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Do you know how conjugated electrons flow in molecules?
Conjugated electron systems are attractive in chemistry not only because they lower the overall energy of a molecule but also because this flow of electrons promotes molecular stability. A conjugated system consists of linked p orbitals, and the electrons in these p orbitals can flow freely within the molecule. Under what specific circumstances does this flow of electrons improve the stability and reactivity of molecules?
The presence of a conjugated system means that the π electrons do not belong to a single bond or atom, but to a group of atoms.
Conjugated systems designed to lower energy are usually presented in the form of alternating single and double bonds, and in addition, lone electron pairs, free radicals, or carbenium ions can also participate in such systems. Once we get into systems like these, especially common organic molecules like benzene and 1,3-butadiene, we see structural features that are closely related to the flow of electrons.
Chemical bonding and conjugated systems
The possibilities for conjugation come not only from alternating single and double bonds. As long as adjacent atoms in a chain have available p orbitals, the system can be considered conjugated. For example, furan is a five-membered ring containing two alternating double bonds and an oxygen atom whose lone pair of electrons can participate in conjugation, making the entire system stable.
Any sp2 or sp hybridized carbon or other atom, including atoms with empty orbitals or lone pairs of electrons, can participate in a conjugated system.
The formation of a conjugated system requires the overlap of atomic orbitals, so the conjugated system is usually required to be planar. This means that the lone electron pair participating in the conjugation will occupy a pure p orbital, rather than the usual spn hybrid orbital location. This is crucial when studying chemical bonding, especially in computational chemistry and molecular orbital theory.
Stability and its energy
The stability of the conjugated structure is closely related to its resonance energy. This stability is found when calculating the energy between real molecules and molecules simplified from the traditional chemical viewpoint (i.e., localized pi bonds). If we consider the influence of external factors, the resonance energy is significantly higher for cationic systems than for neutral systems, while the estimated values for aromatic molecules range from 36 to 73 kcal/mol, indicating their special stability.
The π electrons in a conjugated system are shared by all adjacent sp2 and sp hybridized atoms, structurally forming an overall bonding system that is larger than a molecule.
It is worth noting that non-aromatic or anti-aromatic compounds, even if they have alternating double and single bonds, are not necessarily equally stable. These molecules differ in their geometry and the degree of overlap of their p orbitals, and thus tend to differ in their reactivity and stability.
The Secret of Color: The Role of Conjugated Systems in Color Perception
When there are enough conjugated bonds in a compound's molecules, it can absorb visible light, which makes it appear colorful to the naked eye. Take beta-carotene, for example. Its long conjugated carbon chain gives it its intense orange color. When the system's electrons absorb photons of the right wavelength, they are boosted to a higher energy level. This process is closely related to the quantum mechanics model, especially from the transition of orbital energy levels, we can understand the electron flow characteristics of π bonds.
The extent of photon absorption is proportional to the length of the conjugated system: the longer the system, the larger the wavelength of the photons that can be captured.
However, not all conjugated systems show absorption of visible light. Compounds containing fewer than eight conjugated double bonds generally absorb ultraviolet light and appear colorless to the human eye. As the number of double bonds increases, the corresponding wavelength of absorbed light also becomes longer, and the color can change from yellow to red, which makes it widely used in dye manufacturing.
Conclusion
The flow of conjugated electrons not only shapes the structure of a molecule, but also influences its chemical properties and color absorption, giving us a deeper, collaborative perspective on the world of chemistry. How do these systems affect things we encounter every day, like color and chemical reactions?
