Reaction pathway tetrahedral

Hydrolysis. Esters are cleaved (hydroly2ed) into an acid and an alcohol through the action of water. This hydrolysis is cataly2ed by acids or bases. The mechanistic aspects of ester hydrolysis have received considerable attention and have been reviewed (16). For most esters only two reaction pathways are important. Both mechanisms involve a tetrahedral intermediate and addition-elimination reactions i7i7... 

Chelating aldehydes such as 2-pyridine carbaldehyde and 2-dimethylamino benzaldehyde improve the stability of the aldehyde complexes via N,0 chelation. NMR studies show that the complexes are present in solution without an excess of aldehyde and can be formed in the presence of donor ligands. The X-ray structures showed longer and weaker Zn—O bonds when more than one chelating ligand was present. IR demonstrates the variation in C=0 bond strengths and how the environment of the zinc ion will influence potential catalytic activity via reaction rates or pathways. Tetrahedral chelate complexes, and octahedral bis- and tris-chelate complexes, were isolated.843... 

The reaction pathway is normally nucleophilic addition/elimination, via a so-called tetrahedral intermediate (157), leading to overall substitution. The difference between the reactions of carboxylic... 

Type II nitrosamines have two reaction pathways. One pathway involves nucleophilic attack at the carbon of C=0 to generate a tetrahedral intermediate which decomposes to an active diazotate ion (R-N=N-0 ). The other pathway involves the nucleophililc attack on the nitrogen of the nitroso group resulting in denitrosation (Scheme 3.5). The nucleophile can be a biologically prevalent thiol, therefore type II compounds are often used as NO donors for the formation of S-nitrosothiols [67, 68]. 

Swanson, B. I. and S. K. Satija. 1977. Molecular vibrations and reaction pathways. Minimum energy coordinates and compliance constants for some tetrahedral and octahedral complexes. J. Am. Chem. Soc. 99 987-991. 

The tetrahedral intermediate in the chymotrypsin reaction pathway, and the second tetrahedral intermediate that forms later, are sometimes referred to as transition states, which can lead to confusion. An intermediate is any chemical species with a finite lifetime, finite being defined as longer than the time required for a molecular vibration ( 10-13 seconds). A transition state is simply the maximum-energy species formed on the reaction coordinate and does not have a finite lifetime. The tetrahedral intermediates formed in the chy-motrypsin reaction closely resemble, both energetically and structurally, the transition states leading to their formation and breakdown. However, the intermediate represents a committed stage of completed... 

The observation of lsO-exchange between the carbonyl oxygen atom of an ester and solvent is evidence for the formation of a tetrahedral addition compound, but not proof that this actually lies on the reaction pathway. But the behaviour of the exchange and hydrolysis reactions is so similar that there can be little doubt that this is, in fact, the case. The evidence is most complete for alkaline hydrolysis (see p. 163). One further piece of evidence obtained under acidic conditions is the observation47 that there is no exchange between the solvent and the carbonyl-lsO-labelled ester when methyl 2,4,6-trimethyl-benzoate is hydrolyzed in 3.09,5.78 and 11.5 M sulphuric acid. Other evidence makes it clear that this ester is hydrolyzed by the Aac1 mechanism, and that no reversible addition of water is expected. 

Cis-trans isomerization can take place either photochemically or in the dark, but the reaction pathways are quite different. In the light-induced process the reaction goes through a tetrahedral intermediate formed from the triplet excited state, whereas the dark reaction involves a dissociation of the complex, followed by recombination. In the latter case the presence of free glycine is demonstrated by the use of radioactive tracers no free glycine appears in the photochemical reaction. 

That a single solvent molecule clustered to a nucleophile can drastically change the reaction pathway has been demonstrated by studying the reaction of phenyl acetate with methoxide ion in the gas phase [671, 672]. Alkaline hydrolysis of esters in solution is known to proceed by attack of the nucleophile at the carbonyl carbon atom to form a tetrahedral intermediate, followed by cleavage of the acyl-oxygen bond (Bac2 mechanism) cf. Eq. (5-138a). 

This reaction had previously been shown to involve an intermediate by the kinetic methods mentioned on p. 1256. Bender and Heck showed that the rate of 0 loss and the value of the partitioning ratio 2/ 3 as determined by the oxygen exchange technique were exactly in accord with these values as previously determined by kinetic methods. Thus the original 0-exchange measurements showed that there is a tetrahedral species present, though not necessarily on the reaction path, while the kinetic experiments showed that there is some intermediate present, though not necessarily tetrahedral. Bender and Heck s results demonstrate that there is a tetrahedral intermediate and that it lies on the reaction pathway. 

Ab initio molecular orbital theory has been applied by Olah and coworkers to investigate the reactions of NO and the protonitrosonium ion HNO with methane. The reaction path was found to involve attack of NO on carbon instead of C-H bond insertion in accord with the studies of Schreiner et al. It was, however, pointed out that this is the consequence of the ambident electrophilic nature of NO and does not represent a general electrophilic reaction pathway for the reactions of methane. In fact, Schreiner and coworkers suggested that the electrophilic substitution of methane occurs by substitution of the nonbonded electron pair of methane instead of insertion of the electrophile into a C-H bond via 3c-2e bonding. Nonbonded electron pair formation in methane, however, can be considered only when methane would tend to flatten out (58) from its tetrahedral form, but this would be prohibitively energetic (>100 kcal mol ) and thus unlikely. 

The reaction pathways can be envisaged to involve isolated tetrahedral Ti sites embedded in an amorphous silica network (Fig. 1.9). In the bulk, the Ti atoms are coordinated by four Si-O ligands (tetrapodal Ti site), but on the surface, the active... 

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