Carbenium ions, hardness

Particularly relevant to the present crmtext is the fact that the olefinic double bond is considered as a soft base in Pearson s theory, while many Lewis acids used in cationic polymerisation (BF3, BCI3, AICI3, etc.) are classed as hard acids. Obviously, n-acceptors like chloranil or tetracyanoethylene are considered as soft acids. Thus, the interactions between Lewis acids and olefins must be considered as very weak in the context of the HSAB theory. This prediction is well substantiated by the tenuous character of the complexes observed in experimental studies (see Chap. IV). On the other hand, carbenium ions are usually placed at the borderline between hard and soft acids and are definitely softer than the Lewis acids mentioned above. Consequently, their interactions with olefins must be rather strong, which suggests that that propagation in cationic polymerisations promoted by Lewis acids should be faster than initiation. 

Acenaphthylene on the other hand polymerises with HQO4 in CH2CI2 with no detectable presence of carbenium ions. It seems that this stem is another example of a pseudocationic polymerisation Finally, in the case of N-vinylcarbazole it is difficult to establish the character of its pcJymerisation by perchloric acid in methylene chloride, since its very hi rate even with less than 10 M of acid did not permit a serious search of ptssible carbenium icms formed during the process The fact that this moncmer can be polymerised by dilute aquecus soluticais of perchloric acid in such nucleophilic scAvents as ethyl acetate and acetcme is hardly compatible with a cationic medianism. The nature of the chain carriers in this bizarre system is totally unclear. 

The initiation scheme preposed by Colclou and Dainton in the presence of added alkyl chlorides and invdeed dso for solvent cocatalysis with ethylene chloride seems most unlikely. Indeed, the possiMity of RCl cocatalysis in these systems is remote, due to the hi instability of the carbenium ions involved which would hardly be expected to form or at least to have a sufficient lifetime to attack the monomer. 

The analysis of the effect that the different substituents may have on the rate coefficients in this case is particularly hard to perform by using simple inductive effects, as described for instance by the Hammett substituent constants. Note that, despite the fact that most of the 28 carbenium ions exhibit para substitution, the general structure on top of Chart 6, shows a complex substitution pattern at the carbocation centre. In order to asses the effect of multiple substitution at this site, we first considered those compounds that have two fixed hydrogen atoms at the -position of the phenyl group, which according to Hammett classification have crp(H) = 0.0, and the third phenyl group substituted at -position with H (compound 96 in Chart 6) OCH3 (compound 97 in... 

Reactions of 2-trialkylsilyl- and trialkylstannyl-substituted furans with benzhydryl cations provided 2,5-disubstituted furans and ipso-substituted furans. Kinetic investigations of the reactions revealed that the monosubstituted product was produced from the protonolysis of the 2,5-disubstituted furylstannane, while the 2,5-disubstituted furan was derived from an electrophilic substitution of the mono-substituted furan. Introduction of a trialkylsilyl and a trialkylstannyl group to the 2-position of furan hardly affected the reactivity of this position towards carbenium ions ipso attack), while the 5-position is somewhat activated <01OL1629,01OL1633>. 

Carbon acids are relatively soft. Owing to the electron-withdrawing property of alkyl groups attached to a 57 -hybridized carbon (16), the hardness sequence of several carbenium ions follows the order Ph > Bu > z-Pr > Et > Me . This sequence is supported by thermodynamic data deduced from the reactions of alcohols with hydrogen sulfide. The validity of the above sequence need not be restricted to free cations, but it implies that the more carbenium character a center attains during a reaction, the harder it will be. The increasing stability of isomeric butanols n < iso < sec < tert) parallels the trend of hardness exhibited by the carbenium ion R which combines with the hard hydroxide ion. This offers an explanation for isomerizations such as... 

The rate ratio phso/ Meoe for substitution varies from 512 for / -methoxy to 5300 for p-nitrobenzyl bromides (17). The former compound exhibits considerable carbenium ion character in the transition state, hence it reacts faster with MeO (hard-hard interaction). 

Scheme 2.2.11 Electrophilic addition of HBrto propene - due to the higher stability of the secondary carbenium ion there is hardly any 1-bromopropane found in the product.

These data permit the conclusion that according to their stabilizing effect on carbenium, and, in particular, on arenium, ions the alkyl groups are arranged in the Baker-Nathan order, the difference in the effect depending on the ion structure. However, the difficxxlty of estimating the relative role of electronic and steric effects requires much care, so one can hardly answer the question as to which of these effects actually determines this sequence (see... 

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