Organic Chemistry

Module 3. Carboxylic acids

Lesson 10 A
SUBSTITUTED CARBOXYLIC ACIDS: HALOGEN, KETO AND HYDROXY ACIDS AND THEIR IMPORTANT REACTIONS

10.1 Effect of Substitution on Acidity of Carboxylic Acids

• Any factor that stabilizes the conjugate anion should increase the acidity
• Any factor that destabilizes the conjugate anion should decrease the acidity
• Electron-withdrawing substituent - F, Cl etc - because inductive effect (-I effect)- disperse the negative charge, stabilized the conjugate anion - therefore increase acidity
• Electron-releasing substituent – alkyl group (e.g. CH₃-) - intensify the negative charge on the anion, destabilize the anion - therefore decrease acidity

Fig. 10.1 Effect of substituent

Table 10.1 Dissociation constants of carboxylic acid

• The inductive effect decreases with distance - i.e. increase in chain length causes inductive elective/group to away from the carboxylic group (position of substituent). (Fig. 10.2)

• Aromatic acids (benzoic acid) are similarly affected by substituent

• -Cl, -NO₂, etc make the acid stronger
• -CH₃, C₂H₅- , etc. display both kind of effect depending upon their position (para or meta) of substitution

-Meta position : electron-withdrawing - makes the acid stronger
-Para position : electron-releasing - makes the acid weaker

• Ortho substitution shows unusual behavior

-Unusually large effect
-Makes the acid stronger irrespective of the type of substituent - electron releasing or electron-withdrawing
-Known as ortho effect-reason not completely understood (may be electronic or kinetic reasons).

10.2 Note

• Inductive effect

The process of electron shift along a chain of atoms due to the presence in it of a polar covalent bond is called inductive effect. When the hetero atom is such that it attracts the electron pair to itself, (e.g. chlorine), it is said to exert a – I effect or electron –withdrawing inductive effect. On the other hand when the hetero atom ( or group of atoms) pushes the electrons away from itself, it exerts a +I effect, or electron-releasing inductive effect.

Table 10.2 Effect of substituent position on the ionization constant