Aromaticity in the context of Olfaction

Petrochemicals (sometimes abbreviated as petchems) are the chemical products obtained from petroleum by refining. Some chemical compounds made from petroleum are also obtained from other fossil fuels, such as coal or natural gas, or renewable sources such as maize, palm fruit or sugar cane.

The two most common petrochemical classes are olefins (including ethylene and propylene) and aromatics (including benzene, toluene and xylene isomers).

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'.

Tea is an aromatic beverage prepared by pouring hot or boiling water over cured or fresh leaves of Camellia sinensis, an evergreen shrub native to East Asia which originated in the borderlands of south-western China, north-east India and northern Myanmar. Tea is also made, but rarely, from the leaves of Camellia taliensis and Camellia formosensis. After plain water, tea is the most widely consumed drink in the world. There are many types of tea; some have a cooling, slightly bitter, and astringent flavour, while others have profiles that include sweet, nutty, floral, or grassy notes. Tea has a stimulating effect in humans, primarily due to its caffeine content.

An early credible record of tea drinking dates to the third century AD, in a medical text written by Chinese physician Hua Tuo. It was popularised as a recreational drink during the Chinese Tang dynasty, and tea drinking spread to other East Asian countries. Portuguese priests and merchants introduced it to Europe during the 16th century. During the 17th century, drinking tea became fashionable among the British, who later started to plant tea on a large scale in India, where it had previously been used only as a medicine.

Naphthalene is an organic compound with the chemical formula C
₁₀H
₈. It is the simplest polycyclic aromatic hydrocarbon, and is a white crystalline solid with a characteristic odor that is detectable at concentrations as low as 0.08 ppm by mass. As an aromatic hydrocarbon, naphthalene's structure consists of a fused pair of benzene rings. It is the main ingredient of traditional mothballs.

In organic chemistry, hydrocarbons (compounds composed solely of carbon and hydrogen) are divided into two classes: aromatic compounds and aliphatic compounds (/ˌælɪˈfætɪk/; G. aleiphar, fat, oil). Aliphatic compounds can be saturated (in which all the C-C bonds are single, requiring the structure to be completed, or 'saturated', by hydrogen) like hexane, or unsaturated, like hexene and hexyne. Open-chain compounds, whether straight or branched, and which contain no rings of any type, are always aliphatic. Cyclic compounds can be aliphatic if they are not aromatic.

In organic chemistry, an alicyclic compound contains one or more all-carbon rings which may be either saturated or unsaturated, but do not have aromatic character. Alicyclic compounds may have one or more aliphatic side chains attached.

In organic chemistry, an aryl is any functional group or substituent derived from an aromatic ring, usually an aromatic hydrocarbon, such as phenyl and naphthyl. "Aryl" is used for the sake of abbreviation or generalization, and "Ar" is used as a placeholder for the aryl group in chemical structure diagrams, analogous to “R” used for any organic substituent. “Ar” is not to be confused with the elemental symbol for argon.

A simple aryl group is phenyl (C₆H₅−), a group derived from benzene. Examples of other aryl groups consist of:

Pyrrole is a heterocyclic, aromatic, organic compound, a five-membered ring with the formula C₄H₄NH. It is a colorless volatile liquid that darkens readily upon exposure to air. Substituted derivatives are also called pyrroles, e.g., N-methylpyrrole, C₄H₄NCH₃. Porphobilinogen, a trisubstituted pyrrole, is the biosynthetic precursor to many natural products such as heme.

Pyrroles are components of more complex macrocycles, including the porphyrinogens and products derived therefrom, including porphyrins of heme, the chlorins, bacteriochlorins, and chlorophylls.

In organic chemistry, Hückel's rule predicts that a planar ring molecule will have aromatic properties if it has 4n + 2 π-electrons, where n is a non-negative integer. The quantum mechanical basis for its formulation was first worked out by physical chemist Erich Hückel in 1931. The succinct expression as the 4n + 2 rule has been attributed to W. v. E. Doering (1951), although several authors were using this form at around the same time.

In agreement with the Möbius–Hückel concept, a cyclic ring molecule follows Hückel's rule when the number of its π-electrons equals 4n + 2, although clearcut examples are really only established for values of n = 0 up to about n = 6. Hückel's rule was originally based on calculations using the Hückel method, although it can also be justified by considering a particle in a ring system, by the LCAO method and by the Pariser–Parr–Pople method.

In the field of organic chemistry, a polycyclic compound is an organic compound featuring several closed rings of atoms, primarily carbon. These ring substructures include cycloalkanes, aromatics, and other ring types. They come in sizes of three atoms and upward, and in combinations of linkages that include tethering (such as in biaryls), fusing (edge-to-edge, such as in anthracene and steroids), links via a single atom (such as in spiro compounds), bridged compounds, and longifolene. Though poly- literally means "many", there is some latitude in determining how many rings are required to be considered polycyclic; many smaller rings are described by specific prefixes (e.g., bicyclic, tricyclic, tetracyclic, etc.), and so while it can refer to these, the title term is used with most specificity when these alternative names and prefixes are unavailable.

In general, the term polycyclic includes polycyclic aromatic compounds, including polycyclic aromatic hydrocarbons, as well as heterocyclic aromatic compounds with multiple rings (where heteroaromatic compounds are aromatic compounds that contain sulfur, nitrogen, oxygen, or another non-carbon atoms in their rings in addition to carbon).

A cyclic compound (or ring compound) is a term for a compound in the field of chemistry in which one or more series of atoms in the compound is connected to form a ring. Rings may vary in size from three to many atoms, and include examples where all the atoms are carbon (i.e., are carbocycles), none of the atoms are carbon (inorganic cyclic compounds), or where both carbon and non-carbon atoms are present (heterocyclic compounds with rings containing both carbon and non-carbon). Depending on the ring size, the bond order of the individual links between ring atoms, and their arrangements within the rings, carbocyclic and heterocyclic compounds may be aromatic or non-aromatic; in the latter case, they may vary from being fully saturated to having varying numbers of multiple bonds between the ring atoms. Because of the tremendous diversity allowed, in combination, by the valences of common atoms and their ability to form rings, the number of possible cyclic structures, even of small size (e.g., < 17 total atoms) numbers in the many billions.

• Cyclic compound examples: All-carbon (carbocyclic) and more complex natural cyclic compounds
• 
  Cycloalkanes, the simplest carbocycles, including cyclopropane, cyclobutane, cyclopentane, and cyclohexane. Note, elsewhere an organic chemistry shorthand is used where hydrogen atoms are inferred as present to fill the carbon's valence of 4 (rather than their being shown explicitly).
• 
  Ingenol, a complex, terpenoid natural product, related to but simpler than the paclitaxel that follows, which displays a complex ring structure including 3-, 5-, and 7-membered non-aromatic, carbocyclic rings.
• 
  Paclitaxel, another complex, plant-derived terpenoid, also a natural product, displaying a complex multi-ring structure including 4-, 6-, and 8-membered rings (carbocyclic and heterocyclic, aromatic and non-aromatic).

Adding to their complexity and number, closing of atoms into rings may lock particular atoms with distinct substitution (by functional groups) such that stereochemistry and chirality of the compound results, including some manifestations that are unique to rings (e.g., configurational isomers). As well, depending on ring size, the three-dimensional shapes of particular cyclic structures – typically rings of five atoms and larger – can vary and interconvert such that conformational isomerism is displayed. Indeed, the development of this important chemical concept arose historically in reference to cyclic compounds. Finally, cyclic compounds, because of the unique shapes, reactivities, properties, and bioactivities that they engender, are the majority of all molecules involved in the biochemistry, structure, and function of living organisms, and in man-made molecules such as drugs, pesticides, etc.

In organic chemistry, the phenyl group, or phenyl ring, is a cyclic group of atoms with the formula C₆H₅−, and is often represented by the symbol Ph (archaically φ) or Ø. The phenyl group is closely related to benzene and can be viewed as a benzene ring, minus a hydrogen atom, which may be replaced by some other element or compound to serve as a functional group. A phenyl group has six carbon atoms bonded together in a hexagonal planar ring, five of which are bonded to individual hydrogen atoms, with the remaining carbon bonded to a substituent. Phenyl groups are commonplace in organic chemistry. Although often depicted with alternating double and single bonds, the phenyl group is chemically aromatic and has equal bond lengths between carbon atoms in the ring.

Methylene diphenyl diisocyanate (MDI) is an aromatic diisocyanate. Three isomers are common, varying by the positions of the isocyanate groups around the rings: 2,2′-MDI, 2,4′-MDI, and 4,4′-MDI. The 4,4′ isomer is most widely used, and is also known as 4,4′-diphenylmethane diisocyanate. This isomer is also known as Pure MDI. MDI reacts with polyols in the manufacture of polyurethane. It is the most produced diisocyanate, accounting for 61.3% of the global market in the year 2000.

Phenol (also known as carbolic acid, phenolic acid, or benzenol) is an aromatic organic compound with the molecular formula C₆H₅OH. It is a white crystalline solid that is volatile and can catch fire.

The molecule consists of a phenyl group (−C₆H₅) bonded to a hydroxy group (−OH). Mildly acidic, it requires careful handling because it can cause chemical burns. It is acutely toxic and is considered a health hazard.