MC

Isomerism: Structural and Stereoisomerism, Conformational Analysis, and Molecular Projections

  • Isomerism: Definition and Types

    • Isomerism occurs when different molecules share the same chemical formula.
    • Structural Isomerism (also known as Constitutional or Substitutional Isomerism)
    • Definition: Molecules have the same chemical formula but different connectivity of atoms.
    • Example 1: Butane and Isobutane.
      • Butane: ext{C}4 ext{H}{10} (straight chain of four carbons).
      • Isobutane: ext{C}4 ext{H}{10} (three carbons in a chain with one methyl branch).
      • Both have formula ext{C}4 ext{H}{10}, but their carbon atoms are connected differently.
    • Example 2: Propanol and Methoxyethane (or Methyl Ethyl Ether).
      • Propanol: ext{C}3 ext{H}8 ext{O} (one oxygen bonded to hydrogen and carbon, e.g., ext{CH}3 ext{CH}2 ext{CH}_2 ext{OH}).
      • Methoxyethane: ext{C}3 ext{H}8 ext{O} (one oxygen bonded between two carbons, e.g., ext{CH}3 ext{OCH}2 ext{CH}_3).
      • Same formula ext{C}3 ext{H}8 ext{O}, but different connectivity (alcohol vs. ether functional group).
    • Stereoisomerism
    • Definition: Molecules have the same chemical formula and the same connectivity of atoms, but a different three-dimensional (3D) orientation of atoms in space.
    • Example: Cis/Trans isomers (geometric isomers).
      • Consider a disubstituted alkene. If two substituents (e.g., "metal" groups) are on the same side of the double bond, it's a cis isomer.
      • If the two substituents are on opposite sides of the double bond, it's a trans isomer.
    • Cis and Trans isomers are stereoisomers because they have the same formula and connectivity but differ in the spatial arrangement of their groups.

  • Conformational Isomerism (a specific type of Stereoisomerism)

    • Definition: Isomerism that arises solely from the rotation around a single bond, leading to different spatial arrangements called conformers or conformations.
    • Example: Ethane ( ext{C}2 ext{H}6)
    • Ethane exists in different conformations due to the rotation around the carbon-carbon single bond.
    • Dihedral Angle (Torsion Angle)
      • Definition: The angle between two bonds on adjacent carbons.
      • Eclipsed Conformation: The dihedral angle between corresponding bonds (e.g., H-H) on adjacent carbons is 0^ ext{o}. Bonds on the front carbon directly align with bonds on the back carbon.
      • Staggered Conformation: The dihedral angle between corresponding bonds (e.g., H-H) on adjacent carbons is typically 60^ ext{o} or 180^ ext{o}. Bonds on the front carbon are positioned between the bonds on the back carbon.
      • The conversion between eclipsed and staggered conformations happens through rotation around the single bond.

  • Bond Angle vs. Dihedral Angle

    • Bond Angle: The angle between two bonds originating from the same atom (e.g., two C-H bonds on the same carbon).
    • Depends on the hybridization of the central atom:
      • ext{sp}^3 hybridization (e.g., in alkanes): Bond angle is approximately 109.5^ ext{o} (often approximated as 109^ ext{o} in the context).
      • ext{sp}^2 hybridization (e.g., in alkenes): Bond angle is approximately 120^ ext{o}.
      • ext{sp} hybridization (e.g., in alkynes): Bond angle is approximately 180^ ext{o}.
    • Dihedral Angle (Torsion Angle): The angle between two bonds on different (adjacent) carbons.
    • Requires visualizing the 3D shape of the molecule (e.g., eclipsed or staggered conformations).

  • Molecular Representations

    • Sawhorse Projection
    • A perspective drawing that shows the viewer looking at the C-C bond from an oblique angle.
    • Clearly shows all bonds and their relative positions in 3D space.
    • Example given: Can show hydrogens on adjacent carbons aligned (eclipsed, 0^ ext{o} dihedral) or opposite (180^ ext{o} dihedral) or at 60^ ext{o} (staggered).
    • Newman Projection
    • A view down a specific carbon-carbon bond.
    • Drawing Rules:
      • The front carbon is represented by a central dot.
      • The back carbon is represented by a circle behind the front carbon.
      • Bonds from the front carbon originate from the center of the circle.
      • Bonds from the back carbon originate from the circumference of the circle.
      • When looking from a specific side (e.g., right or left), wedge bonds (towards you) and dash bonds (away from you) in Sawhorse projections are translated into left/right positions on the Newman projection relative to the vertical axis.
      • For example, if looking from the right and a hydrogen is wedged, it appears on the left side of the front carbon in the Newman projection.
    • Advantages: Easier to visualize and determine dihedral angles.
      • In a Newman projection, identifying a 180^ ext{o} dihedral angle means two groups are directly opposite across the circle.
      • A 60^ ext{o} dihedral angle means two groups are slightly offset from each other.
      • A 0^ ext{o} dihedral angle (eclipsed) means a bond from the back carbon is directly behind a bond from the front carbon, often drawn slightly offset for clarity such that one bond is directly under the other.
    • Conformer Visualization: Newman projections readily depict eclipsed (all bonds aligned, 0^ ext{o} dihedral) and staggered (bonds offset, 60^ ext{o} or 180^ ext{o} dihedral) conformations.

  • Interconversion and Strain

    • Conformational isomers (conformers) can interconvert through rotation around single bonds (e.g., an eclipsed conformer can rotate to become a staggered conformer).
    • The next session will discuss "S strain," which refers to different types of strain (e.g., torsional strain, steric strain) that affect the energy and stability of molecules.