[HDK]

Hello everyone.

I did a lab session on acetals as protecting groups. We had to protect the hydroxyl group of 1-phenylethanol with 3,4-dihydro-2H-pyran. What we expected was a colourless compound which would absorb UV light and hence could be detected by TLC during column, but it turned out that mine was red for some reason. I looked up red acetal compounds and found no such things. The RBF was cleaned with acetone. So i believe it cannot be an impurity. Unless it is a compound that cannot be removed that way.

Also, I noticed that some others also had the same compound, with a clear reddish tinge. Could someone give an explanation for that? Does it have to do with conjugation by any chance?

Thanks is advance, everyone.

[discodermolide]

Did you use acidic conditions to do this reaction?

[HDK]

Yes we did. We used p-toluenesulfonic acid monohydrate.

[discodermolide]

1-Phenyl ethanol is a strange choice for this reaction.
Perhaps you managed to eliminate water from the 1-Phenyl ethanol to give styrene which then polymerised?

[HDK]

May I know why you say its a strange choice? I thought it did not matter, as long as you have the hydroxyl functional group, but am I wrong?

Eliminate water to give styrene, okay, but what conditions would give rise to that?

Also, are you saying that due to the double bond offered by the styrene, there is increased conjugation, which caused it to perhaps absorb blue light, thereby emitting red? But is polystyrene coloured?

[discodermolide]

I have never used 1-Phenyl ethanol for this reaction. But perhaps you wanted a UV chromophore.
Styrene is formed by the acid catalysed elimination of water from the 1-Phenyl ethanol. If it polymerises uncontrollably it could give rise to short polystyrene chains which may be cross linked or anything giving rise to the red colour, which indicates lots of conjugation. It probably would not require much to produce a red colour.
So I think it's just a side reaction of the 1-Phenyl ethanol.
By the way did you get your desired product?

[HDK]

Do correct me if I'm wrong, but Acetone, being organic, would remove only organic impurities yeah? So could it be that inorganic impurities were present, due to its usage in a previous lab session?

What could that possibly be? Its not coppery red, its looked like the red colour of diluted fruit punch.

[discodermolide]

So that colour may indicate a trace impurity. Acetone will not remove inorganic impurities, but they would not interfere in your reaction.
Anyway that's my suggestion.

[HDK]

I think that might have been the reason. Since we used TLC in UV to detect it.

Yes the product formed, but my yield was slight, like 20 %. The general yield among my friends was 30-40%. They warned us that it would be poor due to the reversible nature of the reaction, and to reduce exposure to water as much as possible.

[HDK]

Thank you so much for the explanation and suggestion.

[discodermolide]

My pleasure.