Chapter 4 Carbon and its Compounds

A covalent bond is formed when two atoms share a pair of electrons to achieve a stable electron configuration, typically by completing their outermost shell. Carbon has four electrons in its outermost shell and needs four more to complete it. Carbon cannot form ionic bonds by gaining or losing four electrons because it would require too much energy. Instead, carbon shares electrons with other atoms, forming strong and stable covalent bonds. Carbon can form single, double, or triple covalent bonds. For example, methane (CH₄) has four single bonds, ethylene (C₂H₄) has one double bond between carbons, and acetylene (C₂H₂) has a triple bond. This ability to form multiple bonds makes carbon the backbone of organic chemistry.

Saturated hydrocarbons contain only single bonds between carbon atoms and are called alkanes. They are considered saturated because each carbon is bonded to the maximum number of hydrogen atoms possible. Examples include methane (CH₄), ethane (C₂H₆), and propane (C₃H₈). Unsaturated hydrocarbons contain at least one double or triple bond between carbon atoms. Those with double bonds are alkenes (like ethene, C₂H₄) and those with triple bonds are alkynes (like ethyne, C₂H₂). Unsaturated compounds are more reactive than saturated ones and can undergo addition reactions where hydrogen, halogens, or other small molecules are added across the double or triple bond. Vegetable oils are unsaturated fats while animal fats like butter are saturated.

Ethanol (C₂H₅OH), commonly called alcohol, is a colourless liquid with a pleasant smell and low boiling point of 78°C. It is miscible with water in all proportions. Ethanol is used as a solvent in medicines, perfumes, and paints. It is the alcohol present in alcoholic beverages and is also used as a fuel additive. When ethanol burns, it produces carbon dioxide and water. Ethanoic acid (CH₃COOH), commonly called acetic acid or vinegar (when dilute), has a sour taste and a pungent smell. Its melting point is 17°C, so it freezes in cold weather earning it the name glacial acetic acid. It is used in preserving food, making vinegar, and as a solvent. Both compounds are important in everyday life and industrial processes.

A homologous series is a group of organic compounds that have the same general formula, similar chemical properties, and show a gradation in physical properties with each successive member differing by a CH₂ unit. For example, the alkane series: methane (CH₄), ethane (C₂H₆), propane (C₃H₈), and butane (C₄H₁₀) — each differs by one CH₂ group. Key characteristics include: successive members differ by 14 atomic mass units (one CH₂ unit), all members can be represented by a single general formula, they have similar chemical reactions, and their physical properties like boiling point, melting point, and density change gradually. This gradation helps predict the properties of unknown members and makes studying organic chemistry more systematic and organised.

Saponification is the chemical reaction in which a fat or oil (ester) reacts with a strong base like sodium hydroxide or potassium hydroxide to produce soap (sodium or potassium salt of fatty acid) and glycerol. The word saponification literally means soap making. When vegetable oil or animal fat is heated with concentrated NaOH solution, the ester bonds in the triglycerides break down, releasing glycerol and long-chain fatty acid salts — which are soaps. Sodium soaps are hard soaps used for bathing and washing, while potassium soaps are soft soaps used in shaving creams and liquid soaps. Soap cleans because one end of the molecule (hydrophilic head) attracts water while the other end (hydrophobic tail) attracts oil and grease, forming micelles that trap dirt particles.