Hydrogenation sextet mechanism

For the cases where gas phase cyclohexenes do not appear to be intermediates, the question arises as to the nature of the surface reaction. Thus, does cyclohexane simultaneously lose six hydrogen atoms via the sextet mechanism (T3) originally proposed by Balandin in 1929, or does the reaction take place in a stepwise fashion without desorption of intermediate products According to the sextet theory, the active catalyst unit is an aggregate of metal atoms which must be spaced within certain definite limits consistent with the geometry of the cyclohexane ring. While there... 

The sextet-doublet model was adapted for 1-5 dehydrocyclization, the reverse of cyclopentane hydrogenolysis, and it was proposed that physically adsorbed alkane reacts with chemisorbed hydrogen according to a push-pull mechanism (772) (Scheme 55). 

The sextet is due to the hyperfine interaction between the unpaired electron and two a- and two P-hydrogen nuclei. The shape of the ESR spectra of the scission radicals depends on the conformational structure of the chain end site and on the molecular motion as described in detail in a later section. The spectrum obviously changes with increased temperature and this fact indicates that the majority of the scission radicals are converted to peroxy radicals by heat treatments when the initial mechanical treatment is carried out in liquid nitrogen containing a small amount of... 

Remarkably, the above mechanisms are not incompatible with the bewildering experimental observation that one and the same polymorph is invariably obtained for a simple nearly spherical molecule like benzene and that a consistent aggregation obtains for such a complicated molecule as Dianin s compound, whose crystal structure (Fig. 15.4) is a delicate Venetian lace involving a cyclic sextet of O-H- O hydrogen bonds and a hourglass-shaped cage, with 18 symmetry-related molecules in the unit cell [5]. The structure of the benzene crystal is not the outcome of the most favorable molecule-molecule contact, but of the one and only favorable path toward symmetry periodicization. The complexity of the shape and potential of the Dianin molecule must be a source of structural information rather than of structural randomization. 

---