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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

Representing Organic Compounds

NCERT Reference: Chapter 8 – Organic Chemistry – Pages 191–193

Quick Notes

  • Empirical formula – Simplest whole-number ratio of atoms in a compound. (e.g. CH2 for C2H4)
  • Molecular formula – Actual number of atoms of each element. (e.g. C2H4)
  • Full structural formula – Shows all atoms and all bonds.
  • Condensed structural formula – Groups atoms to simplify notation. (e.g. CH3CH3 for ethane)
  • Skeletal formula – Omits C and H atoms bonded to C; only bonds and functional groups are shown.
  • Stereochemical formula – Shows 3D arrangement (e.g. wedges and dashes); only drawn when required.

Full Notes

Overview of Formula Types

Organic molecules can be written using different formula types and you need to be comfortable using and changing between the following:

Formula Type Description Example (Butane)
Empirical Simplest ratio of atoms C2H5
Molecular Actual number of atoms C4H10
Full Structural Shows all atoms and all covalent bonds H3C–CH2–CH2–CH3
Condensed Structural Atoms grouped for compactness CH3CH2CH2CH3
Skeletal Carbon atoms implied at ends and bends, H not shown Zigzag line (structure only)

Molecular Formula – Shows the actual number of atoms in a molecule.

ExampleHexane → C6H14

NCERT 11 Chemistry molecular formula example of hexane C6H14

Empirical Formula – Shows the simplest whole number ratio of atoms in a compound.

Example Hexane (C6H14) → C3H7

NCERT 11 Chemistry empirical formula example showing hexane simplified to C3H7

Structural Formula – Shows how atoms are arranged in a molecule without drawing bonds.

Example Hexane → CH3CH2CH2CH2CH2CH3

NCERT 11 Chemistry structural formula example for hexane showing CH3CH2 repeats

Displayed Formula – Shows all bonds and atoms.

Example Hexane

NCERT 11 Chemistry displayed formula diagram of hexane with all bonds shown

Skeletal Formula – Simplified representation where carbon atoms are at the ends and bends of lines, and hydrogens bonded to carbon atoms aren’t shown.

Example Hexane

NCERT 11 Chemistry skeletal formula diagram of hexane as a zig-zag line
Photo of Matt
Matt’s exam tip

Matt’s exam tip - if you find drawing skeletal formulas hard, start by drawing the displayed formula with a diagonal pattern between C-C bonds, then remove all the C-H bonds.
CIE A-Level Chemistry tip image showing how to convert from displayed to skeletal by removing C–H bonds

Wedge-Dash Notation

Molecules are 3D and we can represent this using wedge and dash notation to show bond angles.

To capture the 3D geometry on a flat surface:

Example The tetrahedral geometry of methane can be drawn with one hydrogen pointing out and one pointing back using wedges

NCERT 11 Chemistry stereochemical wedge-and-dash diagram showing methane tetrahedral 3D arrangement.

Example Ethene (CH2CH2) is planar (120°) around sp² carbon

NCERT 11 Chemistry stereochemical diagram showing trigonal planar bonding of ethene with double bond.

Summary