Isomerism

• Isomerism


  Compounds with the same molecular formula but different structures
• Constitutional isomerism


  • Not restricted to alkanes and occurs widely in organic chemistry
  • Can be: chain isomers, functional isomers, positional isomers
• Representing conformational isomers of alkanes

  1. Sawhorse representation

  2. Newman projections
• Torsional strain


  Repulsion between orbitals in C-H bonds as they pass by each other
• Steric strain


  Repulsion that occurs when atoms or groups are forced closer together than their atomic radii allow
• Contributors to ring strain

  • Angle strain
  • Torsional strain
  • Steric strain
• Cyclopropane


  • Most strained cycloalkane, hence most reactive
• Cyclohexane


  • Six-membered rings are the most commonly occurring cyclic structures in nature
  • Most stable conformation is the chair conformation
• cis and trans


  Spatial arrangement of substituents in cycloalkanes and alkenes
• Stereoisomers


  Compounds with the same molecular formula and the same attachments to the carbon skeleton but a different spatial arrangement
• Cis


  Same side
• E and Z designation


  Used to determine if the high priority groups are on the same side (Z) or on the opposite sides (E) of the double bond when cis and trans designations are not possible
• Unsaturated fatty acids contain cis alkenes
• Omega (ω) designations


  Commonly used in nutrition literature to indicate the carbon positioning of a double bond in the structure of a fatty acid
• Linoleic and α-linolenic are considered essential fatty acids because our bodies cannot produce them and they must be obtained from diet
• Stereoisomers - chiral and enantiomers


  Identical molecules that are nonsuperimposable mirror images of each other
• Chiral


  Objects that are nonsuperimposable mirror images, like a right-handed and a left-handed baseball glove
• Chiral center


  • A tetrahedral carbon atom bonded to four different atoms or groups of atoms
• Identifying chiral carbons

  1. Step 1: Locate the tetrahedral carbons

  2. Step 2: Inspect the tetrahedral carbons and determine if the four groups attached are different

  3. Step 3: Assign the chiral centers, typically with an asterisk
• R and S notation


  Used to assign the absolute configuration around a stereocenter according to the CIP priority rules