Chem 335 - Winter 1999 - Organic Chemistry II
Portland State University - Dr. Carl C. Wamser

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Chapter 16 - Carboxylic Acids

Carboxylic Acids and Derivatives

distinguished from aldehydes and ketones because one substituent is NOT C or H
(usually Cl, O, or N, but it could also be other F, Br, I, S, P, or many other possibilities)

Nomenclature of Carboxylic Acids

• alkanoic acid (-oic acid suffix)
  when the -COOH is part of the parent chain
  assumed #1 position
  
• cycloalkanecarboxylic acid
  when the -COOH is not part of the parent name
  assumed attached to the #1 position
  
• common names
  formic acid (methanoic acid)
  acetic acid (ethanoic acid)
  notice the prefixes are the same as for aldehydes
  
• acyl group names
  needed for naming some derivatives
  formyl (methanoyl)
  acetyl (ethanoyl)
  
• carboxylate salts
  metal alkanoate (-oate suffix for anion)

Structure and Properties of Carboxylic Acids

• very polar functional group with H-bonding
  typically the boiling points are relatively high
  (formic acid, b.p. 100.7°)

Acidity of Carboxylic Acids

• stronger acids than alcohols
  pKa ~ 5
  (alcohol pKa values about 15-18 )
  
• electron-withdrawing effects of the carbonyl group tends to further polarize the O-H bond so it is more easily ionized
• the carboxylate anion is stabilized by resonance

• substituent effects
  electron-withdrawing groups (like Cl) increase acidity

Carboxylate Salts

• carboxylic acids generally dissolve in weak base (NaHCO3)
• salts are water-soluble for the smaller carboxylic acids
• salts of the large carboxylic acids are soaps
  soaps have a hydrophilic (ionic) part
  and a hydrophobic (hydrocarbon) part

Spectroscopy of Carboxylic Acids

• mass spec - alpha cleavage & McLafferty rearrangement
• NMR - COOH at 10-13 ppm (H) or 160-180 ppm (C-13)
• IR - broad C=O at about 1700 cm-1 and very broad O-H at about 3000-3400 cm-1 (H-bonding)

Synthesis of Carboxylic Acids

• oxidations
  primary alcohols to carboxylic acids using CrO3 , other oxidants
  aldehydes to carboxylic acids using Tollen's reagent (Ag+)
  
• hydrolysis of nitriles
  alkyl halide to nitrile by SN2 substitution
  followed by hydrolysis with acid or base catalysis
  
• addition of CO2 to a Grignard reagent

Reactions of Carboxylic Acids

• reduction to primary alcohols by LiAlH4
  unreactive with catalytic hydrogenation or with NaBH4
  
• conversion to acid chlorides with SOCl2
  conversion to other derivatives via the acid chloride
  
• diazomethane reaction - methyl esters

Fischer esterification

• conversion to esters with alcohol and acid catalysis
  
• a reversible equilibrium
  favored to the right with excess alcohol
  favored to the left with excess water (hydrolysis)
  
• see the complete mechanism in the homework answers

Decarboxylation

• beta-keto acids and beta-dicarboxylic acids lose CO2 readily
  cyclic H-bonded transition state requires a beta C=O