Sjyl reactions

Thus solvolysis of ( + )C6H5CHMeCl, which can form a stabilised benzyl type carbocation (cf. p. 84), leads to 98% racemisation while (-l-)C6Hi3CHMeCl, where no comparable stabilisation can occur, leads to only 34% racemisation. Solvolysis of (-l-lCgHjCHMeCl 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 Sjyl reactions than inversion was for 8 2. 

Y may also be the leaving group in the monomolecular formation of substituted carbocations (60), to yield the saturated derivative 63 (and 63 may equal 62, if Z = Y), as usually indicated in Sjyl reactions. If Z = E = Br, in the dibromo derivative 63, the cation 60 has the same form as that obtained by the bromination of olefins. 

Sjyl reactions always occur with inversion of configuration. 

We have already noticed (p. 86) that the SN2 hydrolysis of 1-bromo-2,2-dimethylpropane (neopentyl bromide, 24) is slow due to steric hindrance. Carrying out the reaction under conditions favouring the Sjyl mode can result in an increased reaction rate but the product alcohol is found to be 2-methylbutan-2-ol (26) and not the expected... 

The Sjyl analogy is reinforced by the fact that added nucleophiles, Cl , Me OH, etc., are found to affect the product composition but not the rate of reaction—just as the above rate law would require. 

Simple kinetic measurements can, however, be an inadequate guide to which of the above two mechanisms, Sjyl or Sjy2, is actually operating in, for example, the hydrolysis of a halide. Thus, as we have seen (p. 45), where the solvent can act as a nucleophile (solvolysis), e.g. H2O, we would expect for an Sjy2 type reaction. 

Changing the solvent in which a reaction is carried out often exerts a profound effect on its rate and may, indeed, even result in a change in its mechanistic pathway. Thus for a halide that undergoes hydrolysis by the S l mode, increase in the polarity of the solvent (i.e. increase in e, the dielectric constant) and/or its ion-solvating ability is found to result in a very marked increase in reaction rate. Thus the rate of solvolysis of the tertiary halide, Me3CBr, is found to be 3 x l(f times faster in 50% aqueous ethanol than in ethanol alone. This occurs because, in the Sjyl mode, charge is developed and concentrated in... 

A kinetic distinction between the operation of the Sjyl and Sjy2 modes can often be made by observing the effect on the overall reaction rate of adding a competing nucleophile, e.g. azide anion, N3 . The total nucleophile concentration is thus increased, and for the S 2 mode where [Nu ] appears in the rate equation, this will result in an increased reaction rate due to the increased [Nu ]. By contrast, for the S vl niode [Nu ] does not appear in the rate equation, i.e. is not involved in the rate-limiting step, and addition of N3 will thus be without significant effect on the observed reaction rate, though it will naturally influence the composition of the product. 

A small jft-carbon-14 isotope effect, (kn/kl4)p = 1.008 0.002, has been found607 in the solvolysis of394 (equation 265) and interpreted as resulting from a mixed S l/El reaction (mostly Sjyl). 

Unimolecular nucleophilic substitution reactions, Sjyl, of tetrahedral molecules follow a pathway (see Figure 18.8) that involves the dissociation of a leaving group X and the production of a planar intermediate which... 

The ionization mechanism has several distinguishing features. The ionization step is rate determining and the reaction exhibits first-order kinetics, with the rate of decomposition of the reactant being independent of the concentration and identity of the nucleophile. The symbol assigned to this mechanism is Sjyl, for substitution, nucleophilic, unimolecular. 

The role of the leaving group in determining the reaction rate is somewhat different from Sjv2 and Sjyl substitution at alkyl groups. In those cases, the bond strength is... 

Sjyl/Sjy2 reactions Relative rates Reactivities... 

Water is assumed to favor ionization and promote Sjyl/El reactions over S]v2/E2 reactions. Other protic solvents tend to favor bimolecular reactions, hut slow ionization is possible with long reaction times. Aprotic solvents favor bimolecular reactions, particularly S]y2 reactions 4,5, 7,8,9,10,11,12,16,17,18,22,23, 24, 26. 

A very nice example is provided by the arylmethyl chlorides ArCH2Cl, where ArH is an even AH. As we have seen, both the Sj l and Sj 2 reactions are accelerated by an increase in the E activity of Ar (Figs. 5.19 and 5.21), but the acceleration is greater in the case of the S yl reaction. Therefore while benzyl chloride hydrolyzes by a mixed mechanism in aqueous ethanol, the corresponding hydrolyses of more reactive arylmethyl chlorides are Sjyl. 

The Ef,l reaction involves the same intermediate carbonium ion as an Sjyl replacement of Y and its rate is governed by similar considerations. Likewise, the ElcB process involves an initial step analogous to a prototropic reaction (Y replacing hydrogen). Both reactions are of EOg type and the effect of structure on their rates can be predicted in the same way as the rates of S,yl reactions (p. 237) or of deprotonations by base (p. 243). Thus the Effl reaction will be favored by -I, E, and —E substituents a to X and the lcB reaction by -H/, E, or -f substituents a to Y. Indeed, elimination reactions involving a proton a to a powerful -f group such as acyl always take place by the ElcB mechanism, as in the conversion of )S-chloroethyl ketones to vinyl ketones. 

The reaction does not involve a normal Sjyl or substitution but takes place "" by the following chain mechanism involving intermediate radical anions ... 

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