Sciencemadness Discussion Board - benzyl alcohol + sulphuric acid = ?

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 -CH₂OH 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.

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.

Good reminder to check things on a small scale

jackchem2001 - 12-9-2025 at 23:39

For future experiments, I wanted to sulfonate benzyl alcohol on the aromatic ring. Even though this reaction seems simple, I always check things on a vial scale for safety. When I added sulfuric acid to benzyl alcohol, I was surprised to see a violent thermal run away. Here are my observations:

When there is a large excess of benzyl alcohol to sulfuric acid (for example 20:1 by volume), when the acid is added to the alcohol a fluffy orange precipitate immediately drops out of solution. There is no biphastic mixture as with toluene and sulfuric acid. This must certainly be the addition product of the alcohol and SO3 (structure given below) as aromatic sulfonation of comparable materials (benzene, toluene) requires heating at >100 deg for several hours.
When heat is applied to the mixture obtained above, the orange material turns a dirty mix of white/green/black (different colours in different regions). When agitated, there is a localised violent reaction.
When there is not so much of an excess of alcohol to sulfuric acid (say 5:1 by volume), when the acid is initially added the fluffy orange addition product precipitates. However, after ~3-5 seconds, a run away reaction begins. Rapid heating occurs and a large amount of steam is given off. There is no odor of benzaldehyde or SO2, so no redox chemistry is involved here.

To me, the only thing that fits what happens here is polymerisation. The solid product is difficult to deform at room temperature but easily deformed when heated. It is also sticky (and annoyingly difficult to get off glass). The product is insoluble in hot acid, neutral, and alkali water, benzyl alcohol, toluene, and isopropyl alcohol. These characteristics are explained by a polymer.

Here is my guess for the most likely mechanism of polymerisation, showing only the important steps. You can draw it through a benzylic cation or SN2' as I have done

[Edited on 13-9-2025 by jackchem2001]

Keras - 13-9-2025 at 00:05

It would be interesting to do this experiment with a non-oxidising acid, such as phosphoric or methanesulphonic acid.

[EDIT]: I've just done that. Mixed benzyl alcohol with almost 100% phosphoric acid (made by mixing 80% and phosphorus pentoxide). Nothing happens. Warmed it somewhat. Still nothing. Nada. Around 100 °C some slight plume of vapour, maybe water? But no precipitate of sorts, nothing but a clear solution.

[Edited on 13-9-2025 by Keras]

Fery - 13-9-2025 at 05:41

When adding only tiny amount of catalytic H2SO4 and using very low boiling point entrainer like hexane/isohexanes for azeotropic distillation to remove reaction water then the esterification reaction is without polymerization / tar. Benzyl alcohol (0,50 mol) 54,1 g + acetic acid (0,75 mol) 45,0 g + 0,25 ml 96% H2SO4. Just some tiny amount of unknown precipitate was observed after mixing all together which later dissolved on heating.
https://www.sciencemadness.org/whisper/viewthread.php?tid=15...

Sir_Gawain - 13-9-2025 at 14:29

I've ran into this exact phenomenon recently while trying to make benzyl acetate. Upon the addition of a few drops of sulfuric acid, a light brown precipitate formed with the release of a lot of heat. The precipitate was very sticky and insoluble in water and acetone. It's gotta be some sort of polymer.

Fery - 14-9-2025 at 02:09

Mine precipitate dissolved after reflux. Final solution was completely clear. Perhaps better to add few drops of H2SO4 into acetic acid and only then mix with benzyl alcohol, then maybe even no precipitate.

bnull - 14-9-2025 at 07:07

Have you tried the same on benzyl alcohol mixed with different but small quantities of aliphatic alcohols?

teodor - 15-9-2025 at 01:06

https://www.vedantu.com/question-answer/benzyl-alcohol-with-...

?

Texium - 15-9-2025 at 07:36

Quote: Originally posted by jackchem2001

Here is my guess for the most likely mechanism of polymerisation, showing only the important steps. You can draw it through a benzylic cation or SN2' as I have done

DraconicAcid already described this occurrence five years ago in this very thread. Your proposed mechanism is less likely than protonation of benzyl alcohol, loss of water to form benzyl cation, and Friedel-Crafts with another benzyl alcohol. There is not a significant amount of free SO₃ in conc. sulfuric acid. Ultimately, it results in the same type of polymer though.

jackchem2001 - 16-9-2025 at 01:04

After thinking about it a bit, I agree. It is worth reiterating, the reason I made the post is to warn about the violent thermal runaway when the ratio of sulfuric acid to benzyl alcohol is greater (even 1:10 by volume is enough). I could definitely see someone getting caught off guard trying to do an esterification, especially if he added the benzyl alcohol to the sulfuric acid rather than the other way around

Texium - 16-9-2025 at 09:25

Sure, though that would be a terrible way to set up an esterification, regardless of the existence of this particular side reaction. The ratio used for a Fisher esterification should typically be more like 1:100, or even lower in sulfuric acid. Earlier in this thread, the successful benzyl esterification that Fery described is around 1:200. And, the sulfuric acid should always be added last.