Methyl carbocation

Methyl carbocation, electrostatic potential map of. 196 Methyl group, 83 chiral, 407... 

Figure 6.10 Electrostatic potential maps for (a) tert-butyl (3°), (b) isopropyl (2°), (c) ethyl (1°), and (d) methyl carbocations show the trend from greater to lesser delocalization (stabilization) of the positive charge. (The structures are mapped on the same scale of electrostatic potential to allow direct comparison.)...

Table 1 Rate and equilibrium constants for partitioning of substituted a-methyl carbocations R (R2)CCH3+ between nucleophilic addition of solvent (ks) and deprotonation (kp) (Scheme 7)°... 

Simple tertiary carbocations represent a benchmark against which to compare the reactivity of other a-methyl carbocations. Therefore, it is necessary to deal with complex questions about the mechanism for substitution and elimination reactions at tertiary aliphatic carbon in order to evaluate the rate constant... 

Table 1 summarizes experimentally determined values of the following rate and equilibrium constants for the reactions of aliphatic and benzylic a-methyl carbocations (Scheme 7). 

Three mechanisms have been proposed for this reaction (Scheme 21). The reaction is first order in each of the reactants. In another study, Reutov and coworkers159 found a large primary hydrogen-deuterium kinetic isotope effect of 3.8 for the reaction of tri-(para-methylphenyl)methyl carbocation with tetrabutyltin. This isotope effect clearly demonstrates that the hydride ion is transferred in the slow step of the reaction. This means that the first step must be rate-determining if the reaction proceeds by either of the stepwise mechanisms in Scheme 21. The primary hydrogen-deuterium kinetic isotope effect is, of course, consistent with the concerted mechanism shown in Scheme 21. 

Thus a tertiary carbocation like the above will give nine resonating structures while a primary will give only two hyperconjugative forms. This explains why tertiary carbocations are more stable than secondary which in turn is more stable than primary. This also explains why ethyl carbocation (CH3CIlf) is more stable than methyl carbocation (CH )-... 

From a number of experiments it has been concluded that with short life carbocations (obtained from secondary and tertiary alkyl deviatives) inversion is generally observed. But in long life carbocation where there is spreading of the charge, the product is a racemic one, as in diphenyl methyl carbocation Ph2CHX. Therefore it also affords a means of estimating the relative stability of carbocations. 

On the other hand, the methyl carbocation is the result of removing a hydride anion (a hydrogen atom and an electron) from methane by fission of the C-H bond so that the two electrons are removed with hydrogen. We can now deduce that carbon has... 

A new Y solvolysis scale has been developed for benzylic species with extensive charge delocalization, based upon the solvolyses of some benzhydryl bromides and /-butyl(2-naphthyl)methyl bromides.39 Chlorides have negative salt effects on the ionization of benzhydryl bromide in 7-butyrolactone.40 The X-ray structure of the dimerization product of l,8-bis(dhnethylammonio)-4-naphthyl(phenyl)methyl carbocation has been determined it appears to be formed via a 4n + 2n-cycloaddition mechanism 41... 

The mechanism for this reaction begins with the generation of a methyl carbocation from methylbromide. The carbocation then reacts with the n electron system of the benzene to form a nonaromatic carbocation that loses a proton to reestablish the aromaticity of the system. 

Methyl substrates are excellent for SN2 reactions. There is no /J-carbon, so elimination cannot occur. Also, the methyl carbocation is very unstable, so the SN1 mechanism does not occur either. 

The exact mechanism of this step remains unclear—with such a good leaving group and a bad nucleophile you might expect S l, butthat would require a methyl carbocation. 

A second explanation for the observed trend in carbocation stability is based on orbital overlap. A 3° carbocation is more stable than a 2°, 1°, or methyl carbocation because the positive charge is delocalized over more than one atom. 

CH3)2CH+> H2C==CH—CH+ CH3CH+ > H2C=CH > Ph > CHj. The stabilities of various carbocations can be determined by reference to the order of stability for alkyl carbocations, 3 > 2° > 1 > CH3. The acetyl cation has a stability similar to that of the r-butyl cation. Secondary carbocations, primary benzylic cations, and primary allylic cations are all more stable than primary alkyl cations. Vinyl, phenyl, and methyl carbocations are less stable than primary alkyl cations. 

The cleavage is an reaction that occurs by protonation of the oxygen atom followed by loss of the stable triaryl methyl carbocation. 

Clearly, the tertiary carbocation is the most stable, as it is surrounded by three other carbon atoms that share the burden of its positive charge. Primary and especially methyl carbocations are rarely seen in organic reactions except under special circumstances. 

The behaviour of methyldiazonium ion in water has been investigated in phosphate buffer solutions at a pH of 7.4. The observations indicate that the methyldiazonium ion exchanges protons with deuterated buffers via an equilibrium involving diazomethane (equation 50). The products were recovered as deuterated methanol when the various labelled diazonium ions decomposed to the methyl carbocation, which reacted with deuterated water to form labelled methanol (equation 51). 

We can get a more quantitative feel for the relative stabilities of alkyl carbocations by examining data for the enthalpy of ionization (gas phase) for various alkyl chlorides Of course, each of these reactions is much more endothermic in the gas phase than it would be in solution, where solvent molecules of appropriate polarity characteristics could help to stabilize the electrically charged products of the ionization reaction. (This is why ionizing solvents are often used for reactions that involve charged intermediates.) Nevertheless, the data clearly reflects the order of carbocation stabihty that we have already established tertiary carbocations are the easiest (least endothermic) to form, the secondary, then primary, and the methyl carbocation is the most difficult to form. 

Tertiary carbocations are the most stable, and the methyl carbocation is the least stable. 

Tertiary carbocations have three carbons with C—H bonds (or, depending on the specific example, C — C bonds instead of C — H) adjacent to the carbocation that can overlap partially with the vacant p orbital. Secondary carbocations have only two adjacent carbons with C — H or C — C bonds to overlap with the carbocation hence, the possibility for hyperconjugation is less and the secondary carbocation is less stable. Primary carbocations have only one adjacent carbon from which to derive hyperconjugative stabilization, and so they are even less stable. A methyl carbocation has no possibility for hyperconjugation, and it is the least stable of all in this series. The following are specific examples ... 

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