Chemical Properties of Carboxylic Acids: The carboxyl group consists of a carbonyl group attached to a hydroxyl group, hence its name carboxyl. The name is normally suffixed as ‘oic acid’ when \( – {\rm{COOH}}\) the group is the principal functional group, and its carbon is counted while numbering the longest chain. The carboxyl carbon is assigned the number \(1\) in IUPAC nomenclature. Carboxylic acids are further classified as mono, di, tri- carboxylic acid. Carboxylic acids may be aliphatic or aromatic. Higher members of saturated aliphatic monocarboxylic acids are also referred to as Fatty Acids.

Structure of Carboxyl Group

Hydrocarbon derivatives with a carboxyl \(\left( {{\rm{COOH}}} \right)\) moiety are known as carboxylic acids. Because carbon has four valence electrons, it requires four more electrons or bonds to complete its octet. In its neutral state, carbon creates four bonds, which might be single bonds or a combination of single and multiple bonds. Because oxygen has six valence electrons, it requires two more electrons or bonds to complete its octet, which can be either single or double \(\left( {{\rm{pi}}} \right)\) bonds.

Carbon creates a double bond to one oxygen atom, generating a carbonyl moiety and a single link to another oxygen atom, forming a hydroxyl group, in the carboxylic acid functional group. The fourth bond is formed with another carbon atom (or hydrogen in the case of formic acid).

Carboxylic acids, like aldehydes, ketones, and alcohols, have a carbonyl group and an alcohol group, hence they share some basic physicochemical features.

Chemical Properties of Carboxylic acid

Chemical reactions of carboxylic acids can be discussed under four categories:

(i) Reactions involving Removal of Proton from O-H Group

The acidity of carboxylic acid

\({\rm{R – }}\mathop {\rm{C}}\limits^{\mathop {{\rm{||}}}\limits^{\rm{O}} } {\rm{ – }}{{\rm{O}}^{\rm{ – }}}\) ion exists as two equivalent canonical structures. This ion is resonance stabilized; hence carboxylic acids are acidic in nature.

Effect of Substituent on Acidity

Any factor that makes the anion more stable than the acid should increase the acidity of carboxylic acid, whereas any factor that makes the anion less stable should decrease it. 

(a) An electron-withdrawing substituent stabilizes the anion by dispersing the \({\rm{ – ve}}\) charge and therefore increases the acidity of carboxylic acid.

(b) An electron releasing group intensifies the \({\rm{ – ve}}\) charge on the anion resulting in a decrease of stability of carboxylate anion and therefore decreases the acidity of the acid.

(c) With an increase in the number of electron-withdrawing groups in the same molecule, acidic character increases. Thus, the following acids are arranged in order to decrease acidity.

If phenyl or vinyl groups are directly attached to the carboxylic acid, then the acidity of carboxylic acid increases. This is due to the greater electronegativity of \({\rm{s}}{{\rm{p}}^{\rm{2}}}\) hybridized carbon to which carboxyl carbon is attached.

Action with Blue Litmus

All carboxylic acids turn blue litmus red.

Reaction with Metals

\(2{\rm{C}}{{\rm{H}}_3}{\rm{COOH}} + 2{\rm{Na}} \to \mathop {2{\rm{C}}{{\rm{H}}_3}{\rm{COONa}}}\limits_{{\rm{ Sodium\, acetate }}} + {{\rm{H}}_2}\)

\(2{\rm{C}}{{\rm{H}}_3}{\rm{COOH}} + {\rm{Zn}} \to \mathop {{{\left( {{\rm{CHCOO}}} \right)}_2}}\limits_{{\rm{Zinc\, actate}}} {\rm{Zn}} + {{\rm{H}}_2}\)

Reaction with Alkalis

\({\rm{C}}{{\rm{H}}_3}{\rm{COOH}} + {\rm{NaOH}} \to {\rm{C}}{{\rm{H}}_3}{\rm{COONa}} + {{\rm{H}}_2}{\rm{O}}\)

Reaction with Bicarbonates and Carbonates

\(2{\rm{C}}{{\rm{H}}_3}{\rm{COOH}} + {\rm{N}}{{\rm{a}}_2}{\rm{C}}{{\rm{O}}_3} \to 2{\rm{C}}{{\rm{H}}_3}{\rm{COONa}} + {\rm{C}}{{\rm{O}}_2} + {{\rm{H}}_2}{\rm{O}}\) \({\rm{C}}{{\rm{H}}_3}{\rm{COOH}} + {\rm{NaHC}}{{\rm{O}}_3} \to {\rm{C}}{{\rm{H}}_3}{\rm{COONa}} + {\rm{C}}{{\rm{O}}_2} + {{\rm{H}}_2}{\rm{O}}\)

Note: Reaction of a carboxylic acid with aqueous sodium carbonate solution produces brisk effervescence. However, phenols do not produce effervescence. Therefore, the reaction may be used to distinguish between carboxylic acids & phenols.

Reaction with Grignard Reagent

\({\rm{RC}}{{\rm{H}}_2}{\rm{MgBr}} + {{\rm{R}}^\prime }{\rm{COOH}}\mathop \to \limits^{{\rm{ ether }}} {\rm{R}} – {\rm{C}}{{\rm{H}}_3} + {{\rm{R}}^\prime }{\rm{COOMgBr}}\)

(ii) Reactions involving Cleavage of C-OH Bond

Formation of Anhydride
Carboxylic acid on treatment with any dehydrating agent as \({{\rm{P}}_2}{{\rm{O}}_5}\) or \({{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}\) conc. form anhydride by elimination of water molecule.

Esterification

Esterification occurs when a carboxylic acid interacts with alcohol in the presence of conc. sulphuric acid to generate an ester.

Mechanism

The esterification of carboxylic acids with alcohol is a kind of nucleophilic acyl substitution. If we follow the forward reactions in this mechanism, we have the mechanism for the acid-catalyzed esterification of an acid. If, however, we follow the reverse reactions, we have the mechanism for the acid-catalyzed hydrolysis of an ester. Which result we obtain will depend on the condition we choose. If we want to esterify an acid, we use an excess of the alcohol and, if possible, remove the water as it is formed. If we want to hydrolyze an ester, we use a large excess of water; that is, we reflux the ester with dilute aqueous \({\rm{HCL}}\) or dilute aqueous \({{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}.\)

Reactions with Chloride Compounds

The carboxylic acid can be converted into its corresponding acid halides by reaction with phosphorous halide \(\left( {{\rm{PC}}{{\rm{l}}_5};{\rm{PC}}{{\rm{l}}_3}} \right.\,{\rm{or}}\,{\rm{PB}}{{\rm{r}}_3})\) or thionyl chloride \(\left( {{\rm{SOC}}{{\rm{l}}_{\rm{2}}}} \right).\)

Note: \({\rm{SOC}}{{\rm{l}}_{\rm{2}}}\) is the best and the mildest reagent for the preparation of acid halide because byproducts are gases \(\left( {{\rm{S}}{{\rm{O}}_2}\,{\rm{\& }}\,{\rm{HCl}}} \right)\)

Reaction with Ammonia

Carboxylic acids react with ammonia and give amides at high temperatures.

(iii) Reactions involving –COOH Group

Reduction

(a)Reduction to alkanes: Carboxylic acids on reduction with hydriodic acid and red phosphorus at \({\rm{437}}\,{\rm{K}}\) give alkanes

E.g.,

In the above reactions, the \( – {\rm{COOH}}\) group as a whole is reduced to a group.

(b)Reduction to alcohols: The reduction of carboxylic acids with lithium aluminium hydride \(\left( {{\rm{LiAl}}{{\rm{H}}_4}} \right)\) or better with diborane \(\left( {{{\rm{B}}_{\rm{2}}}{{\rm{H}}_{\rm{6}}}} \right)\) gives primary alcohols.

E.g.,

Decarboxylation

(a)Using soda-lime. Sodium salts of carboxylic acids, when heated with soda-lime \({\rm{(NaOH + CaO}}\) in the ration \(3:1),\) undergo decarboxylation to yield alkanes.

(b) Electrolytic decarboxylation. Electrolysis of sodium or potassium salts of fatty acids in water produces alkanes with double the amount of carbon atoms as the acid’s alkyl group. The process is called Kolbe’s Electrolytic reaction.

For example:

At anode

\(\mathop {2{\rm{C}}{{\rm{H}}_3}{\rm{CO}}{{\rm{O}}^ – }}\limits_{{\rm{ (unssobe) }}} – 2{\rm{e}} \to 2{\rm{C}}{{\rm{H}}_3}{\rm{COO}} \to 2{\rm{C}}{{\rm{O}}_2} + \mathop {{\rm{C}}{{\rm{H}}_3}}\limits_{{\rm{Ethane}}} – {\rm{C}}{{\rm{H}}_3}\)

At cathode

\(2{{\rm{H}}^ + } + 2{\rm{e}} \to {{\rm{H}}_2}\)

(c) Decarboxylation of silver salts of carboxylic acids in the presence of bromine.

(d) Decomposition of calcium salts of fatty acids. Dry distillation of calcium salts of fatty acids gives aldehydes or ketones. For example, the dry distillation of calcium formate gives formaldehyde, that of calcium acetate gives acetone, while that of a mixture of calcium acetate and calcium formate gives acetaldehyde.

(iv) Substitution Reactions in the Hydrocarbon Part

Halogenation

This reaction is also known as the Hell-Volhard-Zelinsky reaction. In the presence of a small amount of phosphorus, aliphatic carboxylic acids react smoothly with chlorine or bromine to yield a compound in which \(\alpha – \) hydrogen has been replaced by halogen.

Note: HVZ reaction results exclusively in \(\alpha – \) substitution and is therefore given by carboxylic acid having \(\alpha – \) hydrogen.

Mechanism

Ring Substitution

Aromatic acids undergo the usual electrophilic substitution reactions of the benzene ring, such as halogenations, nitration, and sulfonation. Since the \({\rm{COOH}}\) group is electron-withdrawing; therefore, it is meta-directing. Hence, benzoic acid does not undergo the Friedel Craft reaction.

Unique Properties of Formic Acid

1. Formic acid is the strongest acid among all the members of the homologous series. It is due to the fact that it contains both the aldehyde group and carboxyl group.

2. It reduces ammoniacal silver nitrate (Tollen’s reagent)

3. It reduces Fehling solution, i.e., gives red ppt of cuprous oxide

Summary

The carboxyl group is made up of a carbonyl group and a hydroxyl group, hence the name carboxyl. Carboxylic acids, like aldehydes, ketones, and alcohols, have a carbonyl group and an alcohol group. Hence they share some basic physicochemical features. Chemical reactions of carboxylic acids can be discussed under four categories:

(i) Reactions involving Removal of Proton from \({\rm{O – H}}\) Group
(ii) Reactions involving Cleavage of \({\rm{C – OH}}\) Bond
(iii) Reactions involving \({\rm{ – COOH}}\) Group
(iv) Substitution Reactions in the Hydrocarbon Part

FAQs on Chemical Properties of Carboxylic Acids

Q.1. Explain: (a) Acetic acid is a stronger acid than ethyl alcohol. (b) Trichloroacetic acid is a stronger acid than acetic acid.
Ans: (a) The negative charge in acetate ion is delocalized over two oxygen atoms, while the negative charge in ethoxide ion is localized on the single oxygen atom.

Hence, \({\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{CO}}{{\rm{O}}^{\rm{ – }}}\) the ion is more stable as a weaker base than \({\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{{\rm{O}}^ – }\) the ion. As a weaker base, it has a stronger conjugate acid, \({\rm{C}}{{\rm{H}}_3}{\rm{COOH}}\) is a stronger acid than \({\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{OH}}.\)
(b) Electron withdrawing substituents decrease the negative charge on the carboxylate ion and stabilize it, i.e., losing proton becomes relatively easy. Three chlorine atoms are electron-withdrawing substituents in trichloroacetic acid, and thus, it is a stronger acid than acetic acid.

Q.2. Convert acetic acid to propionic acid.
Ans:

Q.3. Discuss the effects of substituents on the acidity of carboxylic acid.
Ans: Any factor that stabilizes the anion more than acid should increase the acidity, and any factor that makes the anion less stable should decrease the acidity of carboxylic acid.
(a) An electron-withdrawing substituent stabilizes the anion by dispersing the \({\rm{ – ve}}\) charge and therefore increases the acidity of carboxylic acid.
(b) An electron releasing group intensifies the \({\rm{ – ve}}\) charge on the anion resulting in a decrease of stability of carboxylate anion and therefore decreases the acidity of the acid.
(c) With the increase in the number of electron-withdrawing groups in the same molecule, acidic character increases.

Q.4. Discuss Hell-Volhard-Zelinsky reaction.
Ans: This reaction is also known as the Hell-Volhard-Zelinsky reaction. In the presence of a small amount of phosphorus, aliphatic carboxylic acids react smoothly with chlorine or bromine to yield a compound in which hydrogen has been replaced by halogen.

Q.5. Give the ring substitution reactions of benzoic acid.
Ans: Aromatic acids undergo the usual electrophilic substitution reactions of the benzene ring, such as halogenations, nitration, and sulfonation. Since the \( – {\rm{COOH}}\) group is electron-withdrawing; therefore, it is meta-directing. Hence, benzoic acid does not undergo the Friedel Craft reaction.

Know About Physical Properties Of Carboxylic Acids