Isomers: Structural and Stereo

Isomers

• Two main categories:
  • Structural isomers
  • Stereoisomers

Structural Isomers

• Have the same molecular formula but different structures.
• Like twins with the same formula but different molecular structures.
• Two or more compounds with the same molecular formula but different molecular structures are isomers.
• Structural isomers have the same chemical formula but their atoms are bonded in different arrangements.
• Structural isomers have different chemical and physical properties.
• The structure of a substance determines its properties.
• Structural isomers for alkanes have a minimum of four carbons.

Molecular Formula

• Example: C5H{12}
• Pentane, 2-methylbutane, and 2,2-dimethylpropane all have the same molecular formula but different structures.

Models Used

• Structural formulas
• Ball and stick model with covalent bonds

Stereoisomers

• Isomers in which all atoms are bonded in the same order but arranged differently in space.
• Two types:
  • Geometric isomers (cis and trans prefixes)
  • Optical isomers

Geometric Isomers

• Occur in alkenes (double bonds).
• Single bond carbons in ethane are free to rotate.
• Double-bonded carbons in ethene resist being rotated; they are locked in place.
• Result from different arrangements of groups around a double bond.
• \"cis\" means the branches are on the same side of the double bond.
• \"trans\" means the branches are across from each other.
• Geometric isomers have different physical and chemical properties.
• Example: Butene
  • Cis-2-butene: Carbons are facing the same side.
  • Trans-2-butene: Carbons are across from each other.
• Example: 2-Pentene
  • Cis-2-pentene: Carbons on the same side of the double bond.
  • Trans-2-pentene: Carbons across from each other.

Optical Isomers (Chirality)

• Discovered by observing two forms of tartaric acid that were mirror images.
• Right and left-handed forms (dextro and levo).
• Chirality: The property in which a molecule exists in right and left-handed forms.
• Asymmetric carbon: A carbon atom that has four different atoms or groups of atoms attached to it.
• Isomers result from different arrangements of four different groups around the same carbon.
• Optical isomers have the same physical and chemical properties, except in reactions where chirality is important (e.g., enzyme-catalyzed reactions).
• Optical isomers affect light passing through them.
• Unpolarized light becomes polarized in one plane.
• They exhibit different angles of rotation.