Arsabenzene
Encyclopedia
Arsabenzene is an organoarsenic heterocyclic compound with the chemical formula C5H5As. It belongs to a group of compounds called heteroarenes that have the general formula C5H5E (E= N, P, As, Sb, Bi).
This air sensitive liquid has an onion odor, and it decomposes on heating. Arsabenzene is also an ambidentate ligand, and it prefers to coordinate using η1(As)- or η6(π)-routes.
The study of arsabenzene and related compounds was an important step in the understanding of compounds that contain multiple bonds between carbon and heavier elements.
The study of heteroarenes was begun by Märkl, with the synthesis of 2,4,6-triphenylphosphabenzene. This is achieved by treating 2,4,6-trisubstituted pyrylium salt
with phosphanes. The first derivative of arsabenzene was 9-arsaanthracene prepared by Jutzi and Bickelhaupt.
NMR spectroscopy
carried out on arsabenzene indicates that it has a diamagnetic ring current.
compound undergoes As/Sn exchange with arsenic trichloride
to give 1-chloroacyclohexadiene, which loses a HCl upon heating, forming the arsabenzene.
at its ortho and para
positions. It also undergoes Friedel-Crafts acylation.
Whereas pyridine
does not normally undergo a Diels-Alder reaction
, arsabenzene behaves as a diene
with hexafluoro-2-butyne
. The corresponding phosphabenzene and benzene
undergo the analogous reaction at 100 °C and 200 °C, respectively. Thus the ability of these heterobenzenes to undergo the Diels Alder reaction with this electrophile increases down the periodic table
. Bismabenzene is so reactive that it exists in equilibrium
with its dimer.
Arsabenzene is far less basic than pyridine, being unreactive with Lewis acids. Trifluoroacetic acid
does not protonate the molecule.
This air sensitive liquid has an onion odor, and it decomposes on heating. Arsabenzene is also an ambidentate ligand, and it prefers to coordinate using η1(As)- or η6(π)-routes.
The study of arsabenzene and related compounds was an important step in the understanding of compounds that contain multiple bonds between carbon and heavier elements.
The study of heteroarenes was begun by Märkl, with the synthesis of 2,4,6-triphenylphosphabenzene. This is achieved by treating 2,4,6-trisubstituted pyrylium salt
Pyrylium salt
A pyrylium salt is a salt containing a pyrylium cation or a derivative of it. The pyrylium cation is a conjugated 6-membered carbon ring system with one carbon atom replaced by a positively charged oxygen atom. It is, like benzene, an aromatic compound...
with phosphanes. The first derivative of arsabenzene was 9-arsaanthracene prepared by Jutzi and Bickelhaupt.
Structure
Arsabenzene is planar. The C—C bond distances of 1.39 Å, the As—C bond has a length of 1.85 Å, this is 6.6% shorter than the normal As—C single bond.NMR spectroscopy
NMR spectroscopy
Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is a research technique that exploits the magnetic properties of certain atomic nuclei to determine physical and chemical properties of atoms or the molecules in which they are contained...
carried out on arsabenzene indicates that it has a diamagnetic ring current.
Synthesis
Arsabenzene is synthesized in a two step process from 1,4-pentadiyne. The diyne reacts with dibutylstannane to give 1,1-dibutylstannacyclohexa-2,5-diene. The organotinOrganotin
Organotin compounds or stannanes are chemical compounds based on tin with hydrocarbon substituents. Organotin chemistry is part of the wider field of organometallic chemistry. The first organotin compound was diethyltin diiodide, discovered by Edward Frankland in 1849...
compound undergoes As/Sn exchange with 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...
to give 1-chloroacyclohexadiene, which loses a HCl upon heating, forming the arsabenzene.
- CH2(CHCH)2SnBu2 + AsCl3 → CH2(CHCH)2AsCl + Bu2SnCl2
- CH2(CHCH)2AsCl → C5H5As + HCl
Reactions
Arsabenzene undergoes electrophilic aromatic substitutionElectrophilic aromatic substitution
Electrophilic aromatic substitution EAS is an organic reaction in which an atom, usually hydrogen, appended to an aromatic system is replaced by an electrophile...
at its ortho and para
Arene substitution patterns
Arene substitution patterns are part of organic chemistry IUPAC nomenclature and pinpoint the position of substituents other than hydrogen in relation to each other on an aromatic hydrocarbon.- Ortho, meta, and para substitution :...
positions. It also undergoes Friedel-Crafts acylation.
Whereas pyridine
Pyridine
Pyridine is a basic heterocyclic organic compound with the chemical formula C5H5N. It is structurally related to benzene, with one C-H group replaced by a nitrogen atom...
does not normally undergo a Diels-Alder reaction
Diels-Alder reaction
The Diels–Alder reaction is an organic chemical reaction between a conjugated diene and a substituted alkene, commonly termed the dienophile, to form a substituted cyclohexene system. The reaction can proceed even if some of the atoms in the newly formed ring are not carbon...
, arsabenzene behaves as a diene
Diene
In organic chemistry a diene or diolefin is a hydrocarbon that contains two carbon double bonds.Conjugated dienes are functional groups, with a general formula of CnH2n-2. Dienes and alkynes are functional isomers...
with hexafluoro-2-butyne
Hexafluoro-2-butyne
Hexafluoro-2-butyne is the fluorocarbon with the formula CF3C≡CCF3. HFB, as it is known also, is a particularly electrophilic acetylene, hence a potent dienophile....
. The corresponding phosphabenzene and 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....
undergo the analogous reaction at 100 °C and 200 °C, respectively. Thus the ability of these heterobenzenes to undergo the Diels Alder reaction with this electrophile increases down the periodic table
Periodic table
The periodic table of the chemical elements is a tabular display of the 118 known chemical elements organized by selected properties of their atomic structures. Elements are presented by increasing atomic number, the number of protons in an atom's atomic nucleus...
. Bismabenzene is so reactive that it exists in equilibrium
Chemical equilibrium
In a chemical reaction, chemical equilibrium is the state in which the concentrations of the reactants and products have not yet changed with time. It occurs only in reversible reactions, and not in irreversible reactions. Usually, this state results when the forward reaction proceeds at the same...
with its dimer.
Arsabenzene is far less basic than pyridine, being unreactive with Lewis acids. Trifluoroacetic acid
Trifluoroacetic acid
Trifluoroacetic acid is the simplest stable perfluorinated carboxylic acid chemical compound, with the formula CF3CO2H. It is a strong carboxylic acid due to the influence of the electronegative trifluoromethyl group. TFA is almost 100,000-fold more acidic than acetic acid...
does not protonate the molecule.