Isomerism

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Structural Isomers

• Isomers: organic molecules with the same molecular formula but a different structural formula
• Structural isomers have:
  • Different names
  • Different shapes
  • Different chemical properties

Geometrical Structural Isomers

• There is no rotation possible around a double bond
• Same molecular and structural formula, but a different spatial arrangement of atoms
• 2 molecules can form where the position of the double bond remains the same but the spatial arrangement across the double bond is different
• Cis isomer: CH3 groups are on the same side of the double bond
• Trans isomer: CH3 groups are on opposite sides of the double bond

Structural Isomerisms

• Different structural formulae
• Chain isomerism
  • Carbon chain is arranged differently, e.g. different shape, L shape instead of chain, cross shape
• Positional isomerism
  • Functional group is attached to a different carbon atom, e.g. 3-chloropentane, 2-chloropentane
  • Also exists in alkenes with 4 or more carbon atoms
• Functional group isomerism
  • Have different functional groups, so are members of different homologous series

Stereoisomerism

• Have same structural formula

• 3d arrangement of atoms is different

• 2 types

• Cis-trans isomerism

• Optical isomerism

  Cis-trans isomerism

  • Molecules can rotate freely around C-C covalent bonds, but not around C=C double bonds
  • Leads to cis-trans isomerism, in which isomers differ in the arrangement of the groups attached to the carbons in the double bonds
  • These isomers cannot be super imposed on each other because the arrangement of the methyl groups is different
  • If an alkyl group or atom other than hydrogen is attached to each carbon then the atoms can be named either cis (on the same side) or trans (on the opposite side)

  Limitations of Cis-Trans isomerism

  • In more complex organic compounds, in which multiple hydrogens have been substituted by different groups, isomers cannot be defined using the cis-trans notation
  • Instead, a different system is used for these type of molecules: E-Z notation

E-Z Notation

• Used to identify stereoisomers that cannot be called cis or trans
• Isomers are identified as either E or Z depending on what “priority” is given to the groups attached to the carbon atoms in the double bond
• Priority of these groups is determined by a series of complex rules
• E represents the German word “entgegen”(opposite) and corresponds to trans isomers
• The highest priority groups are on the opposite side of the double bond
• Z represents the German word “zusammen”(together) and corresponds to cis isomers
• The highest priority groups are on the same side of the double bond
• Highest priority, in this case, refers to the highest atomic number of the atoms

Optical isomerism

• A molecule can exist as 2 isomers that are non-superimposable, mirror images of each other, just like a left hand and right hand
• Optical isomers have the same physical properties but they rotate polarised light in opposite directions

Boiling Point with Isomers

Van der Waal’s forces

• Named after Dutch scientist J D Van der Waal who discovered them
• The forces are created by the constant movements of electrons in atoms of molecules at high speeds
• At any instant in time there is the likelihood that one side of the molecule has a greater proportion of electrons
• This is called a dipole, and causes the molecule to have positively and negatively charged ends
• A temporary dipole or imbalance of charge can attract the electron cloud from a neighbouring atom or molecule, e.g. slightly positive end attracts negatively charged electrons
• The temporary dipole induces an imbalance of charges in neighbouring molecules
• The strength of the VdW force is dependent on the size of the molecules or atoms
• As the size and molecular mass of the molecule increases so does the number of electrons, resulting in a greater imbalance of charge, and therefore a greater VdW attraction
• This is why boiling points increase as the number of carbon atoms increase in alkanes, more energy is required to overcome the VdW forces

Putting it all together

• Branched isomers have a smaller surface area
• They are more compact
• Thus, there is less area of contact between temporary/induced dipoles, and/or molecules
• Thus, dispersion force strength decreases
• Thus, boiling point decreases