lecture 12

Lecture 12 Overview

• Course: CHEM 2321/2324
• Instructor: Dr. Robert Pankow
• Content Usage: For temporary utilization of registered students only, under fair use copyright for educational purposes.
• Copyright Notice: © Organic Chemistry – The University of Texas at El Paso

Today's Lecture Topics

• Strain in organic molecules
• Straight chain hydrocarbons
• Cyclic hydrocarbons
• Conformations and strain in cyclohexane
• Index of Hydrogen Deficiency
• Hydrogenation of Alkenes/Alkynes

Types of Strain

Torsional Strain

• Definition: Strain due to repulsive dispersion/electronic forces that cannot be relieved because of restricted conformational mobility.

Van Der Waals Strain (Steric Hindrance)

• Definition: Strain caused by repulsive interactions between electron clouds of neighboring substituents.

Angle Strain

• Definition: Strain resulting from deviation from ideal bond angles (109.5° for sp³ hybridized carbon) due to inherent structural constraints such as ring size.
  • Example: Cyclopropane exhibits significant angle strain due to its small ring size.

Conformational Changes in Linear Hydrocarbons

Butane Conformations

• Description of Conformations:
  • In butane, there are three staggered conformations observed during a complete rotation:
  • Two gauche conformations
  • One anti conformation
• Angles: Each staggered conformation exhibits a 120° rotation around the internal bond.
• Stability: The staggered anti conformation is more stable than the gauche conformations because the CH₃ groups are positioned opposite each other, avoiding steric strain.
• Steric Strain Energy:
  • Energy introduced in gauche conformations: 3.8 kJ/mol due to proximity of CH₃ groups.

Strain in Cyclopropane

Poor Orbital Overlap

• Poor orbital overlap leads to what is known as "bent bonds."
• Hybrid orbitals in cyclopropane are misaligned relative to the internuclear axis, resulting in weaker C-C bonds (~272 kJ/mol versus ~377 kJ/mol typical for C-C) and higher relative potential energy.
• Bond Lengths:
  • C-C internuclear distance: 1.510 Å (shorter than 1.54 Å in alkanes)
  • C-H bond length in cyclopropane: 1.089 Å (shorter than 1.10 Å in ethane)
  • Interpretation: Indications of difficulty in maintaining sp³ hybridization.

Angle Strain in Cyclopropane

• Bonding factors cause angle strain due to bonding geometry:
  • Ideal tetrahedral angle: 109.5° (for sp³)
  • Cyclopropane geometry: Equilateral triangle with internal angles of 60°
  • Angle Strain Calculation: Deviation of 49.5° from ideal results in compression of internuclear angles.

Torsional Strain in Cyclopropane

• Torsional strain arises due to eclipsed hydrogens in cyclopropane.
• Newman projection analysis confirms eclipsed H along C-C bonds, leading to significant torsional strain due to conformational rigidity.

Cyclobutane

Structure and Strain

• Cyclobutane is not planar; its distorted geometry minimizes severe torsional strain.
• Angle strain: Difference between ideal bond angle (109.5°) and actual internal angle (close to 90°).
• Ring strain results from:
  1. Angle strain
  2. Torsion strain

Conformation of Cyclobutane

• Cyclobutane adopts a bent structure to alleviate torsional strain via slight rotations around C-C bonds that reduce steric interactions.

Chair and Half-Chair Conformations of Cyclohexane

Chair Conformation

• The chair conformation is stable, showing no strain energy.

Half-Chair Conformation

• Definition: An unstable conformation requiring energy to overcome a barrier (45.2 kJ/mol) during conversion to chair form.
• Structural changes: Small rotations lead to angular strain and torsional strain, putting half-chair 45.2 kJ/mol above chair energy level.

Twist Boat Conformation

• Definition: A non-planar structure of cyclohexane due to reduced torsional and steric strain.
• Stability: Estimated to be 29.7 kJ/mol less stable than the chair conformation.
• Structural adjustment through slight rotations provides relief from torsional strains.

Ring Flipping and Substituent Positions

• Ring flipping causes conversion of axial substituents to equatorial positions, which impacts stability.
• 1,3-Diaxial Interactions: Bulky groups prefer equatorial positioning to reduce steric hindrance.

Index of Hydrogen Deficiency (IHD)

Definition and Calculation

• Recall Formula: IHD = CₙH₂ₙ₊₂ for alkanes.
  • Example:
  • C₆H₁₄: Alkaline formula (hexane)
  • C₆H₁₂: Formula for possible compounds (1-hexene or cyclohexane)
  • Index Calculation: IHD = 1 (missing 1 pair of hydrogen)

Direct Determination from Molecular Formula

• Examples:
  • C₄H₆ ⇒ missing 2 H₂
  • C₄H₄ ⇒ missing 3 H₂
  • C₆H₈ ⇒ missing 3 H₂
• Conclusion: IHD can be greater than 1.

Reactivity and Structural Elucidation

Application of Reactivity

• Reactivity patterns can assist in deducing molecular structure through IHD.

Stoichiometry for Structural Determination

• Stoichiometric equivalents are useful for determining molecular structures based on reactivity.

Treatment of IHD in Oxygen and Nitrogen Compounds

Oxygen-Containing Compounds

• Calculation: Ignore Oxygen
  • Example: C₄H₈O ⇒ C₄H₈

Nitrogen-Containing Compounds

• Calculation: Subtract 1 H and ignore Nitrogen
  • Example: C₄H₉N ⇒ C₄H₈

Newman Projections and Misconceptions

• Emphasis on clarity regarding naming conventions in structures, particularly in discussing configurations for specific bonds (e.g., 3-methyl-4-penten-2-ol).

Concluding Notes

• A focus on how to visualize and analyze organic structures can lead to better understanding of molecular interactions and stability.

Next Lecture Brief

• Topics to include cis-trans isomerism of cyclohexanes in detail.
• Clarification that ring flipping does not alter isomerism, maintaining stability throughout the process.

Important Chemistry: Hydrogenation

• Applications of hydrogenation in various sectors like food (vegetable oils), petrochemicals, agrochemicals, etc.

END OF LECTURE 12

• © Organic Chemistry – The University of Texas at El Paso