Multiple bonding and stereochemistry

Multiple bonding

Multiple bonding occurs when atoms share more than one pair of electrons, typically forming double or triple bonds. This additional electron sharing shortens the bond length by drawing the atoms closer together and increases the bond energy, making the bonds stronger. Moreover, multiple bonding imposes significant rigidity on a molecule’s structure because double and triple bonds restrict the free rotation that single bonds allow. Even bonds with partial double bond character, as seen in the peptide bond, prevent free rotation, thereby contributing to a fixed molecular geometry.

Stereochemistry of covalently bonded molecules

Isomers are compounds that share the same molecular formula but differ in their structural arrangement. This means they can be identical in composition yet distinct in the way their atoms are connected or arranged in space.

Structural isomers

• Positional isomers have identical functional groups located at different positions within the molecule.
• Constitutional isomers (or structural isomers) share the same formula but differ in the connectivity of atoms.
• Functional isomers possess different functional groups despite having the same molecular formula.

Geometric isomers

Geometric isomers have the same molecular formula and connectivity but differ in the spatial orientation of groups around a double bond:

• When both sides of the double bond contain the same two groups, the designations cis (same side) and trans (opposite sides) are used.
• If different groups are attached to each side, the terms Z (higher priority groups on the same side) and E (higher priority groups on opposite sides) are applied based on the Cahn-Ingold-Prelog rules.

Stereoisomers

Stereoisomers have the same molecular formula and atom connectivity but differ in the three-dimensional arrangement of their atoms:

• Enantiomers are non-superimposable mirror images, where every chiral center (an atom bonded to four distinct groups) is inverted.
• Diastereomers occur when molecules with multiple chiral centers differ in the configuration of some, but not all, centers. In cyclic compounds, cis/trans notation is often used, where cis indicates substituents on the same side of the ring and trans indicates those on opposite sides.
• Meso compounds contain chiral centers but are overall achiral and optically inactive, reducing the total number of possible stereoisomers.

Conformational Isomers

Conformers are different spatial orientations of the same molecule that arise from rotation around single bonds:

• In eclipsed conformations, groups align closely, leading to increased torsional strain and reduced stability. Specific eclipsed forms like the syn-periplanar conformation are particularly strained.
• Staggered conformations minimize torsional strain:
  • Gauche conformations feature groups separated by approximately 60°, providing moderate stability.
  • Anti conformations, with groups positioned 180° apart, are the most stable.
• In cyclic molecules, such as hexoses, the chair conformation is favored for its staggered arrangement, while the boat conformation is less stable due to greater eclipsing.
• Steric interactions further influence stability:
  • Axial positions often result in clashes between bulky groups.
  • Equatorial positions allow substituents to extend outward, reducing repulsion and enhancing overall stability.

Understanding these aspects of stereochemistry is crucial, as the three-dimensional arrangement of atoms greatly influences a molecule’s physical and chemical properties.

Polarization of light and specific rotation

Light is an electromagnetic wave characterized by oscillating electric and magnetic fields that are in phase and perpendicular to each other as well as to the direction of propagation. In unaltered light, these fields vibrate in every direction along a plane that is 360° around the propagation axis. However, when light is polarized, its fields are confined to a single direction.

Specific rotation is the property of chiral molecules—when present as a single enantiomer—to rotate the plane of polarized light. This optical activity means that such molecules can twist the light either to the left or to the right. A leftward rotation is described as negative, l, or levorotatory, while a rightward rotation is noted as positive, d, or dextrorotatory. It is important to remember that these rotation indicators do not correspond to R/S configurations, and that the lower-case d and l are distinct from the upper-case D and L used to denote absolute configurations in sugars.