Organic Conformations and Stereochemistry

Shapes of Molecules

• Key Concepts:

  • Bond Lengths: Average distance between the nuclei of two bonded atoms.

    • Determined by:

      • Atoms involved (single or multiple bonds)

      • Bond angles (VSEPR theory, orbital hybridisation)

      • Bond rotation (multiplicity, substituents)

Bond Length

• CI - CI Bond Length

• An experimental measurement of associated distances.

Bond Lengths in Different Wolcules

• H - CI Bond Length

• Cl - Cl Bond Length

Summary of Bond Lengths

• Bond lengths are determined using X-ray diffraction or microwave spectroscopy.

  • Typical bond lengths: 100-200 pm (1-2 Å).

• Atoms vibrate due to thermal energy, leading to average measurements.

General Trends in Bond Lengths

• Decreases across a period (atomic radii decrease).

• Increases down a group (atomic radii increase).

  • Example:

    • CH₄ (1.09 Å), NH₃ (1.01 Å), H₂O (0.96 Å), HF (0.92 Å)

    • SiH₄ (1.48 Å), PH₃ (1.42 Å), H₂S (1.34 Å), HCl (1.27 Å)

    • GeH₄ (1.53 Å), AsH₃ (1.52 Å), H₂Se (1.46 Å), HBr (1.41 Å)

• Increasing Bond Multiplicity:

  • H₃C–CH₃ (1.54 Å), H₂C=CH₂ (1.33 Å), HC≡CH (1.20 Å)

  • C–O (diethyl ether 1.40 Å), C=O (acetone 1.21 Å)

Bond Angles

• Definition: Angle between two bonds joined by a common central atom.

Explanations for Bond Angles

• Bond angles explained by:

  1. VSEPR Theory

  2. Hybridization of Atomic Orbitals

VSEPR Theory (Valence Shell Electron Pair Repulsion)

• Characteristics:

  • Qualitative model, very reliable for elements of the first and second periods.

  • Regions of negative charge repel each other to maximize distance.

Considerations in VSEPR Theory

• Only outer shell electrons are accounted for.

• Molecular geometries consider the number of atoms and electron lone pairs.

• Coordination Number: Number of σ-bonds between a central atom and ligands.

• Lone Pair: Unshared set of two electrons within valence shell.

Determining Molecular Geometry

• Depends on valence electron pairs.

• Example:

• H₂O has four pairs of valence electrons, adopts tetrahedral geometry.

Geometry of Water (H₂O)

• Lone pairs are not visible, resulting in a bent structure.

• H–O–H bond angle: 104.5° (compressed from ideal tetrahedral angle of 109.5°).

Geometry of Ammonia (NH₃)

• Nitrogen has four pairs of valence electrons.

• Adopt tetrahedral geometry, represented as trigonal pyramidal due to a lone pair.

• H–N–H bond angles = 106.6° (compressed from ideal tetrahedral angle).

Examples of Molecular Geometry Predictions

• Methane (CH₄): Tetrahedral geometry, predicted angles: 109.5°.

• Boron Trifluoride (BF₃): Adopts planar geometry with F–B–F bond angles of 120°.

Summary of Coordination Numbers and Geometries

• Coordination 2:

  • 2: Linear (180°), e.g., CO₂, HCN

  • 4: Bent (<109.5°), e.g., H₂O

• Coordination 3:

  • 3: Trigonal planar (120°), e.g., BF₃

  • 4: Trigonal pyramidal (<109.5°), e.g., NH₃

• Coordination 4:

  • Tetrahedral (109.5°), e.g., CH₄, SiCl₄

  • Trigonal bipyramidal (90°, 120°, 180°), e.g., PCl₅

  • Octahedral (90°, 180°), e.g., SF₆

VSEPR Theory - Key Points

• The VSEPR model posits that regions of negative charge repel each other to minimize proximity.

• Main geometries without lone pair electrons: linear, trigonal, tetrahedral, trigonal bipyramidal, octahedral.

• Key Terms:

  • VSEPR Theory: Predicts the shape of molecules based on electron-pair repulsion.