Carbocations reactive intermediate functions

In contrast to this mechanism, the one proposed in our work operates direct from the oxidation state of the alkane feedstock. The same alkyl cation intermediate can lead to both alkane isomerization (an alkyl cation is widely accepted as the reactive intermediate in these reactions) and we have shown in this paper that a mechanistically viable dehydrocyclization route is feasible starting with the identical cation. Furthermore, the relative calculated barrier for each of the above processes is in accord with the experimental finding of Davis, i.e. that isomerization of a pure alkane feedstock, n-octane, with a dual function catalyst (carbocation intermediate) leads to an equilibration with isooctanes at a faster rate than the dehydrocyclization reaction of these octane isomers (8). 

In the absence of either HPI or Co(acac)2, no appreciable conversion into products was obtained. EPR evidence for the formation of an aminoxyl radical intermediate was acquired, and a KIE of 3.8 determined . Generation of PINO in situ as the reactive intermediate was postulated . A subtle alternative enables the functionalization of hydrocarbons through the formation of carbocations as transient intermediates whenever PINO is formed and reacts in the presence of NO (e.g. 1 atm) . 

Usually, mechanistic proposals based on product studies are supplemented by independent evidence from similar reactions (i.e. arguments based on analogies). Such proposals might also be supported by a knowledge of characteristic reactions of functional groups and properties of reactive intermediates (e.g. acidities of C—H groups, or relative stabilities of carbocations), and the proposals are a basis for the more detailed investigations by methods discussed in later chapters. 

Transient Three-membered Ring Compounds.—The existence of stable protonated alkylcyclopropanes as reactive intermediates has been discussed in detail. The collisional activation spectra of the gaseous ions formed upon protonation of pro-pene and cyclopropane are identical and the low activation energy for the conversion of protonated cyclopropane into the 2-propyl carbocation thus indicated is inconsistent with MINDO/3 calculations. Protonated cyclopropanes have been implicated in the reduction of (225) in the presence of strong acids and in the decomposition of cyclopropane on Zeolite HY. The involvement of the 3-methyl function in the deamination of 3-methyl-2-aminobutane has been evident for some time. However, studies with optically active and labelled substrates have now confirmed... 

Carboxylic acids are oxidized by lead tetraacetate. Decarboxylation occurs and the product may be an alkene, alkane or acetate ester, or under modified conditions a halide. A free radical mechanism operates and the product composition depends on the fate of the radical intermediate.267 The reaction is catalyzed by cupric salts, which function by oxidizing the intermediate radical to a carbocation (Step 3b in the mechanism). Cu(II) is more reactive than Pb(OAc)4 in this step. 

The reaction starts of with a protonation - use the catalyst. Resist the urge to protonate the 4-hydroxyl, but go for the one at position 1 that has the added functionality of the hemiacetal linkage. It is going to be the more reactive one. Protonation is followed by loss of water as leaving group. The intermediate oxonium cation shown is actually a resonance form of the simpler carbocation now you can see the role of the adjacent oxygen. The reaction is completed by attack of the nucleophile, the 4-hydroxyl of another molecule. This is not special, but is merely another version of the hemiacetal synthesis done in part (a). 

The alkenes most reactive to cationic polymerization contain electron-donating functional groups that can stabilize the carbocation intermediate. The reactivity order of substituents in cationic polymerization is similar to the reactivity order of substituted benzenes in electrophilic aromatic substitution reactions. 

Carbanions (b and h), carbocations (c), free radical.s (d). and eaibenc.s (g) are reactive species of high energy that generally can t be "put into a bottle because of their instability. They can, however, function as short-lived reaction "intermediates."... 

It is clear that atoms other than hydrogen can be electron deficient and function as electron pair acceptors. Can a carbon atom function as a Lewis acid The answer is yes, if the definition is modified somewhat. Various reactions generate carbocation intermediates (see 55 and 58) and a Lewis base can certainly donate electrons to that positive carbon. A species that donates electrons to carbon is called a nucleophile (see Section 6.7), so an electron donor that reacts with 55 or with 58 is a nucleophile. In addition to carbocations, which are charged species, the carbon atom in a polarized bond is electron deficient, and a nucleophile could donate electrons to the 6+ carbon. This is the basis of many organic reactions to be discussed, particularly in Chapter 11. The fundamental concept of a species donating electrons to a carbon is introduced in this section, with the goal of relating this chemical reactivity to the Lewis acid-Lewis base definitions used in previous sections. 

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