monofunctional carbanion chemistry

monofunctional carbanion chemistry - Monofunctional...

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Monofunctional Carbanion Chemistry This guide to the lecture material and content of monofunctional carbanion chemistry has been broken up into several broad categories which have then been arranged by page. This should NOT be used instead of lecture material, or relied solely upon for information, and I take no responsibility for any errors contained herein. 1. Contents and Nomenclature 2. Introduction to Carbanions Stability Wittig 4. Enols and enolates Formation Enolizability of carbonyls 5. Ways of Stabilising Enolates and modifying reactivity Silyl enol ethers Enamines 7. Alkylation of enolates Enolate addition Aldol reaction Cross-aldol, and ways of preventing self-condensation Reformatsky and special aldol variants 11. Other reactions of enols and enolates Halogenation Deuteration 12. Organometallics in Chemistry Transmetallation Kinetic and Thermodynamic products Important Nomenclature before beginning H O alpha beta gamma H O 1 2 3 4 1 The left diagram shows the nomenclature that you should properly familiarise yourself with before reading this document. In the case of calling carbons alpha, beta or gamma, I will use their Greek letter counterparts; α, β, and γ- carbons.
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Carbanions Negatively Charged Carbon “C - General Structure : “R 3 C - The extent to which the negative charge on carbon is stabilised is affected by several factors – Hybridisation H sp hybridised pka= ~25 R R' R'' sp2 hybridised pka=~44 R R' R'' R'''' R''' sp3 hybridised pka=~52 As orbitals gain more s-character, the charge is held more tightly toward the carbon nucleus, and hence gives a more stabilised charge. Inductive effects H Cl Cl Cl H pKa=~52 pKa=~40 Cl Cl Cl Electronegative chlorine pulls electron density away from the carbon, making the carbanion more stable. Resonance delocalisation H H B pKa=~52 pKa=~40 The ability to place the charge on four other carbon atoms somewhat stabilises the removal of a proton. Delocalisation onto electronegative atoms via conjugation The ability to delocalise charge onto electronegative atoms allows for another increase in stabilisation. The electronegativity of, for example, oxygen in a ketone, allows for a lower energy delocalisation to occur and a lower energy intermediate to be formed. 2 Because of their negative charge, carbanions are basic, which is a thermodynamic property. They are also nucleophilic, which is a kinetic property.
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O H B O pKa=~20 O This is a very important reaction, and many reactions seen in organic chemistry involve reactivity at the α-carbon next to a carbonyl, or carbonyl analogue (e.g. imines, nitro groups and nitrile-groups.) The α and β-carbons in the enolate are both sp 2 hybridised, and hence the negative charge may be shared across the three atoms (though by resonance, it is only on the oxygen and α-carbon). In enolates, the α-carbon becomes nucleophilic, allowing for reactions to occur here.
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This note was uploaded on 02/25/2012 for the course ECO 101 taught by Professor Staff during the Fall '10 term at Texas State.

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monofunctional carbanion chemistry - Monofunctional...

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