Organic Chemistry

Module 3. Carboxylic acids

Lesson 7
IONIZATION CONSTANT AND STRENGTH OF CARBOXYLIC ACIDS

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 K_a – 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 K_a stronger will be the acid
• K_a would be a true index of the acidity – acid strength
• Now a days K_a values are replaced by pK_a values ; pK_a= -logK_a
• Each carboxylic acid has a characteristic value of K_a or pK_a
• Simple unsubstituted aliphatic and aromatic have K_a = about 10-5

These acids, compared with mineral acids (K_a = 108) are weak