Conjugated system in the context of "Diels–Alder reaction"

Porphyrins (/ˈpɔːrfərɪns/ POR-fər-ins) are heterocyclic, macrocyclic, organic compounds, composed of four modified pyrrole subunits interconnected at their α carbon atoms via methine bridges (=CH−). In vertebrates, an essential member of the porphyrin group is heme, which is a component of hemoproteins, whose functions include carrying oxygen in the bloodstream. In plants, an essential porphyrin derivative is chlorophyll, which is involved in light harvesting and electron transfer in photosynthesis.

The parent of porphyrins is porphine, a rare chemical compound of exclusively theoretical interest. Substituted porphines are called porphyrins. With a total of 26 π-electrons the porphyrin ring structure is a coordinated aromatic system. One result of the large conjugated system is that porphyrins absorb strongly in the visible region of the electromagnetic spectrum, i.e. they are deeply colored. The name "porphyrin" derives from Greek πορφύρα (porphyra) 'purple'.

Aromatic compounds or arenes are organic compounds "with a chemistry typified by benzene" and "cyclically conjugated."The word "aromatic" originates from the past grouping of molecules based on odor, before their general chemical properties were understood. The current definition of aromatic compounds does not have any relation to their odor. Aromatic compounds are now defined as cyclic compounds satisfying Hückel's rule.Aromatic compounds have the following general properties:

• Typically unreactive
• Often non polar and hydrophobic
• High carbon-hydrogen ratio
• Burn with a strong sooty yellow flame, due to high C:H ratio
• Undergo electrophilic substitution reactions and nucleophilic aromatic substitutions

Arenes are typically split into two categories - benzoids, that contain a benzene derivative and follow the benzene ring model, and non-benzoids that contain other aromatic cyclic derivatives. Aromatic compounds are commonly used in organic synthesis and are involved in many reaction types, following both additions and removals, as well as saturation and dearomatization.

In organic chemistry, aromaticity is a chemical property describing the way in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibits a stabilization stronger than would be expected from conjugation alone. The earliest use of the term was in an article by August Wilhelm Hofmann in 1855. There is no general relationship between aromaticity as a chemical property and the olfactory properties of such compounds.

Aromaticity can also be considered a manifestation of cyclic delocalization and of resonance. This is usually considered to be because electrons are free to cycle around circular arrangements of atoms that are alternately single- and double-bonded to one another. This commonly seen model of aromatic rings, namely the idea that benzene was formed from a six-membered carbon ring with alternating single and double bonds (cyclohexatriene), was developed by Kekulé (see History section below). Each bond may be seen as a hybrid of a single bond and a double bond, every bond in the ring identical to every other. The model for benzene consists of two resonance forms, which corresponds to the double and single bonds superimposing to give rise to six one-and-a-half bonds. Benzene is a more stable molecule than would be expected without accounting for charge delocalization.

Glycocholic acid, or cholylglycine, is a crystalline bile acid involved in the emulsification of fats. It occurs as a sodium salt in the bile of mammals. It is a conjugate of cholic acid with glycine. Its anion is called glycocholate.

In a prospective study, positive associations were observed between prediagnostic plasma levels of seven conjugated bile acid metabolites, including glycocholic acid, and colon cancer risk. These findings support experimental data suggesting that high circulating bile acids promote colon cancer risk.

In organic chemistry, polyenes are polyunsaturated organic compounds that contain multiple carbon–carbon double bonds (C=C). Some sources consider dienes to be polyenes, whereas others require polyenes to contain three carbon–carbon double bonds (trienes) or more.

Conjugated polyenes contain a conjugated system of alternating single and double carbon–carbon bonds, with characteristic optical properties.