Leaving groups ionization path

After rate-determining ionization of a good leaving group, path Dn, the cation is deprotonated by path Dg to produce a pi bond. This elimination is the reverse of the AdgZ addition. As might be expected, the El process competes with the SnI process. If A acts as a base, El occurs if it acts as a nucleophile, SnI occurs. Section 9.5 will discuss such decisions in detail. 

The electrophile is the acylium ion, R-C +, generated by Lewis acid-catalyzed ionization of a leaving group (path Dn) from acyl halides or acid anhydrides (shown in the previous section). The proton that is lost comes from the same carbon that the electrophile attacked. The reaction fails for deactivated rings (Ai wg, meta directors). After the electrophile adds it deactivates the ring toward further attack. No rearrangement of the electrophile occurs. 

The stability of both path Dn product carbocations must be checked the lone-pair-stabilized tertiary cation is much more stable and therefore is favored over the unstabilized primary cation. Ionization of the leaving group (path Dn) creates a somewhat less stable cation than the protonated ketal, so ionization of the leaving group is uphill in energy. 

Solvolysis in Aqueous Systems. In weakly acidic hydroxylic solvents, nitrite ion is a better leaving group than acetate ion, and l,li-adducts typically follow an aromatization path that leads to the formation of an aryl acetate and the elements of nitrous acid. Figure 2. Kinetic studies of solvolysis of the adducts derived from hemimellitene reveal the characteristics of El eliminations. Rates of solvolysis of both the cis and trans-isomers can be correlated to measures of solvent ionizing power such as Grunwald-Winstein Y values. Figure if. Both isomers... 

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