Unselective reactions

Also, by the very nature of chemical transformations, there are almost always unused chemicals remaining. These chemical leftovers include contaminants in the raw materials, incompletely converted raw materials, unavoidable coproducts, unselective reaction by-products, spent catalysts, and solvents. There have long been efforts to minimize the production of such waste products, and to recover and reuse those that cannot be eliminated. For those that cannot be reused, some different use has been sought, and as a last resort, efforts have been made to safely dispose of whatever remains. The same efforts apply to any leftovers from the production of the energy from the fuels produced or consumed by the processing industries. Of particular immediate and increasing concern are the potential detrimental effects of carbon dioxide emissions to the atmosphere from fossil fuel combustion, as discussed further in Chapters 9 and 10. 

The above definition of S is employed independently of the actual kinetic order of the product-determining step. Solvolytic reactions may be of higher kinetic order, or it could be argued that their kinetic order is ill-defined. There is no need to derive Equation 2.9 - it can stand alone as the definition of S. To illustrate how the equation works, suppose that the solvent is an equimolar mixture of alcohol and water, so the solvent mole ratio is 1.0. If the product mole ratio is also 1.0, then S = 1.0, and the product-forming reactions are unselective (so log S = 0 for an unselective reaction). If twice as much ether is formed as alcohol in an equimolar solvent mixture, then S = 2. In practice, many solvent compositions are employed, and the equation corrects automatically for variations in solvent composition. 

Early examples of reactivity-selectivity relationships in aromatic substitutions are limited since, in the absence of absolute rate data, it is often difficult to assign relative reactivity to the different electrophiles. For certain cases where the relative reactivity order may be assumed, a reactivity-selectivity relationship was noted. For example, bromina-tion with the reactive species Br+ results in lower selectivity than with the less reactive species Br2 (de la Mare and Harvey, 1956 Brown, 1957). However, it appears that no general reactivity-selectivity relationship exists in electrophilic aromatic substitution reactions, for there exist slow, unselective reactions such as aromatic... 

In contrast to NiX, NiY showed very poor selectivity for dimerization the major products resulted from hydrodimerization and cracking reactions. It was concluded that these unselective reactions occurred over strong acid sites which were present on all the studied catalysts with the exception of NiX. 

Despite much research the direct oxidation of propylene on silver catalysts remains an unselective reaction yielding primarily carbon oxides and a range of... 

In Chapter 52 you met Bakellte, the first synthetic polymer, which results from unselective reactions between these two compounds. 

The presence of a large fraction of adsorbed species on the catalyst surface thtis not only decreases the number of available active sites for alkane selective oxidation, but also decreases their s >ecific turnover number, thus increasing the surface lifeiime of adsorbed intermediates on the PVO deactivated surface. This increases the probability of parallel unselective reactions with a decrease in the selectivity to anhydrides. 

The achiral ethyl ketone 218 (Scheme 9-59) has been used in the synthesis of epothilone B [79]. Here the asymmetric induction comes from aldehyde 219 alone and, at very low temperature, is surprisingly high (85%ds). It appears the unsaturated sidechain of the aldehyde plays an important role, as aldol addition to saturated aldehyde 220 led to an unselective reaction. 

The selective dehydrogenation of alcohols involves hydrogen abstraction and hence atomic oxygen becomes the selective species on copper. On gold, oxygen is not required for the abstraction reaction, while the mechanism on silver is probably similar to that on copper. The weakly adsorbed, possibly molecular, oxygen species appears to be responsible for the unselective reaction which proceeds by oxygen addition. 

Conversely, for very exothermic reactions, changes in enthalpy of reaction will have little effect on the activation energy, giving unselective reactions. The corresponding fluorination of propane (reaction 3.4) is highly exothermic AH = —159 kJ mol-1 at the 1-position and —173 kJ mol 1 at the 2-position. However, with an early transition state and a very small activation energy for both reactions, there is little difference in rate for attack at the two positions and the reaction is very unselective. 

Such a mechanism has been hypothesized for the reaction of aryl halides substituted by strong electron-donating groups.838,845 4-Chloroaniline (239), for example, reacts in the triplet state to give a phenyl cation 240, apparently of triplet character (Jt501), which has a selective reactivity toward Jt-, but not n-, nucleophiles (Scheme 6.94), in contrast to the unselective reactions of common singlet aryl cations.847 Interestingly, the cation is added to an alkene to yield 241 even in nucleophilic methanol. 

Acyclic and cyclic allylic alcohols arc attacked by phase-transfer-generated dichlorocarbene with surprisingly high diastereofacial selectivity24,25. The formation of the major diastereo-mers is consistent with carbene delivery from the side on which the most stable conformer bears the hydroxy group. The unselective reaction of 3-methyl-3-buten-2-ol might be due to an unfavorable allylic 1,3-strain 26 in this conformer. 

Relative reactivity data for nitration must be treated with special caution because of the possibility of encounter control. An example of this can be seen in Part A of Table 9.7, where no difference in reactivity between mesitylene and xylene is found in H2SO4-HNO3 nitration, whereas in HNO3-CH3NO2 the rates differ by a factor of more than 2. Encounter-control prevails in the former case. In general, nitration is a relatively unselective reaction with toluene being about 50-60, as shown in... 

The Wurtz reaction constitutes the direct coupling of the nucleophilic carbon of an alkylsodium compound and the electrophilic carbon in a haloalkane. It is an unselective reaction. There is no way to control it to prevent coupling of two like alkyl groups while attempting to couple two different ones. In other words, the reaction between chloroethane and 1-chloropropane gives a statistical mixture of butane (from two ethyls), pentane (from an ethyl and a propyl), and hexane (from two propyls). 

Figure 20 A metal containing MIP catalyst with a selective (left) and an unselective reaction site (right). Upon addition of a catalyst poison , the less selective but more reactive site is occupied.

Secondly, the product of unselective NH oxidation reaction may itself be NOx (indeed, the oxidation of NH to NO is the first step in the industrial preparation of HNOj). Therefore, an unselective reaction renders the entire selective catalytic reduction (SCR)-NHj deNOx process an expensive method of increasing the NOx concentration of the emitted gas, NH.+ O. N.ORNO. 

Because of the highly unselective reactions involved in the photocatalytic processes, these have been widely studied in environment remediation, in which organic and inorganic pollutants in hquid and gas phases are totally degraded to innocuous substances. 

Most interestingly, allylsilanes, stannanes, and silyl enolethers function as nucleophiles in the reactions described here and are subject to the same stereochemical preferences as alcohols [148, 149]. Thus, unselective reactions are observed with a per-O-benzyl mannopyranosyl donor, p-selective couplings are seen with a 4,6-0-benzylidene protected mannopyranosyl donor, and ot-selective reactions with the corresponding glucopyranosyl donor (Scheme 46). 

A special but important case where the SCR reacting system includes NO2 in signiflcant amounts is represented by the new generation of urea-SCR converters for diesel vehicles integrated with an upstream preoxidation catalyst, which partially oxidizes NO to NO2 (10,11). In such devices, the so-called fast SCR reaction 3, which may be over ten times faster than the standard SCR reaction 1 at low temperatures, plays a critical role in boosting the DeNOx activity at 180 -300°C (12). However, large NO2 feed contents result also in the occurrence of two additional unselective reactions, not observed in the presence of NO-NH3 only, namely the formation of ammonium nitrate, which is critical below 180°C,... 

We will consider both selective and unselective reactions in the next few sections of this chapter. The only criterion will be the nature of the substrate which is catalytically oxidized. The substrates considered will have systems of p or tt electrons by which they are known to react readily with many of the metal complexes used as catalysts. Those instances which are clear examples of coordination catalysis will serve to illustrate principles of catalytic oxygen activation. Those instances in which coordination catalysis plays a less important role may serve to illustrate areas in which future work could provide improvements by means of selective activation and transfer of dioxygen. 

Quinones have been used as oxidants but the early studies indicated rather unselective reactions in allylic oxidation. This is in clear contrast to our results, which use... 

The reactivity of nitrates has to do not only with the selective (to N2) and desired Fast and NO2 SCR reactions, but also with the unselective reactions responsible for the formation of undesired by-products. 

In contrast to heterogeneous catalysis, classical homogeneous catalysis takes place in the bulk of a solvent. However, due to the lack of a phase boundary it is much more complicated in this case to isolate the fully dissolved transition metal complex from the desired reaction product. Often, distillation fails due to the thermal instability of the dissolved catalyst or the fact that unselective reactions occur in the bottom of the distillation column. Catalyst recovery and recycling strategies for homogeneous catalytic processes can therefore sometimes be rather complicated... 

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