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Carbocations solvolysis

A substantial body of evidence indicates that allylic carbocations are more stable than simple alkyl cations For example the rate of solvolysis of a chlonde that is both tertiary and allylic is much faster than that of a typical tertiary alkyl chloride... [Pg.391]

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. [Pg.306]

Nucleophilic substitution reactions that occur imder conditions of amine diazotization often have significantly different stereochemisby, as compared with that in halide or sulfonate solvolysis. Diazotization generates an alkyl diazonium ion, which rapidly decomposes to a carbocation, molecular nitrogen, and water ... [Pg.306]

Negative evidence for a common intermediate is just as important, for it can thereby eliminate a contending mechanism. The solvolysis of 2-halo-2,3,3-trimethylbutanes in methanol provides such an example.17 If it occurs by the elimination of a carbocation, the intermediate should undergo elimination and substitution reactions independent of the identity of the halide. These are shown as follows ... [Pg.111]

An example of a reaction series in which large deviations are shown by — R para-substituents is provided by the rate constants for the solvolysis of substituted t-cumyl chlorides, ArCMe2Cl54. This reaction follows an SN1 mechanism, with intermediate formation of the cation ArCMe2 +. A —R para-substituent such as OMe may stabilize the activated complex, which resembles the carbocation-chloride ion pair, through delocalization involving structure 21. Such delocalization will clearly be more pronounced than in the species involved in the ionization of p-methoxybenzoic acid, which has a reaction center of feeble + R type (22). The effective a value for p-OMe in the solvolysis of t-cumyl chloride is thus — 0.78, compared with the value of — 0.27 based on the ionization of benzoic acids. [Pg.496]

Non-Kolbe electrolysis may lead to a large product spectrum, especially when there are equilibrating cations of about equal energy involved. However, in cases where the further reaction path leads to a particularly stabilized carbocation and either elimination or solvolysis can be favored, then non-Kolbe electrolysis can become an effi-yient synthetic method. This is demonstrated in the following chapters. [Pg.117]

The rearrangement of the intermediate alkyl cation by hydrogen or methyl shift and the cyclization to a cyclopropane by a CH-insertion has been studied by deuterium labelling [298]. The electrolysis of cyclopropylacetic acid, allylacetic acid or cyclo-butanecarboxylic acid leads to mixtures of cyclopropylcarbinyl-, cyclobutyl- and butenylacetamides [299]. The results are interpreted in terms of a rapid isomerization of the carbocation as long as it is adsorbed at the electrode, whilst isomerization is inhibited by desorption, which is followed by fast solvolysis. [Pg.125]

Additional evidence for the SnI mechanism, in particular, for the intermediacy of carbocations, is that solvolysis rates of alkyl chlorides in ethanol parallel carbocation stabilities as determined by heats of ionization mea.sured in superacid solutions (p. 219). It is important to note that some solvolysis reactions proceed by an Sn2 mechanism." ... [Pg.397]

A bridged carbocation with a two-electron, three-centre bond was proposed as early as 1939 (Nevell et al., 1939) for the 2-norbornyl cation [lO ] as a reactive intermediate in the solvolysis of 2-norbornyl system (see also Winstein and Trifan, 1949). It has now been isolated as the SbFe salt and the bridged structure is accounted for using solid-state nmr studies... [Pg.177]

The structure of nonclassical carbocations, such as norbomenyl 3, has been the subject of debate since the 1950s when Saul Winstein published his milestone studies on the solvolysis of tosylated norbomenyl compounds. It was proposed that the norbomenyl cation should be represented as the nonclassical structure 4+, with a 3-center, 2-electron cyclic system (3c-2e), rather than as the classical equilibrium... [Pg.279]

The kinetics of deuterium isotope exchange between diphenyl phosphine and t-butylthiol have been studied by H n.m.r. spectroscopy.274 A negative temperature coefficient was observed for the reaction of a perf1uoroalky1 phosphite with a fluorinated aldehyde.275 The kinetics for the reaction of alcohols with phosphoryl trichloride bore strong similarities to those of carboxylic acid derivatives.276 An interesting report desribed the solvolysis of ary 1 hydroxymethyl-phosphonates. It was shown that a phosphoryl group does not prevent carbocation formation on an immediately adjacent carbon atom.277... [Pg.416]

Thus solvolysis of (+)C6HsCHMeCl, which can form a stabilised benzyl type carbocation (cf. p. 84), leads to 98% racemisation while (+)C6H13CHMeCl, where no comparable stabilisation can occur, leads to only 34% racemisation. Solvolysis of ( + )C6H5CHMeCl in 80 % acetone/20 % water leads to 98 % racemisation (above), but in the more nucleophilic water alone to only 80% racemisation. The same general considerations apply to nucleophilic displacement reactions by Nu as to solvolysis, except that R may persist a little further along the sequence because part at least of the solvent envelope has to be stripped away before Nu can get at R . It is important to notice that racemisation is clearly very much less of a stereochemical requirement for S l reactions than inversion was for SN2. [Pg.91]

Reference has already been made in the last chapter to the generation of carbocations, in ion pairs, as intermediates in some displacement reactions at a saturated carbon atom, e.g. the solvolysis of an alkyl halide via the SN1 mechanism. Carbocations are, however, fairly widespread in occurrence and, although their existence is often only transient, they are of considerable importance in a wide variety of chemical reactions. [Pg.101]

Such species with a bridging phenyl group are known as phenonium ions. The neighbouring group effect is even more pronounced with an OH rather than an OMe substituent in the p-position. Solvolysis is found to occur % 106 times more rapidly under comparable conditions, and matters can be so arranged as to make possible the isolation of a bridged intermediate (5), albeit not now a carbocation ... [Pg.105]

The carbocation (8) is identical with that from SN1 solvolysis... [Pg.248]

Solvolysis involves the initial formation of a carbocation and the subsequent reaction of that cation with a molecule of the solvent ... [Pg.249]

The interpretation of product data for competitive solvolysis and elimination reactions requires that the mechanism for these reactions be known. Two experiments are sufficient to show that the formation of solvolysis and elimination products occurs by partitioning of a common carbocation intermediate (Scheme 3 a) rather than by competing bimolecular reactions of the substrate (Scheme 3b).3... [Pg.74]

The demonstration that formation of the nucleophile adduct R-Nu results in the same proportional decrease in the yields of the alkene and solvent adducts, so that the ratio of the yields of these reaction products is independent of [Nu-]. If the solvolysis and elimination reactions proceed by competing stepwise and concerted pathways, respectively, then the yield of R-OSolv will decrease with increasing trapping of the carbocation intermediate by added nucleophile, while the yield of alkene from elimination will remain constant, so that the ratio [R-OSolv]/[Alkene] will decrease as [Nu ] is increased. [Pg.74]

There are at least two other studies of competitive reactions to form the products of solvolysis and elimination reactions that may provide insight into the relationships between carbocation structure and reactivity toward nucleophile addition and deprotonation. [Pg.109]


See other pages where Carbocations solvolysis is mentioned: [Pg.141]    [Pg.445]    [Pg.163]    [Pg.213]    [Pg.258]    [Pg.289]    [Pg.315]    [Pg.525]    [Pg.445]    [Pg.170]    [Pg.133]    [Pg.399]    [Pg.402]    [Pg.409]    [Pg.412]    [Pg.432]    [Pg.435]    [Pg.579]    [Pg.768]    [Pg.1379]    [Pg.166]    [Pg.167]    [Pg.87]    [Pg.393]    [Pg.76]    [Pg.210]   


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