Two Reaction Directions

From the colorless state it can be switched with light of short wavelength (A = 380 nm) via an electrocycHc ring opening and cis/trans rotation of one half of the molecule into a state with violet/purple color. The reverse reaction is effected by visible light (A = 580 nm). Since the system is metastable, one of the two reaction directions is matched by a rival thermal reaction, the thermoreversion. This progresses, however, in the case of benzospiropyran, at room temperature by a factor of 10 slower than the light-induced reaction. 

Aromatization. The two reactions directly responsible for enriching naphtha with aromatics are the dehydrogenation of naphthenes and the dehydrocyclization of paraffins. The first reaction can he represented hy the dehydrogenation of cyclohexane to benzene. 

The formation of the stable betaine system P+—C—O—B is the driving force for this reaction. With 4,6-disubstituted ammonium 1,3,2,5-dioxaborataphosphorinanes (108), there is the possibility of making a choice between two reaction directions. The reaction product formed by the phosphorane transition state, as 1,3,2,5-dioxaborataphosphoniarinane... 

We cannot compare the rate constants for these two reactions directly because they are expressed in different units. The intramolecular reaction [equation (1)] is kinetically first order, whereas the intermolecular reaction [equation (2)] is second order. But suppose the two molecules of acetic acid that enter into reaction (2) are labeled isotopically to make them distinguishable and that one type of molecule is present in great excess over the other. The process is then kinetically first order in the concentration of the limiting reactant. To make the rate constant the same as for reaction (1), the more abundant species has to be present at a concentration of 3 x 105 m This is far above any concentration that can actually be obtained. 

In practical applications, it is most unlikely that pure CO would be used as fuel. It is more likely that the fuel gas would contain both H2O and CO, and in such cases the electrochemical oxidation of the CO would probably proceed via the water-gas shift reaction (equation 7.3), a fast reaction that occurs on the nickel anode electrocatalyst. The shift reaction converts CO and steam to hydrogen, which then oxidises rapidly on the anode. The two reactions (direct oxidation of CO or shift reaction and then the oxidation of H2) are entirely equivalent. 

Commercially, urea is produced by the direct dehydration of ammonium carbamate, NH2COONH4, at elevated temperature and pressure. Ammonium carbamate is obtained by direct reaction of ammonia and carbon dioxide. The two reactions are usually carried out simultaneously in a high pressure reactor. Recendy, urea has been used commercially as a catde-feed supplement (see Feeds and feed additives). Other important appHcations are the manufacture of resins (see Amino resins and plastics), glues, solvents, and some medicinals. Urea is classified as a nontoxic compound. 

Figure 4.9 Mechanisms of the reactions catalyzed by the enzymes mandelate racemase (a) and muconate lactonizing enzyme (b). The two overall reactions are quite different a change of configuration of a carbon atom for mandelate racemase versus ring closure for the lactonizing enzyme. However, one crucial step (red) in the two reactions is the same addition of a proton (blue) to an intermediate of the substrate (red) from a lysine residue of the enzyme (E) or. In the reverse direction, formation of an intermediate by proton abstraction from the carbon atom adjacent to the carboxylate group.

Addition and elimination processes are the reverse of one another in a formal sense. There is also a close mechanistic relationship between the two reactions, and in many systems reaction can occur in either direction. For example, hydration of alkenes and dehydration of alcohols are both familiar reactions that are related as an addition-elimination pair. 

This theory has stimulated activity in two main directions, suggestions for changes in detail in the steps of processes for particular alkaloids, and work on laboratory syntheses of known alkaloids, using the reactions specified and operated under conditions which might obtain in a plant, i.e., under what are now described as physiological conditions. All the results indicate that the theory is well-founded, and it seems possible that a technique may eventually be found by which the process may be observed in operation, direetly or indirectly, in situ, say in a solanaceous plant. 

When potassium fluoride is combined with a variety of quaternary ammonium salts its reaction rate is accelerated and the overall yields of a vanety of halogen displacements are improved [57, p 112ff. Variables like catalyst type and moisture content of the alkali metal fluoride need to be optimized. In addition, the maximum yield is a function of two parallel reactions direct fluorination and catalyst decomposition due to its low thermal stability in the presence of fluoride ion [5,8, 59, 60] One example is trimethylsilyl fluoride, which can be prepared from the chloride by using either 18-crown-6 (Procedure 3, p 192) or Aliquot 336 in wet chlorobenzene, as illustrated in equation 35 [61],... 

Except for the biochemical exanple just cited, the stnactures of all of the alcohols in Section 5.9 (including those in Problem 5.13) were such that each one could give only a single alkene by p elimination. What about elimination in alcohols such as 2-methyl-2-butanol, in which dehydration can occur in two different directions to give alkenes that are constitutional isomers Here, a double bond can be generated between C-1 and C-2 or between C-2 and C-3. Both processes occur but not nearly to the sane extent. Under the usual reaction conditions 2-methyl-2-butene is the major product, and 2-methyl-1-butene the minor one. 

The ring closure of a diene to a cyclobutene can occur with rotation of the two termini in the same conrotatory) or opposite disrotatory) directions. For suitable substituted compounds, these two reaction modes lead to products with different stereochemistry. 

Taylor has reported a number examples of intramolecular variations directed towards heterocyclic systems.The following two reactions are representative. Intramolecular addition of triazine 79, after loss of nitrogen afforded 80. Alternatively, triazine 81 generated bicyclic systems 82 which could be oxidized to 83. 

The second reaction is the enthalpy of combustion of diamond. It does not directly give Af// of C02, since diamond is not the stable form of carbon. We can, however, get Af// for diamond by subtracting the two reactions 1... 

The last two reactions are useful for esr studies involving free radicals. Until recently, the only trialkyltin radical that had been observed directly, in solution, by esr was MesSn- (295), but many more have now been reported (e.g., EtsSn-, PrsSn-, and BusSn ) (296). Bulky ligands [e g., (PhCMejCHjlaSn ] increase the persistence of the radicals, so that esr observation is easier (297), and tris(2,3,5-trimethylphenyl)tin and tris(2,3,5-triethylphenyl)tin radicals, at 180° and 100°, respectively, are in thermal equilibrium with the corresponding hexaaryldi-tins (298). 

Solution Two independent reactions are needed that involve all four components. A systematic way of doing this begins with the formation reactions but, for the present, fairly simple case. Figure 7.5 includes two reactions that can be used directly ... 

Cytochrome PTSOiy, carries out comparable reactions for removal of the side chain of pregnenolone, and two reactions have been described both of which involved loss of acetate—17a-hydroxylation and formation of the 17-keto compound, and direct formation of the A -ene (Figure 3.19c) (Akhtar et al. 1994). 

It is sometimes said that this electrode is reversible with respect to the anion. This claim must be examined in more detail. An electrode potential that depends on anion activity still constitutes no evidence that the anions are direct reactants. Two reaction mechanisms are possible at this electrode, a direct transfer of chloride ions across the interface in accordance with Eq. (3.34) or the combination of the electrode reaction... 

A simplified scheme of the dual pathway electrochemical methanol oxidation on Pt resulting from recent advances in the understanding of the reaction mechanism [Cao et al., 2005 Housmans et al, 2006] is shown in Fig. 15.10. The term dual pathway encompasses two reaction routes one ( indirect ) occurring via the intermediate formation of COads. and the other ( direct ) proceeding through partial oxidation products such as formaldehyde. 

The stereoselectivity then depends on the conformation of the enone and the location of substituents that establish a steric bias for one of the two potential directions of approach. In the ketone 11, the preferred approach is from the (3-face, since this permits maintaining a chair conformation as the reaction proceeds.132... 

These two oppositely directed reactions are represented by a single equation as ... 

To make the two systems directly analogous, Van Niel<19> suggested that the reaction for photosynthesis in green plants should be written as... 

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