Carbon and its Compounds

1. Chapter at a glance

• Carbon forms covalent bonds by sharing valence electrons to attain noble gas configuration, resulting in molecules with low melting/boiling points and poor electrical conductivity.
• Catenation (self-linking of carbon atoms) and tetravalency enable formation of long chains, branched chains, rings and millions of carbon compounds.
• Carbon compounds are classified as saturated (only single C–C bonds) or unsaturated (double or triple C–C bonds).
• A homologous series is a family of compounds with the same functional group that differ by a –CH₂– unit and show gradation in physical properties but similar chemical properties.
• Functional groups (–OH, –CHO, –CO–, –COOH, halogens) confer characteristic properties regardless of chain length.
• Chemical reactions include combustion (complete/incomplete), oxidation, addition (hydrogenation) and substitution (chlorination in sunlight).
• Ethanol and ethanoic acid are important carbon compounds; soaps form micelles in water while detergents work in hard water.

2. Definitions and laws (exact NCERT wording)
- “Such bonds which are formed by the sharing of an electron pair between two atoms are known as covalent bonds.”
- “Compounds of carbon, which are linked by only single bonds between the carbon atoms are called saturated compounds.”
- “Compounds of carbon having double or triple bonds between their carbon atoms are called unsaturated compounds.”
- “This property is called catenation.” (ability of carbon to form bonds with other carbon atoms).
- “Such a series of compounds in which the same functional group substitutes for hydrogen in a carbon chain is called a homologous series.”
- “Compounds with identical molecular formula but different structures are called structural isomers.”
- “The charged ends of these compounds do not form insoluble precipitates with the calcium and magnesium ions in hard water.” (detergents).
- General formula for alkenes: CₙH₂ₙ (n = 2, 3, 4…).
- No SI units are given for physical constants in the chapter.

3. Important diagrams and activities

• Fig. 4.1–4.5: Electron-dot structures of H₂, Cl₂, O₂ (double bond), N₂ (triple bond) and CH₄ – demonstrate covalent bond formation.
• Fig. 4.6–4.7: Step-wise structures of ethane and ethene – show single vs double bonds.
• Fig. 4.8–4.10: Carbon skeletons and structures of butane isomers, cyclohexane and benzene – illustrate branching and rings.
• Structures of diamond, graphite and C-60 fullerene – show allotropes arising from different bonding arrangements.
• Fig. 4.12–4.13: Soap micelle and cleaning action – demonstrate emulsification of oil.
• Activities 4.3 & 4.4: Burning carbon compounds – distinguish clean vs sooty flame (saturated vs unsaturated).
• Activity 4.5: Alkaline KMnO₄ with ethanol – shows oxidation of alcohol to acid.
• Activity 4.6: Sodium with ethanol – evolution of H₂.
• Activity 4.10–4.12: Soap/detergent with oil and hard water – illustrate micelle formation and action in hard water.

4. Common misconceptions and exam pitfalls

• Assuming all carbon compounds are good conductors (text states they are poor conductors because no ions are formed).
• Confusing saturated (single bonds only) with unsaturated (double/triple bonds) compounds.
• Writing incorrect general formulae (alkanes CₙH₂ₙ+2, alkenes CₙH₂ₙ, alkynes CₙH₂ₙ–2).
• Forgetting that catenation is shown to the maximum extent by carbon only.
• Missing that addition reactions occur only with unsaturated compounds while substitution occurs with saturated ones in sunlight.
• Not balancing combustion equations or omitting “in presence of sunlight” for substitution.

5. Formula sheet

Key reactions (NCERT):
C + O₂ → CO₂ + heat + light
CH₄ + 2O₂ → CO₂ + 2H₂O + heat + light
C₂H₅OH → C₂H₄ + H₂O (conc. H₂SO₄, 443 K)
C₂H₄ + H₂ → C₂H₆ (Ni/Pd catalyst)
CH₄ + Cl₂ → CH₃Cl + HCl (sunlight)
2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂