Sciencemadness Discussion Board

benzyl alcohol + sulphuric acid = ?

Keras - 3-4-2020 at 07:56

I just happened to let a few drop of 95/96% concentrated sulphuric acid fall into a small grad tube with a bit of benzyl alcohol in. It immediately gave rise to a sort of brown compound. Any idea what that can be? Some sort of ether, or an ester?

Just curious if anyone has an idea.

Syn the Sizer - 3-4-2020 at 08:53

Is it possible that it is benzyl sulfate? I was at first thinking of a dehydration but I am sure that wouldn't work with benzyl alcohol.

[Edited on 3-4-2020 by Syn the Sizer]

DraconicAcid - 3-4-2020 at 09:12

Adding sulphuric acid to benzyl alcohol is going to give you benzyl cations (at least in low concentrations). This could react with the alcohol part of other benzyl alcohol molecules to give an ether, but it could also do a Friedels-Craft alkylation to put the benzyl group on the o/p position of another ring. If that happens on the ortho position, it could do it twice to give you dihydroanthracene.

Sigmatropic - 3-4-2020 at 09:17

Probably similar to what happens to paramethoxybenzyl protecting groups during the deprotection under extremely acidic conditions (neat trifluoroacetic acid sometimes with catalytic triflic acid).
It polymerizes by FC alkylation forming a very apolar and strongly colored polymer.

TheMrbunGee - 3-4-2020 at 09:32

And when equal volumes of liquids are combined in a test tube, the reaction is explosive, polymer formed is quite hard to wash off all kinds of surfaces. Thankfully sulfuric acid seemed to be consumed by reaction, because no organic fabrics were damaged around splashes (the sulfuric acid way).

Keras - 3-4-2020 at 11:16

Yeah, it was a bit of a pain to wash the polymer off. Since I was doing the experiment in a TPX grad cylinder, I was a bit nervous to use a piranha solution, lest the cylinder would be dissolved with the rest. I tried sodium percarbonate + sulphuric acid (r.t.) and it produced some sort of nasty brownish compound with fortunately I was able to dilute with acetone, and finally I got my cylinder back clean and intact.

Thanks for suggesting FC alkylation. I hadn't even thought about that. Has the -CHâ‚‚OH group a directive effect? Logically, the C is slightly positive, so this should induce a negative charge in ortho/para positions, no?

unionised - 3-4-2020 at 11:24

benzyl alcohol + sulphuric acid = tar.

[Edited on 3-4-20 by unionised]

Metacelsus - 3-4-2020 at 16:29

Quote: Originally posted by unionised  
benzyl alcohol + sulphuric acid = tar.


Ah yes. Tar, the organic chemist's constant companion.

I posted a few minutes ago that I missed working on fluorophore synthesis. But I definitely won't miss the tar! (Even successful reactions with 90% yield gave brown gunk as the crude product. The other 10% was "tar".)

Keras - 4-4-2020 at 00:38

Quote: Originally posted by Metacelsus  
Quote: Originally posted by unionised  
benzyl alcohol + sulphuric acid = tar.


Ah yes. Tar, the organic chemist's constant companion.

I posted a few minutes ago that I missed working on fluorophore synthesis. But I definitely won't miss the tar! (Even successful reactions with 90% yield gave brown gunk as the crude product. The other 10% was "tar".)


Yet coal tar is such a bountiful source of useful reagents!

Cou - 5-4-2020 at 17:12

Sometimes when doing fischer esterifications, the reaction mixture turns brown due to reaction of sulfuric acid with organic impurities.