science

Wanzlick equilibrium

Wanzlick equilibrium is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Wanzlick equilibrium rather than just read about it. In short: The Wanzlick equilibrium is a chemical equilibrium between a relatively stable carbene compound and its 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.

Key takeaways

Wanzlick equilibrium belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Wanzlick equilibrium to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Wanzlick equilibrium from memory before moving on to harder problems.

Reference excerpt

The Wanzlick equilibrium is a chemical equilibrium between a relatively stable carbene compound and its 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 and E. Schikora proposed that carbenes 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 electrophiles (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.

Conjecture challenged Wanzlick's hypothesis of a carbene-dimer equilibrium was tested by David M. Lemal and others. Heating mixtures of tetraaminoethylene 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 tetraaminoethylenes into 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.

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 (THF) as a solvent. With toluene 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 unhindered 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 dimers).

References

Illustrations

Wanzlick equilibrium: Dimer cross-over experiment — Dimer cross-over experiment

Worked examples

Example 1 — a first encounter with Wanzlick equilibrium

Start with the simplest possible case. Write down what Wanzlick equilibrium claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Wanzlick equilibrium before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Wanzlick equilibrium ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Wanzlick equilibrium

In research: Wanzlick equilibrium appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Wanzlick equilibrium in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Wanzlick equilibrium is common in secondary-school and first-year university syllabi. It links to neighbouring topics Carbenes, so understanding it makes those chapters shorter.
In everyday life: Look for Wanzlick equilibrium outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Wanzlick equilibrium in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Wanzlick equilibrium means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Wanzlick equilibrium out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Wanzlick equilibrium in simple terms?

Why does Wanzlick equilibrium matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Wanzlick equilibrium?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Wanzlick equilibrium.