Isomers

• Isomers


  Molecules with the same molecular formula but different structural formulas
• Types of isomerism


  • Structural isomerism
  • Stereoisomerism
• Structural isomers


  • Have the same molecular formula but different structural formulas
  • Have the atoms organised differently
• Stereoisomers


  • Have the same structural formula but different spatial arrangements or geometries
• Subtypes of structural isomerism

  • Chain isomerism
  • Position isomerism
  • Functional group isomerism
• Chain isomers


  Have different carbon skeletons or chain lengths
• Position isomers


  Have the functional group attached at a different position on the main carbon chain
• Functional group isomers


  Have the same molecular formula but different functional groups
• Subtypes of stereoisomerism


  • Cis-trans (geometric) isomerism
  • Optical isomerism
• Optical isomerism will not be covered until year 2
• Identifying structural isomers


  1. Consider chain length

  2. Consider position of functional groups

  3. Consider type of functional groups
• Chain isomerism


  Molecules have the same molecular formula but different arrangements of the carbon atoms
• Functional group isomerism


  Molecules have the same molecular formula but different functional groups
• Position isomerism


  Molecules have the same molecular formula but the functional group is located at a different position on the carbon chain
• Stereoisomerism


  Molecules have the same structural formula but different spatial arrangements
• Cis-trans (E-Z) isomerism


  Type of stereoisomerism where rotation around a carbon-carbon double bond is restricted, resulting in different spatial arrangements
• Cis isomers have the same substituents on the same side of the double bond
• Trans (E) isomers have the same substituents on opposite sides of the double bond
• Cis-trans (E-Z) isomerism occurs in alkenes due to restricted rotation around the carbon-carbon double bond
• Alkanes can undergo free rotation around carbon-carbon single bonds, whereas alkenes cannot rotate freely around carbon-carbon double bonds
• The number of structural isomers for an alkane with n carbon atoms is 2^(n-1)
• The number of structural isomers for an alkene with n carbon atoms is 2^(n-2)
• The number of structural isomers for a cycloalkane with n carbon atoms is n
• Restricted rotation


  Double bond can't rotate in the way that the alkyne one can
• Alkene


  • Can't rotate
  • Can get two forms of the same molecule that look really similar but are different
• Z isomer


  Priority atoms are on the same side
• E isomer


  Priority atoms are on opposite sides
• Cis and trans isomers are sometimes used interchangeably with E and Z isomers
• Cahn-Ingold-Prelog priority rules


  Determine which isomer is Z or E based on relative atomic mass of atoms bonded to the double bond carbons
• Skeletal formula


  Easier way to draw alkenes and show E/Z isomerism
• Z isomers


  More polar than E isomers due to electronegative groups being on the same side