Departure of the leaving group

Substitution nucleophilic bimolecular (Sn2) mechanism (Sec tions 4 12 and 8 3) Concerted mechanism for nucleophilic substitution in which the nucleophile attacks carbon from the side opposite the bond to the leaving group and assists the departure of the leaving group... 

As depicted, the E2 mechanism involves a bimolecular transition state in which removal of a proton to the leaving group is concerted with departure of the leaving group. In contrast, the rate-determining step in the El mechanism is the unimolecular ionization of... 

The factors that determine whether syn or anti elimination predominates are still subject to investigation. One factor that is believed to be important is whether the base is free or present in an ion pair. The evidence is that an ion pair promotes syn elimination of anionic leaving groups. This effect can be explained by proposing a transition state in which the anion functions as a base and the cation assists in the departure of the leaving group. 

In discussing nonclassical carbocations we must be careful to make the distinction between neighboring-group participation and the existence of nonclassical carbocations. ° If a nonclassical carbocation exists in any reaction, then an ion with electron delocalization, as shown in the above examples, is a discrete reaction intermediate. If a carbon-carbon double or single bond participates in the departure of the leaving group to form a carbocation, it may be that a nonclassical carbocation is involved, but there is no necessary relation. In any particular case, either or both of these possibilities can be taking place. 

Furthermore, 48 solvolyzed 350 times faster than its endo isomer 51. Similar high exo/endo rate ratios have been found in many other [2.2.1] systems. These two results—(1) that solvolysis of an optically active exo isomer gave only racemic exo isomers and (2) the high exo/endo rate ratio—were interpreted by Winstein and Trifan as indicating that the 1,6 bond assists in the departure of the leaving group and that a nonclassical intermediate (52) is involved. They reasoned that solvolysis of the endo isomer 51 is not assisted by the 1,6 bond because it is not in a favorable position for backside attack, and that consequently solvolysis of 51 takes... 

In the case of La3 + - and Zn2+-catalyzed methanolysis of the phosphorothioate esters the observed / ig values of —0.87 and —0.74 also signify an associative mechanism with some departure of the leaving group, but it is difficult to assign the extent of the bond cleavage since the /ieq value is not known for the phosphoryl transfer between thiol and oxygen nucleophiles. 

The ElcB mechanism has the same features as the E2 mechanism except that proton abstraction by the base proceeds essentially to completion prior to departure of the leaving group. A variant of this mechanism may intervene whenever the leaving group is a poor leaving group or an exceptionally stable carbanion may be formed (i.e., due to the presence of Z substituents in addition to the polar a bond and/or a hybridization effect). The factors which lead to stabilization of carbanions have been discussed in Chapter 7. 

The initial trigonal bipyramid (TBP) can be formed in two ways from the tetrahedral phosphorus atom either by nucleophilic attack of the hydroxide ion at any of the six edges of the tetrahedron, leaving the nucleophile 1 in an equatorial position (equatorial attack), or by attack at any of the four faces of the tetrahedron, which would set the nucleophile in an apical position (apical attack). Because apical bonds are longer, and therefore weaker, than equatorial bonds, apical attack is favoured since the formation energy of the initial TBP is then lower. In the same way, since apical bonds are weaker they should be more easily broken than their equatorial counterparts, hence the apical departure of the leaving group 2 is favoured. 

Although the reactions shown in Equations 6.3 and 6.4, and in some equations and schemes found later in this chapter depict a primary carbocation as an intermediate, it is not certain whether these highly unstable species exist in solution. Both reactions may involve migration concerted with departure of the leaving group. See (a) P. Ausloos, R. E. Rebbert, L. W. Sieck, and T. O. Tiernan, J. Amer. Chem. Soc., 94, 8939 (1972) and references therein (b) P. C. Hariharan, L. Radom, J. A. Pople, and P. v. R. Schleyer, J. Amer. Chem. Soc., 96, 599 (1974). 

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