Chapter 4: Carbon and Its Compounds

1. Introduction

Carbon is one of the most important elements on Earth. It forms a large number of compounds—more than any other element. Everything around us, from fuel and plastics to food and medicines, contains carbon. Compounds that contain carbon are called organic compounds.

Carbon forms so many compounds because of its special properties — catenation, tetravalency, and ability to form single, double, and triple bonds.

2. Bonding in Carbon

a. Valency of Carbon

Carbon has atomic number 6 and electronic configuration 2, 4.
 It has 4 electrons in its outermost shell, so it needs 4 more to complete its octet.
 Carbon can neither lose nor gain 4 electrons easily:

  • Losing 4 electrons would make it a C⁴⁺ ion, which requires too much energy.
  • Gaining 4 electrons would make it a C⁴⁻ ion, which is unstable.

Hence, carbon shares its four electrons with other atoms to form covalent bonds.

3. Covalent Bonding

A covalent bond is formed when two atoms share one or more pairs of electrons.
 Each shared pair represents one covalent bond.

Examples:
    • Methane (CH₄): Carbon shares its 4 electrons with 4 hydrogen atoms.
    • Oxygen (O₂): Each oxygen shares two pairs of electrons, forming a double bond.
    • Nitrogen (N₂): Each nitrogen shares three pairs, forming a triple bond.

Covalent bonds are strong but the compounds formed are usually:

    • Low melting and boiling
    • Poor conductors of electricity because they do not have ions.

4. Catenation

Catenation is the ability of carbon atoms to form long chains by bonding with other carbon atoms.

For example:

    • Chains: C–C–C–C

Branched chains:
   C

   |

C–C–C

    • Rings: C₆H₆ (Benzene)

Carbon can form single, double, or triple bonds with itself:

  • Single bond → Alkane
  • Double bond → Alkene
  • Triple bond → Alkyne

This makes carbon compounds extremely versatile and complex.

5. Versatility of Carbon

The two main reasons for carbon’s vast number of compounds are:

  1. Tetravalency – Carbon forms 4 covalent bonds.
  2. Catenation – Carbon forms long stable chains and rings.

Because of these properties, carbon compounds can be straight-chain, branched, or cyclic and may contain single, double, or triple bonds.

6. Hydrocarbons

Hydrocarbons are compounds made up of only carbon and hydrogen

a. Types of Hydrocarbons:

  1. Alkanes – Saturated hydrocarbons (single bonds only).
     General formula: CₙH₂ₙ₊₂
     Example: Methane (CH₄), Ethane (C₂H₆), Propane (C₃H₈)

  2. Alkenes – Unsaturated hydrocarbons (at least one double bond).
     General formula: CₙH₂ₙ
     Example: Ethene (C₂H₄), Propene (C₃H₆)

  3. Alkynes – Unsaturated hydrocarbons with at least one triple bond.
     General formula: CₙH₂ₙ₋₂
     Example: Ethyne (C₂H₂), Propyne (C₃H₄)

7. Functional Groups

Sometimes, one or more hydrogen atoms in a hydrocarbon are replaced by other atoms or groups of atoms. These atoms or groups are called functional groups because they decide the chemical properties of the compound.

Functional Group

Formula

Example

Name

Alcohol

–OH

CH₃OH

Methanol

Aldehyde

–CHO

CH₃CHO

Ethanal

Ketone

–CO–

CH₃COCH₃

Propanone

Carboxylic acid

–COOH

CH₃COOH

Ethanoic acid

Halogen

–Cl, –Br, –I

CH₃Cl

Chloromethane0

8. Homologous Series

A homologous series is a family of organic compounds having:

  • The same functional group
  • Similar chemical properties
  • A difference of –CH₂– (14 mass units) between consecutive members.

Example: Alcohol series
 CH₃OH (Methanol) → C₂H₅OH (Ethanol) → C₃H₇OH (Propanol)

All have the same functional group (–OH), and each differs by a –CH₂– group.

9. Nomenclature of Carbon Compounds

Naming organic compounds follows the IUPAC system.

Steps to name a compound:

  • Find the longest carbon chain (root name):

  • 1 carbon → Meth-
  • 2 carbon → Eth-
  • 3 carbon → Prop-
  • 4 carbon → But-
  • 5 carbon → Pent-, etc.

 

  • Identify the type of bonds:

  • Single → –ane
  • Double → –ene
  • Triple → –yne

 

  • Identify and name the functional group and its position.

Examples:

  • CH₄ → Methane
  • C₂H₆ → Ethane
  • CH₂=CH₂ → Ethene
  • CH≡CH → Ethyne
  • CH₃OH → Methanol
  • CH₃COOH → Ethanoic acid

10. Properties of Carbon Compounds

a. Physical Properties

  • Most carbon compounds are liquid or gas at room temperature.
  • They have low melting and boiling points.
  • They are poor conductors of electricity (no ions).

b. Chemical Properties

  1. Combustion:
     Carbon and its compounds burn in oxygen to produce carbon dioxide, water, and heat.
     Example:
     CH₄ + 2O₂ → CO₂ + 2H₂O + Heat
     (Used in fuels like LPG, CNG, petrol, etc.)

  2. Oxidation:
     Some compounds can be oxidized using oxidizing agents like alkaline KMnO₄ or acidified K₂Cr₂O₇.
     Example:
     Ethanol → Ethanoic acid (by oxidation)

  3. Addition Reaction:
     Occurs in unsaturated hydrocarbons (alkenes, alkynes).
     Hydrogen is added in the presence of catalysts (Ni or Pt).
     Example:
     CH₂=CH₂ + H₂ → CH₃–CH₃ (Ethene to Ethane)

  4. Substitution Reaction:
     Occurs in saturated hydrocarbons (alkanes).
     One or more hydrogen atoms are replaced by other atoms like chlorine.
     Example:
     CH₄ + Cl₂ → CH₃Cl + HCl (in sunlight)

11. Important Compounds of Carbon

(i) Ethanol (C₂H₅OH)

  • Commonly called alcohol or ethyl alcohol.
  • Colourless liquid, soluble in water.
  • Used in wine, beer, and spirits (drinking alcohol), fuels, and sanitizers.

Reactions:

  1. Combustion:
     C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O + Heat
  2. Oxidation:
     C₂H₅OH → CH₃COOH (Ethanoic acid)
  3. Dehydration:
     C₂H₅OH → C₂H₄ + H₂O (in presence of conc. H₂SO₄)

Important Notes:

  • Drinking pure ethanol can damage the liver and nervous system.

Denatured alcohol: Ethanol mixed with methanol or other poisonous substances to make it unfit for drinking.

(ii) Ethanoic Acid (CH₃COOH)

  • Commonly called acetic acid.
  • Sour-tasting, gives vinegar its flavor (vinegar = 5–8% acetic acid).
  • Weak acid, reacts with bases and carbonates.

Reactions:

  1. With base (Neutralization):
     CH₃COOH + NaOH → CH₃COONa + H₂O
  2. With carbonates:
     2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂

With alcohol (Esterification):
 CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O
 (Ethyl ethanoate, a pleasant-smelling ester)

12. Soap and Detergents

Soaps are sodium or potassium salts of fatty acids (e.g., sodium stearate).
 They are made by saponification, the reaction of fat with caustic soda (NaOH).

Reaction: Fat + NaOH → Soap + Glycerol

Working of Soap:

  • Soap molecules have two ends:
    • Hydrophobic tail (repels water, attracts oil)
    • Hydrophilic head (attracts water)
  • When soap is added to dirty water, the tails attach to grease, forming micelles.
  • Water washes away the micelles, removing dirt.

Limitation:
 Soaps don’t work well in hard water (containing Ca²⁺, Mg²⁺), as they form insoluble salts (scum).

Detergents:

  • Synthetic cleansing agents that work even in hard water.
  • Example: Sodium lauryl sulfate.

13. Summary

  1. Carbon forms covalent bonds due to its tetravalency.
  2. It forms millions of compounds due to catenation.
  3. Hydrocarbons are classified as alkanes, alkenes, and alkynes.
  4. Functional groups decide chemical behavior.
  5. Organic compounds show combustion, oxidation, addition, and substitution reactions.
  6. Ethanol and ethanoic acid are important carbon compounds.
  7. Soaps and detergents help in cleaning but have different working conditions.

14. Key Terms

  • Catenation – Self-linking property of carbon.
  • Covalent bond – Bond by sharing electrons.
  • Functional group – Reactive atom/group in a compound.
  • Homologous series – Series differing by –CH₂–.
  • Esterification – Reaction between acid and alcohol to form ester.
  • Saponification – Formation of soap from fats.

15.⚡ Quick Recap

  • Carbon forms 4 covalent bonds.
  • Forms millions of compounds due to catenation.
  • Ethanol and Ethanoic acid are the main carbon compounds.
  • Soaps clean in soft water; detergents in both soft and hard water.
  • Covalent compounds are non-conductors with low melting points.
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