Organic Chemistry Fundamentals: Isomers, Acids/Bases, and Green Chemistry Principles

Isomers: Constitutional, Conformational, and Stereoisomers

• Cis/Trans Isomers (Stereoisomers) in Cyclic Compounds:

  • Cyclic compounds like $1,2$-dimethylcyclopentane can exist as cis or trans isomers.

  • Cis implies both methyl groups are on the same side (e.g., both up).

  • Trans implies one methyl group is up and the other is down.

  • $1,1$-dimethylcyclopentane is neither cis nor trans because both methyl groups are on the same carbon, thus no relative orientation across the ring is possible.

  • Stereoisomers are molecules with the same connectivity but different three-dimensional arrangements of atoms in space.

  • Cis/trans isomers are a type of stereoisomer, specifically diastereomers (non-superimposable and not mirror images).

• Constitutional (Structural) Isomers:

  • Molecules that have the same number of each type of atom (same elemental composition) but different connectivity.

  • Example: Butane (straight chain $C-C-C-C$) versus 2-methylpropane ($C-C(C)-C$). Their list of atom-to-atom connections differs.

• Conformers:

  • Represent different spatial orientations of a single molecule, resulting from rotation around single bonds.

  • Involves changes in the three-dimensional arrangement but not the connectivity.

  • Have low energy barriers for interconversion, meaning they can easily convert from one form to another at normal temperatures.

  • Examples:

    • Butane: Newman projections show different conformers like gauche (methyls close) and anti (methyls opposite) by rotation around a $C-C$ bond.

    • Cyclohexane: Chair and boat forms are conformers, interconvertible by 'wiggling' atoms around.

    • A 20-amino acid protein can have thousands of conformations.

• Distinguishing Stereoisomers from Conformers:

  • Conformers have different arrangements in space, but can interconvert easily by spinning around single bonds (low energy barrier).

  • Stereoisomers also have different arrangements in space, but cannot be easily interconverted by bond rotation. Interconversion typically requires breaking and reforming covalent bonds (high energy barrier). This allows stereoisomers to be isolated as distinct molecules (e.g., a jar of cis and a jar of trans).

• Cis/Trans in Cyclohexanes and Axial/Equatorial Positions:

  • Axial and equatorial describe the position of a substituent relative to the ring plane, not implicitly its cis/trans relationship to another substituent.

  • Cis/trans describes the relationship of two substituents relative to each other and the ring.

  • Example: A methyl group pointing straight up (axial) and a bromine pointing slightly up and out (equatorial) on adjacent carbons are cis because both are