Carbon and its Compounds

1. Introduction to Carbon

• Versatility: Carbon is a versatile element found in food, clothes, medicines, books, and all living structures.
• Occurrence:
  • Earth's Crust: Contains only 0.02% carbon in the form of minerals (carbonates, hydrogen-carbonates, coal, petroleum).
  • Atmosphere: Contains 0.03% carbon dioxide.
• Despite its small percentage in nature, its importance is immense.

2. Bonding in Carbon – The Covalent Bond

Carbon forms covalent bonds rather than ionic bonds due to its atomic structure (Atomic Number: 6; Configuration: 2, 4).

• Why not Ionic?
  • Gaining 4 electrons (C^4-): It is difficult for the nucleus with 6 protons to hold on to 10 electrons.
  • Losing 4 electrons (C⁴⁺): It requires a large amount of energy to remove 4 electrons.
• Solution (Sharing): Carbon shares valence electrons with other atoms to attain noble gas configuration. This sharing forms a Covalent Bond.
• Properties of Covalent Compounds:
  • Low melting and boiling points (weak intermolecular forces).
  • Poor conductors of electricity (no ions formed).
• Examples of Covalent Molecules:
  • Hydrogen (H₂): Single bond.
  • Oxygen (O₂): Double bond.
  • Nitrogen (N₂): Triple bond.
  • Methane (CH₄): Carbon shares 4 electrons with 4 Hydrogen atoms.

3. Allotropes of Carbon

The element carbon occurs in different physical forms known as allotropes:

• Diamond: Each carbon atom is bonded to four other carbon atoms forming a rigid three-dimensional structure. It is the hardest known substance and a poor conductor.
• Graphite: Each carbon atom is bonded to three others in the same plane, forming hexagonal arrays placed in layers. It is smooth, slippery, and a good conductor of electricity.
• Fullerenes: Carbon atoms arranged in the shape of a football (e.g., C-60).

4. Versatile Nature of Carbon

Two unique factors allow carbon to form millions of compounds:

• Catenation: The unique ability of carbon to form bonds with other carbon atoms, giving rise to large molecules (long chains, branched chains, or rings). Carbon-Carbon bonds are very strong and stable.
• Tetravalency: With a valency of 4, carbon is capable of bonding with four other atoms of carbon or atoms of other mono-valent elements (like Oxygen, Hydrogen, Nitrogen, Sulphur, Chlorine).

5. Types of Carbon Compounds

Saturated vs. Unsaturated

• Saturated Compounds (Alkanes): Carbon atoms linked by single bonds only. They are generally not very reactive.
• Unsaturated Compounds (Alkenes/Alkynes): Carbon atoms linked by double or triple bonds. They are more reactive than saturated compounds.

Structural Forms

• Chains: Straight chains of carbon (e.g., Propane, Butane).
• Branches: Carbon skeletons that branch off.
• Isomers: Compounds with the identical molecular formula but different structures (e.g., n-butane and iso-butane).
• Rings: Carbon atoms arranged in a ring (e.g., Cyclohexane - saturated; Benzene - unsaturated).

6. Functional Groups and Homologous Series

• Heteroatoms: Elements like oxygen, nitrogen, or halogens that replace hydrogen in a hydrocarbon chain.
• Functional Groups: Specific groups of atoms that confer specific properties to the compound, regardless of carbon chain length.
  • Halogens: Chloro-, Bromo-
  • Alcohol: -OH
  • Aldehyde: -CHO
  • Ketone: -CO-
  • Carboxylic Acid: -COOH
• Homologous Series: A series of compounds in which the same functional group substitutes for hydrogen in a carbon chain.
  • Successive members differ by a –CH₂– unit.
  • Successive members differ by 14 u in molecular mass.
  • Properties: Chemical properties remain similar; physical properties (like melting/boiling points) show a gradation (increase with mass).

7. Chemical Properties of Carbon Compounds

• Combustion:
  • Burning in oxygen gives CO₂, heat, and light.
  • Saturated Hydrocarbons: Clean blue flame.
  • Unsaturated Hydrocarbons: Yellow sooty flame (black smoke).
  • Note: Even saturated hydrocarbons give a sooty flame if air supply is limited.
• Oxidation:
  • Alcohols can be converted to carboxylic acids using oxidising agents like alkaline KMnO₄ or acidified K₂Cr₂O₇.
• Addition Reaction:
  • Unsaturated hydrocarbons add hydrogen in the presence of catalysts (Pd or Ni) to become saturated.
  • Used to hydrogenate vegetable oils (liquid) into vegetable ghee (solid).
• Substitution Reaction:
  • In sunlight, chlorine replaces hydrogen atoms in saturated hydrocarbons one by one.

8. Important Carbon Compounds

Ethanol (Alcohol)

• Liquid at room temperature, good solvent, soluble in water.
• Reaction with Sodium: Produces Sodium Ethoxide and Hydrogen gas.
• Dehydration: Heating with concentrated H₂SO₄ removes water to form Ethene.

Ethanoic Acid (Acetic Acid)

• 5-8% solution in water is Vinegar.
• Pure acid freezes in winter (Glacial Acetic Acid). It is a weak acid.
• Esterification: Reacts with alcohol to form sweet-smelling Esters (used in perfumes/flavoring).
• Saponification: Esters react with alkali to give back alcohol and sodium salt of acid (soap).
• Reaction with Carbonates/Bicarbonates: Produces salt, water, and Carbon Dioxide (brisk effervescence).

9. Soaps and Detergents

• Soap Molecules: Sodium or potassium salts of long-chain carboxylic acids.
  • Ionic End: Hydrophilic (dissolves in water).
  • Carbon Chain: Hydrophobic (dissolves in oil).
• Cleaning Action (Micelles):
  • In water, soap forms structures called micelles.
  • The hydrophobic tail attaches to dirt (oil), and the ionic head faces outward into the water.
  • This forms an emulsion, allowing dirt to be washed away.
• Scum: In hard water (containing Ca and Mg salts), soap forms an insoluble precipitate called scum, making it ineffective.
• Detergents: Ammonium or sulphonate salts of long-chain carboxylic acids. They do not form insoluble precipitates with Ca and Mg ions, making them effective in hard water.