[yitzik]

hello!
while performing the experiment of benzoin condensation using thiamine as catalist(without heating, just leaving the reaction for a few days).
i have noticed that the solution of thiamin hidrochloride in water was colorless. but when adding the NaOH the solution became yellowish. i guess that the tihamine changed it's color but i'm not sure for the reason. could it be that because of the deprotonation of the amine group in basic conditions from NH3 to NH2 and consequently conjugation, there is a change in color? (notice that in the attached picture the amine group appears as NH2 even in the accidic form, shouldn't it be NH3+ WHILE acidic?)

another question- we had to put the reaction mixture in a sealed container in a dark room for at least  2 days. why dark room? what is sensitive to light here and why? i mean, could it be that the thiamine is sensitive to light? and how can you see that from it's structure?

thanks

[wildfyr]

Your first theory seems right on the money for color change.

And yes thiamine is sensitive to light. I couldn't quickly track down the precise reaction it undergoes upon exposure to UV light, which is present in small amounts in house lighting and larger amounts in sunlight. But its sensitivity is well known. Its not very easy to tell if something will be light sensitive. Sometimes there are functional groups that give it away (like aryl ketones), other times it has to be tested out.

[wildfyr]

Also, benzoin itself is a pretty good free radical photoiniator (presuming you mean the aryl ketone, not the natural resin)

[Babcock_Hall]

When a chemical species absorbs light, the electron is promoted into a higher energy orbital.  This could easily bring about its participation in some kind of chemistry.

[yitzik]

thanks you