[lukas.stib]

Hello chemists,

I have one problem with distillation. I weighed 10 grams of salicylic acid and 37 grams of benzyl alcohol + a small amount of sulfuric acid. I refluxed, neutralized with soda solution, extracted with dichloromethane, dried CaCl2 and distilled. When the temperature of the product was approaching, a solid, orange compound formed in the cooking flask, distillation stopped and the stirrer did not work. It seems to have crystallized, and I don't know how to get benzyl salicylate and what´s this compound?

Please help me. Thank you, Lukáš S.

[rolnor]

Is this a published procedure? It should be a liquid.

https://en.wikipedia.org/wiki/Benzyl_salicylate

[lukas.stib]

Yes, benzyl salicylate is a liquid. But the problem is that there was a foreign substance precipitating, and I don't know what is it, and distillation of benzyl salicylate did not work because it interferes with this solid.

[hollytara]

I have never had much luck with Fischer esterification of salicylic acids. 

Maybe you distilled off excess benzyl alcohol and this is salicylic acid coming back out. 

[rolnor]

TLC would be great here, but the best is a litterature procedure.

[pgk]

1). Salicylic acid and derivatives are easily oxidized in air at such a reflux temperature (benzyl alcohol, bp = 205°C).
2). Orange coloration indicates quinone derivatives.
3). Distillation stopping indicates quinone-hydroquinone dimers (quinhydrone derivatives).
So what can be done, is:
Neutralizing the overall obtained product with dilute aqueous NaOH, removing quinone derivatives by solvent extraction, re-reacidification and distillation. But it may not worth the try.
Alternatively, you can repeat the esterification in inert atmosphere (N2) and in presence of a little amount of BHA or BHT. Besides, using TsOH instead of H2SO4 is preferred, as well as using toluene as a solvent to reduce the reaction temperature.
If the latter does not work, select another synthetic method.
You can check the purity of salicylic acid by TLC or even better, by HPLC. If not adequately pure, recrystallize it before the reaction.

Wet Oxidation of Salicylic Acid Solutions, Environmental Science & Technology, 44(22), 8629-8635, (2010)
https://pubs.acs.org/doi/abs/10.1021/es1021944
Preparation of Quinhydrone-type Complexes Composed of Quinone Dimer and Trimers, 55th Symposium on Organic Reactions, Session ID: P22, (2005)
https://www.jstage.jst.go.jp/article/kisoyuki/55/0/55_0_48/_article
Oxidation of Hydroxy- and Dihydroxybenzoic Acids Under the Udenfriend's Conditions. An HPLC Study, Open Med Chem J., 12, 13-22, (2018)
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5815051/

[lukas.stib]

Thank you very much for your comprehensive answer, PGK, which is very interesting. I just tried to keep it warm, and I couldn't tell what the substance was, because it was still dripping and the temperature kept rising up to 300°C, there was a risk of thermometer rupture, so I finished it. Distilled liquid with a very high refractive index and strange odor, which turned red to pink in 2 days. I destroyed everything. Thank you very much again for your reply. And it is interesting, what compounds was forming.

[AWK]

What pressure did you use for distillation?

[lukas.stib]

I did use normal distillation at normal pressure.

[pgk]

1). Do not worry about destroying your product. It happens to all of us from time to time.
2). You can distill under reduced pressure but you have to gradually brake the vacuum under inert atmosphere (N2) at the end, otherwise atmospheric oxygen will violently penetrate therein and will oxidize everything.

[lukas.stib]

Yes, thank you very much