B.Sc. 2nd Year Chemistry – Detailed and Exam-Oriented Notes
Carboxylic acids are an important class of organic compounds containing the carboxyl functional group, –COOH. The carboxyl group consists of a carbonyl group (>C=O) and a hydroxyl group (–OH) attached to the same carbon atom.
Carboxylic acids are widely distributed in nature. Examples include formic acid in ants, acetic acid in vinegar, benzoic acid in some natural products and fatty acids in fats and oils.
The acidic character of these compounds is mainly due to the ability of the carboxyl group to lose a proton and form a resonance-stabilized carboxylate ion.
The carboxyl group contains two important functional components: a carbonyl group and a hydroxyl group.
The carbon atom of the carboxyl group is approximately sp2-hybridized. The group is approximately planar because of conjugation between the carbonyl system and the hydroxyl oxygen.
When a carboxylic acid loses a proton, the resulting carboxylate ion is stabilized by resonance.
The negative charge is delocalized over the two oxygen atoms, making the carboxylate ion considerably more stable than a localized alkoxide ion.
Compounds containing one carboxyl group are called monocarboxylic acids.
Examples: Methanoic acid and ethanoic acid.
Compounds containing two carboxyl groups are called dicarboxylic acids.
Examples: Oxalic acid and malonic acid.
Compounds containing more than two carboxyl groups are called polycarboxylic acids.
The carboxyl group is attached to an aliphatic carbon chain.
Example: CH3COOH.
The carboxyl group is directly attached to an aromatic ring.
Example: Benzoic acid, C6H5COOH.
In IUPAC nomenclature, the longest carbon chain containing the carboxyl carbon is selected and the suffix -oic acid is used.
| Common name | IUPAC name | Formula |
|---|---|---|
| Formic acid | Methanoic acid | HCOOH |
| Acetic acid | Ethanoic acid | CH3COOH |
| Propionic acid | Propanoic acid | CH3CH2COOH |
| Butyric acid | Butanoic acid | CH3CH2CH2COOH |
| Benzoic acid | Benzoic acid | C6H5COOH |
The carbon atom of the carboxyl group is assigned position 1 when numbering the parent chain.
Primary alcohols can be oxidized first to aldehydes and then further to carboxylic acids using suitable oxidizing agents.
Aldehydes are readily oxidized to corresponding carboxylic acids.
Nitriles can be hydrolyzed under acidic or alkaline conditions to produce carboxylic acids or their salts, followed by acidification.
Esters undergo hydrolysis to give a carboxylic acid and an alcohol under acidic conditions.
Amides can undergo hydrolysis under suitable acidic or basic conditions to give carboxylic acids or carboxylate salts.
Grignard reagents react with carbon dioxide to form magnesium carboxylates, which give carboxylic acids on hydrolysis.
Carboxylic acids show strong intermolecular association because they can form hydrogen bonds.
Carboxylic acids commonly form hydrogen-bonded dimers, especially in non-polar solvents and vapor phase.
The formation of dimers contributes to their relatively high boiling points.
Carboxylic acids are weak acids but are considerably stronger acids than alcohols and many simple hydrocarbons.
Their acidic strength is explained by the stability of the conjugate base, the carboxylate ion.
The negative charge of the carboxylate ion is delocalized over the two oxygen atoms.
Because the negative charge is distributed over two electronegative oxygen atoms, the carboxylate ion is stabilized.
Alkoxide ions formed from alcohols do not have equivalent resonance stabilization. Therefore, alcohols are generally much weaker acids than carboxylic acids.
The acidity of a carboxylic acid is affected by electron-withdrawing and electron-donating groups attached to the carbon chain.
Electron-withdrawing groups such as halogens generally increase acidity by stabilizing the negative charge of the carboxylate ion through the inductive effect.
Alkyl groups generally show an electron-donating inductive effect and tend to decrease acidity relative to the parent acid.
Carboxylic acids react with active metals such as sodium to form carboxylate salts with liberation of hydrogen gas.
Evolution of hydrogen gas provides evidence of the acidic nature of carboxylic acids.
Carboxylic acids react with sodium hydroxide and other strong bases to form salts and water.
This is a neutralization reaction.
Carboxylic acids react with sodium carbonate or sodium bicarbonate to produce carbon dioxide gas.
Carboxylic acids react with alcohols in the presence of a strong acid catalyst, commonly concentrated sulfuric acid, to form esters.
This reaction is called Fischer esterification. The reaction is reversible and is often driven toward ester formation by removing water or using an excess of one reactant.
Many esters have pleasant fruity or floral odors and are important in flavor and fragrance chemistry.
Carboxylic acids react with chlorinating agents such as thionyl chloride to form acid chlorides.
Acid chlorides are highly reactive carboxylic acid derivatives and are useful intermediates in organic synthesis.
Carboxylic acids can form amides through reactions involving ammonia or amines under appropriate conditions.
Initially, an ammonium carboxylate may be formed. Heating can promote dehydration to give an amide.
Carboxylic acids can be reduced to primary alcohols using strong reducing agents such as lithium aluminium hydride.
Reduction of the carboxyl group to an alcohol is an important transformation in organic synthesis.
Decarboxylation is the removal of carbon dioxide from a carboxylic acid or its salt.
Sodium salts of carboxylic acids undergo decarboxylation when heated with soda lime, producing an alkane containing one carbon atom fewer than the original acid.
Soda lime is generally a mixture of sodium hydroxide and calcium oxide.
Carboxylic acids containing an alpha-hydrogen can undergo alpha-halogenation in the presence of halogen and phosphorus-based reagents.
The reaction introduces a halogen atom at the alpha carbon of the carboxylic acid.
The reaction is useful for preparing alpha-halo carboxylic acids, which are valuable intermediates in organic synthesis.
Important derivatives of carboxylic acids are compounds in which the hydroxyl group of the carboxyl group is replaced by another group.
| Derivative | General formula | Example |
|---|---|---|
| Acid chloride | RCOCl | CH3COCl |
| Acid anhydride | (RCO)2O | (CH3CO)2O |
| Ester | RCOOR′ | CH3COOCH3 |
| Amide | RCONH2 | CH3CONH2 |
Carboxylic acid derivatives differ in their reactivity toward nucleophilic acyl substitution.
Acid chlorides are highly reactive because chloride ion is a good leaving group and the carbonyl carbon remains strongly electrophilic. Amides are comparatively less reactive because the nitrogen lone pair participates in resonance with the carbonyl group.
Formic acid, HCOOH, is the simplest carboxylic acid. It occurs naturally in the venom of ants and certain other organisms.
It is a colorless liquid with a pungent odor and has applications in leather processing, textiles and preservation.
Acetic acid, CH3COOH, is an important industrial and biological carboxylic acid. Dilute aqueous acetic acid is the principal acid present in vinegar.
Pure acetic acid freezes near room temperature and is therefore commonly referred to as glacial acetic acid when highly concentrated.
Oxalic acid is a dicarboxylic acid with the formula HOOC–COOH.
It occurs naturally in some plants and can form insoluble salts with certain metal ions.
Oxalic acid and oxalates are also important in analytical chemistry and laboratory applications.
Benzoic acid, C6H5COOH, is the simplest aromatic carboxylic acid.
It is a crystalline solid and has important applications in food preservation and organic synthesis.
| Acid | Structure |
|---|---|
| Oxalic acid | HOOC–COOH |
| Malonic acid | HOOC–CH2–COOH |
| Succinic acid | HOOC–CH2–CH2–COOH |
| Glutaric acid | HOOC–(CH2)3–COOH |
| Adipic acid | HOOC–(CH2)4–COOH |
Several dicarboxylic acids are important intermediates in polymer production and biochemical pathways.
| Property | Carboxylic acids | Alcohols |
|---|---|---|
| Functional group | –COOH | –OH |
| Acidity | Higher | Much lower |
| Conjugate base | Resonance-stabilized carboxylate ion | Alkoxide ion |
| Reaction with NaHCO3 | CO2 evolved | Generally no reaction |
| Typical reaction | Esterification | Oxidation, substitution, dehydration |
Carboxylic acids are widely distributed in biological systems.
Carboxylic acids contain the –COOH functional group and are among the most important oxygen-containing organic compounds.
Their acidic nature results from the resonance stabilization of the carboxylate ion formed after loss of a proton.
They can be prepared by oxidation of primary alcohols and aldehydes, hydrolysis of nitriles, esters and amides, and reaction of Grignard reagents with carbon dioxide.
Important reactions include neutralization, reaction with metals, reaction with carbonates, esterification, formation of acid chlorides and amides, reduction and decarboxylation.
Important reactions for examination include Fischer esterification, soda-lime decarboxylation and the Hell-Volhard-Zelinsky reaction.
Carboxylic acids and their derivatives have extensive applications in food chemistry, pharmaceuticals, polymers, biological systems and industrial organic synthesis.