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Transcript
Reactions at Benzylic Carbons Oxidation: KMnO4 will convert any benzylic carbon to –COOH. KMnO4, -OH 100 °C Reduction: Benzylic functional groups readily reduced all the way to hydrocarbon. (X = halogen, -OH, -OR) H2, Pd-C or LiAlH4 Carboxylic Acids and Reduction/Oxidation • Addition of H2 (or “H-”) • Loss of O2 or O Reduction carboxylic acid aldehyde (or ketone) • Loss of H2 • Addition of O2 or O Neither oxidation nor reduction: alcohol alkane Oxidation Addition or loss of H+, H2O, or HX Review: Carboxylic Acids by Oxidation Chromate oxidation of primary alcohols: Na2Cr2O7 unavoidably H2SO4 aldehyde Oxidative cleavage of alkynes: 1. KMnO4 KOH, 2. H3O+ internal alkyne (or O3, H2O) Purifying Carboxylic Acids by Acid-Base Extraction + unreacted Na2Cr2O7 Let’s say we run a reaction. Na+ H2SO4 Cr(H2O)63+ + organic side products How do we isolate our product from the other reaction contents? a separatory (sept) funnel H2O CHCl3 densities: (H2O) = 1 g/mL (CHCl3) = 1.48 g/mL HSO4- Na+ Cr(H2O)63+ + organic side products HSO4- HSO4- Na+ Cr(H2O)63+ discard H2O layer + organic side products + organic side products add NaOH add acidic H2O (HCl) pure!! discard CHCl3 layer + organic side products Nucleophilic Addition to Carbonyls Heteroatom electron donors add reversibly to C=O groups. Equilibrium driven to the left by: + • strength of C=O double bond true for Cl , HS , NH3… • entropy Carbon anions add irreversibly to C=O groups. Reaction driven to the right by: + • instability of charge on RCH2- true for all carbon anions. Relative Stabilities of Carbanions least stable, most reactive most stable, least reactive < sp3 < < < Increasing substitution pushes electron density onto carbon, destabilizes anion (and makes it more basic, and more reactive). sp2 < sp < < All of these anions add irreversibly to carbonyls, + Organometallic Reagents are Synthetically Equivalent to Carbanions alkyllithium (organolithium) Grignard reagent (organomagnesium halide) alkynylsodium Computed electronic structure of CH3Li All of these react as “R - ” Generating Organometallic Reagents 1. Deprotonation. Not common. Works for alkynyl, some sp2 anions, but not for sp3 (because alkanes are not acidic enough). Examples: + + NaNH2 hexane + or (BuLi) (Et2O) + NH3 Generating Organometallic Reagents 2. Metalation of an alkyl halide. Common for Grignard reagents, sp3 alkyllithiums. Examples: + Mg (s) + 2 Li (s) Et2O hexane + LiBr solid metals Generating Organometallic Reagents 3. Lithium-halogen exchange. Common for sp2 alkyllithiums. Not useful for sp3 alkyllithiums. Example: + + 2 commercially available Reaction is driven by formation of more stable carbanion: not commercially available more stable than + + LiBr