Reaction, identity pathway

Like the previous reaction, multiple pathways in the HO2 - - O3 reaction arise because of the participation of identical atoms. However, in this case the two... 

The reaction of cycloheptaamylose with diaryl carbonates and with diaryl methylphosphonates provides a system in which a carboxylic acid derivative can be directly compared with a structurally analogous organo-phosphorus compound (Brass and Bender, 1972). The alkaline hydrolysis of these materials proceeds in twro steps, each of which is associated with the appearance of one mole of phenol (Scheme Y). The relative rates of the two steps, however, are reversed. Whereas the alkaline hydrolysis of carbonate diesters proceeds with the release of two moles of phenol in a first-order process (kh > fca), the hydrolysis of methylphosphonate diesters proceeds with the release of only one mole of phenol to produce a relatively stable aryl methylphosphonate intermediate (fca > kb), In contrast, kinetically identical pathways are observed for the reaction of cycloheptaamylose with these different substrates—in both cases, two moles of phenol are released in a first-order process.3 Maximal catalytic rate constants for the appearance of phenol are presented in Table XI. Unlike the reaction of cycloheptaamylose with m- and with p-nitrophenyl methylphosphonate discussed earlier, the reaction of cycloheptaamylose with diaryl methylphosphonates... 

The principle of microscopic reversibility requires that any reversible reaction must have identical pathways for the forward and reverse reactions, simply proceeding in opposite directions. (This principle is similar to the idea that the lowest pathway over a mountain chain must be the same regardless of the direction of travel.) If the forward reaction is carbonyl migration (Mechanism 2), the reverse reaction must proceed by loss of a CO ligand, followed by migration of CO from the acyl ligand to the empty site. Because this migration is unlikely to occur to a trans position, all the product should be... 

When the addition and elimination reactions are mechanically reversible, they proceed by identical mechanistic paths but in opposite directions. In these circumstances, mechanistic conclusions about the addition reaction are applicable to the elimination reaction and vice versa. The principle of microscopic reversibility states that the mechanism (pathway) traversed in a reversible reaction is the same in the reverse as in the forward direction. Thus, if an addition-elimination system proceeds by a reversible mechanism, the intermediates and transition states involved in the addition process are the same as... 

In an alkaline medium the condensation of carbonyl and amino groups of the reactants seems to be more probable (pathway 1), although pathway 2, which is identical to the reactions of enaminoketones, is also possible. 

As a rule, the anabolic pathway by which a substance is made is not the reverse of the catabolic pathway by which the same substance is degraded. The two paths must differ in some respects for both to be energetically favorable. Thus, the y3-oxidation pathway for converting fatty acids into acetyl CoA and the biosynthesis of fatty acids from acetyl CoA are related but are not exact opposites. Differences include the identity of the acvl-group carrier, the stereochemistry of the / -hydroxyacyl reaction intermediate, and the identity of the redox coenzyme. FAD is used to introduce a double bond in jS-oxidalion, while NADPH is used to reduce the double bond in fatty-acid biosynthesis. 

We now introduce the principle of microscopic reversibility. This states that the transition states for any pathway for an elementary reaction in forward and reverse directions are related as mirror images. The atoms are in the same places but the momentum vectors are, of course, reversed since in general the transition state is proceeding in one direction only. In other words, the forward and reverse mechanisms are identical, according to this principle. 

Multiple pathways leading to the same product channel can also be observed in a reaction when there are a sufficient number of identical atoms, thereby allowing different intermediate structures to yield the same products. In these cases, the mechanisms in the two pathways are often quite similar, but involve differing positions of identical atoms on the reactants. The different pathways often involve formation of ring intermediates in which the rings have different sizes. A simple example of this class is the photodissociation of vinyl chloride [9]... 

It is challenging experimentally to study two pathways leading to a single product channel for the simple reason that the products in either case are structurally identical. Nevertheless, there are several methods, each applicable to certain classes of reactions, that can distinguish the presence of multiple pathways. 

The final example of a reaction in which multiple pathways arise from the participation of identical atoms is the reaction HCCO + O2. This reaction demonstrates another general feature of pathway competition when unsaturated species are involved, namely, the possibilities for formation of ring intermediates of different sizes. 

Most of the subsequent steps of tetrapyrrole synthesis are identical in plants, animals, and bacteria. The pathway includes synthesis of the monopyrrole porphobilinogen from two molecules of ALA by the action of ALA dehydratase with the elimination of two molecules of water, followed by the assembling of a linear tetrapyrrole hydroxymethylbilane from fonr molecnles of porphobilinogen, ring closure and two modification reactions of side chains. This produces the first tetrapyrrole macrocycle, uroporphyrinogen HI. Therefore, eight molecules of ALA are necessary to form one tetrapyrrole. 

In catalysis active sites are operative that allow for an alternative reaction path. For a satisfactory catalyst this alternative pathway leads to higher rates and higher selectivity. In heterogeneous catalysis reactant molecules adsorb at active sites on the catalyst surface at the surface sites reactions occur and products are desorbed subsequently. After desorption, active sites are again available for reactant molecules and the cycle is closed. In homogeneous catalysis the situation is essentially identical. Here complexation and decomplexation occur. A complication in heterogeneous catalysis is the need for mass transfer into and out of the catalyst particle, which is usually porous with the major part of the active sites at the interior surface. 

Here it is found that the rate of loss of optical activity and the rate of isomerisation are identical, and if the reaction is carried out in the presence of D20 (five moles per mole of substrate) no deuterium is incorporated into the product. The reaction is thus wholly intramolecular under these conditions—no carbanion is involved—and is believed to proceed via a bridged T.S. such as (30). With a number of substrates features of both inter- and intra-molecular pathways are observed, the relative proportions being dependent not only on the substrate, but to a considerable extent on the base and solvent employed also. 

Synthetic pathways with repeating reaction sequences are also attractive for the preparation of linear aliphatic molecules constructed of identical building... 

The correlation of deposition rate with disilane concentration and the zero-barrier of the reaction of silane with silylene to disilane lead to the conclusion that the latter reaction is the dominant subsequent pathway following the silane fragmentation. Disilane shows two characteristic relaxation times, the slower being identical with the relaxation time of silane. In conclusion, the formation of... 

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