Carbon and its Compounds

• Why carbon forms such a huge number of compounds (covalent bonding, catenation, tetravalency)
• The difference between saturated (alkanes) and unsaturated (alkenes, alkynes) hydrocarbons
• Homologous series and how a functional group decides chemical behaviour
• Common reactions of carbon compounds: combustion, oxidation, addition, substitution
• Everyday chemistry of ethanol, ethanoic acid, soaps and detergents

1. Covalent bonding: Carbon has 4 valence electrons. Instead of losing/gaining electrons (which would need a lot of energy for a small atom), it shares electron pairs with other atoms to complete its octet, forming strong covalent bonds. This gives carbon compounds low melting/boiling points and generally poor electrical conductivity.
2. Catenation: Carbon atoms can link to other carbon atoms through single, double, or triple bonds to form long chains, branched chains, and rings. This self-linking ability (catenation) is stronger in carbon than in any other element, which is the main reason for the huge number of carbon compounds.
3. Tetravalency: Because carbon has 4 valence electrons, it forms 4 covalent bonds, allowing it to bond with atoms of carbon, hydrogen, oxygen, nitrogen, sulfur, chlorine and other elements, producing compounds with widely different properties.
4. Saturated hydrocarbons (alkanes): Contain only single covalent bonds between carbon atoms, general formula CnH2n+2 (e.g. methane CH4, ethane C2H6). They burn with a clean blue flame and mainly undergo substitution reactions.
5. Unsaturated hydrocarbons (alkenes and alkynes): Alkenes contain at least one C=C double bond, general formula CnH2n (e.g. ethene C2H4). Alkynes contain at least one C≡C triple bond, general formula CnH2n-2 (e.g. ethyne C2H2). They burn with a sooty, yellow flame and readily undergo addition reactions.
6. Homologous series: A family of compounds with the same general formula and same functional group, where each successive member differs from the previous one by a -CH2- unit (14 u mass difference). Members show a gradual change in physical properties (e.g. increasing boiling point) but similar chemical properties.
7. Functional groups: An atom or group of atoms that decides the characteristic chemical properties of a compound. Common ones: -OH (alcohol), -CHO (aldehyde), -CO- (ketone), -COOH (carboxylic acid), -X (halide, X = Cl/Br/I), -NH2 (amine).
8. Nomenclature basics: Name = (prefix for branches/substituents) + (stem indicating number of carbon atoms: meth-1, eth-2, prop-3, but-4...) + (suffix for functional group/unsaturation: -ane for single bonds, -ene for double bond, -yne for triple bond, -ol for -OH, -al for -CHO, -oic acid for -COOH).
9. Chemical properties:
   • Combustion: Carbon compounds burn in oxygen to give CO2, H2O and release heat and light — this is why hydrocarbons like LPG and CNG are used as fuels.
   • Oxidation: Alcohols can be oxidised to carboxylic acids using oxidising agents like alkaline KMnO4 or acidified K2Cr2O7 (e.g. ethanol → ethanoic acid).
   • Addition reaction: Unsaturated hydrocarbons add hydrogen in the presence of a catalyst (Ni/Pd) to form saturated hydrocarbons — used in hydrogenation of vegetable oils (making vanaspati ghee).
   • Substitution reaction: Saturated hydrocarbons react with chlorine in the presence of sunlight; hydrogen atoms are replaced one by one by chlorine atoms (e.g. CH4 + Cl2 → CH3Cl + HCl).
10. Ethanol (C2H5OH): A common alcohol, liquid at room temperature, miscible with water, used as a solvent and in medicines. Reacts with sodium to release hydrogen gas, and undergoes dehydration with hot concentrated H2SO4 (at 443 K) to form ethene.
11. Ethanoic acid (CH3COOH): A weak carboxylic acid, present in vinegar (5-8% solution). Turns blue litmus red, reacts with sodium carbonate/bicarbonate to release CO2, and reacts with alcohols in the presence of an acid catalyst to form a sweet-smelling ester (esterification).
12. Soaps and detergents: Soaps are sodium or potassium salts of long-chain carboxylic acids (fatty acids). Each soap molecule has a hydrophilic (water-loving) -COO⁻Na⁺ head and a hydrophobic (water-repelling) hydrocarbon tail. In water, soap molecules cluster with tails inward and heads outward around oil/grease droplets to form a micelle, which keeps the dirt suspended (emulsified) so it can be washed away. Detergents work similarly but are effective even in hard water, unlike soaps which form insoluble scum with Ca²⁺/Mg²⁺ ions.

Write the balanced equation for the complete combustion of ethane (C2H6) and identify the type of reaction.

2 C2H6 + 7 O2 -> 4 CO2 + 6 H2O

This is a combustion reaction (an oxidation reaction) — ethane, a saturated hydrocarbon, burns completely in excess oxygen to give carbon dioxide, water, and release heat and light.

• Confusing addition reactions (only for unsaturated compounds, no new molecule lost) with substitution reactions (for saturated compounds, one atom replaces another, releasing a small molecule like HCl).
• Writing the general formula of alkynes as CnH2n-2 correctly, but forgetting alkenes are CnH2n and alkanes are CnH2n+2 — mixing these up is a very common exam error.
• Thinking soap "kills" or "dissolves" dirt chemically — actually it physically traps grease inside a micelle so it can mix with water and be rinsed away.
• Believing ethanol and ethanoic acid are the same functional group — ethanol has -OH (alcohol), ethanoic acid has -COOH (carboxylic acid), and they have very different reactivity (e.g. only ethanoic acid reacts with NaHCO3 to release CO2 gas).

• Why does carbon form covalent bonds instead of ionic bonds?
• What is a micelle, and why is it important for the cleaning action of soap?

Open the Practice tab for graded questions on Carbon and its Compounds.