Chemical Properties of Carboxylic Acid Preparation and Properties
→ Alkanoic acid have both alkyl group and carboxylic group. Thus, its chemical properties can be classified in two groups
- Reactions of alkyl group
- Reaction of carboxylic/group
→ Reactions of Alkyl group (Halogenatlon): Except formic acid on which alkyl group is not present other carboxylic acid on treatment with chlorine or bromine in the presence of small amount of red phosphorus to give α-chloro or β-Bromo acid. This reaction is called Hell-voihard Zelinkey (HVZ) reaction.
→ Second and third a-hydrogen atoms are substituted respectively. If more than one mol chlorine or bromine is used.
Reaction of Carboxylic Group : Reactions involving Carboxylic group as a whole Carboxvlic acid and its salt shows many such reactions in which carboxylic group either eliminate or dissociates. Some of the reaction are as follows:
→ Reduction into alkanes : Carboxylic acid heated with HI and Red P at high pressure reduces and give alkanes.
→ In above reaction —COOH group is completely reduced Lo—CH3 group.
→ Reduction with alcohols : Alkanoic acid are reduced to primary alcohols by lithium aluminium hydride or with diborane (B2H6).
→ By using Sodalime : The sodium salt of carboxylic acid when heated with sodalime (NaOH and CaO in the ratio of 3: 1) decarboxylated and give alkanes.
→ On the other hand the gem dicarboxylic acid in which two carboxyl group are attach to same carbon atom, decarboxylate only on heating.
→ Electrolytic Decarboxylatlon : Alkali metal salts of carboxylic acids also undergo decarboxylation on electrolysis of their aqueous solutions and form hydro carbons having twice the number of carbon atoms present in the a1ky group of the acid. The reaction is known as Kolbe’s electrolysis or Kolbe’s reaction.
→ Decarboxylation of silver salts of carboxylic acids in presence of bromine : The silver salts of the carboxylic acid on treatment with broniine decarboxylated and form alkyl/aryl bromide having one carbon atom less than the parent acid.
This reaction is called hundsdiecker reaction.
→ Schmidt Readhon : Carboxylic acid reacts with hydrozoic acid (N3H) and give primary alkanamine which have one carbon atom less than alkanoic acid. This reaction is called Schmidt reaction.
Reaction involving cleavage of C—OH bond of Carboxylic group:
→ Eaterification: Carboxylic acid reacts with alcohols or phenol in the presence of cone. Sulphuric acid or hydrogen éhloride gas to form esters.
Mechanism of Esterification of Carboxylic Acids : It is nucleophilic acyl substitution type of reaction.
→ Step 1 : Protonation of Carboxyl Group : In the presence of conc. H2SO4 or HCl gas carbonyl oxygen of carboxylic acid accepts a proton and form protonated carboxylic acid.
→ Step 2 : Nucleophilic attack by the alcohol Molecules : Electron rich oxygen atom of alcohol molecule attaiks on positively charged carbon atom to form tetrahedral intermediate.
→ Step 3 : Transfer of Proton : In tetrahedral intermediate the shift of a proton converts hydroxy group to —OH2+. group which is a good leaving group.
→ Step 4 : Loss of Water Molecule : Due to a good leaving group the above intermediate loses a neutral water molecule.
→ Step 5: Loss of Proton : The protonated ester formed above loses a proton to form a ester.
→ Formation of Anhydride : Carboxylic acid on heating with mineral acid like H2SO4 or P2O5 gives corresponding anhydride.
→ ReactIon with PCl5, PCl3 and SOCl2 : The hydroxyl group of carboxylic acid behaves like the hydroxyl group of alcohols and substituted by chlorine atom when reacts with phosphorus pentachioride (PCl5), Phosphorus t.richloride (PCl3) or Thionyl Chloride (SOCl2).
→ Thionyl chloride is prefered for the reaction because two product in this reaction are gaseous in nature and removes from the reaction mixture which make easier purfication of product obtain.
→ Reaction’ with Ammonia : Carboxylic acid reacts with ammonia to form ammonium salt which on heating at high temperature forms amide.
→ Formation of carbonyl Compounds : Dry distilation of calcium salt of alkanoic acid gives carbonyl compound.
Reaction involving Cleavage of bond of carboxylic group
→ Reaction with metals : Carboxylic acid reacts with active metal like : Na, K. Mg, Ca, Zn etc ta form salts and releases hydrogen gas
→ Reaction with Carbonate and Bicarbonate : Acid reacts with carbonate and hydrogen carbonate and releases carbon dioxide gas.
→ Reaction with Base : Carboxylic acid reacts with base to form salts.
All the above reaction shows acidic nature of carboxylic acid.