Carbocations varied

Stabilization over the unsubstituted carbocation varies from 25 kcal mol" for R = H to 10 kcal mol" for R = CH3. 

A closer examination of the data in Table 5.4 indicates that not only does carbocation stability vary according to the familiar pattern 3° > 2° > 1°, but the stability for each t)rpe of carbocation varies according to the number of atoms in the ion. What is the mathematical relationship between the molecular mass of an ion and its thermodynamic stability in the gas phase, and how can you rationalize this pattern ... 

My research during the Cleveland years continued and extended the study of carbocations in varied superacidic systems as well as exploration of the broader chemistry of superacids, involving varied ionic systems and reagents. I had made the discovery of how to prepare and study long-lived cations of hydrocarbons while working for Dow in 1959-1960. After my return to academic life in Cleveland, a main... 

The C-NMR chemical shift of the trivalent carbon is a sensitive indicator of carbocation structure. Given below are the data for three carbocations with varying aryl substituents. Generally, the larger the chemical shift, the lower is the electron density at the carbon atom. 

Scheme 10. Mechanislic possibililies for PF condensalion. Mechanism a involves an SN2-like attack of a phenolic ring on a methylol. This attack would be face-on. Such a mechanism is necessarily second-order. Mechanism b involves formation of a quinone methide intermediate and should be Hrst-order. The quinone methide should react with any nucleophile and should show ethers through both the phenolic and hydroxymethyl oxygens. Reaction c would not be likely in an alkaline solution and is probably illustrative of the mechanism for novolac condensation. The slow step should be formation of the benzyl carbocation. Therefore, this should be a first-order reaction also. Though carbocation formation responds to proton concentration, the effects of acidity will not usually be seen in the reaction kinetics in a given experiment because proton concentration will not vary.

Conjugated dienes undergo several reactions not observed for nonconjugated dienes. One is the 1,4-addition of electrophiles. When a conjugated diene is treated with an electrophile such as HCl, 1,2- and 1,4-addition products are formed. Both are formed from the same resonance-stabilized allylic carbocation intermediate and are produced in varying amounts depending on the reaction conditions. The L,2 adduct is usually formed faster and is said to be the product of kinetic control. The 1,4 adduct is usually more stable and is said to be the product of thermodynamic control. 

The latter three of the above points are dealt with in the following parts (see parts 4.2-4.5). Experimental investigations of the inner structure of the cations can be supplemented by quantum chemical calculations 104 106). For example, the MINDO/3 method allows the heats of formation of carbocations to be calculated 107). A comparison of some calculated and experimental values (Fig. 6) shows that the reproduction quality of MINDO/3 varies. 

Most of these results have been obtained in methanol but some of them can be extrapolated to other solvents, if the following solvent effects are considered. Bromine bridging has been shown to be hardly solvent-dependent.2 Therefore, the selectivities related to this feature of bromination intermediates do not significantly depend on the solvent. When the intermediates are carbocations, the stereoselectivity can vary (ref. 23) widely with the solvent (ref. 24), insofar as the conformational equilibrium of these cations is solvent-dependent. Nevertheless, this equilibration can be locked in a nucleophilic solvent when it nucleophilically assists the formation of the intermediate. Therefore, as exemplified in methylstyrene bromination, a carbocation can react 100 % stereoselectivity. 

Thus in the above case the elimination product is found to contain 82 % of (7). Unexpected alkenes may arise, however, from rearrangement of the initial carbocationic intermediate before loss of proton. El elimination reactions have been shown as involving a dissociated carbocation they may in fact often involve ion pairs, of varying degrees of intimacy depending on the nature of the solvent (cf. SN1, p. 90). 

Finally, as is the case for the secondary a-deuterium KIEs, the /3-deuterium KIE is assumed to vary in magnitude from near unity for a reactant-like transition state to a maximal value for a transition state resembling the carbocation formed in an SN1 reaction. The experimentally determined KIE may, therefore, be used as a measure of transition state structure provided that the maximum value of the KIE, i.e. the EIE for the formation of the carbocation, is known. 

The products are then formed by loss of a proton from this carbocation, with a choice of protons that may be lost, so that a mixture of products in varying proportions results. P-Ionone is the predominant product. This is the most substituted alkene, and has the added stability conferred by extending conjugation with the unsaturated ketone (see Section 2.8). 

Water, alcohols, acids, anhydrides, and esters have varying chain-transfer properties [Mathie-son, 1963]. The presence of any of these transfer agents in sufficient concentrations results in Reaction 5-28 becoming the dominant mode of termination. Termination by these compounds involves transfer of HO, RO, or RCOO anion to the propagating carbocation. Aromatics, ethers, and alkyl halides are relatively weak chain-transfer agents. Transfer to aromatics occurs by alkylation of the aromatic ring. 

The benzylic substrates X-l-Y and X-2-Y have provided a useful platform for examining the changes in reaction mechanism for nucleophilic substitution that occur as the lifetime of the carbocation intermediate is decreased systematically by varying the meta- and para- aromatic ring substituents. When X is strongly resonance electron-donating, X-l-Y and X-2-Y react by a stepwise mechan-... 

Jencks and Richard, and others, had pioneered the use of the azide elock to quantitatively assess the lifetime of carbenium ions generated under solvolytic conditions. The method relies on the use of product yield data collected at varying [N3 ] to determine the N /solvent selectivity, expressed as the ratio of the second-order rate constant for trapping of the ion by N3 and the pseudo-lirst-order rate constant for trapping of the ion by solvent k Jk. The assumption is made that k z is diffusion limited at ca. 5 x 10 M"" s This assumption allows k to be estimated, and l/kg provides the lifetime of the ion in the solvent in the absence of added nucleophiles. McClelland and Steenken showed by direct measurement of k. , for a series of diarylmethyl and triarylmethyl carbocations that k is approximately constant at... 

Apart from some experiments with methyl and /i-chloroethyl vinyl ethers the initiator concentrations employed were such that the initiating cations, and presumably the propagating species, were essentially dissociated from the corresponding counterion. Once again therefore this data is a measure of the reactivity of the free polymeric cations derived from the various monomers. Isobutyl vinyl ether is the monomer most widely studied, and as would be anticipated for free cationic reactivities, the data varies little with the counterion employed (SbClg or BF4), or indeed with the carbocation used as initiator (C7H7 or Ph3C+) under similar experimental conditions. 

The tertiary 5-methyl and 5-phenyl-2,4-dehydro-5-homoadamantyl cations, 26a, were prepared57 by two different routes either from the corresponding alcohols or the corresponding 2-enclassical carbocations with varying degrees of charge delocalization. 

We will deal more briefly with reactions of carbocations with nucleophiles other than water, and then consider correlations in which the nucleophile rather than (as hitherto) the carbocation is varied. Fig. 7 shows a plot of... 

Swain and Scott found satisfactory correlations with Equation (27) which provided 5 values for a number of reactants. However, as indicated in Scheme 33, for the limited number of substrates conveniently studied,158,186 variations in 5 did not show a clearly discernible pattern (and no obvious correlation with reactivity). Moreover, Pearson and Songstad demonstrated that the correlations break down if extended to extremes of soft and hard electrophilic centers such as platinum, in the substitution of trara,s-[Pt(pyridine)2Cl2], or hydrogen in proton transfer reactions.255 Despite this, Swain and Scott s equation has stood the test of time and it is noteworthy that a serious breakdown in the correlations occurs only when the reacting atoms of both nucleophile and electrophile are varied. In this chapter we will restrict ourselves to carbon as an electrophilic center, and particularly, although not exclusively, to carbocations. 

It seems clear that for reactions of carbocations with nucleophiles or bases in which the structure of the carbocation is varied, we can expect compensating changes in intrinsic barrier and thermodynamic driving force to lead to relationships between rate and equilibrium constants which have the form of extended linear plots of log k against log K. However, this will be strictly true only for structurally homogeneous groups of cations. There is ample evidence that for wider structural variations, for example, between benzyl, benzhydryl, and trityl cations, there are variations in intrinsic barrier particularly reflecting steric effects which lead to dispersion between families of cations. 

---