Carbo resonance stabilized

Another linear correlation between A// values and between AG values has been proposed to correlate the heats of heterolysis for the carbon-carbon <7 bond with p Cr+ values of the cations and values of the conjugate acids of the anions by Arnett et al. (1987a, 1990a). From the results of calorimetry for the coordination of resonance-stabilized carbo-cations and carbanions in sulfolane or acetonitrile, these workers demonstrated that (28) and (29), for secondary and tertitu cations, respectively, can be used for predicting heats of heterolysis of the carbon-carbon a bond. 

Acyliiim ion (Section 16.3) A resonance-stabilized carbo-cation in which the positive charge is located at a carbonyl-... 

To understand why some substituents make a benzene ring react faster than benzene itself (activators), whereas others make it react slower (deactivators), we must evaluate the rate-determining step (the first step) of the mechanism. Recall from Section 18.2 that the first step in electrophilic aromatic substitution is the addition of an electrophile (E ) to form a resonance-stabilized carbo-cation. The Hammond postulate (Section 7.15) makes it pos.sible to predict the relative rate of the reaction by looking at the stability of the carbocation intermediate. 

Resonance-stabilized carbo.vy]ate ion (two equivalent resonance forms)... 

Resonance stabilization of the phenox d anion shows the negative charge distrih over the one para and two ortho carbo UtC ... 

Proton abstraction from the chiral center yields a resonance stabilized carbo-anion... 

On the other hand, in the alkoxide ion, RO , the negative charge is not delocalized and is concentrated on the single ojq gen atom. This anion, therefore, is not as stable as the resonance stabilized carboxylate anion. The resonance stabilization promotes dissociation in the carbo llc acids making them stronger in relation to the organic acids where lack of resonance stabilization decreases dissociation. 

The mechanism of the Nicholas reaction is best described as an SnI process. Protonation of the alcohol in 4 followed by loss of water from cation 8 yields cobalt-stabilized carbocation 5. Friedel-Crafts reaction of this electrophile with anisole provides resonance-stabilized carbocation 9 which, upon removal of a proton, furnishes the substitution product 6. In addition to electron rich aromatics like anisole, a variety of neutral carbo- and heterocyclic nucleophiles react successfully with the carbocation... 

In the a-donating (zwitterionic) resonance form of the latter heteroatom-cumu-lated free ligands, all the atoms satisfy the octet rule. Two kinds of all-carbon versions can be distinguished those that are 71-conjugated to a remote heteroatom and those that are not [14—19]. The former are largely exemplified by aminoalle-nylidenes, in which the a-donating resonance form of the free ligand is also stabilized by the octet rule (they are functional carbo-m cs of the Fischer-type aminocarbenes). The second kind is represented by C-substituted allenylidenes. 

How can we account for the formation of 1,4-addition products The answer is that allylic carbocations are involved as intermediates. When 1,3-butadiene is protonated, two carbocation intermediates are possible a primary carbo-cation and a secondary allylic cation (recall that allylic means next to a double bond ). Since an allylic cation is stabilized by resonance between two forms (Section 11.9), it is more stable and forms faster than a nonallylic carbocation. 

We have seen throughout the past several sections that acid chlorides are most reactive toward nucleophilic acyl substitution, followed by acid anhydrides and esters the least reactive are amides. Carboxylate anions are negatively charged and therefore repel nucleophiles the resonance in these species is quite stabilizing. Both of these factors make carbo>qrlate anions essentially inert to nucleophilic acyl substitution (hence, we have not examined them to this point in the chapter). Another useful way to think about the reactions of the functional derivatives of carboxylic acids is summarized in Figure 18.2. 

Aliphatic primary amines also give diazonium salts with nitrous acid. However, in this case the diazonium ion is not stabilized by resonance structures as in the case of arylamine, and decomposition to nitrogen and carbo-nium ions takes place. During the performance of the reaction the evolu-... 

The structure of an orthoester reminds us of that of acetals and ketals, compounds that are easily cleaved by dilute acids. The mechanism of hydrolysis for orthoester 4 should be similar to that of the acetals and ketals, starting with the protonation of one of the oxygen atoms and followed by the formation of a carbo-cation greatly stabilized by resonance. The overall process is a SnI mechanism that ends up by addition of water. Two different species 8 and 10 can be formed... 

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