The Unimolecular Elimination Reaction El

This is a stepwise mechanism involving an initial cleavage of the carbon-halogen bond to form an intermediate carbocation. The reaction is first order, or unimolecular, as the rate depends on the concentration of only the alkyl halide (and not the base). 

The reaction does not require a particular geometry the intermediate carbocation can lose any proton from an adjacent position. 

Tertiary halides react more rapidly than primary halides, because the intermediate carbocation is more stable and therefore more readily formed. 

El eliminations give predominantly the more stable (or more substituted) alkene. The transition state (for the loss of a proton from the intermediate carbocation) leading to the more substituted alkene will be lower in energy. 

El eliminations give predominantly the /Calkene rather than the Z-alkene. The /C alkene is more stable than the Z-alkene for steric reasons (i.e. the bulky substituents are further apart in /Calkenes). The transition state leading to /Calkcncs will therefore be lower in energy. 

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We note that in Eq. 13-11 we have introduced the El (elimination, unimolecular) reaction, which commonly competes with the SN1 reaction provided that an adjacent carbon atom carries one or several hydrogen atoms that may dissociate. We also note that similar to what we have stated earlier for nucleophilic substitution reactions, elimination reactions may occur by mechanisms between the E2 and El extremes. 

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