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In my undergraduate Mechanisms-class the formation of tetranitromethane from acetic Anhydride with fuming $\ce{HNO3}$ was listed as optional knowledge in a chapter about the reactions of enols, enamines, etc. I can't find it in my notes or in the recommended literature, neither can I find information on molbase, and the two papers I can find, the original 1910 synthesis and This, are not mentioning any reaction mechanism.

I guess it somehow decomposes at the stage when the acetic anhydride is tri-substituted on one side, and somewhere there is a trinitromethane-anion involved which forms the product with $\ce{NO2+}$?

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    $\begingroup$ Yeah, probably something like that. $\endgroup$ – Mithoron Aug 4 at 19:48
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The Organic Synthesis preparation you cited for the preparation of tetranitromethane has some clues that are useful in providing a viable mechanism. Upon slowly adding acetic anhydride (Ac2O) to anhydrous HNO3 with cooling an exotherm is produced. The reaction mixture is allowed to sit at room temperature for 7 days. Assuming that they investigators did not go on vacation for a week, I presume that the formation of tetranitromethane (TNM) is slow and that the exotherm is due to the formation of the mixed anhydride 2. [The nitronium cation is also a possible electrophile as well as the mixed anhydride 2.] The enol of acetic anhydride can undergo substitution to afford species 3 and then repetitively on to the trinitro mixed anhydride 4. Anhydride exchange with acetic acid (or nitric acid) affords trinitroacetic acid 5 which decarboxylates to enol 6 (aci-form). Tautomerization of 6 can form trinitromethane. Footnote 3 in the Organic Synthesis link mentions that trinitromethane 7 can be converted to tetranitromethane 8 with HNO3. The transformation of 6 to 8 does not preclude nitration via mixed anhydride 2.

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