MC

Molecular Strain - Types, Causes, and Minimization

Molecular Strain: Unstable Conformers and Their Energy

Introduction to Strain

  • Definition: Strain refers to a molecule possessing high potential energy due to structural distortion or a deviation from its ideal geometry. Simply put, it's considered "bad energy" within a molecule.
  • Consequences: When a molecule experiences strain, it is unstable and under stress, leading to high potential energy.

Types of Strain

1. Angle Strain

  • Definition: Angle strain occurs when a molecule's bond angles deviate from the normal, ideal bond angles predicted by its hybridization.
  • Example: Cyclopropane
    • Hybridization: sp^3 for carbon, ideal angle is 109^[circ].
    • Actual Angle: In cyclopropane, the bond angle is 60^[circ].
    • Deviation: (109^[circ] - 60^[circ]) = 49^[circ]. This significant deviation results in high angle strain, making the molecule unstable and under stress.
  • Example: Cyclobutane
    • Hybridization: sp^3 for carbon, ideal angle is 109^[circ].
    • Actual Angle: In cyclobutane, the bond angle is 90^[circ].
    • Deviation: (109^[circ] - 90^[circ]) = 19^[circ]. This molecule also experiences angle strain.
  • Comparison: Cyclopropane (49 degree deviation) has significantly more angle strain than cyclobutane (19 degree deviation), making cyclopropane more unstable.
  • Occurrence: Angle strain is predominantly observed in cyclic molecules, not typically in linear molecules.

2. Steric Strain (Non-Bonded Interaction)

  • Definition: Steric strain arises from repulsive forces between large-sized, bulky groups that are too close in space. There isn't enough space for these groups.
  • Mechanism: Large groups attempting to occupy the same space leads to an increase in potential energy.
  • Example: Two methyl (CH3) groups positioned very close to each other.
  • Energy Contribution: A specific instance mentioned indicates 12 ext{ kcal/mol} of steric strain.

3. Torsional Strain (Bonded Interaction)

  • Definition: Torsional strain is the energy cost associated with repulsive forces between electrons residing in bonds on adjacent atoms, particularly in eclipsed conformations.
  • Mechanism: Repulsion between electron clouds of bonds that are aligned (e.g., in an eclipsed arrangement).
  • Combined Strain: When a molecule has large groups in an eclipsed conformation, it can experience both steric strain (due to the size of the groups) and torsional strain (due to the electron-electron repulsion of the eclipsed bonds).

Minimizing Strain: Molecular Rotation

  • Molecules undergo rotation around single bonds to minimize strain, reduce overall energy, and lessen repulsive forces (both torsional and steric). This movement allows bulky groups to move further apart and eclipsed bonds to adopt staggered arrangements.

Strain in Acyclic Alkanes (Ethane, Propane, Butane)

Ethane ( ext{CH}3 ext{CH}3)

  • Angle Strain: None, as it's an acyclic molecule, and carbons are sp^3 hybridized with ideal 109^[circ] angles.
  • Steric Strain: None, as only small hydrogen atoms are present.
  • Torsional Strain: Present in the eclipsed conformation due to electron-electron repulsive forces between adjacent C-H bonds.
  • Conformational Change: Ethane rotates to its staggered conformation to minimize torsional strain, leading to a more stable, lower-energy state.
  • Energy Difference: The energy difference between eclipsed and staggered ethane is approximately 3 ext{ kcal/mol} (staggered is more stable).

Propane ( ext{CH}3 ext{CH}2 ext{CH}_3)

  • Newman Projection (C1-C2 view): C1 has three hydrogens, C2 has two hydrogens and one methyl group.
  • Strain in Eclipse: In its eclipsed conformation, propane experiences:
    • Torsional Strain: Due to electron-electron repulsion between eclipsed C-H bonds.
    • Steric Strain: Due to the interaction between the larger methyl group and adjacent hydrogen atoms in the eclipsed position.
  • Energy Difference: The energy difference between eclipsed and staggered propane is slightly higher than ethane, around 3.4 ext{ kcal/mol}. This increase is attributed to the additional steric strain involving the methyl group.

Butane ( ext{CH}3 ext{CH}2 ext{CH}2 ext{CH}3)

  • Conformers (Newman Projection C2-C3 view): Butane has several distinct conformers:
    • Eclipsed Conformations (High Energy):
      • Methyl-Methyl Eclipsed (Worst): Highest energy and most unstable due to direct eclipse of two large methyl groups. This is the most unstable conformer.
      • Methyl-Hydrogen Eclipsed: Lower energy than methyl-methyl eclipsed, but still unstable.
    • Staggered Conformations (Low Energy):
      • Gauche: Two large groups (methyls) are 60^[circ] apart. This is a relatively stable, low-energy conformer, but not the most stable.
      • Anti (Best): Two large groups (methyls) are 180^[circ] apart (opposite sides). This is the most stable and lowest-energy conformer for butane, as both steric and torsional strains are minimized.
  • Energy Diagram: The energy diagram shows anti as the lowest energy point, gauche as slightly higher, and the various eclipsed forms (with methyl-methyl eclipsed being the highest peak) representing unstable, high-energy states.
  • Example Application: To find the lowest energy conformation for a molecule (e.g., viewing between C3 and C4), position the largest groups (e.g., two ethyl groups) in an anti relationship (180^[circ] apart) to minimize steric interactions.

Strain in Cycloalkanes

Cyclopropane

  • Angle Strain: Significant, as carbons are sp^3 (109^[circ] ideal) but bond angles are forced to 60^[circ].
  • Torsional Strain: All hydrogens are in an eclipsed conformation, contributing to significant torsional strain.
  • Overall Instability: Cyclopropane is a highly unstable molecule due to combined angle and torsional strains.
  • Strain Energy: Total strain energy is approximately 27 ext{ kcal/mol}.

Cyclobutane

  • Angle Strain: Present, as carbons are sp^3 (109^[circ] ideal) but bond angles are around 90^[circ].
  • Torsional Strain: If cyclobutane were planar, all hydrogens would be eclipsed, leading to high torsional strain.
  • Conformational Adjustment: To minimize torsional strain, cyclobutane is not planar. It adopts a bent or "butterfly" shape. This puckering reduces the eclipse interactions between hydrogens.
  • Strain Energy: Total strain energy is approximately 26 ext{ kcal/mol} (slightly less than cyclopropane, making it relatively more stable).

Cyclopentane

  • Angle Strain: Minimal or negligible. The ideal angle for a planar pentagon is 108^[circ], which is very close to the sp^3 ideal angle of 109^[circ].
  • Torsional Strain: If cyclopentane were planar, it would experience considerable torsional strain from all its hydrogens being eclipsed.
  • Conformational Adjustment: Cyclopentane is not planar. It adopts an "envelope" conformation, where one carbon atom is out of the plane of the other four. This rotation and out-of-plane movement minimizes the number of eclipsed hydrogen interactions, thereby reducing torsional strain.
  • Strain Energy: Total strain energy is significantly lower, around 6 ext{ kcal/mol}, making it much more stable than cyclopropane or cyclobutane.

Next Session

  • The next session will delve into the detailed discussion of cyclohexane conformations, which is particularly important given its prevalence in natural products, drugs, and medicinal chemistry products.