Kekulé's 1872 modification of his 1865 theory, illustrating rapid alternation of double bonds Critics pointed out a problem with Kekulé's original (1865) structure for benzene: Whenever benzene underwent substitution at the ortho position, two distinguishable isomers should have resulted, depending on whether a double bond or a single bond existed between the carbon atoms to which the substituents were attached; however, no such isomers were observed.  In 1872, Kekulé suggested that benzene had two complementary structures and that these forms rapidly interconverted, so that if there were a double bond between any pair of carbon atoms at one instant, that double bond would become a single bond at the next instant (and vice versa).  To provide a mechanism for the conversion process, Kekulé proposed that the valency of an atom is determined by the frequency with which it collided with its neighbors in a molecule.  As the carbon atoms in the benzene ring collided with each other, each carbon atom would collide twice with one neighbor during a given interval and then twice with its other neighbor during the next interval.  Thus, a double bond would exist with one neighbor during the first interval and with the other neighbor during the next interval.  Therefore, between the carbon atoms of benzene there were no fixed (i.e., constant) and distinct single or double bonds; instead, the bonds between the carbon atoms were identical.  See pages 86–89  of Auguste Kekulé (1872) "Ueber einige Condensationsprodukte des Aldehyds" (On some condensation products of aldehydes), Liebig's Annalen der Chemie und Pharmacie, 162(1): 77–124, 309–320.  From p. 89:  "Das einfachste Mittel aller Stöße eines Kohlenstoffatoms ergiebt sich aus der Summe der Stöße der beiden ersten Zeiteinheiten, die sich dann periodisch wiederholen.  … man sieht daher, daß jedes Kohlenstoffatom mit den beiden anderen, … daß diese Verschiedenheit nur eine scheinbare, aber keine wirkliche ist." (The simplest average of all the collisions of a carbon atom [in benzene] comes from the sum of the collisions during the first two units of time, which then periodically repeat.  … thus one sees that each carbon atom collides equally often with the two others against which it bumps, [and] thus stands in exactly the same relation with its two neighbors.  The usual structural formula for benzene expresses, of course, only the collisions that occur during one unit of time, thus during one phase, and so one is led to the view [that] doubly substituted derivatives [of benzene] must be different at positions 1,2 and 1,6 [of the benzene ring].  If the idea [that was] just presented—or a similar one—can be regarded as correct, then [it] follows therefrom that this difference [between the bonds at positions 1,2 and 1,6] is only an apparent [one], not a real [one].)

Aromatic organic chemical compound with formula C9H6O2.

- Coumarin

220 related topics



Lactones are cyclic carboxylic esters, containing a 1-oxacycloalkan-2-one structure (\sC(\dO)\sO\s), or analogues having unsaturation or heteroatoms replacing one or more carbon atoms of the ring.

Lactone nomenclature: α-acetolactone, β-propiolactone, γ-butyrolactone, and δ-valerolactone
Oxandrolone synthesis
γ-Lactone synthesis from fatty alcohols and acrylic acid
<center>γ-butyrolactone (GBL)</center>
<center> D -glucono-δ-lactone (E575)</center>

Of the naturally occurring bicyclic lactones, phthalides are responsible for the odors of celery and lovage oils, and coumarin for woodruff.

Vitamin K

Vitamin K refers to structurally similar, fat-soluble vitamers found in foods and marketed as dietary supplements.

Vitamin K structures.
Vitamin K1 (phylloquinone) – both forms of the vitamin contain a functional naphthoquinone ring and an aliphatic side chain. Phylloquinone has a phytyl side chain.
Vitamin K2 (menaquinone). In menaquinone, the side chain is composed of a varying number of isoprenoid residues. The most common number of these residues is four, since animal enzymes normally produce menaquinone-4 from plant phylloquinone.
A sample of phytomenadione for injection, also called phylloquinone
Cyclic mechanism of action of vitamin K

Coumarin is used in the pharmaceutical industry as a precursor reagent in the synthesis of a number of synthetic anticoagulant pharmaceuticals.


Anticoagulants, commonly known as blood thinners, are chemical substances that prevent or reduce coagulation of blood, prolonging the clotting time.

Coagulation Cascade and Major Classes of Anticoagulants

These oral anticoagulants are derived from coumarin, which is found in many plants.


Medication that is used as an anticoagulant .

Vitamin K1-warfarin interaction effect. When warfarin levels are high, people have more risk of bleeding. Conversely, lower levels of warfarin lead to increased risk of blood clots. There is a narrow range where the benefits of warfarin are greater than the risks, its therapeutic window. Certain drugs, herbal medicines, and foods can interact with warfarin, increasing or decreasing a previously stable warfarin level.
Acyclic tautomer (left) and cyclic hemiketal tautomer (right)
3 mg (blue), 5 mg (pink) and 1 mg (brown) warfarin tablets (UK colours)
Warning label on a tube of rat poison laid on a dike of the Scheldt river in Steendorp, Belgium. The tube contains bromadiolone, a second-generation ("super-warfarin") anticoagulant.

In some countries, other coumarins are used instead of warfarin, such as acenocoumarol and phenprocoumon.

Anthoxanthum odoratum

Short-lived perennial grass that is native to acidic grassland in Eurasia and northern Africa.

The scent is particularly strong when dried, and is due to coumarin, a glycoside, and benzoic acid – it smells like fresh hay with a hint of vanilla.

Cinnamomum cassia

Evergreen tree originating in southern China, and widely cultivated there and elsewhere in South and Southeast Asia .

Cassia (C. cassia) essential oil
Young Indonesian cinnamon tree, Indonesia
C. cassia (top left) depicted by Michał Boym (1655)
Dried cassia bark

Due to a blood-thinning component called coumarin that could damage the liver if consumed in larger amounts, European health agencies have warned against consuming high amounts of cassia.

Dipteryx odorata

Species of flowering tree in the pea family, Fabaceae.

The smooth brown inside of the tonka bean

They have a strong fragrance similar to sweet woodruff due to their high content of coumarin.


Genus in the family Fabaceae (the same family that also includes the Trifolium clovers).

Melilotus albus

This legume is commonly named for its sweet smell, which is due to the presence of coumarin in its tissues.

Galium odoratum

Flowering perennial plant in the family Rubiaceae, native to much of Europe from Spain and Ireland to Russia, as well as Western Siberia, Turkey, Iran, the Caucasus, China and Japan.


It owes its sweet smell to the odoriferous agent coumarin, and is sometimes used as a flavouring agent due to its chemical content.

Cinnamic acid

Organic compound with the formula C6H5-CH=CH-COOH.

Methane, CH4; is among the simplest organic compounds.

Cinnamic acid is a central intermediate in the biosynthesis of a myriad of natural products including lignols (precursors to lignin and lignocellulose), flavonoids, isoflavonoids, coumarins, aurones, stilbenes, catechin, and phenylpropanoids.