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*Revision Materials* 1 Atomic Structure 2 Atoms, molecules and stoichiometry 3 Chemical Bonding 4 States of matter 5 Chemical energetics 6 Electrochemistry 7 Equilibria 8 Reaction kinetics 9 The Periodic Table, chemical periodicity 10 Group 2 11 Group 17 12 Nitrogen and sulfur 13 Organic 14 Hydrocarbons 15 Halogen compounds 16 Hydroxy compounds 17 Carbonyl compounds 18 Carboxylic acids and derivatives 19 Nitrogen compounds 20 Polymerisation 21 Organic synthesis 22 Analytical techniques 23 Chemical energetics 24 Electrochemistry 25 Equilibria 26 Reaction kinetics 27 Group 2 28 Chemistry of transition elements 29 Organic 30 Hydrocarbons 31 Halogen compounds 32 Hydroxy compounds 33 Carboxylic acids and derivatives 34 Nitrogen compounds 35 Polymerisation 36 Organic synthesis 37 Analytical techniques

13 Organic

13.1 Formulas, functional groups and the naming of organic compounds 13.2 Characteristic organic reactions 13.3 Shapes of organic molecules; σ and π bonds 13.4 Isomerism, structural isomerism and stereoisomerism

Shapes of organic molecules; σ and π bonds

Specification Reference Organic Chemistry, An introduction to AS level organic chemistry 13.3

Quick Notes

  • Organic molecules can be straight-chained, branched, or cyclic.
  • Bond angles and shape depend on hybridisation of carbon atoms:
    • sp → linear (bond angle of 180°)
    • sp² → trigonal planar (bond angles of 120°)
    • sp³ → tetrahedral (bond angles of 109.5°)
  • σ (sigma) bonds form by end-on orbital overlap
  • π (pi) bonds form by sideways overlap of p orbitals
  • π bonds restrict rotation and are weaker than σ bonds.
  • Molecules like ethene are planar due to sp² hybridisation

Full Notes

Molecular Types: Straight-Chain, Branched, and Cyclic

Organic molecules can be classified by their carbon backbone:

CIE A-Level Chemistry diagram showing straight-chain, branched, and cyclic carbon backbones.

These variations can affect physical properties and reactivity.

Hybridisation and Molecular Shape

Hybridisation is the mixing of atomic orbitals to form new, equivalent hybrid orbitals. Carbon is often used as an example for the forming of hybrid orbitals as it reacts.

CIE A-Level Chemistry diagram showing sp, sp2 and sp3 hybridisation of carbon orbitals.

Carbon atoms can form different shapes depending on their hybridisation:

Hybridisation Bonding Shape Bond Angle
sp Found in molecules with triple bonds (C≡C) Linear 180°
sp² Found in molecules with double bonds (C=C) Trigonal planar 120°
sp³ Found in single-bonded carbon atoms Tetrahedral 109.5°

Sigma (σ) and Pi (π) Bonds

σ and π bonding has been covered in more detail here

All single covalent bonds are σ bonds, with double and triple bonds also containing π bonds:

σ (Sigma) Bonds

Formed by end-on overlap of orbitals (e.g. s–s, s–p, or p–p)

CIE A-Level Chemistry diagram showing sigma bond formation from end-on orbital overlap.

π (Pi) Bonds

Formed by sideways overlap of unhybridised p orbitals

CIE A-Level Chemistry diagram showing pi bond formation from sideways overlap of p orbitals.

Examples:

CIE A-Level Chemistry diagram comparing sigma and pi bonding in organic molecules.

Use of the Term "Planar"

Planar means flat, i.e. all atoms lie in the same plane.
Molecules with sp² hybridised atoms (e.g. ethene) are planar

CIE A-Level Chemistry diagram showing planar geometry of sp2 hybridised atoms.

The double bond prevents rotation, giving a fixed geometry and leading to stereoisomerism for some molecules (see Isomerism).

Example Ethene (CH2=CH2):

CIE A-Level Chemistry diagram of ethene molecule showing sp2 hybridisation and planar structure.

Summary