CHEMICAL PROPERTIES OF CARBOXYLIC ACID 1. Acidity of Carboxylic Acid Acidity refers to the ease with which a substance donates a proton, resulting in the formation of an anion. The strength of an acid is determined by the disparity in stability between the acid and its corresponding anion. [ 86] R Θ OH [ H R 8] R OH Non-equivalent structures of Resonance hybrid R 2. Equivalent structures of Resonance hybrid Both the acid and its anion benefit from resonance stabilization, but the anion experiences significantly greater stabilization because it can generate two identical resonating structures. Effect of Substituents on Acidity Any factor that provides more stability to the anion compared to the acid will enhance the acidity, while any factor that reduces the stability of the anion will lower the acidity of the carboxylic acid. (a) Electron-withdrawing substituents enhance anion stability by dispersing the negative charge, thereby increasing the acidity of the carboxylic acid. (b) Electron-releasing substituents increase the negative charge on the anion, reducing the stability of the carboxylate anion, and consequently decreasing the acidity of the acid. Carboxylic acids are considered weak acids, and their carboxylate ions are strong conjugate bases. They exhibit slight alkalinity due to the hydrolysis of the carboxylate anion in comparison to other species. The order of acidity and basicity of their corresponding conjugate bases is as follows. Acidity -RCOOH > HOH > ROH>CH=CH NH3 > RH Basicity --RCOO-OH-HCC- 1 CH2-CH2-COOH ẞ-chloro propanoic acid electron withdrawing effect less 3. Ortho Effect R R The Ortho-substituted benzoic acid, regardless of whether the substituent is electron-withdrawing or electron-releasing, exhibits higher acidity compared to its para and meta isomers. This phenomenon is referred to as the ortho effect, and it arises from the combined effects of steric hindrance and intramolecular hydrogen bonding. This effect occurs whenever the opportunity arises to stabilize the carboxylate anion due to the proximity of the substituent. Groups such as -OH, -Cl, and -NO2 provide even greater stabilization to the anion through direct interaction via intramolecular hydrogen bonding. OH -OH H Intra molecular H-bonding stabilization O-hydroxy benzoic acid (Salicylic acid) COOH OH > Carboxylate aninon COOH COOH OH Order of acidic character OH Reaction due to cleavage of --O--H bond as acid 0 -C-OH(1) Reaction with active metals [alkali and alkaline metal] 0 OH + Na 0 RLONa + H₂ (1) (ii) Reaction with Cao 2R-COOH + CaO (RCOO)2Ca + H2O (iii) Reaction with Bicarbonates and Carbonates Carboxylic acid reacts with carbonates and bicarbonates to liberate CO2 gas 0 R-COOH NaHCO CaCO3 R-ONa+ CO2 + H₂O 0 Ca+ CO2 + H₂O R--C-O Reaction Involving Cleavage of-Oh Group Esterification When carboxylic acid reacts with alcohol in the presence of conc. H2SO4 to form ester, it is known as esterification R-C-OH+R-OH Conc. H2SO4 R-COR+H₂O Mechanism H2SO4 H+ R-C-O-H+H+ R-C-O-H OH OH OH OH R-C-O-H+ROH R-C-OH OR' OR OH R-C-OHR-CR-CR-C-OR' OH ! 0 !! The reactivity of alcohols in the formation of esters is significantly influenced by their structure. When there are bulkier substituents near the -OH group, the reaction proceeds more slowly. This observation holds true for acid-catalyzed reactions as well. CH3OH > CH3CH2OH > (CH3)2 CHOH > (CH3)3 COH H-COOH > CH3COOH > (CH3)2CHCOOH > (CH3)3CCOOH Formation of Acid Chloride + PCIs R-COCI+ HCI + POCI C6H5N R-COOH +SOCI R-COCI+SO+HCI CsHsN CSHSN +SO₂Clz R-COCI + SO3+ HCI Formation of Acid Anhydride When carboxylic acids are subjected to a dehydrating agent such as P2Os, they undergo a reaction that leads to the formation of an anhydride through the elimination of a water molecule. 0 2R-C-OH P2O5 0 R R 0 O+H₂O CH2COOH CH₂CO CH2COOH CH2CO Succinic acid P2O5 O+H₂O Succinic anhydride Formation of Amides R-C-OH+MeNH R-C-0 [σ] NH Me Heat R-C-NH Me The lone pair of electrons on the nitrogen atom participates in resonance, resulting in the presence of a partial positive charge on the nitrogen atom. 0 R-C-NH Me R-C-N-H Me As a result, the degree of hydrogen bonding increases, leading to higher boiling and melting points for amides compared to other types of acid derivatives. 4. Reactions Involving - COOH Group Schmidt Reaction Carboxylic acids, when combined with hydrazoic acid in the presence of concentrated H2SO4 at 90°C, result in the production of primary amines. Conc.H SO R-COOH + N3H R-NH2 + N2 + CO2 Mechanism OH COH O-C+H R-Ce HN R-C-OH он -H₂O R-C-- R-NC-0 -N R-C ← isocyanate -H2O N-N H -NN R-NH2 + CO2 5. Soda lime Decarboxylation When carboxylic acids are subjected to heating with soda lime (a mixture of NaOH and CaO), they undergo a transformation, resulting in the formation of an alkane with one fewer carbon atom than the original acid. Hunsdicker Reaction: R-COOH R-COOH Soda Hme A R--H + CO2 R--COO-Ag+ R--Br+AgBr + CO2 in CCla Mechanism Reaction proceeds through free radical mechanism in various steps. (1) Chain initiation R- BR- R- (ii) Chain propagation step. R+CO R 0 (iii) Chain termination -O-BR+R R Br- R R+R R-R