Stereochemistry: Isomers and Chirality

Molecular Models and Stereoisomers

Isomers: A Foundation

  • Definition of Isomerism: Compounds that share the same molecular formula but possess different structural arrangements. This concept was previously covered.

  • Two Main Types of Isomers:

    • Constitutional Isomers: Differ in the sequence of atomic attachment (their bond connectivity) and, consequently, their IUPAC names and chemical properties.

      • Example 1: Pentane vs. 2-Methylbutane

        • Both have the molecular formula C5H{12}.

        • Pentane (n-pentane): Linear chain of five carbons (C1-C2-C3-C4-C_5).

        • 2-Methylbutane: A four-carbon chain with a methyl group (CH3) at the second carbon (attached to C2). The fifth carbon is attached to \text{number } 2.

        • These are two distinct compounds with different connectivity.

      • Example 2: Ethanol vs. Dimethyl Ether

        • Both have the molecular formula C2H6O.

        • Ethanol: Contains an alcohol functional group (OH); chemical structure is CH3CH2OH.

        • Dimethyl Ether: Contains an ether functional group (ROR'); chemical structure is CH3OCH3.

        • They represent two distinct classes of organic compounds (alcohols vs. ethers) due to different atomic attachments.

Stereoisomers: Spatial Arrangement Differences

  • Definition of Stereoisomerism: Compounds that have the same molecular formula and the same sequence of atomic attachment (bond connectivity), but differ in the three-dimensional (3D) arrangement of their atoms in space.

  • Key Characteristic: Non-Interconvertibility: Stereoisomers cannot be converted into one another simply by bond rotation or ring inversion. If they could, they would be conformational isomers (conformations) of the same compound, not different compounds.

    • Contrast with Conformations: Conformations are interconvertible forms of a single molecule. For instance, cyclohexane can undergo ring inversion (chair-to-chair flip) where axial groups become equatorial and vice versa. Similarly, rotation around single carbon-carbon bonds (e.g., in ethane) allows for different eclipsed and staggered conformations. These are temporary spatial arrangements of the same molecule and do not break bonds.

  • Examples of Stereoisomers:

    • Cyclopropane Derivatives: e.g., 1,2-dimethylcyclopropane

      • The rigid structure of the cyclopropane ring (due to high ring strain) restricts bond rotation around the carbon-carbon bonds within the ring.

      • Cis-1,2-dimethylcyclopropane: Both methyl groups (CH_3) are on the same side (face) of the ring (e.g., both