3.1.3

Isomerism

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Structural Isomerism

Structural isomers are molecules which have the same molecular formula but a different arrangement of atoms in space.

Chain isomers

Chain isomers

  • Chain isomers are molecules that have the same molecular formula but one is linear and one is branched. For example:
    • Butane is a linear molecule with the molecular formula C4H10.
    • Methylpropane is a branched molecule with the molecular formula C4H10.
Position isomers

Position isomers

  • Position isomers are molecules that have the same molecular formula but have their functional group in different places. For example:
    • Butan-1-ol has a hydroxyl group attached to carbon-1.
    • Butan-2-ol has a hydroxyl group attached to carbon-2.
Functional group isomers

Functional group isomers

  • Functional group isomers are isomers that have had their structures changed so that they have different functional groups. For example:
    • Propanoic acid.
    • Methyl ethanoate.

Stereoisomerism

Stereoisomers are molecules that have the same molecular formula, same functional groups and same functional group positions but different arrangements of atoms in space.

Alkene stereoisomerism

Alkene stereoisomerism

  • Stereoisomerism can be seen in alkenes.
  • It is possible in alkenes because of the presence of double bonds:
    • Unlike single bonds, double bonds are rigid and you cannot rotate around them.
    • So alkenes can display stereoisomerism at the double bond.
An example

An example

  • The simplest way to understand what this means is with an example.
    • Above are the two different alkenes, (E)-but-2-ene and (Z)-but-2-ene.
      • The molecules are the same in every way, except for the atoms' position in space.
What do E and Z mean?

What do E and Z mean?

  • E and Z come from the German words 'entgegen' and 'zusammen' for 'opposite' and 'together'.
    • An E-alkene has the larger groups on opposite sides.
    • A Z-alkene has the larger groups on the same side.
  • A good way to remember this is that Z alkenes have large groups on the 'zame zide'.

Cahn-Ingold Prelog Rules

The Cahn-Ingold Prelog rules help us assign E/Z isomerism to more complicated molecules.

The Cahn-Ingold Prelog rules

The Cahn-Ingold Prelog rules

  • They help to distinguish between E and Z isomers for more complicated cases.
  • The rules work by numbering the four directly connecting atoms in order of their atomic mass number. From this, we give them a 'priority'.
    • The highest atomic number gets priority 1.
    • The second highest atomic number gets priority 2, and so on.
E vs Z

E vs Z

  • E:
    • Priority 1 and 2 are on different sides of the double bond.
      • I.e. The two atoms with the highest atomic mass number are on different sides.
  • Z:
    • Priority 1 and 2 are on the same side of the double bond.
      • I.e. The two atoms with the highest atomic mass number are on the same side.
An example: E

An example: E

  • The shown but-2-ene is an E geometry because the two methyl groups are on opposite sides of the double bond.
An example: Z

An example: Z

  • The shown but-2-ene is a Z geometry because the two methyl groups are on the same side of the double bond.
Jump to other topics
1

Physical Chemistry

1.1

Atoms, Molecules & Stoichiometry

1.1.1Relative Masses1.1.2The Mole1.1.3Mass Spectrometry1.1.4Empirical & Molecular Formulae1.1.5Balanced Equations1.1.6Molar Volume1.1.7End of Topic Test - Amount of Substance
1.2

Atomic Structure

1.2.1Fundamental Particles1.2.2Isotopes & Mass Number1.2.3Electron Shells, Sub-Shells & Orbitals1.2.4Electron Configuration1.2.5Ionisation Energy1.2.6Factors Affecting Ionisation Energies1.2.7Trends of Ionisation1.2.8Specific Impacts on Ionisation Energies1.2.9Electron Affinity1.2.10End of Topic Test - Atomic Structure1.2.11A-A* (AO2/3) - Atomic Structure
1.3

Chemical Bonding

1.3.1Ionic Bonding1.3.2Covalent & Dative Bonding1.3.3Shapes of Molecules1.3.4Intermolecular Forces1.3.5Intermolecular Forces 21.3.6Electronegativity1.3.7Bond Length, Bond Energy, & Bond Polarity1.3.8Metallic Bonding1.3.9Physical Properties1.3.10End of Topic Test - Bonding1.3.11A-A* (AO2/3) - Bonding
1.4

States of Matter

1.4.1Simple Covalent Molecules1.4.2The Ideal Gas Equation1.4.3States of Matter1.4.4Particle Diagrams1.4.5The Liquid State1.4.6Giant Ionic Lattices1.4.7Giant Metallic Lattices1.4.8Giant Covalent Structures1.4.9Graphene & Fullerene1.4.10Recycling
1.5

Chemical Energetics

1.5.1Calorimetry1.5.2Bond Enthalpies1.5.3Enthalpy Changes1.5.4Hess' Law1.5.5Born-Haber Cycles1.5.6Born-Haber Cycle Calculations1.5.7Entropy1.5.8Free Energy1.5.9End of Topic Test - Energetics
1.6

Electrochemistry

1.6.1Electrochemical Cells1.6.2Electrochemical Series1.6.3Commercial Application1.6.4Nernst Equation1.6.5Redox
1.7

Equilibria

1.7.1Dynamic Equilibrium & Le Chatelier1.7.2Kc1.7.3Kp1.7.4pH1.7.5The Ionic Product of Water1.7.6Weak Acids & Bases1.7.7Introduction to Solubility Equilibria1.7.8Solubility Equilibria Calculations1.7.9Free Energy of Dissolution1.7.10pH and Solubility1.7.11Common-Ion Effect1.7.12End of Topic Test - Kp & Electrochemistry1.7.13A-A* (AO2/3) - Electrochemical Cells
1.8

Partition Coefficient

1.8.1Kpc
1.9

Reaction Kinetics

1.9.1Collision Theory1.9.2Orders, Rate Constants & Equations1.9.3Rate Graphs1.9.4Rate Determining Step1.9.5Maxwell-Boltzmann Distribution1.9.6Catalysts1.9.7Homogeneous Catalysts1.9.8Heterogeneous Catalysts1.9.9End of Topic Test - Kinetics1.9.10End of Topic Test - Rate Equations1.9.11A-A* (AO2/3) - Rate Equations
2

Inorganic Chemistry

2.1

The Periodic Table

2.1.1The Periodic Table2.1.2Trends in the Periodic Table2.1.3End of Topic Test - Periodicity & Trends2.1.4Ceramics2.1.5Period 3 Elements & Oxides2.1.6Period 3 Oxides2.1.7End of Topic Test - Period 3
2.2

Group 2

2.2.1Group 2 Chemistry
2.3

Group 17

2.3.1Introduction to Halogens2.3.2Reactions of Halogens2.3.3Chlorine & Chlorate(I)2.3.4End of Topic Test - Group 172.3.5A-A* (AO2/3) - Periodicity, Group 2 & 17
2.4

Transition Metals

2.4.1General Properties2.4.2Titrations2.4.3Shapes of Complex Ions2.4.4Variable Oxidation States2.4.5Ligands2.4.6Colours of Ions2.4.7Stereoisomerism of Complex Ions2.4.8Cisplatin2.4.9Ligand Substitutions & Kstab2.4.10End of Topic Test - Transition Metals2.4.11A-A* (AO2/3) - Transition Metals
2.5

Nitrogen & Sulfur

2.5.1Nitrogen & Ammonia2.5.2Formulated vs Manure Fertilisers2.5.3Nitrogen Containing Fertilisers2.5.4Fertilizers & Eutrophication2.5.5Common Atmospheric Pollutants & Their Properties2.5.6Catalytic Converters2.5.7Atmospheric Oxides
3

Organic Chemistry & Analysis

3.1

Introduction to Organic Chemistry

3.1.1Naming Conventions3.1.2Mechanism3.1.3Isomerism3.1.4End of Topic Test - Introduction to Organic Chem
3.2

Hydrocarbons

3.2.1Fractional Distillation3.2.2Cracking3.2.3Combustion3.2.4Chlorination3.2.5End of Topic Test - Alkanes3.2.6Introduction to Alkenes3.2.7Reactions of Alkenes3.2.8Polymerisation Reactions3.2.9End of Topic Test - Alkenes3.2.10Arenes3.2.11Evidence for Structure of Arenes3.2.12Reactions of Benzene3.2.13End of Topic Test -Arenes
3.3

Halogen Derivatives

3.3.1Properties of Haloalkanes3.3.2Substitution Reactions of Haloalkanes3.3.3Environmental Impacts of Haloalkanes3.3.4End of Topic Test - Halogenoalkanes
3.4

Hydroxy Compounds

3.4.1Types of Alcohol3.4.2Oxidation 13.4.3Oxidation 23.4.4Elimination3.4.5Acids & Esters3.4.6Acyl Chlorides & Acylation3.4.7Phenols & Their Reactions3.4.8End of Topic Test - Alcohols
3.5

Carbonyl Compounds

3.5.1Carbonyls3.5.2Hydrogen Bonding & Carbonyls3.5.3Aldehydes & Ketones3.5.4Tri-Iodomethane Reaction3.5.5End of Topic Test -Carbonyls
3.6

Carboxylic Acids & Derivatives

3.6.1Properties of Carboxylic Acids3.6.2Esters3.6.3Acyl Chlorides3.6.4End of Topic Test - Carboxylic Acids
3.7

Nitrogen Compounds

3.7.1Nature & Preparation of Amines3.7.2Basicity & Nucleophilicity3.7.3Amides3.7.4Reactions of Amino Acids3.7.5Amino Acids
3.8

Polymerisation

3.8.1Properties of Addition Polymers3.8.2Condensation Polymerisation3.8.3Effects of Side-Chains on Polymers3.8.4Hydrolysis of Ester & Amide Groups3.8.5Proteins3.8.6DNA3.8.7Adhesive & Conducting Polymers3.8.8End of Topic Test - Amines & Proteins
3.9

Analytical Techniques

3.9.1Chromatography3.9.2High-Performance Liquid Chromatography3.9.3Gas Chromatography3.9.4IR Spectroscopy3.9.5Uses of IR Spectroscopy3.9.6Mass Spectrometry3.9.7Mass Spectrometry Analysis3.9.8Nuclear Magnetic Resonance3.9.9Carbon-13 NMR3.9.10Proton NMR I3.9.11Proton NMR II3.9.12End of Topic Test - Analytical Techniques3.9.13A-A* (AO2/3) - Analytical Techniques
3.10

Organic Synthesis

3.10.1Chirality & Drug Synthesis3.10.2Organic Synthesis3.10.3Examples of Synthetic Routes3.10.4End of Topic Test - Organic Synthesis

Practice questions on Isomerism

Can you answer these? Test yourself with free interactive practice on Seneca — used by over 10 million students.

  1. 1
    Three types of structural isomerism:Fill in the list
  2. 2
    What do position isomers have?Multiple choice
  3. 3
    Which of the following is NOT a chain isomer of pentane?Multiple choice
  4. 4
    What do stereoisomers NOT have in common?Multiple choice
  5. 5
    E-alkene isomers are identified by which of the following?Multiple choice
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Mechanism

End of Topic Test - Introduction to Organic Chem