CPR Organic More Elimination

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Chemical Processes and Reactions  - Organic Chemistry - More Elimination

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E1 - Elimination

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Two step reaction – group leaves before deprotonation

Carbocation intermediate depicted 

Substrates with stabilised C+ at leaving group site (3°, Bn C+)

favoured by good leaving group/ poor base

polar solventstabilises C+ 

Compare to S 1

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E1 Energy Profile

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E1 Energies

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Proceeds via cation intermediate - Favoured by a weak base (slows E2),

good leaving group (speeds up C+ formation), Stabilised C+ promotes E1 – in this case benzyl cation

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But also tertiary cation

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Stereochemistry of E1

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E1 gives predominantly trans double bond - thermodynamically more stable double bond.

Loss of H+ step requires C-H bond line up with vacant p-orbital

– i.e. periplanar but it doesn’t matter which way

But also loss of H+ is from low energy intermediate conformation which

keeps substituents furthest apart – gives trans double bond.

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E1cb Elimination Unimolecular from Conjugate Base

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Two step reaction – group leaves after  base rapidly and reversibly deprotonates

Carbanion intermediate depicted

Favoured by a strong base and poor leaving group

Stabilised C- at   −H carbon promotes E1cb

In this case F- is a bad LG but also e- withdrawing so helps stabilise carbanion

Other end of spectrum from E1

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E1cb Energy Profile

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Where Does E1cb Apply?

E1cb applies to substrates with stabilised carbanions – and poor LG

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Base Catalysed Aldol Reaction

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Hoffman versus Saytzev Elimination

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Hoffman – 1851 working on RNMe + - alkene will predominate that has the least substituents on the double bond carbons.

Saytzev – 1875 working on RBr compounds - alkene will predominate that has the most substituents on the double bond carbons.

In general more substitited double bond favoured except when large, +ve charged LG then we get less substituted.

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Steric Factors of Hoffman vs Saytzev

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Electronic Factors of Hoffman vs Saytzev

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Eliminations Without an External Base

Concerted syn Eliminations

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Cope elimination

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