Electron Pushing and Definitions

An example of an E2 mechanism. All bond breaking and formation occur in one single step. 

Acid-catalyzed tautomerization of an aldehyde (see Chapter 11) gives an enol as the starting point. Protonation of the alcohol attached to the (i-carbon makes it a good leaving group. In the second step, a lone pair on the enol OH is used as an electron source to expel the water. Deprotonation of the carbonyl yields the final product in a last step. 

There are also a series of more complex eliminations that you should be aware of (Eqs. 10.63-10.67), although we are not going to look at these in any detail. One is the elimination of 1,2-dihaloalkanes and 1,4-dihaloalkanes (the Grob fragmentation) using Zn to create al-kenes or dienes (Eqs. 10.63 and 10.64, respectively). The first step in both reactions involves the oxidative addition of Zn to a C-X bond, a reaction we will cover in detail in Chapter 12. Other eliminations involve y-amino alkyl halides, which can spontaneously undergo elimination (Eq. 10.65), and the base-induced eliminations of both (3-hydroxyketones (Eq. 10.66, the reverse aldol reaction) and 8-ketoketones (Eq. 10.67, the reverse Michael addition). 

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