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Lecture Organic chemistry - Chapter 19: Carboxylic acids

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Lecture Organic chemistry - Chapter 19: Carboxylic acids. The following will be discussed in this chapter: Naming: alkanoic acids, resonance, acidity, oxidation of primary alcohols and aldehydes, carbonation: organometallic reagents and carbon dioxide, nitrile hydrolysis,.and other contents. | Chapter 19: Carboxylic Acids Carboxy group: -COOH, -CO2H, Carboxylic acids take precedence over other groups: Naming: Alkanoic Acids IUPAC: Replace –e of alkane name with –oic acid 4-Methylhexanoic acid C1 Cyclic: Cycloalkanecarboxylic acids Cyclohexanecarboxylic acid 1-Naphthalenecarboxylic acid C1, as in cyclic aldehydes HCOOH Methanoic acid—Formic acid accepted CH3COOH Ethanoic acid—Acetic acid accepted CH3CH2COOH Propanoic acid Etc. Resonance Ketones versus Carboxylic Acids: Acidity The carboxy group is relatively acidic: Acetate ion Reasons: 1. Carbonyl carbon is inductively strongly electron withdrawing, 2. Carboxylate ion is stabilized by resonance Compare 2-Propenyl (allyl) BH + pKa ~ 40 + Electron withdrawing groups increase the acidity (decrease pKa): Distance affects acidity: pKa 4.19 pKa 3.98 CH3COOH pKa = 4.76 CF3COOH pKa = 0.23 Basicity Protonated on the carbonyl oxygen: Allows for allylic resonance 1. Oxidation of primary alcohols and aldehydes With KMnO4; or CrO3, . | Chapter 19: Carboxylic Acids Carboxy group: -COOH, -CO2H, Carboxylic acids take precedence over other groups: Naming: Alkanoic Acids IUPAC: Replace –e of alkane name with –oic acid 4-Methylhexanoic acid C1 Cyclic: Cycloalkanecarboxylic acids Cyclohexanecarboxylic acid 1-Naphthalenecarboxylic acid C1, as in cyclic aldehydes HCOOH Methanoic acid—Formic acid accepted CH3COOH Ethanoic acid—Acetic acid accepted CH3CH2COOH Propanoic acid Etc. Resonance Ketones versus Carboxylic Acids: Acidity The carboxy group is relatively acidic: Acetate ion Reasons: 1. Carbonyl carbon is inductively strongly electron withdrawing, 2. Carboxylate ion is stabilized by resonance Compare 2-Propenyl (allyl) BH + pKa ~ 40 + Electron withdrawing groups increase the acidity (decrease pKa): Distance affects acidity: pKa 4.19 pKa 3.98 CH3COOH pKa = 4.76 CF3COOH pKa = 0.23 Basicity Protonated on the carbonyl oxygen: Allows for allylic resonance 1. Oxidation of primary alcohols and aldehydes With KMnO4; or CrO3, H2O; or HNO3; or H2O2 Recall Cr(VI) oxidation: Preparation In H2O: Forms hydrate, which oxidizes to acid 2. Carbonation: Organometallic reagents and carbon dioxide Example: Synthetic strategy: RH → RX → RMgBr → RCO2H 3. Nitrile hydrolysis Mechanisms later, but for the time being, think of it as a triple hydration of C N: With aqueous acid or base, e.g. NaOH, H2O: Cyanohydrin-hydrolysis: 2-Hydroxy acids Reactions Nucleophilic substitution occurs by addition-elimination Lead to carboxylic acid derivatives: General: :Nu E+ Leaving group Elimination Nucleophilic Substitution by Addition- Elimination Tetrahedral intermediate Addition : : Potential problem: Acidity Acid or base catalyzed Nu- acting as a base Base Catalyzed Mechanism Must not compete with :Nu- Acid Catalyzed Mechanism 1. –H+ 2. +H+ Synthesis of Carboxylic Acid Derivatives A. Acyl Halides: + -Cl + -OH More stable Less stable Poor Nu Bad leaving group, strong base, good Nu uphill Therefore use other reagents: SOCl2, PCl5, PBr3 .