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Org. Synth. 1956, 36, 56
DOI: 10.15227/orgsyn.036.0056
α-NAPHTHYL ISOTHIOCYANATE
[Isothiocyanic acid, 1-naphthyl ester]
Submitted by J. Cymerman-Craig, M. Moyle, and R. A. White1.
Checked by N. J. Leonard and F. H. Owens.
1. Procedure
In a 500-ml. round-bottomed flask fitted with a reflux condenser are placed 16.2 g. (0.08 mole) of dry α-naphthylthiourea (Note 1) and 180 ml. of redistilled chlorobenzene. The flask is heated at the reflux temperature by means of an electric heating mantle. Evolution of ammonia begins almost at once, and all of the solid dissolves after 30–45 minutes. The solution is maintained at reflux for 8 hours (Note 2) and then is evaporated on a steam bath at water-pump pressure to remove all of the chlorobenzene. The residue crystallizes on cooling and is extracted with four 30-ml. portions of boiling hexane (Note 3). Removal of solvent from the combined hexane extracts affords pale yellow crystals of α-naphthyl isothiocyanate, m.p. 58–59°. The yield is 12.7–13.0 g. (86–88%). Recrystallization from hexane (9 ml. of hexane for 1 g. of solute) gives colorless needles, melting point unchanged (Note 4).
2. Notes
1. α-Naphthylthiourea may be prepared by the method of Frank and Smith, Org. Syntheses Coll. Vol. 3, 735 (1955), or by the method of de Clermont, Ber., 9, 446 (1876), and Bertram, Ber., 25, 48 (1892).
2. The checkers found that a 24-hour heating period increased the yield of α-naphthyl isothiocyanate to 95%.
3. A fifth extraction yields no further product. The residue insoluble in hexane was found by the submitters to consist of 1.3–1.9 g. of a mixture of equal parts of di-α-naphthylthiourea and α-naphthylthiourea, m.p. 178–181°.
4. The method is generally applicable to the preparation of aryl isothiocyanates. Using this procedure, the submitters have prepared the following isothiocyanates, with the yields and times of refluxing indicated: phenyl, 44%, 8 hours; o-chlorophenyl, 46, 8; p-bromophenyl, 73, 8; p-biphenylyl, 49, 6; β-naphthyl, 70, 10; 9-phenanthryl, 70, 10; 1-pyrenyl, 72, 10.
3. Discussion
Aryl isothiocyanates can be prepared by the action of thiophosgene on the arylamine2 (this reaction fails with naphthyl compounds),3 by fission of a sym-diarylthiourea with acidic reagents4 (this reaction involves the loss of half the amine used), and by the decomposition of an ammonium aryldithiocarbamate5 (low yields are reported for naphthyl and other compounds).5,6 The procedure described here is that of Baxter, Cymerman-Craig, Moyle, and White.7

References and Notes
  1. University of Sydney, Sydney, Australia.
  2. Org. Syntheses Coll. Vol. 1, 165 (1941).
  3. Connolly and Dyson, J. Chem. Soc., 1935, 679.
  4. Werner, J. Chem. Soc., 59, 396 (1891).
  5. Org. Syntheses Coll. Vol. 1, 447 (1941).
  6. Dains, Brewster, and Olander, Univ. Kansas Sci. Bull., 13, 1 (1922) [C. A., 17, 543 (1923)].
  7. Baxter, Cymerman-Craig, Moyle, and White, Chem. & Ind. (London), 1954, 785; J. Chem. Soc., 1956, 659.

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

α-naphthylthiourea

ammonia (7664-41-7)

chlorobenzene (108-90-7)

Thiophosgene (463-71-8)

PHENYL ISOTHIOCYANATE (103-72-0)

hexane (110-54-3)

α-Naphthyl isothiocyanate,
Isothiocyanic acid, 1-naphthyl ester (551-06-4)

di-α-naphthylthiourea

p-bromophenyl isothiocyanate (1985-12-2)

o-chlorophenyl isothiocyanate (2740-81-0)

p-biphenylyl isothiocyanate

2-naphthyl isothiocyanate

9-phenanthryl isothiocyanate

1-pyrenyl isothiocyanate