Isomers

• Physical properties: melting point, boiling point, solubility, odor, color
• Chemical properties:
  • Reactivity of molecule
  • Changes chemical composition
  • Dictated by functional groups

Conformational isomers

• Differ in rotation around single (σ) bonds
• Rotation around single bonds produces strain

Newman projection

• Straight-chain representation
• Minimize steric repulsion → carbons 180° apart
• Staggered conformation = anti
• Gauche = groups 60° apart
• Eclipsed = groups in line (120° apart)
• When groups are close, energy ↑ → molecule rotates to lower energy

Cyclic conformations

• Types of strain:
  • Angle (deviation from ideal bond angles)
  • Torsional (eclipsing or gauche interactions)
  • Nonbonded (groups competing for same space; flagpole interactions in cyclohexane)
• Cycloalkane conformations:
  • Cyclobutane → puckered
  • Cyclopentane → envelope
  • Cyclohexane → chair, boat, twist (skew) boat
• Chair = most stable (minimizes all three strains)
• Axial and equatorial positions alternate
• Bulkiest group prefers equatorial

Chirality

• A chiral object is not superimposable; asymmetrical
• Mirror image = enantiomer
• Diastereomers differ at one or more (but not all) chiral centers

Enantiomers

• Optically active
• Rotate plane-polarized light
• Polarizer allows only specific light orientations
• Rotate right (clockwise) = dextrorotatory (D), “+”
• Rotate left (counterclockwise) = levorotatory (L), “−”

Specific rotation equation

• $[\alpha] = \frac{\alpha_\text{obs}}{c \times l}$
  • $[\alpha]$ = rotation in degrees
  • $\alpha_\text{obs}$ = observed rotation
  • $c$ = concentration (g/mL)
  • $l$ = path length (dm)
• Racemic mixture: equal + and − enantiomers → optical activity cancels
• Can separate enantiomers by reacting them with a single enantiomer of another compound → forms diastereomers → separated via crystallization, filtration, distillation

Diastereomers

• Non–mirror-image configurational isomers
• Multiple chiral centers
• Formula: $2^n$ stereoisomers
• Can rotate light, but rotation relationships not predictable

Cis–trans isomers

• Previously “geometric isomers”
• Differ in position around double or triple bonds

Meso compounds

• Have chiral centers and a plane of symmetry
• Not optically active
• Molecular equivalent of a racemic mixture

E and Z (alkenes)

• Z = higher-priority groups on same side
• E = higher-priority groups on opposite sides
• E/Z are diastereomers

R and S (chiral centers)

• R = clockwise rotation
• S = counterclockwise rotation

Fischer projections

• Horizontal lines = out of page (wedges)
• Vertical lines = into page (dashes)
• Intersection = carbon atom
• Switching two pairs of substituents = rotation 180° → stereochemistry retained
• Switching one pair = inversion of stereochemistry
• Rotating Fischer projection 90° also inverts configuration

Relative vs. absolute configuration

• Relative configuration retained when stereocenter bonds aren’t broken
• Joining molecules keeps groups around chiral centers the same
• Absolute configuration retained if R or S designation is preserved
• Adding two R molecules gives an R product