Chlorine-Chlorosulfuric Acid

Cl2 + ClSO3H

[7782-50-5]  · Cl2  · Chlorine-Chlorosulfuric Acid  · (MW 70.91) (ClSO3H)

[7790-94-5]  · ClHO3S  · Chlorine-Chlorosulfuric Acid  · (MW 116.53)

(effective agent for a-halogenation of aliphatic acids; ClSO3H + Br2 or ClSO3H + I2 are similar)

Alternate Name: chlorine-chlorosulfonic acid.

Physical Data: ClSO3H: mp -80 °C, bp 152 °C; n20D 1.433; d at 18 °C, 1.766 g cm-3; for Cl2 see Chlorine.

Form Supplied in: transparent liquid; widely available.

Purification: commercial chlorosulfuric acid is usually contaminated with sulfuric acid and hydrogen chloride, and should be distilled before use (bp 86-88 °C at 33 mmHg).

Handling, Storage, and Precautions: chlorosulfuric acid is poisonous, and a strong oxidant and skin irritant: handle very carefully with gloves and a protective face shield; store in isolation from water, as it reacts to form H2SO4 and HCl; prolonged heating causes decomposition to H2SO4, SO2, and Cl2.

Chlorination of Carboxylic Acids.

Chlorine (or bromine or iodine) mixed with Chlorosulfonic Acid is conveniently used for the corresponding a-halogenation of aliphatic acids. The radical chlorination of aliphatic acids, e.g. photochlorination, occurs at random positions, while acid-catalyzed chlorination (e.g. the present reaction, as well as Hell-Volhard-Zelinsky procedures using PCl3) gives exclusively a-chloro acids.1,2 Molecular oxygen and Chloranil,3 as well as 7,7,8,8-tetracyanoquinodimethane,4 are effective radical trappers. Chlorosulfuric acid is much more effective than sulfuric acid, as it forms homogeneous mixtures and has a stronger acidity.

The mechanism for the chlorosulfuric acid-mediated halogenation of an aliphatic acid involves the initial conversion of the aliphatic acid into an enolized monoacyl sulfate, followed by the addition of molecular halogen to give an a-haloacyl halide (eq 1).5

Examples of chlorination of aliphatic acids with 4:1 molar ratios of aliphatic acid:ClSO3H and with 2:1 molar ratios of Cl2:O2 are given in Table 1.3,4,6,8,9

Bromine- or Iodine-Chlorosulfuric Acid Reagents.

Similarly, bromination or iodination of aliphatic acids is possible by using the reagents chlorosulfuric acid-halogen in dichloroethane without any radical trapper. Examples of brominations with 1:1:1 molar ratios of aliphatic acid:ClSO3H:Br2 are given in eqs 2 and 3.10

Examples of iodination with molar ratios of 65:32:6.5 of aliphatic acid:ClSO3H:I2 are shown in eqs 4 and 5.11 Here, excess ClSO3H converts the HI evolved to I2, leading to the stoichiometry as shown in eq 6.12

The similar oxidation of HBr to Br2 is observed, but not that of HCl.13 The yields for iodination of acetic acid (12%) and a,a-disubstituted acids (e.g. isobutyric acid, 14%) are poor, but in general acids give 93-98% yields.11

The effect of substituents on the relative rate constant krel of bromination and iodination fits the Taft equation (eq 7),14 where ρ* is the polar reaction constant, &SSigma;s* is the sum of polar substituent constants, d is the steric reaction constant, and &SSigma;Es is the sum of steric substituent constants. The observed ρ* values are negative for both bromination and iodination, suggesting an electrophilic attack of the positively polarized halogen molecule on the double bond of the enol.

Related Reagents.

Chlorine; Chlorosulfonic Acid.

1. Guest, H. H.; Goddard, C. M., Jr. JACS 1944, 66, 2074.
2. Rodin, R. L.; Gershon, H. JOC 1973, 38, 3919.
3. Ogata, Y.; Harada, T.; Matsuyama, K.; Ikejiri, K. JOC 1975, 40, 2960.
4. Crawford, R. J. JOC 1983, 48, 1364.
5. Ogata, Y.; Adachi, K. JOC 1982, 47, 1182.
6. Ogata, Y.; Ikejiri, T. NKK 1975, 1517.
7. Ogata, Y.; Sugimoto, T.; Inaishi, M. BCJ 1979, 52, 255.
8. Ogata, Y.; Sugimoto, T. CI(L) 1977, 538.
9. (a) Ogata, Y.; Sugimoto, T.; Inaishi, M. OS 1980, 59, 20; (b) OSC 1988, 6, 90.
10. Ogata, Y.; Sugimoto, T. JOC 1978, 43, 3684.
11. Ogata, Y.; Watanabe, S. JOC 1980, 45, 2831.
12. Ogata, Y.; Watanabe, S. JOC 1979, 44, 2768.
13. Ogata, Y.; Watanabe, S. BCJ 1980, 53, 2417.
14. (a) Taft, R. W. JACS 1952, 74, 2729, 3120; (b) Taft, R. W. JACS 1953, 75, 4231, 4534, 4538.

Yoshiro Ogata

Nagoya University, Japan

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