7.2 Superacid Catalysts for the Activation of CH 4 …
167
ratio of [BH
+ ]/[B] (i.e., the ionization ratio) is typically measured using spectroscopic
means. The H 0 value is equal to the pK BH value of BH
+ when half the B in the solution
is protonated, i.e., [B] = [BH
+ ].
Lower values of H 0 correspond to greater acid strength. Thus, the acidity function
H 0 describes the acidity of a solution, meaning that H 0 is not a property of individual
molecules, such as H 2 SO 4 . Obviously, H 0 values are solvent-dependent. The Hammett acidity function is a logarithmic scale. For example, 100% H 2 SO 4 has an H 0
of −11.9 and anhydrous HF has an H 0 of −11.0; subsequently, all Brønsted acids
having an H 0 < −12 are considered to be superacids. The H 0 values of some typical
acid solutions are as follows: 100% H 2 SO 4 : −11.94, CF 3 SO 3 H: −14.1, HSO 3 F: −
15.07, SbF 5 (10 mol%)–HSO 3 F: −18.54, HSO 3 F–SbF 5 (1:1): −23, HF–SbF 5 : −28.
7.2.2 Definitions of Carbocations as Key Reaction
Intermediates
In the superacid-catalyzed conversion of hydrocarbons including CH 4 , carbocations
are typically reaction key intermediates. Carbocations can be generated via several
different routes using homogeneous and heterogeneous catalysts. Olah proposed
that the concept of carbocations should encompass all cations of carbon-containing
compounds [9–11]. The carbocations in superacid-catalyzed reactions can be
differentiated into two distinct classes: carbenium ions and carbonium ions.
(i) Carbenium ions: Trivalent carbenium ions, which are referred to as “classical
carbenium ions,” have an sp
2 -hybridized electron-deficient carbon atom. The
carbenium carbon has six valence electrons and thus is highly electron deficient.
Their structure is adequately described using only two-electron, two-center
bonds. The carbon atom easily becomes planar in the absence of constraints
arising from skeletal rigidity or steric interference.
(ii) Carbonium ions: Penta-coordinate carbonium ions (“non-classical carbonium
ions”) have a five-coordinate (or higher) carbon atom bonded by three single
bonds and a two-electron three-center bond.
+ CH 5 carbonium ions are the parent
ions of carbonium ions. The definition mentioned above can apply to higher
molecular weight carbonium ions that contain a higher than five coordinate
carbon atom. The structure of penta-coordinated or higher molecular weight
carbonium ions cannot be expressed using only a two-electron, two-center single
bond. These “carbonium ions” are electron deficient because of the sharing of
two electrons between three atoms.
As discussed in Chapter 1, in addition to •CH 3 and carbene-like species (CH x ),
+ CH 3 is also an important reaction intermediate for the C–C bond formation in
methane conversion to produce higher hydrocarbons. Throughout this chapter, trivalent carbenium ions and penta-coordinated carbonium ions are referred to simply as
carbenium ions such as
+ CH 3 and carbonium ions such as
+ CH 5 , respectively.
167
ratio of [BH
+ ]/[B] (i.e., the ionization ratio) is typically measured using spectroscopic
means. The H 0 value is equal to the pK BH value of BH
+ when half the B in the solution
is protonated, i.e., [B] = [BH
+ ].
Lower values of H 0 correspond to greater acid strength. Thus, the acidity function
H 0 describes the acidity of a solution, meaning that H 0 is not a property of individual
molecules, such as H 2 SO 4 . Obviously, H 0 values are solvent-dependent. The Hammett acidity function is a logarithmic scale. For example, 100% H 2 SO 4 has an H 0
of −11.9 and anhydrous HF has an H 0 of −11.0; subsequently, all Brønsted acids
having an H 0 < −12 are considered to be superacids. The H 0 values of some typical
acid solutions are as follows: 100% H 2 SO 4 : −11.94, CF 3 SO 3 H: −14.1, HSO 3 F: −
15.07, SbF 5 (10 mol%)–HSO 3 F: −18.54, HSO 3 F–SbF 5 (1:1): −23, HF–SbF 5 : −28.
7.2.2 Definitions of Carbocations as Key Reaction
Intermediates
In the superacid-catalyzed conversion of hydrocarbons including CH 4 , carbocations
are typically reaction key intermediates. Carbocations can be generated via several
different routes using homogeneous and heterogeneous catalysts. Olah proposed
that the concept of carbocations should encompass all cations of carbon-containing
compounds [9–11]. The carbocations in superacid-catalyzed reactions can be
differentiated into two distinct classes: carbenium ions and carbonium ions.
(i) Carbenium ions: Trivalent carbenium ions, which are referred to as “classical
carbenium ions,” have an sp
2 -hybridized electron-deficient carbon atom. The
carbenium carbon has six valence electrons and thus is highly electron deficient.
Their structure is adequately described using only two-electron, two-center
bonds. The carbon atom easily becomes planar in the absence of constraints
arising from skeletal rigidity or steric interference.
(ii) Carbonium ions: Penta-coordinate carbonium ions (“non-classical carbonium
ions”) have a five-coordinate (or higher) carbon atom bonded by three single
bonds and a two-electron three-center bond.
+ CH 5 carbonium ions are the parent
ions of carbonium ions. The definition mentioned above can apply to higher
molecular weight carbonium ions that contain a higher than five coordinate
carbon atom. The structure of penta-coordinated or higher molecular weight
carbonium ions cannot be expressed using only a two-electron, two-center single
bond. These “carbonium ions” are electron deficient because of the sharing of
two electrons between three atoms.
As discussed in Chapter 1, in addition to •CH 3 and carbene-like species (CH x ),
+ CH 3 is also an important reaction intermediate for the C–C bond formation in
methane conversion to produce higher hydrocarbons. Throughout this chapter, trivalent carbenium ions and penta-coordinated carbonium ions are referred to simply as
carbenium ions such as
+ CH 3 and carbonium ions such as
+ CH 5 , respectively.