Carbon has atomic number 6 with electronic configuration 2,4. It has 4 electrons in its outermost shell — it neither loses 4 electrons (too much energy needed) nor gains 4 electrons (nucleus too small to hold 10 electrons). So carbon forms 4 covalent bonds by sharing electrons. Covalent bond: A bond formed by mutual sharing of electron pairs between two atoms. Properties of covalent compounds: (1) Low melting and boiling points (weak intermolecular forces). (2) Poor conductors of electricity (no free ions). (3) Generally soluble in organic solvents (not in water). (4) Mostly gases or liquids or low-melting solids at room temperature. Versatility of carbon (catenation): Carbon atoms can bond with each other to form long chains, branched chains, or ring (cyclic) structures. This is called catenation. No other element shows catenation to such a large extent. Tetravalency: Carbon forms 4 bonds, giving rise to a vast number of compounds with H, O, N, S, halogens. This leads to the entire field of Organic Chemistry with millions of compounds.

Carbon exists in two main types of allotropes (different physical forms of the same element): (1) Diamond: Each carbon atom is bonded to 4 other carbon atoms in a rigid tetrahedral 3D network structure. Hardest natural substance. Very high melting point. Does NOT conduct electricity (no free electrons). Used in cutting tools, drilling, jewellery. (2) Graphite: Each carbon atom is bonded to 3 other carbon atoms in flat hexagonal layers (sheets). The 4th electron is free to move — hence graphite conducts electricity. Layers are held by weak van der Waals forces — they can slide over each other. Graphite is slippery and used as a lubricant and in pencil leads. Used as electrode in electrolysis and batteries. (3) Fullerenes (C₆₀ — Buckminsterfullerene): Recently discovered allotrope shaped like a football (60 carbon atoms in pentagons and hexagons). Used in nanotechnology, drug delivery, lubricants. Why different properties: Diamond (all 4 bonds used — no free electrons, rigid) vs Graphite (3 bonds in layer, 1 free — flexible layers, conducts electricity).

Hydrocarbons are compounds containing only carbon and hydrogen. Types: (1) Saturated hydrocarbons (Alkanes): All C–C bonds are single bonds. General formula: CₙH₂ₙ₊₂. Examples: Methane (CH₄), Ethane (C₂H₆), Propane (C₃H₈), Butane (C₄H₁₀). Alkanes burn with a clean blue flame. (2) Unsaturated hydrocarbons: Contain double or triple bonds between carbon atoms. Alkenes (C=C double bond): General formula CₙH₂ₙ. Examples: Ethene (C₂H₄), Propene (C₃H₆). Alkynes (C≡C triple bond): General formula CₙH₂ₙ₋₂. Example: Ethyne/Acetylene (C₂H₂). Unsaturated compounds burn with sooty/yellow flame due to incomplete combustion. Addition reactions: Unsaturated compounds undergo addition reactions with H₂ (hydrogenation), Cl₂ etc. Alkanes undergo substitution reactions. Homologous series: A series of carbon compounds with the same functional group and similar properties, differing by –CH₂– (14 u). Each member is called a homologue. Properties: (a) Same functional group, same general formula. (b) Similar chemical properties. (c) Show gradation in physical properties (BP increases by ~20°C per CH₂). (d) Can be prepared by the same general method.

Functional groups are atoms or groups of atoms that determine the chemical properties of organic compounds. Main functional groups: (1) Halo group (–Cl, –Br): Haloalkanes (e.g., CH₃Cl — chloromethane). (2) Alcohol group (–OH): Alcohols. Example: CH₃OH (methanol), C₂H₅OH (ethanol/ethyl alcohol). Ethanol: colourless liquid, BP 78°C, soluble in water. Used in medicines, as solvent, in alcoholic drinks. Reacts with Na to give H₂: 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂. Dehydration gives ethene: C₂H₅OH → C₂H₄ + H₂O (with H₂SO₄, 170°C). (3) Aldehyde group (–CHO): Aldehydes. HCHO (methanal/formaldehyde), CH₃CHO (ethanal). (4) Carboxylic acid group (–COOH): Carboxylic acids. CH₃COOH (ethanoic acid/acetic acid) — present in vinegar (3–5%). BP 118°C. Glacial acetic acid = pure CH₃COOH. Esterification: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ (ethyl ethanoate) + H₂O (with conc. H₂SO₄, heat). Esters have sweet fruity smell — used in perfumes, flavouring. Saponification: Ester + NaOH → Soap + Alcohol.

Soaps and detergents are cleansing agents. Soaps: Soaps are sodium or potassium salts of long-chain fatty acids (carboxylic acids). Made by saponification: Fat/Oil + NaOH → Soap + Glycerol. Example: C₁₇H₃₅COONa (sodium stearate). Structure of soap molecule: has a long non-polar (hydrophobic — water-hating) hydrocarbon tail and an ionic polar (hydrophilic — water-loving) head (–COONa). How soap cleans: Forms micelles — soap molecules arrange around a grease droplet with tails inside (in grease) and heads outside (in water), forming a spherical cluster. This emulsifies the grease which is then washed away. Soap does NOT work well in hard water: Hard water contains Ca²⁺ and Mg²⁺ ions which react with soap to form an insoluble scum (calcium/magnesium stearate): Ca²⁺ + 2C₁₇H₃₅COONa → (C₁₇H₃₅COO)₂Ca↓ + 2Na⁺. This scum is a white precipitate and wastes soap. Detergents: Synthetic cleansing agents made from petroleum products. Have a sulphonate/sulphate group instead of carboxylate. Advantages of detergents: (1) Work in hard water (Ca/Mg sulphonates are soluble). (2) Can work in acidic solutions. (3) More effective at lower concentrations. Disadvantage: Some detergents are non-biodegradable — cause water pollution.

Carbon has atomic number 6 (2, 4) and four valence electrons, so it is tetravalent and forms four covalent bonds. Catenation is the unique ability of carbon to form long chains, branched chains and rings by bonding to other carbon atoms; it is due to the strength of the C–C bond. Carbon forms single, double and triple bonds. Because of its small size, carbon also forms strong bonds with hydrogen, oxygen, nitrogen and halogens. These two properties explain the huge number of carbon compounds.

A homologous series is a series of compounds with the same functional group and similar chemical properties, where successive members differ by a –CH₂ unit (14 u). Alkanes CₙH₂ₙ₊₂, alkenes CₙH₂ₙ, alkynes CₙH₂ₙ₋₂. Naming: (1) count the carbon atoms in the longest chain; (2) the root: meth, eth, prop, but, pent, hex; (3) suffix: –ane (single bond), –ene (double), –yne (triple); (4) functional group suffix: –ol (alcohol), –al (aldehyde), –one (ketone), –oic acid (carboxylic acid); (5) halo groups as prefixes: chloro, bromo.

Compounds with the same molecular formula but different structures are isomers. Butane (C₄H₁₀) has two isomers: n-butane (CH₃–CH₂–CH₂–CH₃) and isobutane or 2-methylpropane (CH₃–CH(CH₃)–CH₃). Pentane (C₅H₁₂) has three isomers. Isomers have different physical properties (such as boiling points) and may differ in chemical properties. Isomerism is possible only when there are four or more carbon atoms in an alkane.

(1) Combustion: carbon compounds burn in air to give CO₂, water, heat and light: CH₄ + 2O₂ → CO₂ + 2H₂O + heat. Saturated hydrocarbons burn with a clean blue flame; unsaturated ones burn with a yellow sooty flame. (2) Oxidation: alcohols are oxidised to carboxylic acids by alkaline KMnO₄ or acidified K₂Cr₂O₇: CH₃CH₂OH → CH₃COOH. (3) Addition: unsaturated hydrocarbons add hydrogen in the presence of Ni or Pd: CH₂=CH₂ + H₂ → CH₃CH₃ (hydrogenation of vegetable oils to fats). (4) Substitution: in sunlight, CH₄ + Cl₂ → CH₃Cl + HCl.

Ethanol (C₂H₅OH) is a colourless liquid, soluble in water in all proportions, neutral to litmus. Reactions: with sodium: 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂ (hydrogen evolved); dehydration with hot conc. H₂SO₄ at 443 K: C₂H₅OH → CH₂=CH₂ + H₂O; oxidation to ethanoic acid. Uses: solvent in medicines, tinctures, cough syrups and tonics; fuel. Drinking alcohol harms the nervous system; denatured alcohol (mixed with poisonous methanol and a dye) is made unfit for drinking to prevent misuse.

Ethanoic acid (CH₃COOH, acetic acid) is a carboxylic acid; 5–8% solution in water is vinegar. Pure ethanoic acid freezes at 290 K and is called glacial acetic acid. It is a weak acid. Reactions: with sodium carbonate / hydrogencarbonate: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂; with a base: CH₃COOH + NaOH → CH₃COONa + H₂O; with an alcohol (esterification) in the presence of conc. H₂SO₄: CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O.

Esterification: a carboxylic acid reacts with an alcohol in the presence of a few drops of concentrated sulphuric acid to form a sweet-smelling ester: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ (ethyl ethanoate) + H₂O. Esters are used in perfumes and as flavouring agents. Saponification: esters react with an acid or base to give back the alcohol and carboxylate. With sodium hydroxide: CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH. This reaction is used to make soap from fats and oils.

Soaps are sodium or potassium salts of long-chain carboxylic acids. A soap molecule has a hydrophilic (water-loving) ionic head and a hydrophobic (water-hating) hydrocarbon tail. In water, soap molecules form spherical clusters called micelles with tails inward on the oily dirt and heads outward in water; the dirt is thus removed in rinsing. Soap forms insoluble scum with the calcium and magnesium salts in hard water. Detergents are sodium salts of sulphonic acids or ammonium salts with chloride or bromide ions; they do not form scum with hard water.

Complete combustion in sufficient air gives CO₂ and water; incomplete combustion gives carbon monoxide and soot (carbon). A saturated hydrocarbon burns with a clean blue flame because of complete combustion; unsaturated hydrocarbons burn with a yellow, sooty flame because of the large amount of carbon that is not completely oxidised. Coal and petroleum are fossil fuels formed from the decay of biomass. Burning is exothermic; the heat released per unit mass is the calorific value of the fuel.

Alkanes: methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀, pentane C₅H₁₂, hexane C₆H₁₄. Alkenes: ethene C₂H₄, propene C₃H₆, butene C₄H₈. Alkynes: ethyne C₂H₂, propyne C₃H₄. Alcohols: methanol CH₃OH, ethanol C₂H₅OH. Aldehydes: methanal HCHO, ethanal CH₃CHO. Ketones: propanone CH₃COCH₃. Carboxylic acids: methanoic acid HCOOH, ethanoic acid CH₃COOH. Functional groups: –OH, –CHO, >C=O, –COOH, halo (–Cl, –Br).