure 9.41 (b) shows that all silanols have disappeared. There is no band in the -100 ppm region, and this is consistent with the earlier calculated effectiveness factor of 1. The main feature still is the -36 ppm band, attributed to the primary species. However, a significant band is situated at -60 ppm, indicative for secondary species. The conclusion, based of the stoichiometry factor /, that no secondary species exist on a silica surface pretreated at 973 K, is therefore not entirely correct. Obviously, the bifunctional reaction (O) is highly improbable, so secondary reactions (M) must occur. This is only possible when a certain mobility of the surface species exists on the surface, since -on average- the distance between the surface species is too large to react with each other. 

Based only on stoichiometry and assuming no side reactions, in which case will the current efficiency (defined as the charge employed for a given process divided by the total charge passed through the system) for formaldehyde destruction be higher (Ibanez)... 

In most industrially relevant reacting systems, one main reaction typically makes the desired products and several side reactions make byproducts. The specific rate of production or consumption of a particular component in such a reaction set depends upon the stoichiometry and the rates. For example, assume that the main reaction for making vinyl acetate, Eq. (4.4.1, proceeds with a rate r< (mol/L s) and that the side reaction, Eq. (4.8), proceeds with rate r2 (mol/L s). Then the net consumption of ethylene is (-l)r1 - (-1 )r2 (mol/L s). Similarly, the net consumption of oxygen is (-0.5)fi + (— 3)r2, and the net production of water is (l)r-, + (2)ra. For a given chemistry (stoichiometry), our ability to control the production or consumption of any one component in the reactor is thus limited to how well we can influence the various rates. This boils down to manipulating the reactor temperature and/ or the concentrations of the dominant components. Occasionally, the reaction volume for liquid-phase reactions or the pressure for gas-phase reactions can also be manipulated for overall production control. These are the fundamentals of reactor control. 

The reactions used in the preparation of intermediates are, for the most part, simple operations. Frequently, they proceed quantitatively according to the rules of stoichiometry. In other cases, side reactions are encountered which complicate the reaction and greatly reduce the yield. It is one of the important tasks of the dye chemist to study these undesirable side reactions suflSciently to understand their nature and then, if possible, to select the reaction conditions which will favor only the main reaction leading to the desired intermediate. This end is not always attained, because often the set of conditions which will eliminate the side reactions is not known, but the chemist must always bear in mind the possibihty of achieving diese conditions by further study. The preparation of l,8-aminonaphthol-3,6-disulfonic acid (H acid) illustrates this point. This compound has been known for nearly fifty years and is still being studied extensively in many laboratories, yet to this day has not been prepared in satisfactory yield. 

In the first commercial process, introduced in 1933, maleic anhydride was produced by the catalytic oxidation of benzene with air. Although its appeal declined after the 1970s the benzene process is still operated, particularly where -butane is not available. The catalyst is a mixed oxide (70% V2O5 30% M0O3) deposited on a low surface area carrier to limit side reactions. Atom efficiency is inherently low, as implied by the stoichiometry of the oxidation in which two carbon atoms out of six are lost as CO2 (Equation B4). Molar yields however can be relatively high ca. 73%) and are generally higher than those in the -butane processes. 

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