Side bimolecular

Partial but not complete loss of optical activity m S l reactions probably results from the carbocation not being completely free when it is attacked by the nucleophile Ionization of the alkyl halide gives a carbocation-hahde ion pair as depicted m Figure 8 8 The halide ion shields one side of the carbocation and the nucleophile captures the carbocation faster from the opposite side More product of inverted configuration is formed than product of retained configuration In spite of the observation that the products of S l reactions are only partially racemic the fact that these reactions are not stereospecific is more consistent with a carbocation intermediate than a concerted bimolecular mechanism... 

Substitution nucleophilic bimolecular (Sn2) mechanism (Sec tions 4 12 and 8 3) Concerted mechanism for nucleophilic substitution in which the nucleophile attacks carbon from the side opposite the bond to the leaving group and assists the departure of the leaving group... 

Mass transport selectivity is Ulustrated by a process for disproportionation of toluene catalyzed by HZSM-5 (86). The desired product is -xylene the other isomers are less valuable. The ortho and meta isomers are bulkier than the para isomer and diffuse less readily in the zeoHte pores. This transport restriction favors their conversion to the desired product in the catalyst pores the desired para isomer is formed in excess of the equUibrium concentration. Xylene isomerization is another reaction catalyzed by HZSM-5, and the catalyst is preferred because of restricted transition state selectivity (86). An undesired side reaction, the xylene disproportionation to give toluene and trimethylbenzenes, is suppressed because it is bimolecular and the bulky transition state caimot readily form. 

If the water content of the diazotization system is too high, the halogen atom in halogen-substituted mono- and dinitroanilines may be replaced by a hydroxy group in a bimolecular aromatic substitution. Analogous behaviour was observed in the diazotization of pentafluoroaniline, where the 4-fluoro substituent became hydrolysed (Brooke et al., 1965). As already mentioned in Section 2.1, this side reaction does not take place if the diazotization is conducted in a dichloromethane-aqueous sulfuric acid two-phase system in the presence of tetrakis[3,5-bis(trifluoromethyl)-phenyl]borate (Iwamoto et al., 1983a, 1984). 

One other point should be noted. The dimensions of the right-hand side of Eq. (7-57) are time-. That is, only first-order rate constants appear to be permitted, when in fact, the derivation assumed a bimolecular mechanism. The problem is entirely artificial, arising from the different ways in which concentration units are ordinarily dealt. 

Transition state theory. If the TST equation is applied to a bimolecular reaction, there appears to be a discrepancy in the units the left-hand side has dimensions of concentration-1 time-1, whereas the right-hand side has time-1. Comment. 

Chemically, the membrane is known to consist of phospholipids and proteins, many of which have enzymic properties. The phospholipid molecules are arranged in a bimolecular layer with the polar groups directed outwards on both sides. The structures of some phospholipids found in bacteria are shown in Fig. 1.6. Earlier views held that the protein part ofthe membrane was spread as a continuous sheet on either side ofthe... 

The anthraquinone group of the UAQ sensitizer is intercalated on the 3 -side of its linkage site [15]. Use of UAQ permits assessment of the directionality of long-range radical cation migration. Both AQ and UAQ enable the selective and efficient introduction of a radical cation in duplex DNA, whose lifetime is controlled by its relatively slow bimolecular reaction primarily with H20. 

Normally, only a small stoichiometric excess (2-30 mol%) of silane is necessary to obtain good preparative yields of hydrocarbon products. However, because the capture of carbocation intermediates by silanes is a bimolecular occurrence, in cases where the intermediate may rearrange or undergo other unwanted side reactions such as cationic polymerization, it is sometimes necessary to use a large excess of silane in order to force the reduction to be competitive with alternative reaction pathways. An extreme case that illustrates this is the need for eight equivalents of triethylsilane in the reduction of benzyl alcohol to produce only a 40% yield of toluene the mass of the remainder of the starting alcohol is found to be consumed in the formation of oligomers by bimolecular Friedel-Crafts-type side reactions that compete with the capture of the carbocations by the silane.129... 

Because the polymerization with the thermal iniferters previously described was performed at a high temperature, some side reactions might be unavoidable, e.g., ordinary bimolecular termination between polymer radicals, disproportionation between a polymer radical and a small radical leading to deactivation of the iniferter site, initiation by the radical generated from the iniferter sites, rearrangements of the structure of the iniferter sites, and spontaneous initiation of polymerization. 

Electronic effects. Nucleophilic attack is favoured by electron-withdrawing groups on the amide and the acyloxyl side chains. Interpolated bimolecular rate constants at 308 K for the series of para-substituted /V-acetoxy-/V-butoxybenzamides 25c, 26b-g and 26i (Table 5) gave a weak but positive Hammett correlation with a constants ip = 0.13, r = 0.86).42,43 These Sn2 reactions are analogous to those of aniline and substituted pyridines with phenacyl bromides, which have similar Arrhenius activation energies and entropies of activation in methanol (EA= 14-16 kcal mol-1, AS = — 27 to —31 calK-1 mol-1) and 4-substituted phenacyl halides afforded a similar Hammett correlation with pyridine in methanol (cr, p — 0.25).175... 

Figure 21. Photoinduced electron transfer in the synthetic Ru-phenol-Mn triads. (a) from (185) and (b) from (186,187). In both cases, the Ru(II) is oxidized by photoinduced electron transfer to an extraneous electron acceptor e.g., [Co(NH3)5C1]2+ and the electron is recaptured from the tyrosyl moiety that oxidizes a Mn ion in a bimolecular reaction (left) or intramolecularly as shown on the right-hand side.

Processes such as UOP s Tatoray [2] process are used to increase xylene yield in an aromatics facility by converting two moles of toluene into one mole of xylenes and one mole of benzene. As the reaction is thought to be a bimolecular process, zeolites with side pockets or channel intersections are the most desired (Table 12.12). 

An alkylating agent (ICH2COO ) that acts as a potent irreversible inhibitor of enzymes containing reactive thiol, e-amino, and/or imidazole side-chain groups within their active sites. The carboxymethylation reaction with enzymes is typically a bimolecular process that takes place without rate-saturation behavior ... 

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