Stability ionizing solvents

As a result of the inductive and hyperconjugative effects it is to be expected that tertiary carbonium ions will be more stable than secondary carbonium ions, which in turn will be more stable than primary ions. The stabilization of the corresponding transition states for ionization should be in the same order, since the transition state will somewhat resemble the ion. Thus the first order rate constant for the solvolysis of tert-buty bromide in alkaline 80% aqueous ethanol at 55° is about 4000 times that of isopropyl bromide, while for ethyl and methyl bromides the first order contribution to the hydrolysis rate is imperceptible against the contribution from the bimolecular hydrolysis.217 Formic acid is such a good ionizing solvent that even primary alkyl bromides hydrolyze at a rate nearly independent of water concentration. The relative rates at 100° are tertiary butyl, 108 isopropyl, 44.7 ethyl, 1.71 and methyl, 1.00.218>212 One a-phenyl substituent is about as effective in accelerating the ionization as two a-alkyl groups.212 Thus the reactions of benzyl compounds, like those of secondary alkyl compounds, are of borderline mechanism, while benzhydryl compounds react by the unimolecular ionization mechanism. 

Effect of the Solvent The slow step of the SN1 reaction involves formation of two ions. Solvation of these ions is crucial to stabilizing them and lowering the activation energy for their formation. Very polar ionizing solvents such as water and alcohols are needed for the SN1. The solvent may be heated to reflux (boiling) to provide the energy needed for ionization. 

When allyl bromide is heated with a good ionizing solvent, it ionizes to the allyl cation, an allyl group with a positive charge. More-substituted analogues are called allylic cations. All allylic cations are stabilized by resonance with the adjacent double bond, which delocalizes the positive charge over two carbon atoms. 

The long-term stability of aHF solutions of 02 MF6 salts (M = As, Sb, Bi) at ambient temperatures or below provides for the convenient application of these powerful one-electron oxidizers in an excellent ionizing solvent. A dramatic example is the fast quantitative conversion of Pt(lV) to Pt(V) ... 

We can get a more quantitative feel for the relative stabilities of alkyl carbocations by examining data for the enthalpy of ionization (gas phase) for various alkyl chlorides Of course, each of these reactions is much more endothermic in the gas phase than it would be in solution, where solvent molecules of appropriate polarity characteristics could help to stabilize the electrically charged products of the ionization reaction. (This is why ionizing solvents are often used for reactions that involve charged intermediates.) Nevertheless, the data clearly reflects the order of carbocation stabihty that we have already established tertiary carbocations are the easiest (least endothermic) to form, the secondary, then primary, and the methyl carbocation is the most difficult to form. 

The formation of chloronium compounds or ionization of acceptor chlorides is hardly expected in a solvent of low donor properties. The only exception appears to be phosphorus(V) chloride, because it gives unsolvated [PC1J+ units and thus does not require any stabilization by solvent coordination. 

The chloride ion is a stronger ligand towards class (a) acceptors than the bromide ion. Thus chloro-complexes will be stable in certain solvents, in which bromo-complexes cannot be obtained due to the higher stabilities of solvent-coordinated species. For example cobalt(II) bromide is completely ionized in... 

El reactions involve carbenium ion intermediates, and therefore are facilitated by all the factors that stabilize carbenium ions. These are the same factors that facilitate SnI reactions. Strongly ionizing solvents and substitution of electron donating groups on the carbon undergoing heterolysis are necessary. 

The pairs of chiral polyelectrolytes obtained have similar chemical reactions. Their physical behavior such as potentiometric or optical properties are different in solvents stabilizing associations of lateral functions but become similar in dissociant (acidic medium) or ionizing solvents (basic solvents). However, these changes remain reversible and independent of the molecular weight [89]. ORD curves of the polymer obtained with strong acidic catalyst were similar to that of helical polypeptides. 

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... 

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. 

Stabilization of a carbocation intermediate by benzylic conjugation, as in the 1-phenylethyl system shown in entry 8, leads to substitution with diminished stereosped-ficity. A thorough analysis of stereochemical, kinetic, and isotope effect data on solvolysis reactions of 1-phenylethyl chloride has been carried out. The system has been analyzed in terms of the fate of the intimate ion-pair and solvent-separated ion-pair intermediates. From this analysis, it has been estimated that for every 100 molecules of 1-phenylethyl chloride that undergo ionization to an intimate ion pair (in trifluoroethanol), 80 return to starting material of retained configuration, 7 return to inverted starting material, and 13 go on to the solvent-separated ion pair. 

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