Molecular Strain: Types and Conformations

Definition of Strain (S-ray)

Strain (S-ray): A molecule is said to have strain when it possesses high potential energy due to a structural distortion or a deviation from its ideal geometry.
Simple definition: "Bad energy" within a molecule.
Consequence: Molecules with high strain are unstable.

Types of Strain

There are three main types of molecular strain:

1. Angle Strain

Definition: Occurs when a molecule's bond angles deviate from their normal or ideal values.
Ideal Angle (SP^3 Hybridization): For carbons with SP^3 hybridization (four single bonds), the ideal bond angle is 109.5^ ext{o}.
Cyclopropane Example:
- Hybridization: SP^3.
- Expected angle: 109.5^ ext{o}.
- Actual angle: 60^ ext{o}.
- Deviation: 109.5^ ext{o} - 60^ ext{o} = 49.5^ ext{o}.
- Implication: Cyclopropane experiences significant angle strain, is highly unstable, and has high potential energy.
Cyclobutane Example:
- Hybridization: SP^3.
- Expected angle: 109.5^ ext{o}.
- Actual angle: 90^ ext{o}.
- Deviation: 109.5^ ext{o} - 90^ ext{o} = 19.5^ ext{o}.
- Comparison: Cyclopropane (deviation of 49.5^ ext{o}) has more angle strain than cyclobutane (deviation of 19.5^ ext{o}), making cyclopropane more unstable.
Occurrence: Angle strain is primarily observed in cyclic molecules, not linear molecules.

2. Steric Strain (S-Stress)

Definition: Arises from the repulsive forces between large, bulky groups that are too close in space.
Cause: Insufficient space for two large (bulky) groups, leading to "bad energy."
Example: Two methyl (CH_{3}) groups positioned too close to each other on the same side of a molecule.
Consequence: The molecule is under "S-stress" due to the bulky groups.
Energy cost: Can be significant, e.g., 12 ext{ kcal/mol} for certain methyl-methyl interactions.

3. Torsional Strain

Definition: Energy associated with repulsive forces between electron clouds of bonds that are in close proximity, particularly in eclipsed conformations.
Cause: Repulsion between electron pairs in adjacent bonds (e.g., hydrogen-hydrogen or carbon-hydrogen bond electrons).
Eclipse Form: Occurs when bonds on adjacent carbons are directly aligned (eclipsed) when viewed down the carbon-carbon bond axis.
Example: In an eclipsed conformation, the electron clouds of corresponding bonds on the front and back carbons repel each other.

Combined Strains

Some conformations (e.g., an eclipsed conformation with large groups) can exhibit both torsional strain (due to electron-electron repulsion) and steric strain (due to large group interaction).

Minimizing Strain through Rotation

Mechanism: Molecules can rotate around single bonds (carbon-carbon bonds) to change their conformation and reduce strain.
Goal: To achieve a more stable, lower-energy conformation by minimizing repulsive forces (torsional strain) and increasing the distance between bulky groups (steric strain.
Example: Conversion from an eclipsed form (high energy, high strain) to a staggered form (lower energy, lower strain) through a 60^ ext{o} rotation.

Conformations of Acyclic Alkanes

Ethane (CH3CH3)

Angle Strain: None, as it's a linear molecule (not cyclic) and has ideal 109.5^ ext{o} bond angles.
Steric Strain: None, as it only has small hydrogen atoms attached, not bulky groups.
Torsional Strain: Present in the eclipsed conformation due to electron repulsion between aligned C-H bonds.
Staggered Conformation: After a 60^ ext{o} rotation, the C-H bonds are staggered, minimizing torsional strain. This is the more stable, lower-energy form.
Energy Difference: The energy difference between the eclipsed and staggered conformers of ethane is approximately 3 ext{ kcal/mol}. The staggered conformer is 3 ext{ kcal/mol} more stable.

Propane (CH3CH2CH_3)

Considering the C1-C2 bond (front carbon: two H, one CH_{3}; back carbon: three H):
Strains: Propane exhibits both torsional strain (in eclipsed forms, H-H eclipsing) and steric strain (due to the larger methyl group eclipsing with a hydrogen or another group).
Energy Diagram: Similar pattern of high-energy eclipsed and low-energy staggered conformations.
Energy Difference: The energy difference between staggered and eclipsed forms for propane (approx. 3.4 ext{ kcal/mol}) is slightly greater than ethane (3 ext{ kcal/mol}), illustrating the additional steric interaction from the methyl group.

Butane (CH3CH2CH2CH3)

Considering the C2-C3 bond (front carbon: two H, one CH{3}; back carbon: two H, one CH{3}).
Conformers (Newman Projections):
- Eclipsed Forms:
  - Syn-periplanar (or Fully Eclipsed): Methyl groups are directly aligned (eclipsed) with each other (0^ ext{o} dihedral angle). This is the highest energy (worst) conformation due to maximum steric and torsional strain.
  - Gauche Eclipsed: Methyl groups eclipse hydrogens (120^ ext{o} dihedral angle). These are higher in energy than staggered but lower than syn-periplanar.
- Staggered Forms:
  - Anti-periplanar (Anti): Methyl groups are opposite to each other (180^ ext{o} dihedral angle). This is the lowest energy (best) conformation as steric repulsion is minimized.
  - Gauche: Methyl groups are 60^ ext{o} apart from each other. These are more stable than eclipsed forms but less stable than anti due to some residual steric interaction (gauche interaction).
Energy Diagram: The energy profile for butane clearly shows:
- Syn-periplanar eclipsed: Highest energy.
- Gauche: Intermediate energy (higher than anti, lower than eclipsed).
- Anti: Lowest energy.

Example (Butane Conformation - Carbon 3 and Carbon 4 perspective):

To find the lowest energy conformation for a molecule like 3-ethyl-2-fluoropentane (looking from C3 and C4):
- Identify the largest groups on C3 (ethyl, fluorine) and C4 (ethyl).
- The lowest energy (best) conformation will be when the two largest groups (ethyl on C3 and ethyl on C4) are anti to each other (180^ ext{o} dihedral angle).
- Other smaller groups (e.g., fluorine, hydrogen) would then arrange around them.

Strain in Cyclic Alkanes

Cyclopropane

Angle Strain: Significant deviation from 109.5^ ext{o} (60^ ext{o} bond angles), leading to high angle strain.
Torsional Strain: All C-H bonds are in a fully eclipsed conformation due to the planar triangular structure, resulting in considerable torsional strain.
Overall Strain Energy: Approximately 27 ext{ kcal/mol}.
Implication: Cyclopropane is an unstable molecule due to both high angle and torsional strain.

Cyclobutane

Angle Strain: Deviates from 109.5^ ext{o} (90^ ext{o} bond angles), resulting in angle strain.
Torsional Strain: If planar, all C-H bonds would be eclipsed, leading to significant torsional strain.
Conformation (to minimize strain): Cyclobutane is not planar. It adopts a puckered (or "butterfly") conformation to relieve some of the torsional strain by allowing slight rotation around C-C bonds. This bent structure minimizes eclipse interactions.
Overall Strain Energy: Approximately 26 ext{ kcal/mol} (slightly less unstable than cyclopropane).

Cyclopentane

Angle Strain: Very minimal; the ideal internal angle for a regular pentagon is 108^ ext{o}, which is very close to the ideal SP^3 angle of 109.5^ ext{o}. So, nearly no angle strain.
Torsional Strain: If planar, cyclopentane would have five pairs of eclipsed C-H bonds, leading to significant torsional strain.
Conformation (to minimize strain): Cyclopentane is not planar. It adopts an envelope conformation where one carbon atom is out of the plane of the other four. This minimizes torsional strain by reducing the number of eclipsed C-H interactions.
Overall Strain Energy: Approximately 6 ext{ kcal/mol}.
Comparison: Cyclopentane has significantly less strain energy compared to cyclopropane (27 ext{ kcal/mol}) and cyclobutane (26 ext{ kcal/mol}) primarily because it has negligible angle strain and effectively minimizes torsional strain through puckering.

Upcoming Session

The next session will cover cyclohexane in detail, which is a very important cyclic compound found in many natural products, steroids, and drugs.