In a classical reaction, phenolphthalein is synthetised from phthalic anhydride and phenol with acid catalyst. When one uses substituted phenols, one obtains similar compounds (e. g. cresolphthalein, thymohlphthalein).

However, some acid-base indicators contain a sulfo- group instead of a carboxy- group of phthaleins. Does that mean that they are synthetised from phenols and the anhydride of o-sulfobenzoic acid? Phthalic anhydride is fairly cheap, but how is o-sulfobenzoic anhydride prepared?

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    $\begingroup$ Phthalic anhydride is prepared industrially by catalytic oxidation of naphthalene or o-xylene over vanadium pentoxide thechemco.com/chemical/phthalic-anhydride $\endgroup$
    – Waylander
    Commented Jan 11, 2020 at 11:50
  • $\begingroup$ @Waylander I meant o-sulfobenzoic anhydride, but it wasn't really clear. I edited the question. $\endgroup$
    – FusRoDah
    Commented Jan 11, 2020 at 12:26

2 Answers 2


Now a days, sulfonephthaleins can be prepared by reaction of readily available saccharin and the desired plenol compound (Ref.1 & Ref.2). In this method, active reagent, sulfobenzoic anhydride will be prepared in situ as depicted in following diagram:


In this method, it's described the preparation of phenolsulfonephthalein (Phenol Red). If you use thymol instead of phenol, you will get thymolsulfonephthalein, which can be subsequently brominated by bromine/acetic acid to obtain dibromothymolsulfonephthalein (Bromothymol Blue). This procedure is described in Ref.2.


  1. V. H. Tillu, D. K. Dumbre, H. B. Borate, R. D. Wakharkar, V. R. Choudhary, "Solvent-Free One-Pot Synthesis of Sulfonephthaleins from Saccharin and Phenols," Synthetic Communications 2012, 42(8), 1101-1107 (https://doi.org/10.1080/00397911.2010.535946).
  2. B. S. Rao, J. B. Puschett, B. M. Karandikar, K. Matyjaszewski, "Synthesis of Functional Bromothymol Blue Dyes for Surface Attachment to Optical Fibers," Dyes and Pigments 1991, 16(1), 27-34 (https://doi.org/10.1016/0143-7208(91)87018-I).

o-Sulfobenzoic Anhydride is prepared by cyclisation of ammonium o-sulfobenzoate (itself prepared by cHCl hydrolysis of saccharin) with thionyl chloride in benzene. The Organic Syntheses procedure is here: Org Syn

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