Garin-Gault mechanism

Galvanostatic current-pulse method, 38 29, 33 Galvanostatic technique hydrocarbon adsorption, 30 256 surface coverage, 30 299-300 Gamma emission, 26 124, 125, 127 Garin-Gault mechanism, 30 20, 23 Gas chromatography -atomic emission detection... 

Fig. 2. Mechanisms of bond-shift isomerization on metal catalysts, (a) Anderson-Avery mechanism (63). (b) Muller-Gault mechanism for isomerization of neopentane on Pt (64). (c) McKervey-Rooney-Samman mechanism (69). (d) Muller-Gault mechanism of isomerization of isobutane on Pd (64). (e) Clarke-Rooney mechanism (46). (f) Garin-Gault mechanism (65).

Of the four basic mechanisms, two, the Anderson-Avery mechanism (Scheme 19) and the Garin-Gault mechanism (Scheme 25), should be associated with isomerization and hydrogenolysis two, the Muller-Gault mechanism (Scheme 20) and the Rooney-Samman or the Clarke-Rooney mechanism (Schemes 22 and 28) should promote only isomerization. 

One involving an o,a,P triadsorbed precursor, Anderson-Avery mechanism [5], and the Garin-Gault mechanism involving a metaUacyclobutane intermediate [26], Scheme 17.1. 

The mechanism of bond-shift isomerization of alkanes is still a subject of controversy. Several suggested mechanisms involved an intermediacy of multiply bonded species [Anderson-Avery (63), Muller-Gault (64), and Garin-Gault (65) mechanisms] (Fig. 2). More recent data (51, 52, 66-68) seem to support the 1,2 bond-shift isomerization involving monoadsorbed alkyl species (69). Figure 2 shows these mechanistic ideas. 

Garin and Gault 82) reported a hydrogen order of - 3.4 for position isomerization of n-pentane over alumina-supported platinum between 240° and 300°C. They concluded therefore that a surface species that lost 3-4 hydrogen atoms would cyclize. It is not clear to what extent this discrepancy (cf. 77 and 82) may be attributed to different experimental conditions, to different catalysts, or actually to different mechanisms that is, C5 cyclization with participation of unsaturated intermediates. 

Anderson that at this level the mechanistic details are a matter of opinion (7). There is, however, a difference as far as the number of surface atoms participating in the reaction is concerned. Mechanism A requires more than one Mechanism C, however, requires only one metal atom. Van Schaik et al. 89) reported skeletal isomerization according to Mechanism A over platinum-rich platinum-gold alloys, whereas over gold-rich catalysts, isolated platinum atoms could promote Mechanism C only. Garin and Gault (82) assumed the formation of a C4 cyclic intermediate with the insertion of a platinum atom as the fourth member of the ring. This concept of Mechanism B would also involve one metal atom. 

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