Organic Syntheses, CV 4, 700
Submitted by J. Cymerman-Craig, M. Moyle, and R. A. White
1.
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).
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 arylamine
2 (this reaction fails with naphthyl compounds),
3 by fission of a
sym-diarylthiourea with acidic reagents
4 (this reaction involves the loss of half the amine used), and by the decomposition of an ammonium aryldithiocarbamate
5 (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
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