Module 3. Carboxylic acids

Lesson 7

7.1 Introduction
  • Organic compounds containing carboxylic (-COOH) group (s) are called carboxylic acids
  • They are represented by general formula R-COOH, where R may be
  • Aliphatic, alicyclic or aromatic
  • Saturated or unsaturated
  • Substituted or unsubstituted
  • The common names of aliphatic acids are generally related to their source rather than to their structure e.g.
Formic acid = Latin: formica means red ants
Acetic acid = Latin : acetum means vinegar (Acetobacter acetii)
Caprylic, caproic and capric acids = Latin: caper means goat
  • Branch-chain acids and substituted acids = named as derivatives of straight-chain acids
  • The position of the branch/substituent are indicated by Greek prefixes α-,β-,γ-, δ-, etc (Fig. 7.1: Trivial names)

7.2 Structure Of Carboxylate Ion And Carboxylic Acid
  • A carboxylic acid is a resonance hybrid of structure I & II

Fig. 7.2 Resonance structure

  • Due to positive charge on the oxygen atom of the hydroxyl group there would be a displacement of the electron pair of the O-H bond towards the oxygen atom
  • This would facilitate the release of hydrogen as a proton

Fig. 7.3 Acidic nature

  • Since similar resonance is not possible in alcohols, the proton in alcohol would be released with greater difficulty
  • Therefore alcohols are much weaker acids than carboxylic acids
  • The carboxylate anion is also resonance-stabilized

Fig. 7.4 Resonance stability of carboxylate ion

  • Since the resonance hybrid of the carboxylate anion involves two equivalent contributing structures, the negative charge is evenly distributed over both the oxygen atoms.
  • The carbon in carboxylate anion is literally joined to each oxygen atom “by one and a half” bond. Neither it is a pure C-O single bond nor it is >C=O double bond.

Fig. 7.5 Resonance hybrid structure
  • This view is supported by the evidence of bond length
  • Both the carbon-oxygen bonds in the carboxylate ion has same bond length (1.27 °A) rather than 1.21°A for C=O and 1.41 °A for C-OH
  • As the double bond character of the C=O linkage in carboxylic acid and carboxylate is reduced considerably, they do not show characteristic of the carbonyl group
  • Do not form oximes, phenylhydrazones, semicarbazones, etc.
7.3 Acidity Of Carboxylic Acids


Fig. 7.6 Resonating structures

  • Carboxylate ion is resonance stabilized
  • Both the structure are equivalent, the resonance energy of the anion is much greater than that of the acid
  • Loss of a proton from the carboxylic acid produces a more stable structure
  • Thus accounts for an easy release of the protein from the acid
  • Ionization of carboxylic acid

According to law of mass action, the equilibrium constant, K is expressed as


Fig. 7.7 Acidity of carboxylic acid

  • The value of Ka – ionization constant of the acid
  • Directly proportional to ionized entity - [RCOO-] and [H+]
  • Inversely proportional to unionized acid - [RCOOH]
  • Greater the concentration of H+ - large the value of Ka stronger will be the acid
  • Ka would be a true index of the acidity – acid strength
  • Now a days Ka values are replaced by pKa values ; pKa= -logKa
  • Each carboxylic acid has a characteristic value of Ka or pKa
  • Simple unsubstituted aliphatic and aromatic have Ka = about 10-5
These acids, compared with mineral acids (Ka = 108) are weak

Last modified: Thursday, 27 September 2012, 6:26 AM