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From single bonds to double bonds: How do conjugated systems work?
In chemical theory, a conjugated system is a set of connected p orbitals with delocalized electrons that overall lowers the energy of the molecule and increases its stability. These systems are usually represented as alternating single and multiple bonds and may include lone pairs of electrons, free radicals, or carbene ions, and may be cyclic, acyclic, linear, or mixed structures. As an important illustration of this field, the term "conjugation" was first coined by German chemist Johannes Thiele in 1899.
The key to conjugation is the overlap between one p orbital and another p orbital on an adjacent sigma bond.
The presence of a conjugated system allows the π electrons to be delocalized across all adjacent aligned p orbitals, which means that these π electrons do not belong to a single bond or atom, but to a group of atoms. In chemistry, these molecules containing conjugated systems are often called conjugated molecules. Representative conjugated molecules include 1,3-butadiene, benzene and alkenyl cations. Very large conjugated systems can be found in graphene, graphite, conducting polymers and carbon nanotubes.
Chemical bonding in conjugated systems
Conjugation is achieved by alternating single and double bonds, with each atom providing a p orbital perpendicular to the plane of the molecule. However, this is not the only way to achieve conjugation. As long as every adjacent atom in the chain has an available p orbital, the system can be considered conjugated. For example, furan is a five-membered ring with two alternating double bonds and lone pairs on the oxygen atoms, one of which occupies a p orbital perpendicular to that position of the ring, thus maintaining the five-membered ring. Conjugation of the ring.
In a conjugated system, overlap of p orbitals is the fundamental requirement to make conjugation possible.
A conjugated system must be planar or nearly planar to satisfy the overlap requirement. This means that the lone pair of electrons involved in conjugation will occupy orbitals of pure p nature rather than the usual hybrid orbitals. The most common model of conjugated molecules is a treatment that combines valence bond theory and Huckel molecular orbital theory. In this framework, the σ framework of a molecule is separated from its π system (or systems).
Stabilization Energy
The stabilization energy accumulated in a conjugated system, usually described as the resonance energy, is the energy difference between the actual chemical species and a hypothetical chemical species with local π bonding. Although this energy cannot be measured, it can be roughly estimated, showing the important effect of conjugation on the stability of some molecules.
In general, cationic systems are more stable than neutral systems.
For example, in 1,3-butadiene, the activation energy for rotating the C2-C3 bond is about 6 kcal/mol, and resonance stabilization is presumed to be part of this. In cycloalkanes, such as benzene, the resonance energy range has been estimated to be between 36 and 73 kcal/mol, demonstrating the surprising stability of conjugated systems for aroma compounds.
Conjugated cyclic compounds
Cyclic compounds may be partially or fully conjugated. Completely conjugated monocyclic hydrocarbons are called cycloalkenes. Compounds of this type are considered aromatic if they have a planar conjugated system that satisfies the (4n + 2) π electron structure, as is the case with benzene. The numerous conjugation pathways are closely related to the electrical and optical properties of the molecules.
The conjugated p-system enables the molecule to grab photons of specific wavelengths, displaying different colors.
For example, the long conjugated carbon-hydrogen chain of beta-carotene gives it its intense orange color. This not only affects the color of the molecules, but also their optical properties and applications, including various types of synthetic dyes in the field of photochemistry.
As we gain a deeper understanding of conjugated systems, do you seem to be able to feel the beauty and power hidden in these chemical structures?
