A Publication
of Reliable Methods
for the Preparation
of Organic Compounds
Annual Volume
Org. Synth. 1932, 12, 44
DOI: 10.15227/orgsyn.012.0044
[Thiophene, 2-iodo-]
Submitted by Wesley Minnis
Checked by Roger Adams and H. D. Cogan.
1. Procedure
In a glass-stoppered, wide-mouthed bottle cooled by ice water are placed 35 g. (0.42 mole) of thiophene (p. 578) and 50 cc. of benzene (Note 1). With constant shaking (Note 2), and cooling when necessary, 75 g. (0.35 mole) of yellow mercuric oxide and 109 g. (0.43 mole) of iodine are added alternately in small amounts during a period of fifteen to twenty minutes. The yellow mercuric oxide changes to crimson mercuric iodide. The mixture is filtered, and the residue is washed with three 25-cc. portions of ether. The ether-benzene filtrate is shaken with a dilute solution of sodium thiosulfate to remove excess iodine and then dried over 5 g. of calcium chloride and filtered. The ether and benzene are removed by distillation on a steam bath (Note 3), and the residue is fractionally distilled under reduced pressure. 2-Iodothiophene distils at 73°/15 mm.; 80–81°/20 mm.; 90–94°/34–38 mm. (Note 4). The yield is 63–66 g. (72–75 per cent of the theoretical amount) (Note 5). If the iodothiophene is still colored by traces of iodine, the color may be removed by shaking with a small amount of mercuric oxide.
2. Notes
1. Ligroin (b.p. 100–120°) may be substituted for benzene.
2. Better yields are obtained when the mixture is vigorously shaken by hand than when mechanical stirring is used. The ordinary stirrer will not keep the mercuric oxide in suspension.
3. Unreacted thiophene can be recovered from the ether-benzene distillate by treating the latter with mercuric oxide and dilute acetic acid, collecting the white precipitate [C4H2S(HgOCOCH3)HgOH] on a Büchner funnel, and decomposing it with concentrated hydrochloric acid.1 In checking this preparation there was not enough unreacted thiophene to be recovered.
4. A small amount of 2,5-diiodothiophene is formed in the reaction. About 4 g. of crystalline diiodothiophene, m.p. 40–41°, can be isolated from the residue remaining after distillation of the 2-iodothiophene (b.p. 65.5–66.5°/9 mm.). (O. Ivan Lee, private communication.)
5. 2-Iodothiophene reacts with magnesium to form a Grignard reagent and is hence useful in the preparation of other thiophene derivatives.
3. Discussion
2-Iodothiophene has been prepared only by the action of iodine and mercuric oxide on thiophene.2
This preparation is referenced from:

References and Notes
  1. Dimroth, Ber. 32, 759 (1899).
  2. Meyer and Kreis, ibid. 17, 1558 (1884); Thyssen, J. prakt. Chem. (2) 65, 5 (1902).

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

calcium chloride (10043-52-4)

hydrochloric acid (7647-01-0)

acetic acid (64-19-7)

Benzene (71-43-2)

ether (60-29-7)

magnesium (7439-95-4)

sodium thiosulfate (7772-98-7)

mercuric oxide (21908-53-2)

iodine (7553-56-2)

mercuric iodide (7774-29-0)

Thiophene (110-02-1)

Thiophene, 2-iodo-,
iodothiophene (3437-95-4)

2,5-diiodothiophene (625-88-7)