E1 elimination

Thomson MCW Click Organic Process to view an animation showing the mechanism of an E1 elimination reaction. 

Fig. 9.1. Simplified reaction mechanisms in the hydrolytic decomposition of organic nitrates. Pathway a Solvolytic reaction (Reaction a) with formation of a carbonium ion, which subsequently undergoes SN1 addition of a nucleophile (e.g., HO ) (Reaction b) or proton E1 elimination to form an olefin (Reaction c). Pathway b HO -catalyzed hydrolysis (,SN2). Pathway c The bimolecular carbonyl-elimination reaction, as catalyzed by a strong base (e.g., HO or RO ), which forms a carbonyl derivative and nitrite.

The product is predominantly the A2-olefin, which is neither the more stable isomer (the equilibrated mixture is 2 1, A3 A2) nor the product expected from E1 elimination (this would be the A3-olefin). It is noteworthy that this elimination was conducted in the absence of pyridine. 

Dehydration of alcohols (Sections 5.9-5.13) Dehydration requires an acid catalyst the order of reactivity of alcohols is tertiary > secondary > primary. Elimination is regioselective and proceeds in the direction that produces the most highly substituted double bond. When stereoisomeric alkenes are possible, the more stable one is formed in greater amounts. An E1 (elimination unimolecular) mechanism via a carbo-cation intermediate is followed with secondary and tertiary alcohols. Primary alcohols react by an E2 (elimination bimolecular) mechanism. Sometimes elimination is accompanied by rearrangement. 

EXAMPLE E1 elimination of bromocyclohexane in methanol. Step 1 Ionization gives a carbocation and bromide ion in a slow step. 

Alcohol dehydrations usually go through E1 elimination of the protonated alcohol. 

E1 elimination from quaternary ammonium ions is also known. Competing with either E2 or El elimination there is, as usual, substitution either 8 2 or 8 1. (Problem What products would you expect from substitution )... 

One of the most commoidy used methods for forming carbon-carbon double bonds is by -elimination reactions of the types shown in Scheme 2.1, where X = e.g. OH, OCOR, halogen, OSO2R, NRa, etc. Included among these reactions are acid-catalysed dehydrations of alcohols, solvolytic and base-induced eliminations from alkyl halides or sulfonates and the Hofmann elimination from quaternary ammonium salts. They proceed by both E2 (elimination bimolecular) and E1 (elimination... 

We have left detailed discussion of the formation of alkenes until this chapter, but we used the term elimination in Chapters 10 and 11 to describe the loss of a leaving group from a tetrahedral intermediate. For example, the final steps of acid-catalysed ester hydrolysis involve E1 elimination of ROH to leave a double bond C=0 rather than C=C. 

In Chapter 11 you even saw an E1 elimination giving an alkene. That alkene was an enamine—here is the reaction. 

This explanation of both stereo and regioselectivity in E1 reactions is based on kinetic arguments— which alkene forms faster. But it is also true that some E1 eliminations are reversible the alkenes may be protonated in acid to re-form carbocations, as you will see in the next chapter. This reprotonation allows the more stable product to form preferentially under thermodynamic contio. In any individual case, it may not be clear which is operating. However, with E2 reactions, which follow, only kinetic control applies E2 reactions are never reversible. 

Dehydration results from E1 elimination of the protonated alcohol Step I Protonation converts the hydroxyl group to a good leaving group. 

The E1 Elimination Reaction Dehydration of 2-Butanol to Yield 1 -Butene, trans-2-Butene, and cis-2-Butene... 

With a weak base that is a good to moderate nucleophile, Sf 2 will dominate. Examples are those nucleophiles that have conjugate acids with pA//s below 11. However, because the alkyl group is secondary, the Sf 2 reaction may be sluggish, and Sj. 1 and E2/E1 elimination pathways may compete to a small extent. 

Like E1 elimination, E1cB requires two steps. Unlike E1, though, the intermediate in E1 cB is a carbanion, not a carbocation. E1cB stands for Elimination, unimolecular, conjugate base. 

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