Arsole

Arsole, also called arsenole or arsacyclopentadiene, is an organoarsenic compound with the formula C₄H₅As. It is classified as a metallole and is valence isoelectronic to and related to pyrrole except that an arsenic atom is substituted for the nitrogen 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, with about 40% the aromaticity of pyrrole.

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 system approved by IUPAC, as summarized below:

{| class="wikitable" style="text-align:center"

!Ring size!!Unsaturated ring!!Saturated ring

|-

| 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

{|class="wikitable" style="text-align:center; float:right;margin-left:0.5em"

|+Calculated geometry and inversion barrier energy E for some C₄H₄MH molecules

!M !! d(M-C), 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 fluorine. 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 between the two configurations. Aromaticity of arsole and its derivatives has been debated for years both from experimental and theoretical points 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. or 1,4-dilithio-1,2,3,4-tetraphenylbutadiene with phenylarsenous dichloride (C₆H₅AsCl₂) in ether.

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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.

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See also

• Pyrrole, a nitrogen analog.
• Furan, an oxygen analog.
• Thiophene, a sulfur analog.
• Simple aromatic rings
• Varsol, a petroleum distillate with a boiling range of 150–200 °C.

References