p-Toluenesulfonic acid

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p-Toluenesulfonic acid[1]
P-Toluenesulfonic acid
CAS number 104-15-4 YesY
6192-52-5 (monohydrate)
PubChem 6101
ChemSpider 5876 YesY
DrugBank DB03120
KEGG C06677 YesY
Jmol-3D images Image 1
Molecular formula CH3C6H4SO3H
Molar mass 172.20 g/mol (anhydrous)
190.22 g/mol (monohydrate)
Appearance colorless (white) solid
Density 1.24 g/cm3
Melting point 38 °C (100 °F; 311 K) (anhydrous)[2]
103 to 106 °C (217 to 223 °F; 376 to 379 K) (monohydrate)
Boiling point 140 °C (284 °F; 413 K) at 20 mmHg
Solubility in water 67 g/100 mL
Acidity (pKa) -2.8 (water),[3]

8.5 (acetonitrile)[4]

Molecular shape tetrahedral at S
MSDS External MSDS
R-phrases R36/37/38
S-phrases S26
Main hazards skin irritant
Related compounds
Related sulfonic acids Benzenesulfonic acid
Sulfanilic acid
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 N (verify) (what is: YesY/N?)
Infobox references

p-Toluenesulfonic acid (PTSA or pTsOH) or tosylic acid (TsOH) is an organic compound with the formula CH3C6H4SO3H. It is a white solid that is soluble in water, alcohols, and other polar organic solvents. The 4-CH3C6H4SO2- group is known as tosyl group and is often abbreviated as Ts or Tos. Most often, TsOH refers to the monohydrate, TsOH.H2O.

As with other sulfonic acids, TsOH is a strong organic acid. It is about one million times stronger than benzoic acid.[citation needed] It is one of the few strong acids that is solid and, hence, conveniently weighed. Also, unlike some strong mineral acids (especially nitric acid, sulfuric acid, and perchloric acid), TsOH is non-oxidizing.

Preparation and handling[edit]

TsOH is prepared on an industrial scale by the sulfonation of toluene. It hydrates readily. Common impurities include benzenesulfonic acid and sulfuric acid. Impurities can be removed by recrystallization from its concentrated aqueous solution followed by azeotropic drying with toluene.[5]

TsOH finds use in organic synthesis as an "organic-soluble" acid catalyst. Examples of uses include:

Tosylate esters[edit]

Ball-and-stick model of the tosylate anion

Tosylate esters are used as alkylating agents because the tosyl group is electron-withdrawing, which makes the tosylate anion a good leaving group. The tosyl group is also a protecting group for alcohols and amines, prepared by combining the alcohol with 4-toluenesulfonyl chloride, usually in an aprotic solvent, often pyridine, the basicity of which activates the reaction.[9] Toluenesulfonate esters undergo nucleophilic attack or elimination. Reduction of tosylate esters gives the hydrocarbon. Thus, tosylation followed by reduction allows for the deoxygenation of alcohols.


  • When TsOH is heated with acid and water, a hydrolysis reaction takes place and toluene is formed:
CH3C6H4SO3H + H2O → C6H5CH3 + H2SO4

This reaction is general for aryl sulfonic acids, but the rate at which it occurs depends upon the structure of the acid, the temperature and the nature of the catalyzing acid. For example, TsOH is unaffected by cold concentrated hydrochloric acid, but hydrolyzes when heated to 186 °C in concentrated phosphoric acid.[11][12]

See also[edit]


  1. ^ Merck Index, 11th Edition, 9459.
  2. ^ Armarego, Wilfred (2003). Purification of Laboratory Chemicals. Elsevier Science. p. 370. ISBN 0-7506-7571-3. 
  3. ^ Guthrie, J. P. Hydrolysis of esters of oxy acids: pKa values for strong acids. Can. J. Chem. 1978, 56, 2342-2354.
  4. ^ Eckert, F.; Leito, I.; Kaljurand, I.; Kütt, A.; Klamt, A.; Diedenhofen, M. Prediction of Acidity in Acetonitrile Solution with COSMO-RS. J. Comput. Chem. 2009, 30, 799-810. doi:10.1002/jcc.21103
  5. ^ Perrin, D. D. and Armarego, W. L. F. (1988). Purification of Laboratory Chemicals. Oxford: Pergamon Press. 
  6. ^ H. Griesser, H.; Öhrlein, R.; Schwab, W.; Ehrler, R.; Jäger, V. (2004), "3-Nitropropanal, 3-Nitropropanol, and 3-Nitropropanal Dimethyl Acetal", Org. Synth. ; Coll. Vol. 10: 577 
  7. ^ Furuta, K. Gao, Q.-z.; Yamamoto, H. (1998), "Chiral (Acyloxy)borane Complex-catalyzed Asymmetric Diels-Alder Reaction: (1R)-1,3,4-Trimethyl-3-cyclohexene-1-carboxaldehyde", Org. Synth. ; Coll. Vol. 9: 722 
  8. ^ Imwinkelried, R.; Schiess, M.; Seebach, D. (1993), "Diisopropyl (2S,3S)-2,3-O-isopropylidenetartrate", Org. Synth. ; Coll. Vol. 8: 201 
  9. ^ "Nucleophilic Substitution". 
  10. ^ L. Field and J. W. McFarland (1963), "p-Toluenesulfonic Anhydride", Org. Synth. ; Coll. Vol. 4: 940 
  11. ^ C. M. Suter (1944). The Organic Chemistry of Sulfur. New York: John Wiley & Sons. pp. 387–388. 
  12. ^ J. M. Crafts (1901). "Catalysis in concentrated solutions". J. Am. Chem. Soc. 23: 236–249.