Everything about Aryne totally explained
In
chemistry, an
aryne is an uncharged
reactive intermediate derived from an
aromatic system by removal of two
ortho substituents, leaving two
orbitals with two
electrons distributed between them.. In analogy with
carbenes and
nitrenes, an aryne has a
singlet state and a
triplet state.
The simplest aryne, C
6H
4 (labeled
1 in the diagram to the right), is sometimes called
benzyne. However, this name is open for criticism because it implies a triple bond which would be a special case of triple bonds, so a better name is
didehydrobenzene. Benzyne is stabilized by
resonance between structures
1 and
2—a better representation of the electronic structure is
3. The "extra"
pi bond (
4b) is localised and
orthogonal to the other pi bonds making up the aromatic ring (
4a). Benzyne can also be drawn as a diradical: the pi bond
4b splits homolytically, leaving one electron on each of the two atoms that are formally part of that bond.
Benzyne is an extremely reactive species due to the nature of its
triple bond. In normal acetylenic species (such as the simplest,
ethyne) the unhybridized p orbitals are parallel to one another above and below the molecular axis. This facilitates maximum orbital overlap. In benzyne, however, the p orbtials are distorted to accommodate the
triple bond within the ring system, reducing their effective overlap. A suitable
chemical trap for benzyne is a
cyclopentadiene.
There are three possible diradical didehydrobenzenes: 1,2-didehydrobenzene, 1,3-didehydrobenzene and 1,4-didehydrobenzene. Their energies
in silico are respectively 106, 122, and 138 kcal/mol (444, 510, and 577 kJ/mol).
»
Aryne chemistry
Arynes are often prepared from aryl halides in presence of a strong base. The most prominent aryne reactions are
Diels-Alder reactions with dienes. Tetrabromobenzene reacts with
butyllithium and
furan to form a tetrahydroanthracene . The mixture of
syn and
anti stereoisomers can be separated based on difference in
methanol solubility.
Anthracene is converted to a
triptycene by Diels-Alder reaction of an aryne with the central benzene ring . A pentiptycene is the anthracene analogue after the reaction with 1,2,4,5-tetrabromobenzene and
butyllithium.
Aryne reactivity can also be extended to carbon insertion reactions into substrates that can react both as a
nucleophile and as an
electrophile with for instance a
malonic acid ester . The precursor of benzyne in this reaction is 2-(trimethylsilyl)phenyl triflate.
Didehydrobenzene interconversions
A 1,2- to 1,3-didehydrobenzene conversion has been postulated to occur in the
pyrolysis (900°C) of the
phenyl substituted aryne precursor
1 :
Further Information
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