Key Mechanism The E1 Reaction

The abbreviation El staixls for Elimination, unimolecular. The mechanism is called unimolecular because the rate-limiting transition state involves a single molecule rather than a collision between two molecules. The slow step of an EI reaction is the same as in the SnI reaction unimolecular ionization to form a carbocation. In a fast second step, a base abstracts a proton fr n the carbon atom adjacent to the C . The electrons that once fwmed the carbon-hydrogen bond now form a pi bond between two carbon attxns. The general mechanism for the E1 reaction is shown in the following Key Mechanism box. 

The El reaction requires ionization to a carbocation intermediate like the 1, so it fol- 

Step 1 Unimolecular ionization to give a carbocation (rate-limiting). 

Step 2 Deprotonation by a weak base (often the solvent) gives the alkene (fast). 

EXAMPLE El elimination of bromocyclohexane in methanoi. Step 1 Ionization gives a carbocation and bromide ion. 

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