Caibocation stability

Concepts such as relative aadity and caibocation stability can be fundamentally related to hardness and electronegativity as defined by DFT. 

A wide range of caibocation stability data has been obtained by measuring the heat of ionization of a series of chlorides and cafbinols in nonnucleophilic solvents in the presence of Lewis acids. Some representative data are given in Table 5.4 These data include the diarylmediyl and triarylmethyl systems for which pX R+ data are available (Table 5.1) and give some basis for comparison of the stabilities of secondary and tertiary alkyl carbocations with those of the more stable aryl-substituted ions. 

To summarize, the most important factor to consider in assessing caibocation stability is the degree of substitution at the positively charged carbon. 

One important experimental fact is that the rate of reaction of alcohols with hydrogen halides increases in the order methyl < primary < secondary < tertiary. This reactivity order parallels the caibocation stability order and is readily accommodated by the mechanism we have outlined. 

CleaiTy, the steric crowding that influences reaction rates in Sn2 processes plays no role in SnI reactions. The order of alkyl halide reactivity in SnI reactions is the same as the order of caibocation stability the more stable the caibocation, the more reactive the alkyl halide. 

Jencks has discussed how the gradation from the 8fjl to the 8n2 mechanism is related to the stability and lifetime of the carbocation intermediate, as illustrated in Fig. 5.6. In the 8n 1 mechanism, the carbocation intermediate has a relatively long lifetime and is equilibrated with solvent prior to capture by a nucleophile. The reaction is clearly a stepwise one, and the energy minimxun in which the caibocation mtermediate resides is significant. As the stability of the carbocation decreases, its lifetime becomes shorter. The barrier to capture by a nucleophile becomes less and eventually disappears. This is described as the imcoupled mechanism. Ionization proceeds without nucleophilic... 

Any structural effect which reduces the electron deficiency at the tricoordinate carbon will have flie effect of stabilizing the caibocation. Allyl cations are stabilized by delocalization involving the adjacent double bond. 

In contiaat, die isomer, in which tire double bond is not in a position to participate in die iooizsdon step, reacts 10 times slower than die anti isomer. The reaction product is derived fiom a rearranged caibocation ion that is stabilized by virtue of being allylic. ... 

Benzyl and allyl alcohols which can generate stabilized caibocations give Friedel-Crafts alkylation products with mild Lewis acid catalysts such as scandium triflate. ... 

We will see numerous reactions that involve caibocation intermediates as we proceed through the text, so it is important to understand how theii structure determines their- stability. 

We have seen this situation before in the reaction of alcohols with hydrogen halides (Section 4.11), in the acid-catalyzed dehydration of alcohols (Section 5.12), and in the conversion of alkyl halides to alkenes by the El mechanism (Section 5.17). As in these other reactions, an electronic effect, specifically, the stabilization of the carbocation intennediate by alkyl substituents, is the decisive factor. The more stable the caibocation, the faster it is fonned. 

Section 11.14 Benzylic caibocations are intermediates in SnI reactions of benzylic halides and are stabilized by electron delocalization. 

All alkyl groups, not just methyl, are activating substituents and ortho, paia directors. This is because any alkyl group, be it methyl, ethyl, isopropyl, rerr-butyl, or any other, stabilizes a caibocation site to which it is directly attached. When R = alkyl. 

Competition studies reported by Kuwajima, ° which also complement the results of Nakai, illustrate the limitations of the -effect as a tool for predicting the outcome of vinylsilane-terminated cyclizations (Scheme 4). Acylium ion initiated cyclizations of (7a) and (7b) gave the expected cyclopentenones (8a) and (8b). However, compound (7c), upon treatment with titanium tetrachloride, gave exclusively the cyclopentenone product (8c) arising from the chemoselective addition on the 1,1-disubstituted alkene followed by protodesilylation of the vinylsilane. The reversal observed in the mode of addition may be a reflection of the relative stabilities of the carbocation intermediates. The internal competition experiments of Kuwajima indicate that secondary B-silyl cations are generated in preference to secondary carbocations (compare Schemes 3 and 4), while tertiary caibocations appear to be more stable than secondary P-silyl carbocations, as judged by the formation of compound 9c). 

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