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Org. Synth. 1964, 44, 62
DOI: 10.15227/orgsyn.044.0062
HOMOPHTHALIC ACID
Submitted by P. A. S. Smith and R. O. Kan1.
Checked by Melvin S. Newman and Bernard Darre.
1. Procedure
Ten grams (0.056 mole) of 2a-thiohomophthalimide2 and a solution of 30 g. of potassium hydroxide in 125 ml. of water are placed in a 300-ml., one-necked, round-bottomed flask (Note 1). The mixture is refluxed for 48 hours, filtered, and acidified with 12N hydrochloric acid. The solid that forms on cooling is collected by filtration and recrystallized from a mixture of 25 ml. of water and as much acetic acid (about 7 ml.) as necessary to dissolve the solid in the boiling solution, with addition of a little activated carbon. The yield of homophthalic acid, m.p. 181° (Note 2), is 6.1–7.5 g. (60–73%) (Note 3).
2. Notes
1. Because base can attack glass vessels, possibly introducing difficultly removable silicates into the reaction mixture, a copper flask is recommended for routine operations.
2. The melting point depends on the rate of heating. When the solid is heated slowly, the melting range can be as low as 172–174°.
3. An alternative procedure involves 3 days of refluxing in a mixture of 75 ml. of glacial acetic acid, 50 ml. of 12N hydrochloric acid, and 30 ml. of water. The product separates on cooling in a slightly lower yield (48%).
3. Discussion
Homophthalic acid may be obtained by the oxidation of indene,3,4 the reduction of phthalonic acid,5,6 and the hydrolysis of o-carboxyphenylacetonitrile.7 Other methods are listed in an earlier volume.3
4. Merits of the Preparation
This is a general method for converting 2a-thiohomophthalimides to homophthalic acids. Since 2a-thiohomophthalimides are readily obtained from phenylacetyl chlorides,2 this is a convenient method for preparing homophthalic acids.
This preparation is referenced from:

References and Notes
  1. Department of Chemistry, University of Michigan, Ann Arbor, Michigan.
  2. P. A. S. Smith and R. O. Kan, this volume, p. 1051.
  3. O. Grummitt, R. Egan, and A. Buck, Org. Syntheses, Coll. Vol. 3, 449 (1955).
  4. W. F. Whitmore and R. C. Cooney, J. Am. Chem. Soc., 66, 1239 (1944).
  5. K. Miescher and J. R. Billeter, Helv. Chim. Acta, 22, 608 (1939).
  6. W. Davies and H. G. Poole, J. Chem. Soc., 1617 (1928).
  7. C. C. Price, Org. Syntheses, 22, 61 (1942).

Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)

phthalonic acid

hydrochloric acid (7647-01-0)

acetic acid (64-19-7)

carbon (7782-42-5)

potassium hydroxide (1310-58-3)

indene (95-13-6)

Homophthalic acid (89-51-0)

thiohomophthalimide

o-CARBOXYPHENYLACETONITRILE (6627-91-4)