Wanzlick equilibrium
Encyclopedia
The Wanzlick equilibrium is a chemical equilibrium
between a relatively stable carbene
compound and its dimer.
and E. Schikora proposed that carbene
s derived from dihydroimidazol-2-ylidene
were generated by vacuum pyrolysis of 2-trichloromethyl dihydroimidazole derivatives, with the loss of chloroform
.
Wanzlick and Schikora believed that once prepared these carbenes existed in an unfavourable equilibrium with their corresponding dimers. This assertion was based on reactivity studies which they believed showed that the free carbene reacted with electrophile
s (E-X). The dimer (a substituted tetraaminoethylene
) was believed to be inactive to the electrophiles (E-X), and thought to merely act as a stable carbene reservoir.
derivatives with different N-aryl substitututions:
This reaction did not produce a mixed dimeric product, and accordingly indicated that a 'carbene-dimer equilibrium' did not exist for these dihydroimidazol-2-ylidene derivatives.
Lemal proposed an alternative mechanism to account for the reactions observed by Wanzlick by considering the reactivity of the electron rich tetraaminoethylene
s — and not the stable carbenes, which at the time were still hypothetical.
Lemal believed that the tetraaminoethylene
, not the carbene, reacted with the electrophile (E-X) to generate a transient cationic species. He proposed that this cation then dissociated into the free carbene plus the resultant salt. The free carbene could then either re-dimerise (regenerating the tetraaminoethylene
starting material) or react with E-X (as Wanzlick originally predicted), with either route eventually giving the same reaction product, the dihydroimidazolium salt.
reinvestigated the cross-over experiments and did find mixed dimers thus supporting the Wanzlick equilibrium. This prompted Lemal to repeat his 1964 experiments. He was able to confirm Denk's findings but only with deuterated tetrahydrofuran
(THF) as a solvent. With toluene
and added KH
as a electrophile quencher, however, the cross-over product was again not observed by Lemal.
In 1999 Lemal and Hahn both investigated an equilibrium between a dibenzotetraazafulvalene derivative and its carbene, which led Böhm & Herrmann to conclude in 2000 that "the Wanzlick equilibrium between a tetraaminoethylene and its corresponding carbene did exist" after all., a notion confirmed in 2010 by Kirmse
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...
between a relatively stable carbene
Persistent carbene
A persistent carbene is a type of carbene demonstrating particular stability. The best-known examples are diaminocarbenes with the general formula 2C:, where the 'R's are various functional groups...
compound and its dimer.
Original conjecture
In 1960, H.-W. WanzlickHans-Werner Wanzlick
Hans-Werner Wanzlick was a professor of chemistry at the Berlin Technical University. He is notable for work on persistent carbenes and for proposing the Wanzlick equilibrium between saturated imidazolin-2-ylidenes and their dimers — which he called "das doppelte Lottchen", after a 1949 novel by...
and E. Schikora proposed that carbene
Carbene
In chemistry, a carbene is a molecule containing a neutral carbon atom with a valence of two and two unshared valence electrons. The general formula is RR'C:, but the carbon can instead be double-bonded to one group. The term "carbene" may also merely refer to the compound H2C:, also called...
s derived from dihydroimidazol-2-ylidene
Dihydroimidazol-2-ylidene
Dihydroimidazol-2-ylidene is a hypothetical organic compound with formula C3H6N2. It would be a heterocyclic compound, formally derived from imidazolidine with two hydrogen atoms removed from carbon number 2, leaving two vacant chemical bonds — which makes it a carbene.Although carbenes in general...
were generated by vacuum pyrolysis of 2-trichloromethyl dihydroimidazole derivatives, with the loss of chloroform
Chloroform
Chloroform is an organic compound with formula CHCl3. It is one of the four chloromethanes. The colorless, sweet-smelling, dense liquid is a trihalomethane, and is considered somewhat hazardous...
.
Electrophile
In general electrophiles are positively charged species that are attracted to an electron rich centre. In chemistry, an electrophile is a reagent attracted to electrons that participates in a chemical reaction by accepting an electron pair in order to bond to a nucleophile...
s (E-X). The dimer (a substituted tetraaminoethylene
Tetraaminoethylene
Tetraaminoethylene is a hypothetical organic compound with formula C2N4H8 or 2C=C2. Like all geminal polyamines, this compound has never been synthesised and is believed to be extremely unstable....
) was believed to be inactive to the electrophiles (E-X), and thought to merely act as a stable carbene reservoir.
Conjecture challenged
Wanzlick’s hypothesis of a carbene-dimer equilibrium was tested by D. Lemal and others, and separately by H. Winberg and others. They heated together two tetraaminoethyleneTetraaminoethylene
Tetraaminoethylene is a hypothetical organic compound with formula C2N4H8 or 2C=C2. Like all geminal polyamines, this compound has never been synthesised and is believed to be extremely unstable....
derivatives with different N-aryl substitututions:
Lemal proposed an alternative mechanism to account for the reactions observed by Wanzlick by considering the reactivity of the electron rich tetraaminoethylene
Tetraaminoethylene
Tetraaminoethylene is a hypothetical organic compound with formula C2N4H8 or 2C=C2. Like all geminal polyamines, this compound has never been synthesised and is believed to be extremely unstable....
s — and not the stable carbenes, which at the time were still hypothetical.
Tetraaminoethylene
Tetraaminoethylene is a hypothetical organic compound with formula C2N4H8 or 2C=C2. Like all geminal polyamines, this compound has never been synthesised and is believed to be extremely unstable....
, not the carbene, reacted with the electrophile (E-X) to generate a transient cationic species. He proposed that this cation then dissociated into the free carbene plus the resultant salt. The free carbene could then either re-dimerise (regenerating the tetraaminoethylene
Tetraaminoethylene
Tetraaminoethylene is a hypothetical organic compound with formula C2N4H8 or 2C=C2. Like all geminal polyamines, this compound has never been synthesised and is believed to be extremely unstable....
starting material) or react with E-X (as Wanzlick originally predicted), with either route eventually giving the same reaction product, the dihydroimidazolium salt.
Conjecture confirmed
In 1999 M. DenkMichael K. Denk
Michael K. Denk is a Professor of chemistry at the University of Guelph, Ontario.Michael Denk obtained his M.Sc. at the Ludwig Maximilian University in Munich, Germany . He got his Ph.D. at the Technical University of Munich , advised by W. Herrmann, with a dissertation on cyclic metalloamides...
reinvestigated the cross-over experiments and did find mixed dimers thus supporting the Wanzlick equilibrium. This prompted Lemal to repeat his 1964 experiments. He was able to confirm Denk's findings but only with deuterated tetrahydrofuran
Tetrahydrofuran
Tetrahydrofuran is a colorless, water-miscible organic liquid with low viscosity at standard temperature and pressure. This heterocyclic compound has the chemical formula 4O. As one of the most polar ethers with a wide liquid range, it is a useful solvent. Its main use, however, is as a precursor...
(THF) as a solvent. With toluene
Toluene
Toluene, formerly known as toluol, is a clear, water-insoluble liquid with the typical smell of paint thinners. It is a mono-substituted benzene derivative, i.e., one in which a single hydrogen atom from the benzene molecule has been replaced by a univalent group, in this case CH3.It is an aromatic...
and added KH
Potassium hydroxide
Potassium hydroxide is an inorganic compound with the formula KOH, commonly called caustic potash.Along with sodium hydroxide , this colorless solid is a prototypical strong base. It has many industrial and niche applications. Most applications exploit its reactivity toward acids and its corrosive...
as a electrophile quencher, however, the cross-over product was again not observed by Lemal.
In 1999 Lemal and Hahn both investigated an equilibrium between a dibenzotetraazafulvalene derivative and its carbene, which led Böhm & Herrmann to conclude in 2000 that "the Wanzlick equilibrium between a tetraaminoethylene and its corresponding carbene did exist" after all., a notion confirmed in 2010 by Kirmse