Reactions secondary

In oxidative degradation, it is convenient to make a distinction between primary chemicals formed by reactions of the polymer peroxy radicals, and secondary products created in subsequent reactions of these primary compounds. Ketones and al- 

It should be emphasized that the reaction scheme of Eq. (49) is unlikely if hydroperoxides are distributed homogeneously over the whole sample. Owing to restricted mobility below Tm, the oxidation products carmot diffuse away. Locally, hydroperoxides can reach sufficiently high concentrations for secondary reactions to occur at a significant rate. In a similar fashion, the formation of carboxylic acids, esters and y-lactones proceeds through a complex series of oxidation reactions of alcohols and ketones. 

The aldehydes and ketones formed by yS-scission can undergo Norrish type I [Eq. (50a)] and Norrish type II reactions [Eq. (50b)] during photodegradation (see Section 15.5.3). 

For a long time, the similarity between thermal and photolytic products was a mystery because Norrish photoprocesses are absent in the former situation. By using low MW model compounds, chemical derivatization, and C-NMR techniques, it has been assessed that a large fraction of the carbonyl ontaining compounds formed during oxidative degradation of PP are a-methylated acids. Such a carboxylic structure can only originate from the oxidation of macroalkyl radicals [Eq. (51)1, which can be formed either by a Norrish I photoprocess or by j -sdssion of alkoxy radicals (Ref. 10, p. 583). 

Once the structure of the carboxylic acids had been elucidated, the IR absorption band which appears as a shoulder at 1740 cm in thermal or photo-oxidation of PP, and was initially attributed to ester functions, was reassigned to an acidic group hydrogen-bonded to a vicinal hydroperoxide (VI). 

Leighton et al. suggested the same expression for the rate of formation of CO, viz. 

Steps (16) and (17) are important only at low temperatures. A steady-state treatment applied to steps (9)-(15) leads to the expression 

Kerr and Trotman-Dickenson extended the investigations up to about 400 °C. They suggested that hydrogen atom may be abstracted also from the alkyl group 

C3H7+C3H7CHO - C3H8 + C3H6CHO CjHgCHO CjHg + CHO 

The photolysis of 2-methylbutanal was investigated by Gruver and Calvert at 3130 A in the absence and presence of iodine. Since the primary processes occurring, and the evidence supporting them, are the same as in the case of n-butyraldehyde photolysis, there will be no detailed discussion. 

Lipids may contribute to food flavor formation through participation in other chemical pathways, most notably, the Maillard reaction. Whitfield [42] has provided a very comprehensive review of how lipids and their degradation products may participate in the Maillard reaction. He lists the primary means of interaction as  

The participation of free radicals from oxidizing lipids in the Maillard reaction. 

The reaction of hydroxy or carbonyl lipid degradation products with free hydrogen sulfide from the Maillard reaction. This is essentially analogous to the participation of lipid derived oxidation products with NH2 from the Maillard reaction. One finds many S-containing heterocyclics in heated foods that must have been derived from lipid sources (long R groups). 

This interdependency of reactions has been most studied in meats, or model meat reaction systems [42,72,81]. Wasserman [82] was amongst the first to find that the lean portion of the meat supplied the meaty, brothy character and the fat provided the species character much of which is due to lipid/Maillard interactions. This knowledge has long been used in the manufacture of process products (meat flavors). Meat process flavors contain approximately the same sugars and amino acids for the basic meat flavor but contain different fats to give the unique pork, beef, or chicken notes. 

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Single reactions. Most reaction systems involve multiple reactions. In practice, the secondary reactions can sometimes be neglected, leaving a single primary reaction to consider. Single reactions are of the type... 

Multiple reactions in parallel producing byproducts. Rather than a single reaction, a system may involve secondary reactions producing (additional) byproducts in parallel with the primary reaction. Multiple reactions in parallel are of the tj ie... 

Some of the benzene formed undergoes a secondary reaction in series to an unwanted byproduct, diphenyl, according to the reaction... 

Cfeed = molar concentration of FEED in the reactor di, 0-2 = constants (order of reaction) for primary and secondary reactions... 

Selectivity for series reactions of the types given in Eqs. (2.7) to (2.9) is increased by low concentrations of reactants involved in the secondary reactions. In the preceding example, this means reactor operation with a low concentration of PRODUCT—in other words, with low conversion. For series reactions, a significant reduction in selectivity is likely as the conversion increases. 

If the secondary reaction is reversible and involves a decrease in the number of moles, such as... 

For all reversible secondary reactions, deliberately feeding BYPRODUCT to the reactor inhibits its formation at the source by shifting the equihbrium of the secondary reaction. This is achieved in practice by separating and recycling BYPRODUCT rather than separating and disposing of it directly. 

An alternative way to improve selectivity for the reaction system in Eq. (2.27) is again to deliberately feed BYPRODUCT to the reactor to shift the equilibrium of the secondary reaction away from BYPRODUCT formation. 

Secondary reactions can occur to higher chlorinated compounds ... 

The secondary reactions are series with respect to the chloromethane but parallel with respect to chlorine. A very large excess of methane (mole ratio of methane to chlorine on the order of 10 1) is used to suppress selectivity losses. The excess of methane has two effects. First, because it is only involved in the primary reaction, it encourages the primary reaction. Second, by diluting the product, chloromethane, it discourages the secondary reactions, which prefer a high concentration of chloromethane. 

Two principal secondary reactions occur, to diethanolamine and triethanolamine ... 

The secondary reactions are parallel with respect to ethylene oxide but series with respect to monoethanolamine. Monoethanolamine is more valuable than both the di- and triethanolamine. As a first step in the flowsheet synthesis, make an initial choice of reactor which will maximize the production of monoethanolamine relative to di- and triethanolamine. 

Further consideration of the reaction system reveals that the ammonia feed takes part only in the primary reaction and in neither of the secondary reactions. Consider the rate equation for the primary reaction ... 

An excess of ammonia in the reactor decreases the concentrations of monoetha-nolamine, diethanolamine, and ethylene oxide and decreases the rates of reaction for both secondary reactions. 

Thus an excess of ammonia in the reactor has a marginal eflFect on the primary reaction but significantly decreases the rate of the secondary reactions. Using excess ammonia also can be thought of as operating the reactor with a low conversion with respect to ammonia. 

An initial guess for the reactor conversion is very difficult to make. A high conversion increases the concentration of monoethanolamine and increases the rates of the secondary reactions. As we shall see later, a low conversion has the effect of decreasing the reactor capital cost but increasing the capital cost of many other items of equipment in the flowsheet. Thus an initial value of 50 percent conversion is probably as good as a guess as can be made at this stage. 

Recycling byproducts for improved selectivity. In systems of multiple reactions, byproducts are sometimes formed in secondary reactions which are reversible, such as... 

The three recycle structures shown in Fig. 4.2 also can be used with this case. Because the BYPRODUCT is now being formed by a secondary reaction which is reversible, its formation can be inhibited by recycling BYPRODUCT as shown in Fig. 4.3a. In Fig. 4.3a, the BYPRODUCT formation is inhibited to the extent that it is effectively stopped. In Fig. 4.36 it is only reduced and the net BYPRODUCT formation removed. Again, the separation configuration will change between different processes as the order of volatility between the components changes. 

Figure 4.3 If a b5rproduct is formed via a reversible secondary reaction, then recycling the byproduct can inhibit its formation at the source.

Where possible, introducing extraneous materials into the process should be avoided, and a material already present in the process should be used. Figure 4.6h illustrates use of the product as the heat carrier. This simplifies the recycle structure of the flowsheet and removes the need for one of the separators (see Fig. 4.66). Use of the product as a heat carrier is obviously restricted to situations where the product does not undergo secondary reactions to unwanted byproducts. Note that the unconverted feed which is recycled also acts as a heat carrier itself. Thus, rather than relying on recycled product to limit the temperature rise (or fall), simply opt for a low conversion, a high recycle of feed, and a resulting small temperature change. 

Secondary reactions can produce waste byproducts for example. 

Reducing waste from multiple reactions producing waste byproducts. In addition to the losses described above for single reactions, multiple reaction systems lead to further waste through the formation of waste byproducts in secondary reactions. Let us briefly review from Chap. 2 what can be done to minimize byproduct formation. 

Assuming that the cis isomer is the reactant, the bans isomer product is expected to be accompanied by others arising from secondary reactions of the biradical, as observed experimentally [58]. 

Acetone in conjunction with benzene as a solvent is widely employed. With cyclohexanone as the hydrogen acceptor, coupled with toluene or xylene as solvent, the use of higher reaction temperatures is possible and consequently the reaction time is considerably reduced furthermore, the excess of cyclohexanone can be easily separated from the reaction product by steam distillation. At least 0 25 mol of alkoxide per mol of alcohol is used however, since an excess of alkoxide has no detrimental effect 1 to 3 mols of aluminium alkoxide is recommended, particularly as water, either present in the reagents or formed during secondary reactions, will remove an equivalent quantity of the reagent. In the oxidation of steroids 50-200 mols of acetone or 10-20 mols of cyclohexanone are generally employed. 

In the synthesis of molecules without functional groups the application of the usual polar synthetic reactions may be cumbersome, since the final elimination of hetero atoms can be difficult. Two solutions for this problem have been given in the previous sections, namely alkylation with nucleophilic carbanions and alkenylation with ylides. Another direct approach is to combine radical synthons in a non-polar reaction. Carbon radicals are. however, inherently short-lived and tend to undergo complex secondary reactions. Escheirmoser s principle (p. 34f) again provides a way out. If one connects both carbon atoms via a metal atom which (i) forms and stabilizes the carbon radicals and (ii) can be easily eliminated, the intermolecular reaction is made intramolecular, and good yields may be obtained. 

Synthetically useful reaction h + - Rate of secondary reactions can be kept comparatively small... 

These results show that in the phenylation of thiazole with benzoyl peroxide two secondary reactions enter in competition the attack of thiazole by benzoyloxy radicals, leading to a mixture of thiazolyl benzoates, and the formation of dithiazolyle through attack of thiazole by the thiazolyl radicals resulting from hydrogen abstraction on the substrate and from the dimerization of these radicals. This last reaction is less important than in the case of thiophene but more important than in the case of pyridine (398). 

The acid function of an aliphatic chain bonded to a thiazole ring can be esterified. The corresponding acid chloride can also be prepared by the action of thionyl chloride, though the reaction is often accompanied by secondary reactions and gives poor yields (49, 74). 

In a simple liquid-liquid extraction the solute is partitioned between two immiscible phases. In most cases one of the phases is aqueous, and the other phase is an organic solvent such as diethyl ether or chloroform. Because the phases are immiscible, they form two layers, with the denser phase on the bottom. The solute is initially present in one phase, but after extraction it is present in both phases. The efficiency of a liquid-liquid extraction is determined by the equilibrium constant for the solute s partitioning between the two phases. Extraction efficiency is also influenced by any secondary reactions involving the solute. Examples of secondary reactions include acid-base and complexation equilibria. 

Scheme for a simple liquid-liquid extraction without any secondary reactions. 

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