Carbon and Its Compounds

Introduction

• Carbon is a versatile element, forming the basis of all living organisms and numerous compounds used in daily life.
• Its significance lies in both elemental forms (e.g., diamond, graphite) and combined forms (e.g., organic compounds).
• Activity 4.1: List ten items used or consumed since morning, categorize them into materials like metal, glass/clay, and others. Items with multiple materials are listed in all relevant categories. This highlights the ubiquity of carbon-based materials (e.g., plastics, food).
• Carbon’s ability to form millions of compounds, far outnumbering those of other elements, is due to its unique chemical properties, explored in this chapter.

Covalent Bonding in Carbon Compounds

• Carbon compounds primarily form covalent bonds, where electrons are shared between atoms to achieve a stable electron configuration.
• Properties:
  • Poor conductors of electricity (no free ions or electrons, unlike ionic compounds).
  • Low melting and boiling points due to weak intermolecular forces (e.g., Table 4.1: Methane: 90 K melting, 111 K boiling; Ethanol: 156 K melting, 351 K boiling).
  • These properties contrast with ionic compounds (Chapter 3), which have high melting points and conduct electricity when molten or dissolved.
• Carbon’s Electron Configuration:
  • Atomic number: 6; Electron configuration: 1s² 2s² 2p² (4 valence electrons).
  • Needs 4 more electrons to achieve noble gas configuration (octet rule).
  • Forming C⁴⁺ (losing 4 electrons) or C⁴⁻ (gaining 4) is energetically unfavorable due to high energy requirements or nuclear charge limitations.
  • Solution: Carbon shares electrons, forming covalent bonds with itself or other elements (e.g., H, O, N).
  • This sharing mechanism allows carbon to form stable, diverse molecules without forming ions, unlike metals or halogens.
• Examples of Covalent Bonding:
  • Hydrogen (H₂): Each H atom (1 electron) shares one electron, forming a single bond (Fig. 4.2).
  • Oxygen (O₂): Each O atom (6 valence electrons) shares two electrons, forming a double bond (Fig. 4.3).
  • Nitrogen (N₂): Each N atom (5 valence electrons) shares three electrons, forming a triple bond (Fig. 4.4).
  • Water (H₂O): Oxygen shares one electron with each H atom, forming two single bonds.
  • Ammonia (NH₃): Nitrogen shares one electron with each of three H atoms, forming three single bonds, with one lone pair.
  • Methane (CH₄): Carbon shares one electron with each of four H atoms, forming four single bonds (Fig. 4.5).
  • These examples illustrate how covalent bonds satisfy the valence requirements of atoms, leading to stable molecules.

Allotropes of Carbon

• Carbon exists in multiple forms (allotropes) with distinct structures and properties, despite identical chemical composition.
• Diamond:
  • Structure: Each carbon atom is tetrahedrally bonded to four others in a 3D lattice.
  • Properties: Hardest known substance, high melting point, non-conductor of electricity (no free electrons).
  • Uses: Cutting tools, jewelry.
• Graphite:
  • Structure: Carbon atoms form hexagonal layers with three bonds each (one double bond); layers are weakly bonded.
  • Properties: Soft, slippery, good conductor of electricity (due to delocalized electrons in layers).
  • Uses: Pencils, lubricants, electrodes.
• Fullerenes (C-60):
  • Structure: 60 carbon atoms arranged in a spherical, football-like structure (Buckminsterfullerene).
  • Properties: Unique molecular structure, potential applications in nanotechnology.
  • The structural differences in allotropes arise from the arrangement of covalent bonds, showcasing carbon’s versatility.

Versatile Nature of Carbon

• Carbon forms millions of compounds due to two key properties:
• Catenation:
  • Ability to form long chains, branched chains, or rings by bonding with other carbon atoms.
  • Chains can have single, double, or triple bonds, leading to diverse structures.
  • Unlike silicon, which forms less stable chains, carbon’s strong C-C bonds ensure stability.
• Tetravalency:
  • Carbon’s four valence electrons allow bonding with up to four other atoms (C or other elements like H, O, N, Cl).
  • Small atomic size ensures strong bonds, as the nucleus holds shared electrons tightly.
  • This enables carbon to form complex molecules with varied properties, unlike larger atoms with weaker bonds.
• Organic Chemistry:
  • Originally thought to require a “vital force,” disproved by Wöhler’s synthesis of urea (1828).
  • Excludes carbides, oxides, carbonates, and hydrogencarbonates, focusing on hydrocarbons and derivatives.

Saturated and Unsaturated Carbon Compounds

• Saturated Compounds:
  • Contain only single bonds between carbon atoms (e.g., alkanes: methane, ethane).
  • Structure: Carbon atoms are linked, and remaining valencies are satisfied by hydrogen (e.g., Ethane, C₂H₆, Fig. 4.6).
  • Properties: Less reactive due to stable single bonds.
• Unsaturated Compounds:
  • Contain double or triple bonds (e.g., alkenes: ethene, C₂H₄; alkynes: ethyne, C₂H₂).
  • Structure: Double/triple bonds satisfy carbon’s valency with fewer atoms (Fig. 4.7).
  • Properties: More reactive due to the presence of pi bonds, which are easier to break.
  • The reactivity difference is key in applications like hydrogenation of oils.

Chains, Branches, and Rings

• Carbon chains vary in length and structure:
  • Straight Chains: E.g., Methane (C₁), Ethane (C₂), Propane (C₃).
  • Branched Chains: E.g., Isobutane (C₄H₁₀, Fig. 4.8).
  • Rings: E.g., Cyclohexane (C₆H₁₂).
• Structural Isomers:
  • Compounds with the same molecular formula but different structures (e.g., n-butane and isobutane, both C₄H₁₀).
  • Different structures lead to different physical and chemical properties.
  • Isomerism increases the diversity of carbon compounds, impacting their applications.

Functional Groups and Homologous Series

• Functional Groups:
  • Atoms or groups replacing hydrogen in hydrocarbons, conferring specific properties (Table 4.3).
  • Examples:
    • Haloalkanes: -Cl, -Br.
    • Alcohol: -OH.
    • Aldehyde: -CHO.
    • Ketone: -CO-.
    • Carboxylic acid: -COOH.
  • Functional groups determine the reactivity and applications of compounds, e.g., alcohols in solvents, carboxylic acids in preservatives.
• Homologous Series:
  • Series of compounds with the same functional group and similar chemical properties, differing by a -CH₂- unit.
  • Example: Alkanes (CH₄, C₂H₆, C₃H₈); Alkenes (C₂H₄, C₃H₆).
  • Molecular mass difference: 14 u (C: 12 u, 2H: 2 u).
  • Homologous series exhibit gradual changes in physical properties (e.g., boiling points increase with chain length).

Carbon Compounds as Fuels

• Carbon compounds (e.g., coal, petroleum, natural gas) are major energy sources due to high energy release during combustion.
• Flame vs. Non-Flame Combustion:
  • Gaseous fuels (e.g., LPG, candle wax vapor) burn with a flame due to vaporized volatile substances.
  • Solid fuels (e.g., coal, charcoal) glow red without a flame, as they lack sufficient volatile components.
  • Yellow candle flame: Due to incomplete combustion producing carbon (soot).
• Fossil Fuels:
  • Formed from biomass (plants, marine organisms) under high pressure and temperature over millions of years.
  • Coal: From terrestrial plants; Oil/Gas: From marine organisms trapped in porous rock.
  • Environmental Impact: Combustion produces SO₂, NOₓ (pollutants).
  • Fossil fuels are non-renewable, driving research into sustainable alternatives.

Chemical Reactions of Carbon Compounds

Oxidation

• Carbon compounds undergo oxidation, adding oxygen or removing hydrogen.
• Example: Ethanol to Ethanoic acid (Activity 4.5):
  • CH₃CH₂OH → CH₃COOH (using alkaline KMnO₄ or acidified K₂Cr₂O₇).
  • Oxidizing agents (KMnO₄, K₂Cr₂O₇) facilitate oxygen addition.
• Oxidation is critical in industrial processes and metabolic pathways.

Addition Reaction

• Unsaturated hydrocarbons add hydrogen across double/triple bonds, forming saturated compounds.
• Example: Ethene + H₂ → Ethane (with Ni/Pd catalyst).
• Application: Hydrogenation of vegetable oils to produce solid fats (e.g., margarine).
• Unsaturated oils are healthier due to lower saturated fat content.

Substitution Reaction

• Saturated hydrocarbons react with halogens (e.g., Cl₂) in sunlight, replacing H with Cl.
• Example: CH₄ + Cl₂ → CH₃Cl + HCl.
• Substitution reactions are stepwise, producing multiple products with higher homologues.

Important Carbon Compounds

Ethanol (C₂H₅OH)

• Properties:
  • Liquid at room temperature (melting: 156 K, boiling: 351 K).
  • Soluble in water (miscible in all proportions).
  • Used in alcoholic beverages, solvents, medicines.
  • Health Effects: Causes drunkenness, impairs coordination, long-term use leads to health issues.
  • Denatured Alcohol: Mixed with methanol/dyes to prevent misuse.
• Reactions:
  • With Sodium (Activity 4.6): 2Na + 2CH₃CH₂OH → 2CH₃CH₂O⁻Na⁺ + H₂.
  • Dehydration: CH₃CH₂OH → CH₂=CH₂ + H₂O (with hot conc. H₂SO₄).
• Ethanol’s solvent properties and reactivity make it industrially significant.

Ethanoic Acid (CH₃COOH)

• Properties:
  • Weak acid (partially ionizes, unlike strong acids like HCl).
  • 5-8% solution (vinegar) used as a preservative.
  • Melting point: 290 K (freezes in cold climates, hence “glacial”).
• Reactions:
  • Esterification (Activity 4.8): CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O (sweet-smelling ester).
  • Saponification: CH₃COOC₂H₅ + NaOH → C₂H₅OH + CH₃COONa.
  • With Base: CH₃COOH + NaOH → CH₃COONa + H₂O.
  • With Carbonates (Activity 4.9): 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂.
• Ethanoic acid’s acidic nature and ester formation are key in food and chemical industries.

Soaps and Detergents

• Soaps:
  • Sodium/potassium salts of long-chain carboxylic acids.
  • Form micelles (Fig. 4.12): Hydrophilic head interacts with water, hydrophobic tail with oil, emulsifying dirt (Activity 4.10).
  • Issue: Forms scum with hard water (Ca²⁺, Mg²⁺ ions) (Activity 4.11).
• Detergents:
  • Sodium salts of sulphonic acids or ammonium salts; effective in hard water (Activity 4.12).
  • Used in shampoos, laundry products.
• The cleaning mechanism relies on the amphiphilic nature of soap/detergent molecules.

Key Questions

1. CO₂ Electron Dot Structure: O=C=O (double bonds, each O has two lone pairs).
2. S₈ Structure: Ring of eight S atoms, each bonded to two others via single bonds.
3. Ethanol to Ethanoic Acid: Oxidation (adds O, removes H); confirmed by color change of KMnO₄.
4. Ethyne Welding: Ethyne + O₂ burns hotter and cleaner than ethyne + air (less N₂, fewer pollutants).
5. Alcohol vs. Carboxylic Acid: Litmus test (acid turns blue litmus red, alcohol neutral); pH (acid lower); esterification (acid forms ester).
6. Structures:
   • Ethanoic Acid: CH₃COOH.
   • Bromopentane: C₅H₁₁Br (multiple isomers possible).
   • Butanone: CH₃COCH₂CH₃.
   • Hexanal: CH₃(CH₂)₄CHO.
7. Naming:
   • CH₃CH₂Br: Bromoethane.
   • HCHO: Methanal.
   • Cyclohexane: C₆H₁₂ (ring structure).