What is meant by Carboxylic Acid and its Derivatives?

They are organic compounds containing carboxyl groups (-COOH). They may be aliphatic (RCOOH) or aromatic (Ar-COOH) depending upon whether the -COOH group is attached to the aliphatic chain (R) or aryl group (Ar) respectively. The general formula of these compounds is C_nH_2n+1COOH. The modified compounds formed by various reactions of carboxylic acids are referred to as its derivatives. They include esters, acid halides, amides and acid anhydrides.

Classifications

• Monocarboxylic acids- They contain one -COOH group per molecule. Examples include formic acid (H-COOH), acetic acid (CH₃-COOH) etc.
• Dicarboxylic acids- They contain two -COOH groups per molecule. One example includes oxalic acid (COOH-COOH).
• Tricarboxylic acids- They contain three -COOH groups per molecule. One example is citric acid.
• Polycarboxylic acids- They contain more than three -COOH groups per molecule.

Preparation

They can be prepared from various organic and inorganic molecules.

• From alcohols- When alcohols are oxidized in the presence of potassium dichromate and concentrated sulfuric acid, they are converted to carboxylic acids.

R-CH₂-OH + [O] → R-COOH

• From cyano-alkanes- They undergo acid hydrolysis by dilute acid to produce organic acids.

R-CN + H₃O⁺ → R-COOH

• From grignard reagents- When dry ice (solid CO₂) is treated with alkyl or aryl magnesium chloride (R/ArMgCl), an addition reaction occurs followed by acid hydrolysis to produce carboxylic acids.

CO₂ + R/ArMgCl → Mg(Cl)(OH) + R-COOH

• From esters- Esters get hydrolyzed to produce acids.

R-COO-CH₂-R + H₂O → R-CH₂-OH + R-COOH

• From haloalkanes- Haloalkanes react with water to produce acids.

RCOX (X= halogen) + H₂O → HX + R-COOH

• From acid anhydrides- They are hydrolyzed to produce acids.

(R-CO)₂O + H₂O → 2R-COOH

• From tri-halo alkanes- Tri-halo alkanes react with alkali (NaOH or KOH) to produce acids and salts.

R-CX₃ + 3NaOH → 3NaX + R-COOH

Physical Properties of Carboxylic Acids

• They are polar compounds and can form H-bonds with water and with each other to form dimers.
• They have high boiling points.
• With the increase in the molecular mass, the solubility of carboxylic acid in water decreases.
• Low molecular weight aliphatic acids are colorless liquid at room temperature with unpleasant odors.
• High molecular weight aliphatic acids are like waxy solids and are odorless due to low volatility.
• They are less acidic than sulfonic acids but more acidic than other functional groups in organic chemistry.

Chemical Reactions of Carboxylic Acids

Hunsdiecker reaction

When silver carboxylate is treated with bromine dissolved in carbon tetrachloride, alkyl bromide is produced along with decarboxylation. Thus carbon dioxide is one of the by-products of this reaction. This is an example of a step-down reaction.

R-COOAg + Br₂ (Anhydrous CCl₄) → AgBr↓ + CO₂ + R-Br

Hell Volhard Zelinsky reaction

Any carboxylic acid that contains alpha hydrogen atoms, when reacted with halogen along with red phosphorus, alpha hydrogens start to get replaced by the halogen atoms. In this manner, the acidic strength of the acid gets increases as the -I effect increases.

RCH₂COOH + Cl₂ → RCCl₂-COOH

Schmidt's reaction

When carboxylic acids are subjected to react with hydrazoic acid (N₃H) in presence of concentrated sulfuric acid, they are converted to primary amines. This is the best way to convert organic acids to amines. Hence, carboxyl is replaced by amine. It is an example of a decarboxylation reaction.

R-COOH + N₃H → N₂ + CO₂ + R-NH₂

Reaction with phosphorus pentachloride and thionyl chloride

Any organic compound containing '-OH' reacts with phosphorus pentachloride (PCl₅) or thionyl chloride to convert the hydroxyl group by chloro group and HCl is produced also. Organic acids undergo this reaction to produce acid chlorides.

R-COOH + PCl₅ → R-CO-Cl + HCl + POCl₃

R-COOH + SOCl₂ → R-CO-Cl + HCl + SO₂

Reaction with ammonia 

Carboxylic acids react with ammonia to produce ammonium carboxylate. This is an acid-base reaction. When ammonium carboxylate is heated, it removes one molecule of water to produce acid amide. When ammonium carboxylate is dehydrated in presence of phosphorus pentoxide, it is converted to cyanide.

R-COOH + NH₃ → R-COO-NH₄

R-COO-NH₄ (heat) → R-CO-NH₂

R-COO-NH₄ +P₂O₅ → R-CN

Reaction with soda-lime

Soda (sodium hydroxide) and lime (calcium oxide) are the substituents of soda-lime. Soda reacts and lime acts as a dehydrating agent. Organic acids react with soda-lime to produce hydrogen gas (in the case of formic acid) or alkanes (in the case of others) along with decarboxylation. Sodium carbonate is produced here.

R-COOH + NaOH/CaO → R₂ + Na₂CO₃

Esterification reaction

Carboxylic acids are heated with alcohols in presence of concentrated sulfuric acid, water is released and ester is formed. This formation of the ester is very important for the identification of ethyl alcohol.

R-COOH + R-CH₂-OH + Conc. H₂SO₄ → R-COO-CH₂-R

Reaction with red phosphorus and hydrogen iodide

Red P and HI reduce organic acids to produce alkanes and water.

R-COOH + P(red) + HI + 6[H] → R-H + 2H₂O

Dehydration

Acids are dehydrated in presence of the dehydrating agent, generally P₂O₅, to produce acid anhydrides.

2R-COOH + P₂O₅ → (R-COO)₂O + H₂O

Uses of Carboxylic Acids

• Formic acid is used in leather tanning, textile, dyeing and finishing.
• Ethanoic acid is used in the production of vinegar, manufacture of rayon, plastics, rubber and silk.
• Benzoic acids and their salts are used as urinary antiseptics.
• Lactic acid is an important fermentation product produced in muscle cells of vertebrates and in certain bacterial species.
• Fatty acids present in the body are none other than carboxylic acids with a long hydrophobic chain, i.e. a large number of carbon atoms.

Carboxylic Acid Derivatives

The acid derivatives include esters, acid halides, acid amides and acid anhydrides.

Esters

These compounds bear the functional group '-COO-'. They can be synthesized by the reaction of alcohol with an organic acid in presence of the dehydrating agent, concentrated sulfuric acid. 

Transesterification reaction: This is basically a process called alcoholysis which is carried out by refluxing esters with a large excess of alcohol, preferably in the presence of a small amount of sodium alkoxide as a catalyst. Alcoholysis is usually effective in replacing higher alcohol with a lower one.

CH₃COOC₄H₉ + C₂H₅OH → CH₃COOC₂H₅ + C₄H₉OH

Acid chlorides

These compounds are designated as RCO-Cl (any acyl chain can be attached). They are synthesized by the reaction of carboxylic acids with PCl₅, PCl₃, sulfuryl chloride or thionyl chloride. 

Acid amides

Acid amides are designated as R-CO-NH2. They are synthesized by heating organic acids with ammonia.

Important uses of the Derivatives of Carboxylic Acids

• These derivatives are used in the manufacture of polymers, biopolymers, coatings and adhesives at the industrial level.
• They are used in the production of pharmaceutical drugs in the manufacturing of medicines.
• Methyl salicylate is a very important compound that is produced by heating methyl alcohol and salicylic acid together in the presence of concentrated sulfuric acid. Methyl salicylate is the principal component of pain reliever balms.
• Ethyl ethanoate is easily identified by its fruity odour, so it is used in the identification of acid or alcohol functional groups in the laboratory scale.
• Beta-lactam ring, a four-membered ring is a cyclic amide (N atom is attached to the beta carbon atom relative to the carbonyl group) that is present in the penicillin that acts as an antibiotic.

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for, Bachelors and Masters in Chemistry and Biochemistry.

Related concepts

• Acid-catalyzed hydrolysis.
• Base-catalyzed hydrolysis.
• Substitution reaction.
• Elimination reaction.
• Carbocations and carbanions.
• Carbenes.