Processes with slow secondary reactions

In this case, the form of Assumptions 4.1 and 4.3 remains the same as in the second section, while Assumption 4.2 implies that the rate constant of the reaction which leads to the formation of the impurity is very small, or fojS = / i , 

Notice that here Assumption 4.2 is expressed as a ratio of the characteristic time for the chemical reaction and the characteristic time for convection, being thus equivalent to considering that the second reaction has a low Damkohler number. 

With the aforementioned assumptions, the dynamic model of the system takes 

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In addition to 2-MN acetylation, secondary reactions of l-AMN, isomerization into 2-AMN, and deacylation, which both increase the selectivity to the desired product 2-AMN were demonstrated to occur on BEA zeolites.1261 This is shown in Figure 3.4 on a HBEA-15 (framework Si/Al ratio of 15). After 45 min of reaction, AA is completely consumed, AMN and acetic acid being the only reaction products (yield in AMN with respect to AA close to 100%). Afterward there is a slow decrease in this yield indicating a deacylation process, and a faster increase in the 2- and 3-AMN... 

Photoreduction of benzophenone by primary and secondary amines leads to the formation of benzpinacol and imines [145]. Quantum yields greater than unity for reduction of benzophenone indicated that the a-aminoalkyl radical could further reduce the ground state of benzophenone. Bhattacharyya and Das confirmed this in a laser-flash photolysis study of the benzophenone-triethylamine system, which showed that ketyl radical anion formation occurs by a fast and a slow process wherein the slow process corresponds to the reaction of a-aminoalkyl radical in the ground state of benzophenone [148]. Direct evidence for similar secondary reduction of benzil [149] and naphthalimides [150] by the a-aminoalkyl radical have also been reported. The secondary dark reaction of a-aminoalkyl radicals in photo-induced electron-transfer reactions with a variety of quinones, dyes, and metal complexes has been studied by Whitten and coworkers [151]. 

When Pt-olefin complexes such as PtCl3(olefin) are treated with a second olefin, replacement of the coordinated olefin by the incoming olefin does not result in either double bond migration or (Z), ( ) isomerization of the displaced olefin. However, the olefin complexes when treated at low temperature with a nucleophile such as pyridine (py) or a secondary amine undergo conversion to a or-complex by a stereospecific trans-process, i.e., frans-addition and tra s-elimination. Treatment of frans-LPtCljCZ)-ethylene-l,2-reversible formation of the carbon (7-bonded complex with slow release of pure (Z)-ethylene-l,2-[Pg.383]

B.ii. The SnI Reaction. The mechanistic rationale described above results from many years of experimental observations about the SnI reaction (statements 1-6 below). 25 i i general, the SnI process occurs in those cases where water is present as a solvent or co-solvent, there is a good leaving group and the substrate is tertiary or secondary. A SnI reaction can occur in any polar protic solvent such as methanol, ethanol, or acetic acid, but ionization is much slower in these solvents relative to water because they are not as efficient for, the separation of ions (sec. 2.7.B). If ionization is relatively slow, faster reactions (Sn2 or elimination processes) often predominate. It is convenient to assume that SnI reactions occur only in aqueous media and use this assumption to predict product distributions for a given set of reaction conditions. As with any... 

Because of the inherently low reactivity of most components, a very large number of other functional groups are tolerated in the reaction. Functionalities that are not compatible within the Passerini reaction include those that are reactive toward activated or unhindered aldehydes or ketones under the mildly acidic conditions, lest such reactivity be competitive with the slow Passerini reaction. As a direct consequence, unprotected primary or secondary amines are not compatible because of the facility by which they form imines and iminiums by acid-catalyzed condensation. Iminiums, themselves are susceptible to nucleophilic attack by isonitriles and the formation of a-acylamino amides by this process is called the Ugi reaction (see chapter 3.6). In reactions where the Ugi and Passerini reactions are possible competitive processes the Ugi products are generally favored to the detriment of any Passerini products. ... 

Since VNS can proceed under kinetic control, namely, initially formed a -adducts can be converted into the products faster than they dissociate, the reaction can serve as a proper tool for determination of electrophilic activities of nitroarenes. Effects of substituents on rates of S Ar was subject of thorough studies [43] however, the results, although useful in practice of synthesis, cannot be considered as a reliable measure of electrophilic activities of nitroarenes because S Ar of halogens is a slow secondary process preceded by a reversible formation of the o -adducts. On the other hand, the rate of VNS reaction under kinetic control reflects the rates of the initial nucleophilic addition of carbanions to nitroaromatic rings, thus can be used as measure of electrophilic activities of these compounds. Particularly convenient and reliable way to determine such effects is the competitive experiments in which two nitroarenes compete for the VNS reaction with carbanion of chloromethyl phenyl sulfone under conditions that assure faster (1-elimination of HCl from the o -adducts than their dissociation [42]. Relative rate constants of the addition of this carbanion to some nitroarenes in relation to nitrobenzene are given in Scheme 11.24. 

The oxidation of alkanes by r-butyl hydroperoxide (TBHP) has been catalysed by titanium alkoxides, producing the corresponding alcohols and ketones. A radical mechanism is proposed in which r-butoxyl radical formed from TBHP and titanium alkoxide initiates the reaction. The evolution of oxygen (from the decomposition of peroxide) and the abstraction of hydrogen from alkane to form alkyl radical occur competitively. A method for the determination of both the primary and secondary KIEs at a reactive centre based on starting-material reactivities allows the determination of the separate KIEs in reactions for which neither product analysis nor absolute rate measurements are applicable. It has been applied to the FeCls-catalysed oxidation of ethylbenzene with TBHP, which exhibits both a primary KIE and a substantial secondary KIE the findings are in accordance with previous mechanistic studies of this reaction. The oxidation of two l-arylazo-2-hydroxynaphthalene-6-sulfonate dyes by peroxy-acids and TBHP catalysed by iron(III) 5,10,15,20-tetra(2,6-dichloro-2-sulfonatophenyl)porphyrin [Fe(ni)P] is a two-step process. In single turnover reactions, dye and Fe(in)P compete for the initially formed OFe(IV)P+ in a fast reaction and OFe(IV)P is produced the peroxy acid dye stoichiometry is 1 1. This is followed by a slow phase with 2 1 peroxy acid dye stoichiometry [equivalent to a... 

The fact that the singlet-triplet mixing in radical pairs becomes faster at high fields, due to the increase of the Zeeman interaction, can also permit modelling of the sequential electron-transfer process of both the primary and secondary pairs. The importance of protein dynamics on the electron-transfer rate was noted in a 95 GHz study of bacterial photosynthetic reaction centres with slow electron-transfer rates. ... 

The relative rates of reaction of 2-bromopropane and 2-bromo-2-methylpropane with water to give the corresponding alcohols are shown in Table 7-1 and are compared with the corresponding rates of hydrolysis of their unbranched counterparts. Although the process gives the products expected from an Sn2 reaction, the order of reactivity is reversed from that found under typical Sn2 conditions. Thus, primary halides are very slow in their reactions with water, secondary halides are more reactive, and tertiary halides are about 1 million times as fast as primary ones. 

Reaction of phthalic anhydride with the secondary hydroxyl groups is so slow that continued polymerization can be carried our as a second step. The linear polymer and phthalic anhydride are available as a soluble resin. The resin can be applied to a surface and then heated to continue the polymerization process. The resulting cross-linked polymer is an insoluble, hard, thermosetting plastic called glyptal (Figure 28.12). 

For deactivated compounds this limitation does not exist, and nitration in sulphuric acid is an excellent method for comparing the reactivities of such compounds. For these, however, there remains the practical difficulty of following slow reactions and the possibility that with such reactions secondary processes might become important. With deactivated compounds, comparisons of reactivities can be made using nitration in concentrated sulphuric acid such comparisons are not accurate because of the behaviour of rate profiles at high acidities ( 2.3.2 figs. 2.1, 2.3). 

A protonic acid derived from a suitable or desired anion would seem to be an ideal initiator, especially if the desired end product is a poly(tetramethylene oxide) glycol. There are, however, a number of drawbacks. The protonated THF, ie, the secondary oxonium ion, is less reactive than the propagating tertiary oxonium ion. This results in a slow initiation process. Also, in the case of several of the readily available acids, eg, CF SO H, FSO H, HCIO4, and H2SO4, there is an ion—ester equiUbrium with the counterion, which further reduces the concentration of the much more reactive ionic species. The reaction is illustrated for CF SO counterion as follows ... 

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