Drawing the Complete Mechanism of an E2 Process

In the previous section, we saw that additional steps can accompany an El process. In contrast, an E2 process consists of one concerted step and is rarely accompanied by any other steps. A carbocation is never formed, and therefore, there is no possibility for a carbocation rearrangement. In addition, E2 conditions generally require the use of a strong base, and an OH group cannot be protonated under such conditions. It is therefore not common to see an E2 process with a proton transfer at the beginning of the mechanism. It is much more common to see a proton transfer at the beginning of an El process (a dehydration reaction). All of the E2 processes that we will encounter in this textbook will be comprised of just one concerted step, as seen in Mechanism 8.2. 

An E2 process is comprised of just one concerted step in which the base abstracts a proton and the ieaving group ieaves at the same time 

The E2 mechanism requires only three curved arrows, all of which must be carefully placed. When drawing the first curved arrow, the tail is placed on a lone pair of the base and the head is drawn pointing to a proton at the (3 position. When drawing the second curved arrow, the tail is placed on the C—H bond that is breaking, and the head is placed on the bond between the a and (3 positions. Finally, the third curved arrow is drawn to show the loss of the leaving group. 

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