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1 Some Basic Concepts of Chemistry 2 Structure of Atom 3 Classification of Elements and Periodicity in Properties 4 Chemical Bonding and Molecular Structure 5 Thermodynamics 6 Equilibrium 7 Redox Reactions 8 Organic Chemistry – Some Basic Principles and Techniques 9 Hydrocarbons

8 Organic Chemistry – Some Basic Principles and Techniques

8.1 General Introduction (Organic Chemistry) 8.2 Tetravalence of Carbon - Shapes of Organic Compounds 8.3 Structural Representations of Organic Compounds 8.4 Classification of Organic Compounds 8.5 Nomenclature of Organic Compounds 8.6 Isomerism 8.7 Fundamental Concepts in Organic Reaction Mechanism 8.8 Methods of Purification of Organic Compounds 8.9 Qualitative Analysis of Organic Compounds 8.10 Quantitative Analysis of Organic Compounds

Tetravalence of Carbon: Shapes of Organic Compounds

NCERT Reference: Chapter 8 – Organic Chemistry– Page 238–239 (Part II)

Quick Notes

  • Tetravalency of Carbon: Carbon has 4 valence electrons and can form 4 covalent bonds.
  • Carbon Hybridisation:
    • sp3 → tetrahedral (e.g., methane)
    • sp2 → planar trigonal (e.g., ethene)
    • sp → linear (e.g., ethyne)
  • π Bonds: Formed by sideways overlap of unhybridized p orbitals and found in double and triple bonds.
  • Bond Characteristics:
    • σ bonds are strong and stable.
    • π bonds restrict rotation, affecting shape & reactivity.

Full Notes

Carbon is central to organic chemistry due to its ability to form four covalent bonds, a property stemming from its electronic configuration (1s2 2s2 2p2).

To achieve a stable octet, it forms four bonds by sharing its valence electrons. This tetravalency leads to a variety of stable and diverse organic molecules.

The Shapes of Carbon Compounds

The geometry of organic molecules is dictated by the type of hybrid orbitals formed by carbon atoms:

Ethane (CH3CH3):
Carbon undergoes sp3 hybridisation, forming four sigma (σ) bonds with four hydrogen atoms.

IB Chemistry NCERT Class 11 diagram showing sp3 hybridisation in methane with a tetrahedral shape and 109.5° bond angles.

Shape around carbon: Tetrahedral
Bond Angle: 109.5°

Ethene (C2H4):
Each carbon is sp2 hybridised — three sp2 orbitals form σ bonds (2 with H, 1 with C), and the unhybridised p orbital on each carbon overlaps sideways to form a π bond.

IB Chemistry NCERT Class 11 diagram showing sp2 hybridisation in ethene with a planar trigonal geometry and 120° bond angles; the p orbitals overlap to form a pi bond.

Shape: Planar
Bond Angle: 120°

Ethyne (C2H2):
Carbon atoms undergo sp hybridisation — one sp orbital forms a σ bond with hydrogen, the other with the other carbon. The two unhybridised p orbitals on each carbon form two π bonds (a triple bond).

IB Chemistry NCERT Class 11 diagram showing sp hybridisation in ethyne with linear geometry and 180° bond angles; two orthogonal p orbitals form two pi bonds.

Shape: Linear
Bond Angle: 180°

These spatial arrangements are critical in determining the chemical and physical properties of organic compounds.

Some Characteristic Features of π Bonds

Formation:
π bonds result from the sideways (lateral) overlap of unhybridized p orbitals on adjacent carbon atoms. Unlike σ bonds, which involve head-on overlap, π bonds are weaker and more reactive.

Presence in Double/Triple Bonds:

Restricted Rotation:
The presence of a π bond restricts the rotation around the bond axis due to the overlap alignment. This affects molecular conformation and is important in understanding stereochemistry (e.g., cis-trans isomerism).

Electron Density:
π electrons are more exposed (above and below the plane of the atoms) making π bonds more reactive in electrophilic addition reactions.

Summary