Change of hybridization

To assess, at least qualitatively, how much of the observed shift in the triphenylcarbonium ion is due to the change of hybridization from to sp and how much to the effect of the positive charge, a comparison of the chemical shifts of the triphenyl-C -carbonium and trimethyl-C -carbonium ions with their parent sp -hybridized covalent precursors and with some C -compounds having p -hybridization is useful. Data of Table 9, indicate that the C -shifts of ap -hybridized compounds (olefins and aromatic hydrocarbons), at least in the molecules studied (Lauterbur, 1957,1962), are very similar and fairly independent of the nature of the molecules. 

As in other series, the Bu group produces compression shifts at nearest carbon atoms without change of hybridization (740MR(6)46l), while an iodine atom exerts the usual large atom shielding effect at the attached carbon. 

Reinvestigation of the excited state acid-base properties of 2-naphthylamine (Schulman and Capomacchia, 1972) showed that a reported change of hybridization from sp3 to sp2 on excitation had little effect on the entropy of protonation of nitrogen in the Sx state and that therefore the Forster cycle was still applicable. A pX (S1 )-value, calculated from the fluorescence maxima of the B and BH+ form, of —8-1 is in poor agreement with the value, —2, obtained from fluorescence titration measurements. From the acidity dependence of fluorescence intensity for 1- and 2-naphthylamine Liedke and Schulman (1973a) found that the decrease in emission of the B form occurred at lower acidities than the appearance of BH+ fluorescence. [A similar titration curve for the fluorescence of the neutral molecule was obtained by Seliskar and Brand (1971), who obtained a value of 0-64 for pkr(S1) from the decrease of the... 

Cyclopropylidenespiropentane (6) contains a central three-membered ring with an exocyclic double bond which is able to conjugate through the spiro center. In 7-cyclopropylidenedispiro[2.0.2.1]heptane (7) and 1-cyclopropylidenedispi-ro[2.0.2.1]-heptane (8) the additional spiro-connected three-membered ring introduces more strain and changes of hybridization. 

A remarkable dependence of the reactivity on ring size has been found in the series of methylenecycloalkanes (Fig. 9) [106]. The exceptionally low rate constant for methylenecyclopropane indicates that the low solvolysis rates of cyclopropyl derivatives [154] are not only caused by the unfavorable change of hybridization of one ring carbon in cyclopropane but also by the low stability of the cyclopropyl cation relative to a compound with the same hybridization (methylenecyclopropane). The destabilization of the cyclopropyl cation must actually be greater than indicated by the numbers in Fig. 9 as the transition state of the electrophilic attack may already profit from the stabilizing ring-opening process (cf., Section III.B.2). 

Macroscopic solvent effects can be described by the dielectric constant of a medium, whereas the effects of polarization, induced dipoles, and specific solvation are examples of microscopic solvent effects. Carbenium ions are very strong electrophiles that interact reversibly with several components of the reaction mixture in addition to undergoing initiation, propagation, transfer, and termination. These interactions may be relatively weak as in dispersive interactions, which last less than it takes for a bond vibration (<10 14 sec), and are thus considered to involve "sticky collisions. Stronger interactions lead to long-lived intermediates and/or complex formation, often with a change of hybridization. For example, onium ions are formed with -donors. Even stable trityl ions react very rapidly with amines to form ammonium ions [41], and with water, alcohol, ethers, and esters to form oxonium ions. Onium ion formation is reversible, with the equilibrium constant depending on the nucleophile, cation, solvent, and temperature (cf., Section IV.C.3). 

In this section, the reactions of 1,3-dithioles and 1,3-dithiolanes, in which at least one ring atom is involved in any chemical transformation or undergoes a change of hybridization (cycloaddition reactions), are described. 

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