Title: Caro's Acid
CAS Name: Peroxymonosulfuric acid
Additional Names: sulfomonoperacid; persulfuric acid
Molecular Formula: H2O5S
Molecular Weight: 114.08
Literature References: Dry reagent is prepd by stirring 10 g potassium persulfate into 11 g concd H2SO4 for 10 min and adding 30 g finely powdered potassium sulfate; liquid reagent is obtained by triturating potassium persulfate with three times as much (by weight) of H2SO4; dil reagent is prepd by stirring 10 g potassium persulfate into 11 g concd H2SO4 and adding 50 cc ice: Baeyer, Villiger, Ber. 32, 3625 (1899).
Properties: The product is a sirupy liquid consisting of about equal amounts of Caro's acid and H2SO4. pK2 of Caro’s acid 9.4 ± 0.1. Oxygen is evolved at room temp; should be stored at dry ice temp.
pKa: pK2 of Caro’s acid 9.4 ± 0.1
CAUTION: Can be dangerously unstable, like most peroxides. Description of explosion at Brown University: J. O. Edwards, Chem. Eng. News 33, 3336 (1955). Explosion at Sun Oil, ibid. 38, 59 (Nov. 21, 1960). May be highly irritating to skin, eyes, mucous membranes.
Use: In prepn of dyes; oxidation of olefins to a-glycols; oxidation of ketones to lactones or esters.

Pelnicki
My other tip: use Caro's acid (H2SO5) for B/V step. Best yields in ballpark of 55 to 65% molar yield (!!! how awesome is that !!!).

The long and the short of it is that Caro's acid can be made from H2SO4 + (NH4)2.S2O8,

Persulfates in the BV reaction were the originally mentioned substrates by B and V themselves.

A number of fellow bees have reported a 50-60% molar yield, which yes, is much better than similar perborate/peracetic/performic experiments.

Baeyer-Villiger Oxidation of Ketones
The reaction of peracids with ketones proceeds relatively slowly but allows for the conversion of ketones to esters in good yield. In particular, the conversion of cyclic ketones to lactones is synthetically useful because only a single product is to be expected. The reaction has been carried out with both percarboxylic acids and Caro's acid (formed by the combination of potassium persulfate with sulfuric acid).

Oxidations have been carried out using suspensions of dry reagent or solutions of persulfuric acid in concentrated or dilute sulfuric acid, in glacial acetic acid, in petroleum ether, and in ethanol-sulfuric acid.

Persulfate oxidation of ketones (general procedure):
The oxidizing agent is prepared in a 500-ml flask equipped with a magnetic stirrer and cooled in an ice bath as follows: In the flask are placed 60 ml of concentrated sulfuric acid and 20 ml of water, and the solution is cooled to 10°. Potassium persulfate (42 g, 0.15 mole) is added slowly to the stirred solution while maintaining the temperature below 10°. The solution is diluted with an additional 65 ml of water maintaining the temperature below 15°. The solution is now cooled to about 7° and 0.08 mole of the ketone is added over 40 minutes. After the addition has been completed, the solution is allowed to come to room temperature and stirring is continued for 20 hours. The solution is diluted carefully with 150 ml of water and extracted twice with 75-ml portions of ether. The ether is washed with sodium bicarbonate solution, followed by water, and the ethereal solution is dried. Removal of the solvent, followed by fractional distillation, affords the product ester.
ref.: Monson, Richard S.; Advanced organic synthesis: methods and techniques (1971), p 11

Fernandez and Schwartz, 3,4,5-Trinitroluene via Caro’s Acid Oxidation, J. Chemical and Engineering Data, Vol 15, No. 3, 1970
Caro’s acid was prepared by the method of Brady and Taylor (J. Chem Soc. 117, 876 (1920), Fieser and Fieser, Reagents for Organic Synthesis, p. 118 (1967)). using 60 grams of finely powdered ammonium persulfate dissolved in 44 ml of cold concentrated sulfuric acid.  The acid slurry and 100 grams of crushed ice were added to 9.95 g (0.0198 mole) of 3,5-dinitro-4-aminotoluene dissolved in 6 ml of concentrated sulfuric acid. . . .

Synthesis of Caro's Acid
1. 145 g (0.64 moles) of ammonium persulfate was added to 54 ml of cold, concentrated H2SO4.
2. The mixture was allowed to stand for about an hour and poured into 355 g of
crushed ice.

Caro acid (peroxymonosulfuric acid)
Ammonium persulfate (23.0 g) was added in small portions to 29.0 g of 85% H2SO4

US Patent 4049786 by Chiang:
High strength peroxymonosulfate is prepared by adding concentrated sulfuric acid to a solution of a soluble peroxydisulfate whereby the heat of solvation of the sulfuric acid hydrolyzes the peroxydisulfate to the peroxymonosulfate; the temperature is controlled to provide a range of about 140° to 160° F. After about 15 to 45 minutes, the solution is cooled rapidly to about room temperature.
. . . .
It has been discovered that the desideratum aforesaid can be realized by a novel modification in the production of peroxymonosulfates from the hydrolysis of peroxydisulfates comprising the steps of:

a. providing a solution of a peroxydisulfate at about 100° F.;

b. mixing the peroxydisulfate solution of step (a) with concentrated sulfuric acid in a volume ratio of 1 to 0.07 to 1 to 0.15 whereby the heat of solvation of the sulfuric acid rapidly hydrolyzes the peroxydisulfate to produce a solution temperature between about 140° to 160° F.;

c. maintaining the resulting temperature of about 140 to 160° F. for about 5 to 45 minutes; and

d. rapidly cooling the solution to about room temperature.
. . . .
In carrying out the invention, a peroxydisulfate solution is prepared by dissolving a soluble peroxydisulfate salt in water and bringing the temperature of the solution to about 100° F. A convenient procedure consists in dissolving the persulfate in hot water at about 140° F. whereby the endothermic heat of dissolution lowers the temperature to the vicinity of 100° F. The concentration of the persulfate is not critical and solutions ranging from 0.1 m/l to saturation give satisfactory results. A preferred concentration is about 2 m/l since this provides minimum volume for easy handling while maximizing peroxymonosulfate yield for a given quantity of sulfuric acid. Any soluble peroxydisulfate is suitable, although for reasons of solubility and economy, sodium and ammonium peroxydisulfate are preferred and ammonium peroxydisulfate is most preferred.

The concentrated sulfuric acid should be sufficiently high to provide the desired heat of hydration directly; a convenient source is commercial 98% acid. To the approximately 100° F. solution of persulfate is added concentrated sulfuric acid in a ratio of 1 volume of the persulfate solution to 0.07 to 0.15 volume of concentrated sulfuric acid under conditions whereby the heat of solvation of the sulfuric acid rapidly initiates the hydrolysis reaction and produces a temperature range of about 140° to 160° F. The hydrolysis of the peroxydisulfate to the peroxymonosulfate proceeds rapidly at the temperature aforesaid, occuring to the extent of 50% to 80% in 15 minutes and is substantially complete in 30 to 45 minutes. Within the first 15 minute reaction period, essentially no hydrogen peroxide is formed. As the conversion proceeds to completion, the hydrogen peroxide content rises, thereby limiting the net conversion to about 80%. Accordingly, the overall practical reaction time can vary from about 5 to 45 minutes while the preferred range is about 15 to 20 minutes.

As above pointed out, the hydrolysis of the peroxydisulfate is conducted under conditions whereby the heat of solvation is utilized to initiate and sustain the reaction and to this end an insulated reaction zone may be used, particularly for small scale reactors where heat losses are proportionately greater than with large vessels. The reaction zone is also provided with cooling means in order to control the upper temperature range. Desirably, the reaction vessel is chosen in which heat losses are minimal while providing the temperature range for the hydrolysis. For laboratory scale runs, a Dewar flask serves as an excellent vessel for retaining reaction heat. An insulated vessel can also be used while large reactors may retain sufficient heat without extraneous insulation. Such thermal characteristics can readily be established by conducting a few trial runs.

After the hydrolysis has reached about 80% conversion, the reaction is rapidly cooled to room temperature and so maintained preparatory to being used.
. . . .
However, when the hydrolysis is carried out in accordance with the present invention utilizing the heat of solvation of sulfuric acid, the reaction is peculiarly speeded up and the desired peroxymonosulfate reaches maximum concentration in less than an hour. If the sulfuric acid and peroxydisulfate solution are mixed under room temperature conditions and then heated rapidly to about 150 F., the yield of peroxymonosulfate is only about 20%. For some reason, conducting the hydrolysis wherein the temperature is provided by the sulfuric acid heat of solvation plus the residual heat of the 100 F. peroxydisulfate solution greatly accelerates the rate of conversion. By way of a theory, it is suggested that the heat of solvation spreads so rapidly throughout the preheated 100 F. reaction zone that all reacting species are heated instantly to the optimum temperature, a condition not realized by heating the reaction zone by external means.

Reference is now made to the following non-limiting examples.

EXAMPLE 1

Hot water at about 140 F. was added to 91.3 g of ammonium peroxydisulfate in a 300 ml tall form beaker with mild agitation. The final solution volume was about 200 ml. The peroxydisulfate was dissolved in about one minute. The resulting solution temperature was about 100 F.

The solution was transferred to a Dewar flask. After allowing for temperature stabilization i.e., about 5 minutes, 20 ml of concentrated sulfuric acid was added using a magnetic stirrer; temperature rose immediately to 140 F. and stabilized at 158 F. after 15 minutes. About 30 minutes after the sulfuric acid addition, the solution was transferred to a 250 ml Erlenmeyer flask and rapidly cooled to room temperature with running tap water.

The solution was analyzed and found to contain 0.04 m/l of peroxydisulfate, 1.41 m/l of peroxymonosulfate and 0.23 m/l of hydrogen peroxide. The net conversion of peroxydisulfate to peroxymonosulfate was calculated to be 77.5%.

EXAMPLE 2

An experiment similar to Example 1 was carried out, except 100 g of sodium peroxydisulfate was used. Similar temperature changes for the solution were observed. After 45 minutes, the solution was cooled to room temperature.

Analysis showed the resulting solution contained 0.03 m/l of peroxydisulfate, 1.55 m/l of peroxymonosulfate and 0.20 m/l of hydrogen peroxide. The net conversion of peroxydisulfate to peroxymonosulfate was calculated to be 81.6%.

EXAMPLE 5

A 4 liter beaker equipped with a stainless steel cooling coil and a heater was used. Ammonium peroxydisulfate, about 912 g, was weighed out and transferred to the beaker. About 1.4 liters of hot tap water (about 140 F.) was added; the final volume was about 2 liters. Peroxydisulfate was found to completely dissolve in about a minute and the final solution temperature was reduced to 99 F. due to endothermic dissolution of peroxydisulfate.

When peroxydisulfate dissolution was completed, about 200 ml concentrated sulfuric acid was added with stirring using a magnetic stirrer. Within about 10 seconds, the solution temperature increased to 140 F. The solution temperature decreased to 136 F. in six minutes due to heat losses through the uninsulated beaker wall. The heater in the beaker was turned on and off to compensate for the heat losses thereby maintaining the solution at 140 F. About 15 minutes after the sulfuric acid addition, the solution was cooled to 77 F. by turning off the heater and running cold tap water through the cooling coil. The cooling took a total of four minutes.

The final solution was analyzed and found to contain 0.86 m/l of peroxydisulfate, 0.96 m/l of peroxymonosulfate and 0.02 m/l of hydrogen peroxide. The conversion from peroxydisulfate to peroxymonosulfate was calculated to be 52.2%.

Rearrangement of aliphatic primary, secondary, and tertiary alkyl hydroperoxides in strong acid, Norman C. Deno, Wilbur E. Billups, Kenneth E. Kramer, and Robert R. Lastomirsky Journal of Organic Chemistry, 35, 3080-3082 (1970)
Solutions of K2S2O8 (effectively H2S05) in 20-60% H2SO4 are advantageous for the Baeyer-Villiger oxidation of ketones. Yields are quantitative and differences in migratory aptitudes are as large as or larger than those found with other peracids.
. . . .
Baeyer-Villiger Oxidation of Ketones.-A review of the Baeyer-Villiger reaction pointed out that only one simple aliphatic ketone had been reported and that other peracids were preferable to H2SO5. It was thus of some interest to find that solutions of K2S2O8 in 50% H2SO4 gave quantitative yields of the Baeyer-Villiger products for a variety of simple aliphatic ketones. The reactions were complete in minutes at 25°, and we cannot understand why this extremely simple procedure has not been utilized. Although K2S2O8 was used to generate H2SO5, addition of H2O2 would undoubtedly have led to identical results.