A comprehensive view of acid catalysis mechanisms: From carbenium over. The “pentonium mechanism” for superacids will be demonstrated in detail for + + to carbonium to supercarbonium as reactive intermediates isomerization involving 6e5c , and alkane disproportionation with 4e5c carbocations. Use of the DH–SiS theory [5] to predict acid strength will be demonstrated for “2D electrolyte solutions” [3] on solid surfaces of working strong acid catalysts. Dan Fraenkel* Eltron Research & Development LLC, 4600 Nautilus Court South, Increased Acid Strength Boulder, CO 80301 (U.S.A.) *[email protected]. Ho -5 -10 -15 Introduction In the absence of unsaturated molecules or intermediate species, low-temperature Superacid skeletal transformations of saturated hydrocarbons through methyl–hydride shifts appear to be catalyzed only by superacids. Such transformations have long been known with Friedel-Crafts systems, e.g., AlCl ·HCl [1] and with fluoro-acids (e.g., ) [2], and more recently, 3 + also with the non-halogen solid acid sulfated zirconia [3] and related systems. Alkane + isomerization was proposed [4] to involve “protonated cyclopropane rings,” but such + + hypothetical species appear to lack logical bonding concepts, have high bond strain, and cannot be expected to form at low temperature. Dimer carbenium intermediates proposed for alkane disproportionation [1] should lead to C2n and to alkenes; but none are formed. + + Cyclopropane-H and C2n species are avoided in a new mechanistic route proposed for superacid catalysis. This route joins those of weaker acids, and is compatible with them carbynium carbenium carbonium “Supercarbonium” through a new intermediate species formalism that is also proposed. (carbunium?)

Integrated Acid Catalysis Mechanism 4e3c 0e1c 2e3c 4e5c Acid strength increase is associated with the size enlargement of the conjugate base (acid anion) and the consequent larger spread between the size of the positively charged proton trienium mononium trionium pentonium and that of the negatively charged anion. According to the DH–SiS theory of strong electrolytes [5], this is causing higher electrostatic repulsion hence higher chemical potential of M-ion T-ion P-ion the proton. In attempt to relieve this repulsion, the proton “inflates” itself through associating with molecular species having even weak electron density. I propose that as the acid becomes Scheme 1. Reactive intermediates in acid-catalyzed reactions, their hierarchy, formalism and stronger, carbocations grow in both size and dimension from the small 1D carbenium ion, relation to acid strength. (Internal lines are C–C bonds; terminal lines, C–C or C–H bonds; through the medium-size 2D , to a large, hyperactive, short-lived, five-center 3D dots, C or H .) New notations are proposed in addition to the existing ones (underlined). [3]. This new hierarchy is presented in Scheme 1, and is placed along the Hammett + acidity scale (H0). In the same way as a σ-bond can be attacked by a carbenium ion (1c ) to Significance form a carbonium ion (3c+) [2], it can also be attacked by a carbonium ion if this ion is The new comprehensive mechanistic picture and formalism as proposed here may stabilized in strong superacidic media and becomes sufficiently long-lived and “strong” for help understand better the fundamentals of acid catalysis, thus helping in the design of such an attack. The resultant superacidic 5c+ carbocation (that I dub “pentonium” vs. the lower superacid catalysts and processes, e.g., for increasing octane rating of refinery alkane streams. rank “trionium” and “mononium,” or P-ion vs. T-ion and M-ion, respectively; Scheme 1) is a penta-coordinate cationic “complex” lacking a central ; but like known penta-coordinate References complexes, e.g., PF5, Fe(CO)5, it undergoes rapid pseudorotation, thus scrambling its member 1. Pines, H., “The Chemistry of Catalytic Hydrocarbon Conversions,” Academic Press, atoms. Cleaving and creating bonds occur through equilibria with carbonium ions and σ-bonds, 1981. which I call P-ion-to-T-ion transitions (or simply, P↔T transitions). In contrast, non- 2. Olah, G.A., J. Org. Chem. 70, 2413 (2005). 3. Fraenkel, D., Jentzsch, N.R., Starr, C.A., and Nikrad, P.V., J. Catal. 247, 29 (2010). superacidic strong-acid reactions occur via T↔M transitions (at H0 ~ -8 to -12). Carbocation speciation in the entire acid strength range is believed to result from an M↔T↔P type 4. Brouwer, D.M. and Hogeveen, H. in “Progress in Physical Organic Chemistry” (A. dynamic equilibrium that shifts to the left (Scheme 1) for lower acid strength, where carbenium Streitwieser, Jr. and R.W. Taft, Eds.), Vol. 9, p. 179. Wiley-Interscience, N.Y., 1972. ions dominate, and to the right for higher acid strength, where pentonium ions eventually take 5. Fraenkel, D., J. Chem. Theory Comput. 11, 178 (2015) and references therein.