Does Propanone trimerise to give 2,6-dimethylhepta-2,5-dien-4-one? [closed]








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Does propanone trimerise to give 2,6-dimethylhepta-2,5-dien-4-one?


How and under what conditions?




As [the comment](https://chemistry.stackexchange.com/questions/66715/does-propanone-trimerise-to-give-2-6-dimethyl-2-5-heptadiene-4-one#comment115229_66715) pointed out, this is an [aldol condensation](https://en.wikipedia.org/wiki/Aldol_condensation).


I shall demonstrated the acid-catalyzed version.


Tautomer
========


![tautomer of propanone with propen-2-ol](https://i.stack.imgur.com/N4ZbT.png)


Propanone exists in [tautomer](https://en.wikipedia.org/wiki/Tautomer) with propen-2-ol. This is an example of [keto-enol tautomerism](https://en.wikipedia.org/wiki/Keto%E2%80%93enol_tautomerism), with "keto" referring to the **keto**ne which is propanone, and "enol" referring to prop**en**-2-**ol**.


Dimer
=====


![Mechanism of formation of dimer of propanone](https://i.stack.imgur.com/Xkso9.png)


One molecule of propanone $(1)$ is converted to its tautomer, propen-2-ol $(2\mathrm a)$, with the help of hydrogen ions.


Another molecule of propanone $(1)$ is protonated to form a cation $(2\mathrm b)$.


The central carbon of the protonated propanone is an electrophile, so it attacks the double-bond in the propen-2-ol. A carbon-carbon bond is formed $(3)$.


After deprotonation, the intermediate 4-hydroxy-4-methylpentan-2-one is formed $(4)$.


It is converted to its keto-enol tautomer and the tertiary hydroxyl group is protonated $(5)$.


Then, after [dehydration](https://en.wikipedia.org/wiki/Dehydration_reaction) $(6)$ and deprotonation, our dimer 4-methylpent-3-en-2-one is formed $(7)$.


This is the first aldol condensation step.


Trimer
======


![Mechanism of formation of trimer of propnanone](https://i.stack.imgur.com/xCY9u.png)


The mechanism of the formation of the trimer is akin to that of the dimer just above.


Similarly, one molecule of propanone $(1\mathrm a)$ is converted to its tautomer, propen-2-ol $(2\mathrm a)$, with the help of hydrogen ions.


A molecule of the dimer, 4-methylpent-3-en-2-one $(1\mathrm b)$ is protonated to form a cation $(2\mathrm b)$.


The central carbon of the protonated propanone is an electrophile, so it attacks the double-bond in the propen-2-ol. A carbon-carbon bond is formed $(3)$.


After deprotonation, the intermediate 6-hydroxy-2-methylhept-2-en-4-one is formed $(4)$.


It is converted to its keto-enol tautomer and the tertiary hydroxyl group is protonated $(5)$.


Then, after [dehydration](https://en.wikipedia.org/wiki/Dehydration_reaction) $(6)$ and deprotonation, our trimer 2,6-dimethylhepta-2,5-dien-4-one is formed $(7)$.


This is the second aldol condensation step.


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