Carboxylic Acid

• Carboxylic acids


  A class of organic compounds with the general formula, RCOOH where R can be an alkyl or aryl group
• Nomenclature


  Carboxylic acids are named using the IUPAC convention
• IUPAC nomenclature for carboxylic acids


  1. Use the name of the alkane that corresponds to the longest continuous chain of carbon atoms

  2. Replace the final -e in the alkane name with the suffix -oic acid

  3. Number the chain starting with the carboxyl carbon atom to give positions of substituents
• The carboxyl group takes priority over any other functional groups in naming
• Naming unsaturated carboxylic acids


  1. Use the name of the corresponding alkene

  2. Replace the final -e with -oic acid

  3. Number the chain starting with the carboxyl carbon and indicate the location of the double bond
• Carboxylic acid is approximately planar with the sp2 hybridised carbonyl carbon atom in a nearly trigonal bond angle
• The O-H bond lies in the same plane as the carbonyl C=O bond in the most stable eclipsed conformation
• Carboxylic acids


  • Bond lengths and angles
• Boiling point


  Carboxylic acids boil at considerably higher temperatures than alcohols, ketones or aldehydes of similar molecular weights due to dimerisation
• Carboxylic acids containing more than 8 carbon atoms are generally solids, unless they contain double bonds
• The boiling point of carboxylic acids increases down the homologous series due to an increase in molecular weight
• The presence of double bonds, especially cis-double bonds in a long chain, impedes the formation of a stable crystal lattice, resulting in a lower melting point
• Stearic acid and linoleic acid


  • Both have 18 carbon atoms but stearic acid melts at 70°C and linoleic acid melts at -5°C
• Solubility


  Lower molecular weight carboxylic acids (up through 4 carbon atoms) are miscible with water due to hydrogen bonding, but solubility decreases as the hydrocarbon chain length increases
• Acidity measurement


  • Carboxylic acids may dissociate in water to yield a proton and a carboxylate ion, with the equilibrium constant called the acid-dissociation constant (Ka)
  • The pKa is the negative logarithm of Ka, with larger Ka indicating greater acidity
• Substituent effects on acidity


  • Electronegative substituents that stabilise the carboxylate ion enhance dissociation and result in a stronger acid
  • Substituents on the α-carbon are most effective in increasing acid strength, with more distant substituents having smaller effects
• Dicarboxylic acids


  Have two carboxyl groups, with two acid dissociation constants (Ka1 and Ka2)
• Preparation of carboxylic acids


  1. Oxidation of primary alcohols to aldehydes then carboxylic acids

  2. Oxidation of secondary alcohols to ketones (not further oxidised)

  3. Carbonation of Grignard reagents

  4. Hydrolysis of nitriles
• Reactions of carboxylic acids


  1. Undergo nucleophilic acyl substitution, unlike aldehydes and ketones which undergo nucleophilic addition

  2. Esterification with alcohols

  3. Conversion to acid chlorides then amides

  4. Reduction to alcohols using metal hydrides like LiAlH4

  5. Reaction with bases to form salts
• Carboxylic acid derivatives


  Obtained by replacing the OH group with various substituents like acid chlorides, anhydrides, esters, and amides