Protonation of a carbocation-anion pair

A Global scheme for solvolysis 2 Clocks for reactions of ion pairs 3 Addition of solvent to carbocation-anion pairs i Protonation of a carbocation-anion pair 11 Isomerization of ion pair reaction intermediates Reorganization of ion pairs in water 13 Internal return of isotopically labeled ion pairs Racemization of ion pairs 22 Concluding remarks 24 Acknowledgements 24 References 24... 

Electrophiles (E) are reagents that are able to accept a pair of electrons whereas nucleophiles (Nu) are reagents that can donate a pair of electrons. Both processes ultimately form a covalent bond. The simplest electrophile is the proton, H", and the simplest nucleophile is the hydride anion, H. Carbocations containing a vacant orbital, and carbanions with their unshared pair of electrons, are the most obvious examples of electrophiles and nucleophiles. 

All elimination routes are improved by this swap of oxygen for carbon (Fig. 4.29). The ElcB proton transfer step now creates a very stable oxyanion, an anion on a very electronegative oxygen atom. The slow step of the El, loss of the leaving group, is also sped up because the oxygen lone pair can stabilize the carbocation formed. All routes have formed a very strong C=0 bond. The primary determinant at this point, since all routes are reasonable, is the pH of the reaction medium. The ElcB is found in base because the oxyanion is basic (pA"abH is about 12 to 16). The El is found in acid because the lone pair stabilized carbocation is just a protonated carbonyl (pA a is near -7). The E2 is found in more neutral media. 

A number of years ago G. N. Lewis extended our understanding of acid-base behavior to include reactions other than proton transfers. According to Lewis, an acid is an electron-pair acceptor and a base is an electron-pair donor. Thus, carbocations are electron-pair acceptors and are Lewis acids. Halide anions are electron-pair donors and are Lewis bases. It is generally true that electrophiles are Lewis acids, and nucleophiles are Lewis bases. 

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