Organic Chemistry: Naming and Isomers

Naming Organic Compounds

Branched Alkanes

  • Number the carbon chain so that alkyl groups have the lowest number possible.
  • Identify alkyl groups (methyl, ethyl, propyl, etc.) and their positions on the carbon chain.
  • Use prefixes (di, tri, tetra, penta, hexa) if there are multiple identical alkyl groups.
  • List alkyl groups alphabetically, separating location from the group name with a dash and locations from each other with commas.
  • End the name with the parent chain name.

Branched Alkenes

  • Alkenes contain C=C double bonds.
  • Find the longest carbon chain that contains the double bond.
  • Number the carbon chain to give the double bond the lowest possible numbering.
  • The alkene is the priority for numbering.
Naming Example (Heptene)
  1. Longest Carbon Chain: 7 carbons (Heptene).
  2. Numbering: Number from the end that gives the double bond the lowest number. If the double bond is between carbon 1 and 2, it’s a 1-heptene.
  3. Alkyl Groups: Identify and name alkyl groups (e.g., ethyl, methyl) and their positions.
    • Example: 3-ethyl-4,6-dimethyl-1-heptene.
  • Each carbon atom has a tetrahedral shape.
Key Differences: Alkanes vs. Alkenes
  • Alkenes end with "-ene."
  • The longest carbon chain must contain the double bond.
  • Numbering prioritizes the double bond.
  • The double bond number indicates the carbon after which the double bond comes. For example, 1-heptene means the double bond is after carbon 1.

Alkynes

  • Alkynes contain C≡C triple bonds.
  • The process is similar to alkenes.
  • The name of the hydrocarbon ends in "-yne" to indicate the presence of a triple bond.
  • Find the longest carbon chain that contains the triple bond.
  • Number the carbon chain to give the triple bond the lowest possible numbering.
Naming Example (Pentyne)
  1. Longest Carbon Chain: 5 carbons (Pentyne).
  2. Numbering: Number to give the triple bond the lowest number. For example, 2-pentyne.
  3. Alkyl Groups: Locate and name alkyl groups, providing the location twice even if on the same carbon.
    • Example: 4,4-dimethyl-2-pentyne.

Isomers

  • Isomers have the same chemical formula but different structures.
  • Different isomers can have significantly different physical and chemical properties.
  • Structure dictates activity, so changing the structure, even by one bond, can greatly affect the molecule's behavior.
Types of Isomers

  1. Structural (Constitutional) Isomers

    • Different order of atom connectivity.
    • Example: Butane ($\$C4H{10}\$) vs. 2-methylpropane.
    • Structural isomerism in alkanes starts with butane.
      • Propane ($\$C3H8\$) has only one possible arrangement.
      • Butane ($\$C4H{10}\$) has two: n-butane and 2-methylpropane.
    • The number of structural isomers increases dramatically with the number of carbon atoms.
      • Pentane ($\$C5H{12}\$) has 3 isomers.
      • Hexane ($\$C6H{14}\$) has 5 isomers.
      • Octane ($\$C8H{18}\$) has 18 isomers.
      • Decane ($\$C{10}H{22}\$) has 75 isomers.
    • Example: Isomers of Pentane ($\$C5H{12}\$)
      • n-Pentane: A straight chain of five carbon atoms.
      • 2-Methylbutane: A four-carbon chain with a methyl group ($\$CH_3\$) on the second carbon.
      • 2,2-Dimethylpropane: A three-carbon chain with two methyl groups on the second carbon.

  2. Stereoisomers

    • Same chemical formula and order of atom connectivity, but different spatial orientation of atoms.
    • Geometric (Cis-Trans) Isomers
      • Different arrangement of atoms around a double bond ($\$C=C\$).
      • Cis: Substituents on the same side of the double bond.
      • Trans: Substituents on opposite sides of the double bond.
      • Double bonds cannot rotate, leading to isomerism.
      • Example: 1,2-Dichloroethene ($\$C2H2Cl_2\$)
        • Cis-1,2-dichloroethene: Chlorines on the same side.
        • Trans-1,2-dichloroethene: Chlorines on opposite sides.
      • Alkenes are flat molecules with trigonal planar geometry.
      • Cis and trans isomers can have different polarities, affecting physical and chemical properties.

  3. Optical Isomers

    • Molecules must be mirror images of each other and non-superimposable.
    • Involve a chiral center (carbon atom with four different substituents attached).
    • Enantiomers: Non-superimposable mirror images.
    • Enantiomers interact with polarized light differently.
    • Structural Feature: Chiral Center.
    • Thalidomide: A drug with two optical isomers, one safe and one causing birth defects.
    • Chiral Carbon: A carbon atom with four different substituents attached.
    • If a molecule can be cut in half so that the left side of the molecule is the mirror image of the right side, it will not show optical isomerism.