Arsole, also called arsenole or arsacyclopentadiene, is an

organoarsenic compound Organoarsenic chemistry is the chemistry of compounds containing a chemical bond between arsenic and carbon. A few organoarsenic compounds, also called "organoarsenicals," are produced industrially with uses as insecticides, herbicides, and fungic ...

with the

formula In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwee ...

C₄H₄AsH. It is classified as a metallole and is isoelectronic to and related to

pyrrole Pyrrole is a heterocyclic aromatic organic compound, a five-membered ring with the formula C4 H4 NH. It is a colorless volatile liquid that darkens readily upon exposure to air. Substituted derivatives are also called pyrroles, e.g., ''N''-meth ...

except that an

arsenic Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, but ...

atom is substituted for the

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

atom. Whereas the pyrrole molecule is planar, the arsole molecule is not, and the hydrogen atom bonded to arsenic extends out of the molecular plane. Arsole is only moderately

aromatic In chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to satur ...

, with about 40% the aromaticity of pyrrole. Arsole itself has not been reported in pure form, but several substituted analogs called arsoles exist. Arsoles and more complex arsole derivatives have similar structure and chemical properties to those of

phosphole Phosphole is the organic compound with the chemical formula ; it is the phosphorus analog of pyrrole. The term phosphole also refers to substituted derivatives of the parent heterocycle. These compounds are of theoretical interest but also serve ...

derivatives. When arsole is fused to a

benzene Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms, ...

ring, this molecule is called arsindole, or benzarsole.

Nomenclature

Arsole belongs to the series of heterocyclic

pnictogen A pnictogen ( or ; from grc, πνῑ́γω "to choke" and -gen, "generator") is any of the chemical elements in group 15 of the periodic table. Group 15 is also known as the nitrogen group or nitrogen family. Group 15 consists of the el ...

compounds. The naming of cyclic

organoarsenic compounds Organoarsenic chemistry is the chemistry of compounds containing a chemical bond between arsenic and carbon. A few organoarsenic compounds, also called "organoarsenicals," are produced industrially with uses as insecticides, herbicides, and fungic ...

such as arsole is based on an extension of the

Hantzsch–Widman nomenclature In organic chemistry, Hantzsch–Widman nomenclature, also called the extended Hantzsch–Widman system (named for Arthur Rudolf Hantzsch and ), is a type of systematic chemical nomenclature used for naming heterocyclic parent hydrides having no ...

system approved by

IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...

, as summarized below: Because of its similarity to the English slang word "

arsehole The word ''asshole'' (in North American English) or arsehole (in all other major varieties of the English language), is a vulgarism used to describe the anus, and often used pejoratively (as a type of synecdoche) to refer to people. History ...

" (in common use outside North America), the name "arsole" has been considered a target of fun, a "silly name", and one of several chemical compounds with an unusual name. However, this "silly name" coincidence has also stimulated detailed scientific studies.

Properties

Arsole itself has not been isolated experimentally yet, but the molecular geometry and electronic configuration of arsole have been studied theoretically. Calculations also addressed properties of simple arsole derivatives, where hydrogen atoms are substituted by other atoms or small hydrocarbon groups, and there are experimental reports on chemical properties of more complex arsole derivatives. The situation is similar for other C₄H₄MH metalloles where M = P, As, Sb and Bi.

Planarity

Calculations suggest that whereas pyrrole (C₄H₄NH) molecule is planar, phosphole (C₄H₄PH) and heavier metalloles are not, and their

-bonded hydrogen atom extends out of plane. A similar tendency is predicted for the fluorinated C₄F₄MH derivatives (M = N, P, As, ..), but the inversion barriers are about 50–100% higher. The planarity is lost even in pyrrole when its nitrogen-bonded hydrogen atom is substituted, e.g., with

fluorine Fluorine is a chemical element with the symbol F and atomic number 9. It is the lightest halogen and exists at standard conditions as a highly toxic, pale yellow diatomic gas. As the most electronegative reactive element, it is extremely reacti ...

. However, the planarity is evaluated in calculation by the energy required to convert between the two configurations where the M-H bond is extending left or right from the molecular plane. However, non-zero (small) value of this energy does not necessarily mean the molecule has low symmetry, because of the possibility of thermal or

quantum tunneling In physics, a quantum (plural quanta) is the minimum amount of any physical entity (physical property) involved in an interaction. The fundamental notion that a physical property can be "quantized" is referred to as "the hypothesis of quantizati ...

between the two configurations.

Aromaticity

Aromaticity In chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to saturate ...

of the arsole manifests itself in

delocalization In chemistry, delocalized electrons are electrons in a molecule, ion or solid metal that are not associated with a single atom or a covalent bond.IUPAC Gold Boo''delocalization''/ref> The term delocalization is general and can have slightly diff ...

and

resonance Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillatin ...

of its ring electrons. It is closely related to planarity in that the more planar the molecule the stronger its aromaticity. Aromaticity of arsole and its derivatives has been debated for years both from experimental and theoretical point of view. A 2005 review combined with quantum chemical calculations concluded that arsole itself is "moderately" aromatic as its ring current is 40% that of pyrrole, which is known to be aromatic. However, comparable ring current was calculated for cyclopentadiene, which has long been regarded as non-aromatic. Other reports suggest that the aromaticity (and planarity) can vary between arsole derivatives.

Chemical properties (arsole derivatives)

Chemical properties of arsole derivatives have been studied experimentally; they are similar to those of

and its derivatives. Substitution of all hydrogen atoms in arsole with

phenyl In organic chemistry, the phenyl group, or phenyl ring, is a cyclic group of atoms with the formula C6 H5, and is often represented by the symbol Ph. Phenyl group is closely related to benzene and can be viewed as a benzene ring, minus a hydrogen ...

groups yields yellow needles of crystalline pentaphenylarsole, which has a melting point of 215 °C. This complex can be prepared, at a yield of 50–93%, by reacting 1,4-diiodo-1,2,3,4-tetraphenylbutadiene or 1,4-dilithio-1,2,3,4-tetraphenylbutadiene with phenylarsenous dichloride (C₆H₅AsCl₂) in

ether In organic chemistry, ethers are a class of compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula , where R and R′ represent the alkyl or aryl groups. Ethers can again be c ...

. :

Substituting in this reaction

arsenic trichloride Arsenic trichloride is an inorganic compound with the formula AsCl3, also known as arsenous chloride or butter of arsenic. This poisonous oil is colourless, although impure samples may appear yellow. It is an intermediate in the manufacture of o ...

for phenylarsenous dichloride yields 1-chloro-2,3,4,5-tetraphenylarsole, which also forms yellow needles but with a lower melting point of 182–184 °C. Pentaphenylarsole can further be oxidized with

hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3%� ...

resulting in yellow crystals with melting point of 252 °C. It can also be reacted with

iron pentacarbonyl Iron pentacarbonyl, also known as iron carbonyl, is the compound with formula . Under standard conditions Fe( CO)5 is a free-flowing, straw-colored liquid with a pungent odour. Older samples appear darker. This compound is a common precursor to ...

(Fe(CO)₅) in

isooctane 2,2,4-Trimethylpentane, also known as isooctane or iso-octane, is an organic compound with the formula (CH3)3CCH2CH(CH3)2. It is one of several isomers of octane (C8H18). This particular isomer is the standard 100 point on the octane rating sc ...

at 150 °C to yield a solid organoarsenic compound with the formula C₃₄H₂₅As,Fe(CO)₃. Reacting pentaphenylarsole with metallic lithium or potassium yields 1,2,3-triphenyl naphthalene. Reaction of phenylarsenous dichloride with linear diphenyls results in 1,2,5-triphenylarsole (see below), a solid with a melting point of about 170 °C. This compound forms various

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

s upon treatment with alkali metals. : Substituted arsole

References