Reaction switchover

Scheme 1. Switchover of the reaction path by acidic molecules to basic catalysis.

Catalytic reaction regulated by coexisting molecules at the catalyst surface, where enhancement of the reaction rate for the formation of P or switchover of the reaction path from the P formation to the... 

Scheme 8.1 Switchover of the reaction paths by weakly adsorbed ethanol.

In a carbonyl-ene reaction of ethyl glyoxylate with a-methylstyrene catalysed by copper triflate-bisoxazoline complexes, ees of up to 100% have been achieved, but a dramatic switchover in stereochemistry is seen for an apparently minor change in bisoxazoline structure.185 A change in the metal geometry is implicated. 

The electrohydrodimerisation of acrylonitrile to give adiponitrile (a one-electron process at high substrate concentrations, Scheme 1.8A and Chapter 6) is an example of how an industrially important electrosynthetic process has been investigated following recent instrumental developments, viz. the application of ultramicroelectrodes at low-voltage sweep rates. Use of conventional electrodes would have required substrate concentrations in the mM range but, under these conditions, acrylonitrile undergoes a different reaction - a two-electron electrochemical reduction of the alkene residue (Scheme 1.8B). The switchover between the two reactions occurs at about 1 mol dm-3 substrate concentration. 

Fig. 25 The series of dyads, 29(n), possessing the oligo-p-phenylenevinylene bridge that were used to investigate the switchover from superexchange characteristics to molecular wire behaviour in the photoinduced electron transfer reaction, from the locally excited state of tetracene (TET) donor to the pyromellitimide (PI) acceptor.148 Also, shown are a schematic of the photoinduced charge separation rate versus, donor-acceptor distance (lower left-hand side) and the LUMO energies of TET and the various bridges (lower right-hand side).

Catalytic reactions on a metal oxide single crystal switchover of the reaction paths in formic acid decomposition on titanium dioxide TiO2(110). J. Am. Chem. Soc., 115, 10460-10461. 

Switchover of reaction paths in the catalytic decomposition of formic acid on TiO2(110) surface./. Catal., 146, 557-567. 

Uemura, Y., Taniike, T., Tada, M., Morikawa, Y. and Iwasawa, Y. (2007) Switchover of reaction mechanism for the catalytic decomposition of HCOOH on a TiO2(110) surface./. Phys. Chem. C, 111, 16379-16386. 

Switchover of reaction pathways on SiOi-attached Nb catalyst... 

An interesting example with a switchover of the stereochemical course was observed during the enantioselective diacetoxylation of alkenes using chiral iodine (III) reagents (Scheme 13) [56]. Enantioselective Prevost and Woodward reaction products were formed through the optically active l,3-dioxolan-2-yl cation intermediates 30. 

Figure 5.13 Plot of a collision trajectory for a reactive coiiision of the "direct" type (the time scale is that for the H + H2 reaction). The collision is direct because of the fast switchover between the old and new bonds. Note also the vibration of the reactant BC molecule prior to the encounter with the reactant atom and the low-amplitude oscillation of the newly formed AB bond, indicating only a minimal product vibrational excitation. To interpret the dynamics recall that the slope of a plot of distance vs. time is the velocity. From Newton s first law, a change in the slope in such a plot indicates that a force is acting.

For a given potential surface (and mass), classical mechanics can be used to illustrate the progress of the reaction through plotting the interatomic distances as a function of time. Typically, for a direct reaction the switchover between the... 

Figure 23-8 shows that at low temperatures, A Gq,) is much more negative than Ai.G( ). This makes Aj.G° for the overall reaction positive and the reaction nonspontaneous. At high temperatures, the situation is reversed A,.G(°a)is more negative than A.G(°b), and the overall reaction is spontaneous. The switchover from nonspontaneous to spontaneous occurs at the point of intersection of the blue line and the red line— about 950 °C. There, AjG for the overall reaction is zero. 

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