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Why so much color change in synthesis of triphenylmethanol?
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JP Highwater
Why so much color change in synthesis of triphenylmethanol?
The colour is because of electron delocalisation over the three rings rather than just one because the cation is planar. (see structure in answer from @ChemistryHelpCenter). Think of particle on a ring, bigger ring , lower energy, thus the absorption of the compound changes from being in the uv (only benzene rings delocalised ) to all three rings, and absorption in the visible. The final product will have a more or less tetrahedral structure so this delocalisation is lost and the colour is again in the uv and outside out visual perception.
The colour is because of electron delocalisation over the three rings rather than just one because the cation is planar. (see structure in answer from @ChemistryHelpCenter). Think of particle on a ring, bigger ring , lower energy, thus the absorption of the compound changes from being in the uv (only benzene rings delocalised ) to all three rings, and absorption in the visible. The final product will have a more or less tetrahedral structure so this delocalisation is lost and the colour is again in the uv and outside out visual perception.
@Mithoron , yes not quite planar as you say, so I should have been more exact in my description. The rings are twisted rather like propellor blades, but nevertheless the delocalisation produces the colour as it were. In fact, the photophysics of these compounds is very interesting showing very non-exponential fluorescence decays and solvent dependent dynamics.More
Well, maybe its again nitpicking, but trityl isnt planar, or one may say only as planar as it can get before steric repulsion of ortho hydrogens blocks further planarisation.More
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It sounds like you might have generated a species with this structural moiety:
This can happen if your solution was concentrated enough or the boiling of your ether was vigorous, so the local overheating caused the formation of $\ce{MgO}$, the guy above, and $\ce{Br-}$, which gives a very distinct bright red-pink color, which is VERY bright and only requires a bit of the side product to form. Though this carbocation is kinda stable, it will find the $\ce{Br-}$ in the solution and form triphenylmethylbromide as a minute impurity.
It sounds like you might have generated a species with this structural moiety:
This can happen if your solution was concentrated enough or the boiling of your ether was vigorous, so the local overheating caused the formation of $\ce{MgO}$, the guy above, and $\ce{Br-}$, which gives a very distinct bright red-pink color, which is VERY bright and only requires a bit of the side product to form. Though this carbocation is kinda stable, it will find the $\ce{Br-}$ in the solution and form triphenylmethylbromide as a minute impurity.
This is a pretty common lab experiment in the undergraduate organic chemistry lab, including in courses I teach. I would love to see a reference for this cation so that I can share it with my students.More
@permeakra, yes, youre right, I didnt mean the cation itself as is, it would need an EDG to give more of a blue hue in low concentrations, or shift towards blue when "stacked" with, say, benzophenone or other aromatix.More
@BenNorris exactly, every other student in my lab observed similar color array and its described in other protocols for synthesis of triphenylmethanol I found online. Neither the TAs nor professor could pinpoint the exact cause of all the short-lived color change.More
The colour is because of electron delocalisation over the three rings rather than just one because the cation is planar. (see structure in answer from @ChemistryHelpCenter). Think of particle on a ring, bigger ring , lower energy, thus the absorption of the compound changes from being in the uv (only benzene rings delocalised ) to all three rings, and absorption in the visible. The final product will have a more or less tetrahedral structure so this delocalisation is lost and the colour is again in the uv and outside out visual perception.
The colour is because of electron delocalisation over the three rings rather than just one because the cation is planar. (see structure in answer from @ChemistryHelpCenter). Think of particle on a ring, bigger ring , lower energy, thus the absorption of the compound changes from being in the uv (only benzene rings delocalised ) to all three rings, and absorption in the visible. The final product will have a more or less tetrahedral structure so this delocalisation is lost and the colour is again in the uv and outside out visual perception.
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It sounds like you might have generated a species with this structural moiety:
This can happen if your solution was concentrated enough or the boiling of your ether was vigorous, so the local overheating caused the formation of $\ce{MgO}$, the guy above, and $\ce{Br-}$, which gives a very distinct bright red-pink color, which is VERY bright and only requires a bit of the side product to form. Though this carbocation is kinda stable, it will find the $\ce{Br-}$ in the solution and form triphenylmethylbromide as a minute impurity.
It sounds like you might have generated a species with this structural moiety:
This can happen if your solution was concentrated enough or the boiling of your ether was vigorous, so the local overheating caused the formation of $\ce{MgO}$, the guy above, and $\ce{Br-}$, which gives a very distinct bright red-pink color, which is VERY bright and only requires a bit of the side product to form. Though this carbocation is kinda stable, it will find the $\ce{Br-}$ in the solution and form triphenylmethylbromide as a minute impurity.
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