Charge intermediate

Ab-initio calculations are particularly usefiil for the prediction of chemical shifts of unusual species". In this context unusual species" means chemical entities that are not frequently found in the available large databases of chemical shifts, e.g., charged intermediates of reactions, radicals, and structures containing elements other than H, C, O, N, S, P, halogens, and a few common metals. 

Sigmatropic rearrangements are normally classified as concerted processes with relatively nonpolar transition states. However, the Fischer cyclization involves rearrangement of a charged intermediate and ring substituents have a significant effect on the rate of the rearrangement. The overall cyclization rate... 

Nucleophilic substitutions on an aromatic ring proceed by the mechanism shown in Figure 16.17. The nucleophile first adds to the electron-deficient aryl halide, forming a resonance-stabilized negatively charged intermediate called a Meisenlieimer complex. Halide ion is then eliminated in the second step. 

It is clear from the results that there is no kinetic isotope effect when deuterium is substituted for hydrogen in various positions in hydrazobenzene and 1,1 -hydrazonaphthalene. This means that the final removal of hydrogen ions from the aromatic rings (which is assisted either by the solvent or anionic base) in a positively charged intermediate or in a concerted process, is not rate-determining (cf. most electrophilic aromatic substitution reactions47). The product distribution... 

Electrophilic substitution begins like electrophilic addition, with an attack on a region of high electron density to form a positively charged intermediate ... 

Notice that there are three curved arrows here. For some reason, students drawing this mechanism commonly forget to draw the third curved arrow (the one that shows the expulsion of Br ). The product of this hrst step is a bridged, positively charged intermediate, called a bromonium ion ( onium because there is a positive charge). In the second step of our mechanism, the bromonium ion gets attacked by Br (formed in the hrst step) ... 

The acid initially, protonates the nitrosamine to give a charged intermediate having significant water solubility. When the agent is gaseous hydrogen chloride or concentrated hydrochloric acid, the protonated intermediate is rapidly... 

As might be expected, the results from both theory and experiment suggest that the solution is more than a simple spectator, and can participate in the surface physicochemical processes in a number of important ways [Cao et al., 2005]. It is well established from physical organic chemistry that the presence of a protic or polar solvent can act to stabilize charged intermediates and transition states. Most C—H, O—H, C—O, and C—C bond breaking processes that occur at the vapor/metal interface are carried out homolytically, whereas, in the presence of aqueous media, the hetero-lytic pathways tend to become more prevalent. Aqueous systems also present the opportunity for rapid proton transfer through the solution phase, which opens up other options in terms of reaction and diffusion. 

For example, the observed decreasing redox peaks in the cyclic voltammogram of Figure 10A is attributed to the consumption of the charged intermediates. The optical, absorption data of Figure 9 also appear to evidence bipolarons (dications) as intermediate and hence, consistent with this mechanism. 

Because the organosilane reduction of ketones passes through a positively charged intermediate via the complexation or protonation of the carbonyl oxygen, the presence of suitably placed C=C functions can lead to cyclizations with the hydride of the silane adding to the C=C group. This strategy applies to... 

A toluene solution of the benzocyclobutene (327) was thermolyzed by heating in a sealed-tube at 180-200 °C for 48 hours to yield the lactone (328), via an intriguing epimerization pathway113), in which the initially formed kinetically controlled trans product was isomerized to the charged intermediate and then recyclized to furnish the thermodynamic m-isomer (328) u3). 

The solvomercuration reaction is thought to be a two-step process. In the first step (equation 147), electrophilic attachment of mercury ion to the alkene produces a positively charged intermediate. In the second step (equation 148), a nucleophile (generally a solvent molecule) reacts with the intermediate leading to the organomercury compound. 

Also, recall that alkaline hydrolysis of organophosphorothioates has been shown to involve a negatively-charged intermediate (v.s. and ]). Such an intermediate would be expected to be less stable in the negatively charged environment of the sediment particle surface. 

In the presence of protons, the initial radical anion A is protonated to a radical D which has two options either to couple with another radical to form a pinacol E, or to accept another electron to form an alcohol F. The pinacol E and the alcohol F may also result from double protonation of the doubly charged intermediates C and B, respectively. 

Because many of them are nearly inert, ionic liquids have been used to stabilize highly polar or ionic transition states. Ionic liquids provide favorable media for the formation and stabilization of intermediates in reactions that proceed through charged intermediates. An example is the Baylis-Hillman reaction catalyzed by 1,4-diazabicyclo (222). octane (DABCO) (Scheme 8) (162). 

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