Carboxylic Acids and Reduction/Oxidation • Addition of H (or “H-”) Reduction 2 • Loss of O2 or O carboxylic aldehyde alcohol alkane acid (or ketone) • Loss of H 2 Oxidation • Addition of O2 or O + Neither oxidation nor reduction: Addition or loss of H , H2O, or HX Carboxylic acids are the most oxidized functional group of carbon. Carboxylic Acids via Oxidation Chromate oxidation of primary alcohols: unavoidably Na2Cr2O7 H2SO4 aldehyde carboxylic alcohol acid Oxidation of aldehydes with Tollens’ reagent: 1. Ag2O, NH3 2. H O+ 3 Selective for aldehydes; will not oxidize alcohols. aldehyde carboxylic acid Carboxylic Acids via Oxidation Oxidative cleavage of alkynes: 1. O3 2. H2O internal alkyne Oxidation of benzylic carbons: KMnO4 Purifying Carboxylic Acids by Acid-Base Extraction Acidic and basic organic molecules can be separated from other substances by manipulating their protonation state and solubility. Let’s say we run a reaction. Na2Cr2O7 + unreacted H2SO4 + Na+ 3+ We want this… + Cr(H2O)6 + organic + HSO - …but not these. side products 4 How do we isolate our desired product from the other materials, without using distillation or chromatography? Acid-Base Extraction Step 1: Remove inorganics from organics via differential solubility. a separatory (sept) funnel - Na+ HSO4 Cr(H O) 3+ H2O 2 6 CHCl3 + organic side densities: products (H2O) = 1 g/mL (CHCl3) = 1.48 g/mL A large density difference ensures the two liquids separate completely and quickly. (Much faster than, say, oil and water.) Step 2: Add basic water to deprotonate carboxylic acid, and transfer it to the aqueous phase. - Na+ HSO4 3+ discard Cr(H2O)6 H2O layer + organic + organic side side products products add discard NaOH/ H2O CHCl3 layer + organic side products Step 3: Re-acidify carboxylate to return it to organic solution. Remove solvent in vacuo to yield pure acid. add HCl/ H2O, fresh CHCl3 discard H2O layer remove solvent in rotary evaporator Chemistry student, removing solvent in a “rotovap” Nucleophilic Addition to Carbonyls Heteroatom electron donors add reversibly to C=O groups. Equilibrium driven + to the left by entropy. Also true for Cl , HS , NH3 nucleophiles. Hydrogen anions (“hydride”) and carbon anions add irreversibly to C=O groups. + Reaction driven to the right by instability of - - charge on H , RCH2 . +.