Can any other common reagent other than than $\ce{H3PO4}$ cause this conversion? What about (conc) $\ce{H2SO4}$ or (conc) $\ce{HNO3}$ or (conc) $\ce{HCl}$?
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2 Answers
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This is an intramolecular Friedel-Crafts acylation. Other sources call it Haworth reaction.
Concentrated phosphoric acid (in its viscous solution up to $\pu{85 m\%}$) is indicated as it is both strong (to generate the acylium cation), as well as removing the water generated in the course of the reaction (which could trap the acylium cation just generated). Hydrochloric acid (up to $\pu{37 m\%}$), in comparison, is a strong Bronsted acid, too, but is lesser hygroscopic than concentrated phosphoric acid.
This does not imply that phosphoric acid will be your solvent of reaction; indeed you may even use polyphosphoric acid (CAS 8017-16-1) -- which is a water free, waxy solid -- to catalyse the reaction. As alternatives to the above route, I suggest
using strongly acidic ion exchangers (like Amberlyst-15), provided you find a way to remove the water generated (Dean-Stark distillation, trapping in an other reaction).
If you change to anhydrous conditions, a Lewis Acid ($\ce{AlCl3}$ is a classic example of the stronger ones) may be used, too. Along this classical route, of course, you would need to generate the acyl chloride first (typically $\ce{SOCl2}$, or $\ce{PCl5}$), to transform the acid in the acyl chloride. And subsequently, you will perform the Friedel-Crafts acylation. Perhaps $\ce{AlCl3}$ may serve in a one-pot reaction, too -- yet you would need a bit more.
answered May 13, 2017 at 13:09
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$\begingroup$ What do you mean by "catch the water" ? Why can phosphoric acid catch the water but not HCl ? $\endgroup$– user38977Commented May 13, 2017 at 13:11
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$\begingroup$ Concentrated $\ce{H3PO4}$ may be up to 85 m% and is highly hygroscopic. Concentrated $\ce{HCl}$ is up to 37 m%, lesser hygroscopic than the former (63 m% of water, seen from the other perspective). In addition, instead of phosphoric acid as solution, you may use polyphosphoric acid (CAS 8017-16-1) which is a water free solid. You need both generation of the acyl cation, and removal of water (because water would react just again on the acyl cation) to drive the reaction forward. Of course this does need not to imply to run the reaction "just in phosphoric acid" (as sole solvent). $\endgroup$ Commented May 13, 2017 at 13:21
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$\begingroup$ Please edit to add this to your answer so that I can upvote it! Thanks $\endgroup$– user38977Commented May 13, 2017 at 13:25
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$\begingroup$ @blue And till some moments ago, I did not know that instead of the more general "intramolecular Friedel-Crafts reaction" this may be called a "Haworth reaction", too. The gain here is (at least) for both of us. $\endgroup$ Commented May 13, 2017 at 13:53
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$\begingroup$ On the wiki page they don't show any conditions required for Haworth reaction. Not even H3PO4. Strange! en.wikipedia.org/wiki/… $\endgroup$– user38977Commented May 13, 2017 at 14:03
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Eaton's reagent - $\ce{P2O5}$ in Methanesulfonic Acid - is a good alternative. JOC 62(1997) 3552
Conc. Nitric acid risks nitrating the aromatic ring. I have never seen conc. Nitric or conc. $\ce{HCl}$ used for this
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answered May 13, 2017 at 13:03
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$\begingroup$ What about HCl, HNO3....? You answered the question only partly. $\endgroup$– user38977Commented May 13, 2017 at 13:04
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$\begingroup$ Edited to include nitric and HCl $\endgroup$ Commented May 13, 2017 at 13:07