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Author: Subject: Why some compounds may disappear in TLC?
RabbitTy
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[*] posted on 29-10-2019 at 00:23
Why some compounds may disappear in TLC?


TLC is always be using as monitor method.I found that some compound would disapear in TLC after several hours.However, The boiling point of these
compounds is not that low.
Can someone help me explain the mechanism?
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Ubya
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[*] posted on 29-10-2019 at 04:28


an example of a compound that does this would be helpful




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Metacelsus
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[*] posted on 29-10-2019 at 04:37


Sometimes compounds react with silica gel and decompose.



As below, so above.
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Cactuar
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[*] posted on 29-10-2019 at 11:53


If you are using fluorescent TLC plates they show spots by glowing under UV-light, but any aromatic compound (or in other way conjugated) will absorb this light, showing dark spots. Any compound with a significant vapor pressure will eventually evaporate if it's not polar enough to be held by the silica. Xylenes boil at around 140 C and would probably disappear within 10 minutes. If you ever eluate anything with toluene you wont be able to read the TLC the first few minutes until it evaporates. When synthesizing small simple aromatics you quickly learn to circle any spots with a pencil.
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RabbitTy
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[*] posted on 29-10-2019 at 16:42


Quote: Originally posted by Metacelsus  
Sometimes compounds react with silica gel and decompose.


O-chlorobenzonitrile is one of them
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RabbitTy
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[*] posted on 29-10-2019 at 16:44


Quote: Originally posted by Ubya  
an example of a compound that does this would be helpful

O-chlorobenzonitrile is one of them
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RabbitTy
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[*] posted on 29-10-2019 at 16:48


Quote: Originally posted by Cactuar  
If you are using fluorescent TLC plates they show spots by glowing under UV-light, but any aromatic compound (or in other way conjugated) will absorb this light, showing dark spots. Any compound with a significant vapor pressure will eventually evaporate if it's not polar enough to be held by the silica. Xylenes boil at around 140 C and would probably disappear within 10 minutes. If you ever eluate anything with toluene you wont be able to read the TLC the first few minutes until it evaporates. When synthesizing small simple aromatics you quickly learn to circle any spots with a pencil.


Thus its not depend on the boiling point solely. Thanks a lot
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RabbitTy
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[*] posted on 29-10-2019 at 17:37


Quote: Originally posted by Ubya  
an example of a compound that does this would be helpful


O-chlorobenzonitrile is solid at r.t.Benzoonitrile is liquid ar r.t.
However, Benzoonitrile wont disappear in TLC, O-chlorobenzonitrile will.
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[*] posted on 30-10-2019 at 01:08


Other possibilities are oxidation or hydrolysis catalysed by SiO2
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