Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: BahamaBob0 on April 26, 2009, 11:50:30 AM
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why is there a diels-alder reaction between anthracene and p-benoquinone when most aromatic substances such as benzene to not go through the reaction, though they possess a cisiod diene?
Thank you!
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What product are you proposing?
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In the lab we used xylene, anthracene, and p-benzoquinone, and heated to produce dihydroxytriptycene. we later converted it to the keto isomer with potasium hydroxide.
As far as I new, benzene rings easily reacted in the diels-alder reaction, but my lab prof said is usually does not, so now I am rather confused.
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I don't think it's the xylene that's reacted here, but can you draw a mechanism?
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i actually just corrected that, mixed xylene up with p-benzoquinone.
i think it is the double bonds of the middle benzene ring of anthracene that are reacting with 2-3 carbon double bond of p-benzoquinone. This leaves the keto form of dihydroxytriptycene. when converted to the enol tautomer with KOH, the ring that had the double bonded oxygens becomes aromatic.
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mechanism is attached.
Please take the effort to post some structures yourself.
i think it is the double bonds of the middle benzene ring of anthracene that are reacting with 2-3 carbon double bond of p-benzoquinone.
This is correct, this is because it is the most stable/thermodynamic product. Since it yields 2 sextets.
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it'll produce the adduct of p-benzoquinone and anthracene, the 9,10-tetrahydro-9,10[1',2']-
benzenoanthracene-1',4'(2'H,3'H)-dione, that is:
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fimg223.imageshack.us%2Fimg223%2F6000%2Fadductofpbenzoquinonean.png&hash=226ca1749cd0072a1445a30a3030c813faa2b4b5)
the synthesis of it it's the project that was given to me on my organic synthesis classes. i had a really hard process to do it, but it's simple after all...
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the xylene it's the solvent of the p-benzoquinone.
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I carried out this reaction in lab, The adduct was crystallized but the experimental proceedure says not to attempt melting point. Any idea why?
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I carried out this reaction in lab, The adduct was crystallized but the experimental proceedure says not to attempt melting point. Any idea why?
The thermal Diels-Alder reaction is reversible. That is in this case at the MPt. you will get the retero-Diels-Alder process proceeding.
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is it wrong if i suggest that a polymerisation side reaction takes place and that because of the complex nature of the compound, it would not have a melting point but a softening point?
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is it wrong if i suggest that a polymerisation side reaction takes place and that because of the complex nature of the compound, it would not have a melting point but a softening point?
Yes, that would be wrong. No polymerization is taking place. A retro Diels-Alder reaction is reforming the starting materials, not a polymerization reaction.
Decompositions are not standard. Some compounds can melt with decomposition and the melting points can be consistent. For others, and I presume this is the case here, the decomposition can occur well below the mp. If that is the case, you could heat it below the melting point and discover a much lower melting point than if you heated it quickly. That is a slow melting point determination will enable a larger portion of the sample to decompose below the melting point. In those cases, it is difficult to predict the mp of the crystal. The greater the decomposition occurs below the melting point, the greater the variability of the apparent decomposition/melting point.
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But then how could the melting point of the adduct be determined?
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But then how could the melting point of the adduct be determined?
It can't be determined by normal methods, perhaps DSC may do it?
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Aromatization in polycyclic systems is a little unusual - these systems tend to behave as if they were made up of discrete benzene rings, with resonance forms made of all the different ways of isolating discrete benzen rings in the system. Your anthracene, for example, could act as if any one of the rings was a benzene ring, with the other two rings being cyclohexadienes. The aromatic character in each ring is much less than if it were three separate benzene rings, and the possibility to form two isolated benzene rings is part of what drives the center ring to be reactive to Diels-Alder reactions. (See Clar's Rule for aromaticity of polycyclic aromatic hydrocarbons).
As for the melting point, as orgopete says, instead of melting you are decomposing the adduct. Look at your components - p-benzoquinone melts around 115C, anthracene around 215C - I don't know what temperature is required to drive the retro Diels-Alder, but you can see how you might have some liquid p-benzoquinone, some solid anthracene, and some of your adduct for a very wide range of temperatures. This is not a melting point.
DSC may be able to give you some numbers for the temperatures at which these transitions occur, but if the retro Diels-Alder is occurring at a lower temperature than the melting point of the adduct, then you simply can't determine a melting point for the adduct, only a decomposition temperature.
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But then how could the melting point of the adduct be determined?
It can't be determined by normal methods, perhaps DSC may do it?
Applying high pressure may prevent the reverse reaction and allow you to get a MPt.
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Thanks that was great help :)