Module 3. Carboxylic acids

Lesson 9


9.1 Introduction
  • Class of organic compounds which are derived from carboxylic acids by replacing –OH of the –COOH group with other atom or group
  • include acid chlorides, acid anhydrides, acid amides and esters
  • Called functional derivatives of carboxylic acids
  • In acyl compounds group is common. Figure_9.1.swf
9.1.1 Nomenclature
  • Names are taken from corresponding carboxylic acid (Fig. 9.2)

9.2 Acid Chlorides
  • Prepared by interaction of carboxylic acids with thionyl chloride (SOCl2), phosphorus thrichloride (PCl3) or phosphorus pentachloride (PCl5)

9.3 Reactions
  • Typically undergo necleophilic substitution
  • Chlorine is expelled as chloride and its place is taken by other basic group
  • Because of carbonyl group these reactions take place much more rapidly than the corresponding alkyl halide
  • Acid chloride are the most reactive of the derivatives of carboxylic acids therefore used as acylating agents (Fig. 9.3)
9.4 Acid Anhydride
  • Acetic anhydride is immensely important
  • Prepared by (Fig. 9.4)
9.4.1 Reactions
  • Undergo the same reactions as acid chlorides, but
  • A little more slowly
  • Yield a molecule of carboxylic acid (instead of HCl)
  • Anhydrides of acids more convenient acylating agents
  • Acetic anhydride is cheap, less volatile and more easily handled than acetyl chloride and does not form corrosive HCl (Fig. 9.5)
9.5 Acid Amides (Fig. 9.6)

9.5.1 Reactions

  • The electron-deficiency of the carbonyl carbon is somewhat made up by the electron donor action of the nitrogen atom

Fig. 9.7 Resonating structures of acid amides

Nucleophilic attack on acyl carbon is difficult than the acid chloride - therefore they are not used as acylating agents
  • They are not even hydrolysed by water
  • However, they are hydrolysed by heating in presence of dilute acid or alkali

Fig. 9.8 Hydrolysis
  • Amides may be expected to show basic character like amines, but they are almost neutral compounds
  • Because the electron pair on N in CONH2 group is not normally available for protonation
9.6 Esters
  • Widely distributed in nature
  • The fragrance of fruits, flowers and essential oils- largely due to esters
  • Fats and oils are esters of higher fatty acids with glycerol
  • Waxes are also high molecule weight esters
9.6.1 Preparation

1. By action of carboxylic acids on alcohol
  • Boiling an acid with an alcohol in presence of acid catalyst (e.g. H2SO4)

Fig. 9.9 Esterification

  • Relative reactivity of carboxylic acids and alcohols – markedly dependent on their structure
  • Greater the bulk of the substituent near –OH and /or –COOH group, the slower the reaction rate – because steric hindrance.
  • Relative reactivity order in esterification
Alcohols – CH3OH > CH3CH2OH > (CH3)2CHOH > (CH3)3COH (1° >2°>3°)

2. From acid chlorides by alcoholysis


Fig. 9.10 Alcoholysis
  • This reaction is not reversible - therefore faster than that of the direct esterification reaction – therefore better yield of the product i.e. ester
  • Phenyl esters and esters of sterically hindered acids and/or alcohols are generally prepared by this method
3. From acid anhydride by alcoholysis


Fig. 9.11 Reaction of acid anhydride with aldohol

4. From esters by trans-esterification
  • In esterification of an acid, an alcohol act as a nucleophilic reagent
  • In hydrolysis of an esters, an alcohol is displaced by a nucleophilic reagent
  • These observation gave an idea – that the alcohol present in an ester can be displaced by another alcohol to form new ester
  • This alcoholysis (cleavage by an alcohol) of an ester is called transesterification

Fig. 9.12 Transesterification

9.6.2 Reactions (Fig. 9.13)

9.7 Lactones

  • Lactones are cyclic esters formed by the intramolecular interaction of ‘-OH’ and ‘-COOH’ groups present on the same molecule- intramolecular ester formation – formation of ester within the molecule
  • In hydroxyl carboxylic acids ‘-OH’ and ‘-COOH’ groups required for formation of ester, are present in the same molecule (Fig. 9.14)
  • Due to steric hindrance lactone smaller than γ-lactone are unstable and not formed
  • Lactones are produced during thermal oxidation of saturated and unsaturated fatty acids
  • High level of lactones in fresh milk is foreign to its natural flavor and considered as flavor defect
  • In beverage milk trace (1-2 ppm) of these odoriferous compounds is optimal
  • In butter 5 to 10 ppm are desirable
  • The development of lactones is stored whole milk powder imparts coconut flavor
  • In confectioneries and candies it is a major source of their unique flavor.
Last modified: Wednesday, 7 November 2012, 5:41 AM