Arsole
Encyclopedia
Arsole, also called arsenole or arsacyclopentadiene, is an organoarsenic compound
with the formula
C4H4AsH. It is classified as a metallole and is isoelectronic to and related to pyrrole
except that an arsenic
atom is substituted for the nitrogen
atom. Whereas the pyrrole molecule is planar, arsole molecule is not, and the hydrogen atom bonded to arsenic extends out of the molecular plane. Arsole is only moderately aromatic, 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
derivatives. When arsole is fused to a benzene
ring, this molecule is called arsindole, or benzarsole.
The naming of cyclic organoarsenic compounds such as arsole is based on an extension of the Hantzsch–Widman nomenclature
system approved by IUPAC, as summarized below:
Because of its similarity to the English slang word "arsehole" (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.
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.
. It should be noted, however, that 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 between the two configurations.
of the arsole manifests itself in delocalization and resonance
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.
and its derivatives. Substitution of all hydrogen atoms in arsole with phenyl 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
.
Substituting in this reaction arsenic trichloride
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
resulting in yellow crystals with melting point of 252 °C. It can also be reacted with iron pentacarbonyl
(Fe(CO)5) in isooctane 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 anions upon treatment with alkali metals.
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 fungicides. In general these applications are declining in...
with the formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
C4H4AsH. It is classified as a metallole and is isoelectronic to and related to pyrrole
Pyrrole
Pyrrole is a heterocyclic aromatic organic compound, a five-membered ring with the formula C4H4NH. It is a colourless volatile liquid that darkens readily upon exposure to air. Substituted derivatives are also called pyrroles, e.g., N-methylpyrrole, C4H4NCH3...
except that an arsenic
Arsenic
Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...
atom is substituted for the nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
atom. Whereas the pyrrole molecule is planar, arsole molecule is not, and the hydrogen atom bonded to arsenic extends out of the molecular plane. Arsole is only moderately aromatic, 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
Phosphole is the organic compound with the chemical formula C4H4PH; it is the phosphorus analog of pyrrole. The term phosphole also refers to substituted derivatives of the parent heterocycle...
derivatives. When arsole is fused to a benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
ring, this molecule is called arsindole, or benzarsole.
Nomenclature
Arsole belongs to the series of heterocyclic pnictogen compounds.The naming of cyclic organoarsenic compounds such as arsole is based on an extension of the Hantzsch–Widman nomenclature
Hantzsch–Widman nomenclature
Hantzsch–Widman nomenclature, also called the extended Hantzsch–Widman system, is a type of systematic chemical nomenclature used for naming heterocyclic parent hydrides having no more than ten ring members....
system approved by IUPAC, as summarized below:
| Ring size | Unsaturated ring Unsaturated hydrocarbon Unsaturated hydrocarbons are hydrocarbons that have double or triple covalent bonds between adjacent carbon atoms. Those with at least one double bond are called alkenes and those with at least one triple bond are called alkynes... | Saturated ring Saturation (chemistry) In chemistry, saturation has six different meanings, all based on reaching a maximum capacity... |
|---|---|---|
| 3 | Arsirene | Arsirane |
| 4 | Arsete | Arsetane |
| 5 | Arsole | Arsolane |
| 6 | Arsinine | Arsinane |
| 7 | Arsepine | Arsepane |
| 8 | Arsocine | Arsocane |
| 9 | Arsonine | Arsonane |
| 10 | Arsecine | Arsecane |
Because of its similarity to the English slang word "arsehole" (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
| M | d(M-C), Å Ångström The angstrom or ångström, is a unit of length equal to 1/10,000,000,000 of a meter . Its symbol is the Swedish letter Å.... | d(M-H), Å | α(C-M-C), ° | E, kJ/mol |
|---|---|---|---|---|
| N | 1.37 | 1.01 | 110 | 0 |
| P | 1.81 | 1.425 | 90.5 | 67 |
| As | 1.94 | 1.53 | 86 | 125 |
| Sb | 2.14 | 1.725 | 80.5 | 160 |
| Bi | 2.24 | 1.82 | 78 | 220 |
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 pnictogen-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 fluorineFluorine
Fluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic...
. It should be noted, however, that 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 between the two configurations.
Aromaticity
AromaticityAromaticity
In organic chemistry, Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone. The earliest use of the term was in an article by August...
of the arsole manifests itself in delocalization and resonance
Resonance (chemistry)
In chemistry, resonance or mesomerism is a way of describing delocalized electrons within certain molecules or polyatomic ions where the bonding cannot be expressed by one single Lewis formula...
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
Cyclopentadiene
Cyclopentadiene is an organic compound with the formula C5H6. This colorless liquid has a strong and unpleasant odor. At room temperature, this cyclic diene dimerizes over the course of hours to give dicyclopentadiene via a Diels–Alder reaction...
, 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 phospholePhosphole
Phosphole is the organic compound with the chemical formula C4H4PH; it is the phosphorus analog of pyrrole. The term phosphole also refers to substituted derivatives of the parent heterocycle...
and its derivatives. Substitution of all hydrogen atoms in arsole with phenyl 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
Ether
Ethers are a class of organic compounds that contain an ether group — an oxygen atom connected to two alkyl or aryl groups — of general formula R–O–R'. A typical example is the solvent and anesthetic diethyl ether, commonly referred to simply as "ether"...
.
Substituting in this reaction arsenic trichloride
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 organoarsenic compounds.-Structure:AsCl3 is a...
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
Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...
resulting in yellow crystals with melting point of 252 °C. It can also be reacted with iron pentacarbonyl
Iron pentacarbonyl
Iron pentacarbonyl, also known as iron carbonyl, is the compound with formula 5. Under standard conditions Fe5 is a free-flowing, straw-colored liquid with a pungent odour. This compound is a common precursor to diverse iron compounds, including many that are useful in organic synthesis. Fe5 is...
(Fe(CO)5) in isooctane 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
1,2,3,4-Tetraphenylnaphthalene
1,2,3,4-Tetraphenylnaphthalene is a polycyclic aromatic hydrocarbon commonly prepared in the undergraduate teaching laboratory as an introduction to the Diels-Alder reaction, in this case between benzyne and tetraphenylcyclopentadienone . It has two crystalline forms, and therefore has two...
.
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 anions upon treatment with alkali metals.
See also
- PyrrolePyrrolePyrrole is a heterocyclic aromatic organic compound, a five-membered ring with the formula C4H4NH. It is a colourless volatile liquid that darkens readily upon exposure to air. Substituted derivatives are also called pyrroles, e.g., N-methylpyrrole, C4H4NCH3...
, a nitrogenNitrogenNitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
analog. - FuranFuranFuran is a heterocyclic organic compound, consisting of a five-membered aromatic ring with four carbon atoms and one oxygen. The class of compounds containing such rings are also referred to as furans....
, an oxygenOxygenOxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
analog. - ThiopheneThiopheneThiophene is a heterocyclic compound with the formula C4H4S. Consisting of a flat five-membered ring, it is aromatic as indicated by its extensive substitution reactions. Related to thiophene are benzothiophene and dibenzothiophene, containing the thiophene ring fused with one and two benzene...
, a sulfurSulfurSulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
analog. - Simple aromatic rings
- Varsol, a petroleumPetroleumPetroleum or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. Petroleum is recovered mostly through oil drilling...
distillateDistillationDistillation is a method of separating mixtures based on differences in volatilities of components in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction....
with a boiling rangeBoiling pointThe boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....
of 150–200 °C.