By John P. Richard (Eds.)
Advances in actual natural Chemistry offers the chemical group with authoritative and important tests of the numerous features of actual natural chemistry. the sphere is a speedily constructing one, with effects and methodologies discovering software from biology to stable country physics. * reports the appliance of quantitative and mathematical tools in the direction of figuring out chemical difficulties * Multidisciplinary volumes conceal natural, organometallic, bioorganic, enzymes and fabrics subject matters
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Thus the nearly invariant pKa for four of the cations arises from compensation between changes in the stability of the cation and of the aromatic molecule. It is noteworthy that as judged by pKR protonated benzene is a particularly stable carbocation. 110 A surprising observation is that benzoannelation is quite strongly destabilizing for the cations. We will return to the significance of this later in the chapter, as well as to the fact that the effect of benzoannelation on the pKas is opposite to that on pKR.
For these cations the equilibrium constant KR refers to equilibria with the corresponding aromatic hydrates, as is illustrated in Scheme 5 for the benzenonium ion (cyclohexadienyl cation) 9 for which the hydrate is cyclohexadienol 10. 104,105 OH + + H+ + H2O 10 9 Scheme 5 OH2+ OH + H Scheme 6 + Slow + + H2O Fast + H3O+ 38 R. MORE O’FERRALL + kA + H3O+ + H2O kp Scheme 7 In principle, a value of Ka for the benzenonium ion may be obtained from the ratio of rate constants for protonation of benzene, kA, and proton loss from the ion, kp, as shown in Scheme 7.
The greatest effects are from a-oxygen substituents but effects of substituents in the aromatic ring of benzylic carbocations are also large. As we shall see, there is a surprising influence of b-oxygen substituents upon the stabilities of arenonium ions. 130 Jencks and Amyes measurements gave ratios kAz =kH2 O for reactions of the carbocations with water and azide ions from which values of kH2 O could be derived in the usual way by assigning a diffusional rate constant, 5Â109 MÀ1 sÀ1, to kAz, save in the case of the methoxymethyl cation for which kH2 O was presumed to have achieved it relaxation limit (1011 sÀ1).
Advances in Physical Organic Chemistry by John P. Richard (Eds.)