Comment on “Activation of methane to CH ₃ ⁺ : A selective industrial route to methanesulfonic acid”

Abstract

Díaz-Urrutia and Ott (Reports, 22 March 2019, p. 1326) report a selective conversion of methane to methanesulfonic acid that is proposed to occur by a cationic chain reaction in which CH3+ adds to sulfur trioxide (SO3) to form CH3–S(O)2O+. This mechanism is not plausible because of the solvent reactivity of CH3+, the non-nucleophilicity of the sulfur atom of SO3, and the high energy of CH3–S(O)2O+.

Synthesis of Methanesulfonic Acid Directly from Methane: The Cation Mechanism or the Radical Mechanism?

“…First, we studied the cation chain process proposed by Diaz-Urrutia and Ott. 39 Similar to the results of Roytman and Singleton, 41 the cationic [H 3 C−SO 3 ] + (1) is confirmed to be a high-energy intermediate, with ΔG = 19 kcal/mol above the starting CH 3 + /SO 3 species. This is expected, because the S atom is not the nucleophilic site of the SO 3 molecule, according to the Lewis structure and our MO analysis (Figure 2).…”

“…This is expected, because the S atom is not the nucleophilic site of the SO 3 molecule, according to the Lewis structure and our MO analysis (Figure 2). On the contrary, a much lower-lying isomer [H 3 C−OSO 2 ] + is located, in which the carbon atom in the CH 41 Since Roytman and Singleton suggested a radical-catalyzed mechanism for the CH 4 + SO 3 reaction, 41 we turned to this radical-catalyzed mechanism, in which no proton H + participates. As shown in Figure 3 kcal/mol.…”

“…Figure 3. Radical-catalyzed mechanism for the CH 4 + SO 3 reaction, based on the suggestions of Roytman and Singleton 41. …”

“…However, Roytman and Singleton forcefully questioned this cationic chain reaction mechanism . Initially, they pointed out that the CH 3 + cation was too reactive with the solvent to be an intermediate in a chain reaction.…”

“…This high barrier is because the sulfur atom of SO 3 is not nucleophilic for connecting CH 3 + . Thus, this cation chain process was precluded by Singleton et al Although Singleton et al stated that the actual mechanism of this transformation remains unknown, they opt for a radical mechanism for the CH 3 ·/SO 3 reaction, which is supposed to occur at approximately a diffusion-controlled rate, potentially complicating the experimental observations. In 2020, Diaz-Urrutia and Ott responded to Roytman and Singleton’s comments.…”