Carbocation Generation The Role of Protonation

There are three methods for generating carbocations ionization of a C X x bond, reaction of a lone pair on the heteroatom in a C=X bond with a Lewis acid, and reaction of a C=C 77 bond with a Lewis acid such as H+ or a carbocation. 

Carbocations can also be generated by one-electron oxidation of C=C tt bonds. The carbocations generated by these methods are radical cations, they have an odd number of electrons. Reactions involving radical cations are discussed more thoroughly in Chapter 5. 

Common error alert The reaction of esters, amides, and carboxylic acids with electrophiles occurs on the carbonyl O. Many more resonance structures can be drawn when the carbonyl O is protonated than when the noncarbonyl heteroatom is protonated. 

Both the first and second methods for generating a carbocation involve the reaction of a heteroatom with H+ or another Lewis acid. The protonation of a heteroatom converts a 77-bond electrophile into a more potent one, or it converts a mediocre leaving group into a better one. In Chapter 2 we saw that deprotonation of a substrate does not always occur at the most acidic site in polar basic mechanisms. Similarly, in polar acidic mechanisms, protonations do not always occur at the most 

A heteroatom in a C=X bond can use its lone pair to react with a Lewis acid to give a product for which a carbocationic resonance structure can be drawn. Protonation of a carbonyl compound belongs in this category. One of the lone pairs on O coordinates to to give a compound with two major resonance structures, one of which is carbocationic. The carbocationic resonance structure is not the best structure, but it tells the most about the reactivity of the ion. If the carbonyl C has heteroatoms directly attached, more resonance structures can be drawn, e.g., for esters and amides. Reactions of imines (Schiff bases) often begin by protonation of N. 

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