Typical Reaction Mechanisms

Pyrolysis of alkanes is referred to as cracking. Alkanes from the paraffins (kerosene) fraction in the vapor state are passed through a metal chamber heated to 400-700°C. Metallic oxides are used as a catalyst. The starting alkanes are broken down into a mixture of smaller alkanes, alkenes, and some hydrogen. 

The reaction proceeds via a free radical mechanism with homolytic fission occuring between the carbon-carbon atoms. The mechanism of reactions for the cracking of propane is  

Vanadium pentoxide, V205, is used as a catalyst in the oxidation of sulfur dioxide. The mechanism involves oxidation-reduction of V205 that exists on the support at operating conditions in the molten state. The mechanism of reaction is  

Studies of ammonia synthesis on iron catalyst suggest that the reaction occurs through surface imine radicals. 

The overall stoichiometric reaction for the oxidation of ammonia to nitric oxide is  

---

Fig. 17 The two typical reaction mechanisms employed by phosphatases. In (a), the phos-phoryl group is transferred directly to a water molecule, which is typically bound to one or two metal ions if the substrate is made chiral at phosphorus, the stereochemical outcome is inversion. In (b), the phosphoryl group is first transferred to an enzymatic residue. In a subsequent step the phosphoenzyme intermediate is hydrolyzed. Since each step occurs with inversion of configuration at phosphorus, the net outcome is retention. Pi inorganic phosphate.

In catalytic cracking many reactions take place simultaneously. Cracking occurs by C-C bond cleavage of paraffins, dealkylation etc. Isomerization and even condensation reactions take place. These reactions occur via positively charged hydrocarbon ions (carbocations). The nature of the carbocations is the subject of debate. For the cracking of paraffinic hydrocarbons it is usually assumed that carbenium ions are the crucial intermediates, which decompose via beta fission into olefins and (smaller) carbenium ions (see Chapter 4, Section 4.4). A typical reaction mechanism for catalytic cracking (and hydrocracking) imder the relatively mild conditions used in FCC is shown overleaf. 

A typical reaction mechanism is shown in Figure 2.17, where one can see the doubling of the profile due to the HS and LS states. The reaction pathway involves three phases (a) a C-H abstraction phase that leads to an alkyl radical coordinated to the iron-hydroxo complex by a weak OH—C hydrogen bond, labeled as C,. 

The typical reaction mechanism for tri-liquid PTC in a batch reactor under agitation is illustrated in the schematic diagram of Fig. 9. Three types of reaction scheme considering the partition of the catalyst in the different phases and the place where the inherent reaction occurred have been proposed [226,227]. For the substitution reaction of alkyl halide (RX) and aqueous reactant metal salt (MY) using quaternary ammonium salt (QX) as the catalyst, the different types of reaction are addressed as follows [226]. 

The reaction mechanism depends on the system and may be based on energy or electron transfer between the naphthalene moieties of the copolymer and the substrate molecule. In the case of oxidations, singlet oxygen, generated by energy transfer from the naphthalene moiety to 02, may be involved. Typical reaction mechanisms are presented in Schemes 14.3 and 14.4. 

For the first time, in 2008, Gokhale et al. [34] proposed the concept of carboxylate mechanism. They did several density functional theory (DFT) calculations to investigate the mechanism of WGSR over Cu(l 11). The typical mechanism involves the oxidation of CO by OH to form carboxyl (COOH) species. The carboxyl so formed may then yield CO2. This mechanism is very similar to the redox mechanism. The typical reaction mechanism is as follows ... 

Aldehydes and ketones react with halogens at the a-position via the enol or enolate, depending on the solution pH (Figure 17.14). For the acid-catalyzed reaction, the acid is usually HBr or HOAc, and a typical reaction mechanism is shown in Figure 17.15. In acid, the enol reacts with bromine in base, the enolate is the reacting species. The enolate, as you might expect (remember that the phenolate anion is a better nucleophile than neutral phenol), is the better nucleophile. However, irrespective of pH, the rate of bromination is not dependent on the concentration of molecular bromine. The RDS is enolization, and subsequent reaction of the enol/enolate with bromine is fast. 

---