Module 92. Organic Chemistry

Organic chemistry is the branch of chemistry that deals with the study of the structure, properties, composition, reactions, and synthesis of compounds containing carbon. These compounds, known as organic compounds, form the basis of all known life and are fundamental to many materials, from fuels and plastics to pharmaceuticals and biomolecules. The subject stands at the intersection of biology, medicine, and industry, making it one of the most dynamic and essential areas of modern science.

Historical Background

The origins of organic chemistry date back to the early nineteenth century when it was believed that organic compounds could only be produced by living organisms, a concept known as vitalism. This notion was challenged in 1828 when the German chemist Friedrich Wöhler synthesised urea (an organic compound found in urine) from inorganic ammonium cyanate. This experiment demonstrated that organic compounds could be created artificially, marking the birth of modern organic chemistry.
Throughout the nineteenth and twentieth centuries, scientists such as August Kekulé, Archibald Scott Couper, and Friedrich August Kekulé von Stradonitz contributed to the understanding of chemical bonding and molecular structures. Kekulé’s discovery of the benzene ring structure revolutionised the understanding of aromatic compounds. Later, advances in spectroscopy, chromatography, and quantum chemistry refined the study of organic molecules, allowing chemists to determine structures and mechanisms with great precision.

Characteristics of Organic Compounds

Organic compounds are primarily composed of carbon, hydrogen, oxygen, nitrogen, sulphur, and halogens. Carbon’s unique ability to form four covalent bonds and create long chains or rings (a property known as catenation) is the foundation of organic chemistry.
Some defining features include:

• Covalent bonding: Carbon atoms bond covalently with other atoms, forming stable compounds.
• Isomerism: Compounds with the same molecular formula but different structures, known as isomers, exhibit distinct physical and chemical properties.
• Functional groups: Specific groups of atoms within molecules that determine characteristic reactions. Examples include hydroxyl (–OH), carbonyl (–CO–), carboxyl (–COOH), and amino (–NH₂) groups.
• Solubility and melting point: Organic compounds often have lower melting and boiling points than inorganic compounds and are typically soluble in organic solvents rather than water.

Classification of Organic Compounds

Organic compounds can be broadly categorised based on their structure and functional groups:

1. Acyclic or open-chain compounds: Consist of straight or branched carbon chains, such as alkanes, alkenes, and alkynes.
2. Cyclic compounds: Contain carbon atoms arranged in rings.
   • Alicyclic compounds (e.g., cyclohexane) resemble aliphatic chains in properties.
   • Aromatic compounds (e.g., benzene) contain delocalised π-electrons and exhibit exceptional stability.
3. Heterocyclic compounds: Contain atoms other than carbon (such as oxygen, nitrogen, or sulphur) in the ring structure; examples include pyridine and furan.
4. Functional derivatives: Compounds containing specific reactive groups, such as alcohols, aldehydes, ketones, carboxylic acids, esters, amides, and halides.

Types of Organic Reactions

Organic chemistry involves various types of chemical reactions, each defined by the process and the nature of the reacting species. The major types include:

• Substitution reactions: One atom or group in a molecule is replaced by another, as in the halogenation of alkanes.
• Addition reactions: Two reactants combine to form a single product, common in alkenes and alkynes.
• Elimination reactions: Elements are removed from a molecule, often forming double or triple bonds.
• Oxidation–reduction (redox) reactions: Involve the gain or loss of electrons or oxygen/hydrogen atoms.
• Condensation reactions: Two molecules combine with the loss of a small molecule such as water.
• Polymerisation reactions: Small monomer units join to form large macromolecules, such as polyethylene or nylon.

Reaction mechanisms, representing step-by-step pathways, are vital in understanding how organic reactions occur. They involve reaction intermediates such as carbocations, carbanions, free radicals, and carbenes.

Structural Representation and Nomenclature

The International Union of Pure and Applied Chemistry (IUPAC) has established systematic rules for naming organic compounds. The nomenclature is based on identifying the parent hydrocarbon and the functional groups attached. For instance, CH₃CH₂OH is named ethanol, indicating a two-carbon chain (ethane) with a hydroxyl group (–OH).
Organic structures can be represented in several ways:

• Molecular formula: Indicates the number of atoms (e.g., C₂H₆O).
• Structural formula: Shows bonding and arrangement.
• Condensed formula: Simplified notation without showing all bonds.
• Line-angle formula: Common in organic chemistry, where carbon atoms are represented by line junctions.

Applications of Organic Chemistry

Organic chemistry underpins numerous scientific and industrial fields:

• Pharmaceuticals: Design and synthesis of drugs such as aspirin, penicillin, and paracetamol.
• Agriculture: Development of pesticides, herbicides, and fertilisers.
• Polymers and plastics: Production of materials like polyethylene, PVC, nylon, and polyester.
• Petrochemical industry: Refining of crude oil into fuels and raw materials for chemical synthesis.
• Biochemistry: Understanding biomolecules such as carbohydrates, proteins, lipids, and nucleic acids.
• Dyes and pigments: Creation of organic colourants for textiles and paints.

The field also plays a crucial role in green chemistry, which aims to design environmentally sustainable processes that minimise waste and use non-toxic reagents.

Analytical and Spectroscopic Techniques

Modern organic chemistry relies on a range of analytical methods to determine molecular structure and purity. Key techniques include:

• Mass spectrometry (MS): Determines molecular mass and composition.
• Infrared spectroscopy (IR): Identifies functional groups through bond vibration frequencies.
• Nuclear magnetic resonance (NMR) spectroscopy: Reveals details about hydrogen and carbon atom environments.
• Ultraviolet-visible (UV-Vis) spectroscopy: Examines electronic transitions in conjugated systems.
• Chromatography: Separates and analyses mixtures based on polarity or molecular size.

These techniques together allow chemists to elucidate complex molecular structures with remarkable accuracy.

Significance and Future Trends

Organic chemistry remains at the forefront of innovation, driving advancements in medicine, materials science, nanotechnology, and energy research. Developments in synthetic organic chemistry enable the creation of new molecules with precise biological or industrial functions. Computational chemistry and artificial intelligence are increasingly used to predict reaction outcomes and design efficient synthetic routes.
Moreover, the emphasis on sustainable practices has led to the growth of bio-based materials, renewable fuels, and biodegradable polymers, aligning organic chemistry with environmental goals.