The Wanzlick equilibrium is a

chemical equilibrium In a chemical reaction, chemical equilibrium is the state in which both the reactants and products are present in concentrations which have no further tendency to change with time, so that there is no observable change in the properties of the sy ...

between a relatively stable carbene compound and its

dimer Dimer may refer to: * Dimer (chemistry), a chemical structure formed from two similar sub-units ** Protein dimer, a protein quaternary structure ** d-dimer * Dimer model, an item in statistical mechanics, based on ''domino tiling'' * Julius Dimer ...

. The equilibrium was proposed to apply to certain electron-rich alkenes, such as tetraminoethylenes, which have been called "carbene dimers." Such equilibria occur, but the mechanism does not proceed simply, but requires catalysts.

Original conjecture

In 1960,

Hans-Werner Wanzlick Hans-Werner Wanzlick (1917-1988) was a German chemist. 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 a ...

and E. Schikora proposed that

carbene In organic 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 or where the R represents substituents or hydrogen atoms. The term "carbene" ma ...

s derived from

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 — ...

were generated by

vacuum pyrolysis The pyrolysis (or devolatilization) process is the thermal decomposition of materials at elevated temperatures, often in an inert atmosphere. It involves a change of chemical composition. The word is coined from the Greek-derived elements ''py ...

of 2-trichloromethyl dihydroimidazole derivatives, with the loss of

chloroform Chloroform, or trichloromethane, is an organic compound with chemical formula, formula Carbon, CHydrogen, HChlorine, Cl3 and a common organic solvent. It is a colorless, strong-smelling, dense liquid produced on a large scale as a precursor to ...

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 In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. Because electrophiles accept electrons, they are Lewis acids. Most electrophiles are positively charged, have an atom that carries ...

s (E-X). The dimer (a substituted

tetraaminoethylene In organic chemistry, tetraaminoethylene is a hypothetical, organic compound with formula or . Like all polyamines that are geminal, this compound has never been synthesised and is believed to be extremely unstable.Stephen A. Lawrence (2004) ...

) 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

David M. Lemal David Markham Lemal (born 1934) is the Albert W. Smith Professor of Chemistry Emeritus and Research Professor of Chemistry at Dartmouth College. He received an A.B. degree (summa) from Amherst College in 1955 and a Ph.D. in Chemistry from Harvard ...

and others. Heating mixtures of

derivatives did not produce mixed dimers: This result indicates that a 'carbene-dimer equilibrium' does not occur for these dihydroimidazol-2-ylidene derivatives. Lemal proposed that Wanzlick's observations could be explained by acid-catalyzed reactions. Lemal proposed that the electrophile converts the

s into

cationic An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

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

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 Michael K. Denk reinvestigated the cross-over experiments that supported the Wanzlick equilibrium. This report prompted Lemal to repeat his 1964 experiments. Denk's findings were confirmed only with deuterated

tetrahydrofuran Tetrahydrofuran (THF), or oxolane, is an organic compound with the formula (CH2)4O. The compound is classified as heterocyclic compound, specifically a cyclic ether. It is a colorless, water-miscible organic liquid with low viscosity. It is ma ...

(THF) as a solvent. With

toluene Toluene (), also known as toluol (), is a substituted aromatic hydrocarbon. It is a colorless, water-insoluble liquid with the smell associated with paint thinners. It is a mono-substituted benzene derivative, consisting of a methyl group (CH3) at ...

and added KH as an electrophile quencher, however, the cross-over product was again not observed. In 1999 Lemal and others investigated an equilibrium between a dibenzotetraazafulvalene derivative and its carbene. These studies led Böhm & Herrmann to conclude in 2000 that "''the Wanzlick equilibrium between a tetraaminoethylene and its corresponding carbene did exist''". This notion was confirmed in 2010 by Kirmse. Others subsequently showed that un hindered diaminocarbenes form dimers by acid-catalysed dimerisation as shown in the Lemal. Sublimation experiments with carbene dimers and their protonated derivatives quantify acid catalysis and corroborate that tetraaminoolefins may dissociate without adventitious protons. Acid catalysis is however required for the dissociation of triaminoolefins ( NHC-

CAAC The Civil Aviation Administration of China (CAAC; ) is the Chinese civil aviation authority under the Ministry of Transport. It oversees civil aviation and investigates aviation accidents and incidents. As the aviation authority responsible f ...

dimers).

References

{{reflist, 2 Carbenes