chapter_6_-2011-posting

chapter_6_-2011-posting - Carbanions and Other Carbon...

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Carbanions and Other Carbon Nucleophiles Chapter 6 Suggested problems: 1-3, 5,6, 9, 10, 12, 13, 14, 20, 21 1
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6.1. Acidity of Hydrocarbons Carbanions, which are the conjugate bases formed by deprotonation at carbon atoms. C-H Carbanions are very important in synthesis because they are good nucleophiles and formation of new carbon-carbon bonds often requires a nucleophilic carbon species. The basicity of a base-solvent system can be specified by a basicity function H_. The value of H_ corresponds essentially to the pH of strongly basic nonaqueous solutions. The larger the value of H_, the greater the proton-abstracting ability of the medium. 2
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The order of decreasing acidity Ph 3 CH >Ph 2 CH 2 >PhCH 3 , for example, reflects the ability of each successive phenyl group to stabilize the negative charge on carbon. This stabilization is a combination of both resonance and the polar EWG effect of the phenyl groups. 4
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6.2. Carbanion Character of Organometallic Compounds Organolithium and Grignard reagents behave as if they were carbanions and they are therefore very strong bases. They react readily with hydrogen atoms attached to oxygen, nitrogen or sulfur, in addition to other acidic hydrogens (water and alcohol solvents cannot be used) Preparation of Grignard and Organolithum Compounds In solution, organolithium compounds exist as aggregates, with the degree of aggregation depending on the structure of the organic group and the solvent. 6 Tetrameric structures
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7 The reactivity of the organolithium compounds is increased by adding molecules capable of solvating the lithium cations. Tetramethylenediamine (TMEDA) is commonly used for organolithium reagents.
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6.3. Carbanions Stabilized by Functional Groups 8 Electron-withdrawing substituents cause very large increases in the acidity of C–H bonds. Among the functional groups that exert a strong stabilizing effect on carbanions are carbonyl, nitro, sulfonyl, and cyano. The anion stabilization order: NO 2 >C=O>CO 2 R∼SO 2 ∼CN>CONR 2
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9 The presence of two EWGs further stabilizes the negative charge. Pentane-2,4- dione, for example, has a pK _______________in water. Deprotonation by base
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10
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11 Rates of enolate formation can be measured by following isotopic exchange using either deuterium or tritium An older technique is to measure the rate of halogenation of the carbonyl compound
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12 Base-promoted halogenation occurs via an enolate, Acid-promoted halogenation occurs via an enol Exercise! please provide a detail mechanism for above transformations Haloform Reaction Reaction of methyl ketones with X2 in the presence of base results in multiple halogenation at the methyl carbon, in aqueous hydroxide the reaction gives a carboxylate anion and a haloform (CX3H) CR' O R 2 CH H CR' OH R 2 CH CR' OH R 2 CH Br 2 Br Br CR' O R 2 C Br CR' O R 2 CH OH CR' O R 2 CH CR' O R 2 CH Br 2 Br Br CR' O R 2 C Br C O CH 3 Ph NaOH, X 2 C O O Ph + HCX 3 Haloform C O CX 3 Ph NaOH
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13 A detail mechanism:
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For alkyl ketones,
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This note was uploaded on 11/27/2011 for the course CHEM 6210 taught by Professor Wang during the Spring '11 term at University of New Orleans.

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chapter_6_-2011-posting - Carbanions and Other Carbon...

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