14.1 Carboxylic acids
🎯 Learning Outcomes
- Describe briefly the nomenclature and isomerism of aliphatic and aromatic carboxylic acids.
- Show the preparation of monocarboxylic acids from: aldehydes, nitriles, dicarboxylic acids, sodium alkoxide, and trihaloalkanes.
- Show the preparation of benzoic acid from alkylbenzene.
- State physical properties of monocarboxylic acids.
- Explain the chemical properties of aliphatic and aromatic carboxylic acids: action with alkalis, metal oxides, metal carbonates, metal bicarbonates, PCl3, LiAlH4, and dehydration of carboxylic acids, Hell–Volhard–Zelinsky reaction.
- Electrophilic substitution reaction of benzoic acid (bromination, nitration, and sulphonation).
- Explain the effect of constituents on the acidic strength of carboxylic acid.
- Describe the abnormal behaviour of methanoic acid.
Introduction to Carboxylic Acids
The organic functional group –COOH is called the carboxyl group because it is formed by the combination of the carbonyl group (>C=O) and hydroxyl group (–OH).
Its structure is:
However, due to resonating structures, compounds containing the carboxyl group have entirely different properties than both carbonyl compounds and alcohols.
Carboxylic acid is one of the most reactive functional groups among the organic compounds.
Nomenclature
Carboxylic acids are named by adding the suffix –oic acid, replacing the final –e in the name of the parent hydrocarbon alkane — i.e., alkanoic acid.
Isomerism
I. Functional Isomerism
Carboxylic acids can form functional isomers with esters.
e.g. C2H4O2 can exist as:
II. Chain Isomerism
Carboxylic acids having more than 3 carbon atoms exhibit chain isomerism.
e.g., the following two isomers are possible for the formula C4H8O2:
Preparation of Monocarboxylic Acids
I. From Trihaloalkanes
Hydrolysis of 1,1,1-trihaloalkanes produces unstable trihydroxy alkanes which undergo dehydration to give monocarboxylic acid.
II. By Oxidation of 1° Alcohols and Aldehydes
(studied in Alcohols and Aldehydes–Ketones chapters already)
Primary alcohols and aldehydes get oxidised in the presence of strong oxidising agents like alkaline or acidified KMnO4, K2Cr2O7, etc., and carboxylic acids are produced.
III. By Hydrolysis of Nitriles
Alkanenitriles on acidic or basic hydrolysis produce acids (via amide and ammonium salt).
IV. From Sodium Alkoxide
Sodium alkoxide, when heated with carbon monoxide under pressure, gives sodium alkanoate, which on acidification gives carboxylic acids.
V. From Dicarboxylic Acids
Dicarboxylic acids, when heated, yield monocarboxylic acids and carbon dioxide gas. This reaction is also known as a decarboxylation reaction.
Preparation of Benzoic Acid
Preparation of Benzoic Acid from Alkylbenzene
Irrespective of the chain length of the alkyl group, alkylbenzenes are oxidised by alkaline KMnO4 or dil. HNO3 as an oxidising agent into benzoic acid.
Manufacture of Benzoic Acid
Benzoic acid is prepared on an industrial scale by bubbling chlorine through boiling toluene in the presence of ultraviolet light. The benzotrichloride obtained is hydrolysed by alkali to benzoic acid.
2. PCl₃
3. PCl₅
4. SOCl₂CH₃CH₂Clalc. KCNCH₃–CH₂–CNOH⁻boil with waterCH₃–CH₂–COOH
Physical Properties
I. State
The first few members are liquids and higher members are waxy solids, because the intermolecular force of attraction increases with increasing molecular mass.
II. Solubility
The lower members of carboxylic acids are highly soluble in water owing to their polar nature and the capacity to form hydrogen bonds with water.
But the solubility decreases gradually moving up the homologous series as the non-polar alkyl part gets bigger and hence dominant.
III. Melting and Boiling Points
The melting and boiling points of carboxylic acids are higher relative to corresponding compounds of other classes of organic compounds of comparable molecular mass. This is because of strong hydrogen bonds formed between two molecules, which result in dimers, making the effective molecular mass double. e.g.
| CH3COOH | CH3CH2CH2OH | CH3CH2CH2CH3 | |
|---|---|---|---|
| Molecular wt. | 60 | 60 | 58 |
| Boiling point | 118°C | 97°C | –1°C |
Chemical Properties
1. Acidic Nature
Reaction with active metal (Na)
Active metals like alkali and alkaline earth metals replace hydrogen from acids.
Reaction with base (NaOH or Na₂O etc.)
Carboxylic acids react with a base to give salt and water.
Reaction with metal carbonates and bicarbonates
Carboxylic acids react with metal carbonates and bicarbonates, evolving carbon dioxide gas. This reaction is used for the test of the carboxylic acid functional group in organic compounds.
2. Reaction Involving the –OH Group
Reaction with PCl₃
The hydroxyl group of the carboxylic acid is replaced by the chloride group, forming acid chloride on treatment with these reagents in the presence of pyridine.
Dehydration — Formation of Acid Anhydrides (reaction with P₂O₅)
The only primary alcohol that gives the iodoform test is ethanol. Therefore, A must be ethanol.
Reduction with LiAlH₄ (partial reduction)
3. Reaction Involving Side Chain (Alkyl Group)
Halogenation of Side Chain (Hell–Volhard–Zelinsky – HVZ reaction)
When carboxylic acids are treated with Cl₂ or Br₂ in the presence of red phosphorus, all the α-hydrogen atoms of carboxylic acids are replaced successively by the halogen atoms.
Abnormal Behaviour of Formic Acid
Formic acid or methanoic acid, apart from being a carboxylic acid, resembles somewhat an aldehyde functional group. Thus, it gives a few reactions which other carboxylic acids don’t.
Therefore, formic acid can be distinguished from other acids by the following chemical reactions.
I. Reaction with Tollens’ reagent
Formic acid gives a silver mirror with ammoniacal silver nitrate (Tollens’ reagent).
II. Reaction with Fehling’s solution
III. Reaction with conc. H₂SO₄
IV. Reaction with acidified KMnO₄
V. Action of heat
Electrophilic Substitution of Benzoic Acid
The carboxylic group is an electron-withdrawing group and meta director; therefore it undergoes nitration, sulphonation, and halogenation reactions at the meta position with difficulty.
Acidity of Carboxylic Acids
The acidity of carboxylic acids can be explained based on the following two facts:
Ionization constant
When a carboxylic acid is dissolved in water, it ionises to give protons to the water molecules. The stronger acids give up most of their ionizable protons, while weaker acids transfer only a small proportion.
The dissociation of a carboxylic acid can be represented as:
The equilibrium constant (Kc) of the process is given by the expression:
Kc = [H₃O⁺][RCOO⁻][RCOOH][H₂O]
Ka = [H₃O⁺][RCOO⁻][RCOOH]
A stronger acid has a larger Ka. Based on the value of Ka, therefore, the acidity decreases in the following order:
Formic acid > Acetic acid > Propionic acid > n-Butyric acid
Trichloroacetic acid > Dichloroacetic acid > Chloroacetic acid > Acetic acid
Resonance stabilization
Carboxylic acids are quite strong acids because of the presence of a polar O–H group. They ionise to give protons.
Carboxylic acids as well as their carboxylate anion are both stabilised by resonance.
However, the carboxylate anion is more stabilised by resonance because its contributing structures are exactly identical and do not include charge separation as in the carboxylic acid. Therefore, the above equilibrium lies very much towards the ionised form. Hence, carboxylic acids behave as fairly strong acids — stronger than alcohols and phenols, which is attributed to the stable resonating structure of the carboxylate ion.
Effects of Substitution on Acidity of Carboxylic Acids
The factors which increase the stability of the carboxylate ion more than the carboxylic acid increase the acidic strength.
Therefore, electron-withdrawing groups stabilise the carboxylate ion by dispersal of the negative charge and increase acid strength. On the other hand, electron-releasing groups destabilise the carboxylate ion and decrease acid strength.
The greater the tendency of an acid to release a proton (H⁺), the stronger the acidic character. In acetic acid, the methyl group is an electron-releasing group; due to this, the proton of the acid does not lose easily. But in chloroacetic acid, chlorine is an electron-withdrawing group and withdraws electrons; due to this, the proton of the acid is lost easily. Therefore, chloroacetic acid is a stronger acid than acetic acid.
The greater the tendency of an acid to release a proton (H⁺), the stronger the acidic character. In acetic acid, the methyl group is an electron-releasing group; due to this, the proton of the acid does not lose easily. Therefore, acetic acid is a weaker acid than formic acid.
In other words, the carboxylate ion of acetic acid is destabilised by the electron-releasing methyl group in comparison to formic acid. That makes acetic acid a weaker acid than formic acid.
📘 Chapter Summary
- The –COOH (carboxyl) group forms from a carbonyl + hydroxyl group; resonance between the two oxygens makes carboxylic acids far more acidic than alcohols.
- Named as alkanoic acids; show functional isomerism with esters and chain isomerism (C₄ and above).
- Monocarboxylic acids are prepared from trihaloalkanes, oxidation of 1° alcohols/aldehydes, hydrolysis of nitriles, sodium alkoxide + CO, and decarboxylation of dicarboxylic acids.
- Benzoic acid is made by oxidising alkylbenzenes, or industrially via chlorination of toluene followed by alkaline hydrolysis.
- Lower acids are liquid and water-soluble; hydrogen-bonded dimer formation raises boiling points well above alcohols of similar mass.
- Chemically: acidic reactions with Na/NaOH/carbonates; –OH group reactions with PCl₃/SOCl₂ (acid chlorides) and P₂O₅ (anhydrides); LiAlH₄ partial reduction; and HVZ α-halogenation of the side chain.
- Formic acid behaves like an aldehyde too — it reduces Tollens’ and Fehling’s reagents, unlike other carboxylic acids.
- –COOH is an electron-withdrawing meta director, so benzoic acid undergoes nitration, halogenation and sulphonation slowly at the meta position.
- Acid strength rises with electron-withdrawing substituents (which stabilise the carboxylate ion) and falls with electron-releasing groups — explaining why formic acid is a stronger acid than acetic acid.