Hydride Bronsted acidity

The Bronsted acidity of transition metal hydrides. R. G. Pearson and P. C. Ford. Comments Inorg. Chem., 1982, 1, 279-291 (33). 

Thus the reactant ions for chemical ionization formed in the methane plasma consists of approximately equal amounts of a strong gaseous Bronsted acid (CH5+) and ions which can act either as Lewis acids or Bronsted acids (C2H5+ + C3H5+). These reactant ions will effect the chemical ionization with an added substance by proton transfer or hydride ion transfer, both of which may be accompanied by fragmentation of the ion initially formed. 

Several reaction pathways for the cracking reaction are discussed in the literature. The commonly accepted mechanisms involve carbocations as intermediates. Reactions probably occur in catalytic cracking are visualized in Figure 4.14 [17,18], In a first step, carbocations are formed by interaction with acid sites in the zeolite. Carbenium ions may form by interaction of a paraffin molecule with a Lewis acid site abstracting a hydride ion from the alkane molecule (1), while carbo-nium ions form by direct protonation of paraffin molecules on Bronsted acid sites (2). A carbonium ion then either may eliminate a H2 molecule (3) or it cracks, releases a short-chain alkane and remains as a carbenium ion (4). The carbenium ion then gets either deprotonated and released as an olefin (5,9) or it isomerizes via a hydride (6) or methyl shift (7) to form more stable isomers. A hydride transfer from a second alkane molecule may then result in a branched alkane chain (8). The... 

Hirschler and Hudson (36/6), however, favor the opinion that Bronsted sites are exclusively responsible for the activity of silica-alumina. In studying the adsorption of perylene and of triphenylmethane, they concluded that carbonium ions are not formed by a hydride abstraction mechanism as claimed by Leftin (362). Instead, triphenylmethane is oxidized by chemisorbed oxygen to triphenylcarbinol in a photo-catalyzed reaction, followed by reaction with a Bronsted acid giving water and a triphenylmethyl carbonium ion. After treatment with anhydrous ammonia, the organic compound was recovered by extraction as triphenylcarbinol. About thirteen molecules of ammonia per assumed Lewis site were required to poison the chemisorption of trityl ions. The authors explain the selective inhibition of certain catalyzed reactions by alkali by assuming that only certain of the acidic protons will ion-exchange with alkali ions. 

Initial methane formation from methanol on the fresh catalyst is proposed to proceed on Bronsted acid sites as a reaction with a hydride donor - in... 

Catalytic and Electron Transfer Properties. The isomerization of cyclopropane on HY zeolites activated at temperatures less than 600° C is attributed to catalysis by Bronsted acid sites (12, 13), and the activation temperature for maximum activity was in the range 300°-400°C (13). On the other hand, rearrangement of protoadamantane to adamantane proceeds by hydride ion abstraction at Lewis acid sites (lfy. Materials B, therefore, appear to have good Bronsted activity (Figure 5) and in view... 

Nonmetals form covalent molecular hydrides, which consist of discrete molecules. These compounds are volatile and many are Bronsted acids. Many are gases, for example, ammonia, the hydrogen halides (HF, HC1, HBr, HI), and the lighter hydrocarbons such as methane, ethane, ethene, and ethyne. Liquid compounds include water and hydrocarbons such as octane and benzene. 

In those instances where (I) (and similar catalysts that require the presence of a neutral Lewis acid or a cationic Lewis or Bronsted acid) does not contain an alkyl or hydride group already bonded to the metal (i.e. neither Q or S is an alkyl or hydride), the neutral Lewis acid or a cationic Lewis or Bronsted acid also alkylates or adds a hydride to the metal, i.e. causes an alkyl group or hydride to become bonded to the metal atom. 

On a molar basis, most organic compounds contain similar amounts of hydrogen and carbon, and processes involving transfer of hydrogen between covalently bound sites rank in importance in organic chemistry second only to those involving the carbon-carbon bond itself. Most commonly, hydrogen is transferred as a proton between atoms with available electron pairs (l), i.e. Bronsted acid/base reactions. The alternative closed shell process, hydride transfer or shift, involves motion of a proton with a pair of electrons between electron deficient sites (2). These processes have four and two electrons respectively to distribute over the three atomic centres in their transition structures. It is the latter process, particularly when the heavy atoms are both first row elements, which is the subject of this review. The terms transfer and shift are used here only to differentiate intermolecu-lar and intramolecular reactions. 

Common superacids in use are the Bronsted acid FSO3H and the Lewis acid SbFj dissolved in SO ClF or mixtures of SOjClF and SO Fj. To be able to study liquid ionic solutions at very low temperatures (ca —160°C), e.g. by nmr spectroscopy, freons like CHCljF may be added to the solution to keep the viscosity at a tolerable level. Superacids can be up to a billion times stronger acids than sulphuric acid. Carbocations are generated in the reactions of e.g. alcohols and olefins with FSO3H and of chlorides with e.g. SbFj or by hydride abstraction. The superacid chemistry has been treated in a number of reviews (e.g. Olah, 1979). A general survey of the chemistry of superacids is given by Olah et al. (1979b). Other reviews have appeared... 

Bronsted acid catalysis in electron transfer described in Section 1.3.1 has also been effective for redox reactions via the electron transfer step. As shown in the case of metal ion-catalyzed hydride transfer reactions (see above), hydride transfer reactions from an NADH analogue to /7-benzoquinones also proceed via Bronsted acid-catalyzed electron transfer [255, 256]. Since NADH and ordinary NADH model compounds are subjected to the acid-catalyzed hydration [98, 257, 258], an acid-stable NADH model compound, 10-methyl-9,10-dihydroacridine (AcrH2), was used as a hydride donor to / -benzoquinone (Eq. 24) ... 

The kinetic deuterium isotope effects (/th/Zcd) are observed in the Bronsted acid-catalyzed hydride transfer reactions from AcrH2 to the hydride acceptors (A) [259]. The pH dependence of the observed kinetic deuterium isotope effects kn/ko) in-... 

The ions CHs and C2H5 are strongly acidic and react with the sample molecule chiefly by proton transfer (as Bronsted acids) or by hydride abstraction (as Lewis acids) stable addition products can also be formed ... 

---