E1+mechanism

Unimolecular Elimination (E1) Mechanism
The E1 mechanism follows two basic steps:
 * 1) leaving group dissociation and carbocation formation;
 * 2) base-induced hydrogen removal and alkene formation.

The first step may also be initiated by acid-catalyzed hydrolysis, as we saw in SN1 reactions.

In fact, the E1 mechanism will typically compete with SN1 if the nucleophile, leaving group, or solvent has some basicity.

Here is a general mechanism for E1:

As shown above, the leaving group (X) undergoes heterolysis to form a carbocation intermediate (in brackets). This is followed by removal of a hydrogen on an adjacent atom by a base, whose electrons collapse towards the carbocation to form an alkene.

Here is another example:



Most nucleophiles can have some basicity to them; only the chloride ion is an exception to this.

Many solvents can also act as bases, especially if they have free lone pairs (e.g. alcohols, water).

Sometimes, just simply heating can do the trick, if the leaving group has basicity.


 * Be aware that it is possible to form both E and Z isomers (depending on structure, of course) in an E1 reaction.**

Here is an E1 mechanism where hydrolysis is the first step:




 * Zaitsev's Rule**

Due to the stability of a more-substituted carbon, Zaitsev's rule states that a hydrogen on a more substituted carbon will be removed more easily by a base. So if given the choice, the alkene will also be more substituted.

Here is an example: