In the evolving narrative of organic chemistry, carboxylic acids and amines hold center stage as functional groups that define acidity and basicity. From the sourness of vinegar to the complexity of neurotransmitters, these compounds form the backbone of both industrial applications and biological functions. Unit 5 explores the principles of acidity, substituent effects, and practical uses of these compounds, linking theory with everyday relevance.
Download UNIT 5 – Carboxylic Acids and Aliphatic Amines — The Chemistry of Acidity and Basicity Notes
Get simplified revision notes for this unit:
Download Unit 5 Notes PDF
Carboxylic Acids: Nature and Acidity
Carboxylic acids are characterized by the –COOH group, which imparts their acidic nature. The release of a proton (H⁺) from the carboxyl group defines their acidity. Unlike mineral acids, their acidic strength is moderate but significant enough to affect reactivity and applications.
Effect of Substituents on Acidity
The acidity of carboxylic acids does not remain constant; it is heavily influenced by substituents on the carbon chain:
Electron-withdrawing groups (–Cl, –NO₂) increase acidity by stabilizing the carboxylate anion through the inductive effect.
Electron-donating groups (–CH₃, –OCH₃) reduce acidity by destabilizing the anion.
This fine balance explains why trichloroacetic acid is stronger than acetic acid, despite having the same functional group.
Qualitative Tests
Carboxylic acids can be identified through simple laboratory techniques:
Effervescence with carbonates or bicarbonates indicates CO₂ evolution.
Litmus test confirms their acidic nature.
Formation of esters with alcohols in the presence of acid provides further evidence.
Notable Carboxylic Acids and Their Uses
The world of carboxylic acids extends far beyond textbooks, weaving into pharmaceuticals, food chemistry, and industry. Some notable members include:
Acetic Acid: The main component of vinegar, used in food preservation, solvents, and as a precursor in synthetic reactions.
Lactic Acid: Found in sour milk and muscles during anaerobic respiration, widely used in food and cosmetics.
Tartaric Acid: Key to baking powders and effervescent tablets.
Citric Acid: Naturally occurring in citrus fruits, employed in soft drinks, preservatives, and as an acidulant.
Succinic Acid: Involved in metabolic cycles, used in resins and polymers.
Oxalic Acid: Present in leafy vegetables, but also used industrially as a cleaning and bleaching agent.
Salicylic Acid: A cornerstone in skincare, known for its role in acne treatment.
Benzoic Acid: Preservative in foods and pharmaceuticals.
Benzyl Benzoate: Used as an antiparasitic agent and in fragrances.
Dimethyl Phthalate: A common plasticizer and insect repellent.
Methyl Salicylate: The familiar “oil of wintergreen,” used in liniments and analgesic balms.
Acetyl Salicylic Acid (Aspirin): One of the most famous pharmaceuticals, used as an analgesic, antipyretic, and anti-inflammatory drug.
These compounds highlight the dual nature of carboxylic acids: vital in biology and indispensable in industry.
Amides and Esters: Extensions of Carboxylic Acids
Carboxylic acids also give rise to amides and esters, derivatives that expand their utility.
Amides often play roles in pharmaceuticals and polymers like nylon.
Esters are renowned for their fruity aromas, making them staples in perfumes, flavorings, and solvents.
Aliphatic Amines: The Basic Counterpart
Switching from acidity to basicity, we encounter aliphatic amines, organic derivatives of ammonia where hydrogen atoms are replaced by alkyl groups. Their basic nature arises from the lone pair of electrons on nitrogen, which can accept a proton.
Factors Affecting Basicity
Electron-donating substituents enhance basicity by increasing electron density on nitrogen.
Electron-withdrawing groups reduce basicity.
Solvent effects also play a role: in water, hydrogen bonding stabilizes certain amines, affecting their apparent basicity.
Qualitative Tests
Amines can be detected through:
Carbylamine Test: Formation of foul-smelling isocyanides confirms primary amines.
Hinsberg Test: Distinguishes between primary, secondary, and tertiary amines.
Notable Aliphatic Amines and Their Applications
Ethanolamine: Used in pharmaceuticals, detergents, and as a gas scrubbing agent.
Ethylenediamine: A chelating agent and precursor in the production of resins and medicines.
Amphetamine: A potent central nervous system stimulant, used therapeutically but also associated with misuse.
These amines showcase how simple structural changes can yield compounds of vast medical and industrial significance.