In
organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, ...
, a diethynylbenzene dianion is an anion consisting of two
ethynyl anions as
substituents on 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. With the
chemical formula
In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbol ...
, three
positional isomer
In chemistry, a structural isomer (or constitutional isomer in the IUPAC nomenclature) of a compound is another compound whose molecule has the same number of atoms of each element, but with logically distinct bonds between them. The term meta ...
s are possible, differing in the
relative positions of the two substituents around the ring:
*''ortho''-diethynylbenzene dianion
*''meta''-diethynylbenzene dianion
*''para''-diethynylbenzene dianion
The gaseous state of all three anions are of theoretical interest. They have been generated by
decarboxylation of benzene di
propynoic acids, using the technique of
mass spectrometry.
The three isomers of the dianion are the three strongest known superbases ever, with the ''ortho'' isomer being the strongest, with a proton affinity of .
The ''meta'' isomer is the second-strongest, and the ''para'' isomer is the third-strongest.
Observation
These dianions were generated in a linear quadrupole ion-trap mass spectrometer.
Electrospray ionization
Electrospray ionization (ESI) is a technique used in mass spectrometry to produce ions using an electrospray in which a high voltage is applied to a liquid to create an aerosol. It is especially useful in producing ions from macromolecules becaus ...
(ESI) of the diacid precursor results in the dicarboxylate dianion
6H4(C3O2)2">6H4(C3O2)2sup>2β by loss of two hydrogen atoms, identified spectrometrically by its
mass-to-charge ratio (''m''/''z'') of 106. This dianion was mass-selected and then subjected to
collision-induced dissociation
Collision-induced dissociation (CID), also known as collisionally activated dissociation (CAD), is a mass spectrometry technique to induce fragmentation of selected ions in the gas phase. The selected ions (typically molecular ions or protonate ...
(CID), resulting in the consecutive loss of two carbon dioxide molecules to form the diethynyl dianion
6H4(C2)2">6H4(C2)2sup>2β at ''m''/''z'' = 62. For the ''ortho'' isomer, the reaction process is as follows, with the other isomers following an analogous process depending on the isomer of the original diacid:
Reactions
Reactions of the gas-phase dianions were studied by reacting with a small quantity of various reagents added to the helium carrier gas in the spectrometer. For example, reaction with deuterium oxide (
heavy water) produced the singly-deuterated monoanion C
6H
4(C
2D)() identified as ''m''/''z'' = 126. Reaction with benzene produced the phenyl anion (''m''/''z'' = 77) highlighting the extreme basicity of the dianion. Attempted reaction with
deuterium
Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two stable isotopes of hydrogen (the other being protium, or hydrogen-1). The nucleus of a deuterium atom, called a deuteron, contains one proton and one ...
gas and deuterated
methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Ea ...
was not successful despite the favourable thermodynamics; the authors attribute this to the high
activation barrier for proton abstraction from those substrates.
[
]
Basicity
All three isomers are superbasic
SuperBASIC is an advanced variant of the BASIC programming language with many structured programming additions. It was developed at Sinclair Research by Jan Jones during the early 1980s. Originally SuperBASIC was intended as the BASIC interprete ...
. According to calculations, ''ortho''-diethynylbenzene dianion is the strongest superbase and has a proton affinity of . The ''meta'' isomer is the second-strongest, and the ''para'' isomer is the third. All three are readily able to accept any proton to its ethynyl tails, from almost any compound. All three isomers function as superbases better than helonium
The helium hydride ion or hydridohelium(1+) ion or helonium is a cation (positively charged ion) with chemical formula HeH+. It consists of a helium atom bonded to a hydrogen atom, with one electron removed. It can also be viewed as protonated ...
does as a superacid.
See also
* Lithium monoxide anion
Lithium monoxide anion (LiOβ) is a superbase existing in the gas phase. It was the strongest known base until 2008, when the isomeric diethynylbenzene dianions were determined to have a higher proton affinity. The methanide ion CH3β was the ...
References
{{Reflist
Acetylides
Anions
Benzene derivatives
Superbases