Mechanism for Synthesis of Tetranitromethane from Acetic Anhydride and Fuming Nitric Acid
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.