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 mechanism is also confirmed by the following new facts. 

On this basis, Taylor denied the sextet mechanism at first (223), but then he.recognized it, and in his paper in the book Chemical Architecture (224) he gives the scheme of this mechanism. The review by Trap-nell (225) is devoted to the multiplet theory. His work (225) gives a rather true and detailed account of the structural aspect of the multiplet theory. Trapnell does not touch upon the energetic aspect of the theory. However, at the same time, he is not right in stating that the theory supposes van der Waals adsorption. The alternation of the sextet model, suggested by him, was considered by the author as early as 1929... 

In this respect the sextet mechanism may be tested from another... 

Cyclohexane dehydrogenation represents another classical example for isotopic studies. Balandin s sextet mechanism predicted direct dehydrogenation of cyclohexane over several metals, assuming a planar reactive chemisorption of the reactant. Cyclohexene is also readily dehydrogenated to benzene. The use of hydrocarbons labelled with established the true reaction pathway. T6tenyi and co-workers[ °di] reacted a mixture of [ 0]-cyclohexane and inactive cyclohexene on different metals and measured the specific radioactivity of the fractions (cyclohexane, cH, cyclohexene, cH= and benzene, Bz) in the product at low conversion values (Table The... 

Although with oxide catalysts the sextet mechanism does not apply, in the case of metals, while some of the evidence cited for the mechanism is incorrect, there is still a great deal in favor. Only two facts oppose ... 

The following mechanism appears reasonable (compare Section VI, 12), It assumes that the function of the aluminium ieri.-butoxide, or other alkoxide. is to provide a source of aluminium ions and that the aluminium salt of the secondary alcohol is the actual reactant. Aluminium with its sextet of electrons has a pronounced tendency to accept a pair of electrons, thus facilitating the initial coordination and the subsequent transfer of a hydride ion ... 

A final statement on the mechanism of the diaziridine formation cannot yet be made. The obvious formulation [Eq. (36) ] as a reaction of the CN double bond with the imen 39 (with an electron sextet) is almost certainly excluded. The formation of 39 as an intermediate has been jiroposed for the Raschig hydrazine synthesis, but has been disjmted. The following facts are against a diaziridine formation corresjionding to Eq. (36) ... 

Carbocations can stabilize themselves in various ways (see p. 227), but for this type of ion the most likely way is by loss of either or . The aromatic sextet is then restored, and in fact this is the second step of the mechanism ... 

The mechanism is generally regarded as involving formation of a carbene. It is the divalent carbon that has the open sextet and to which the migrating group brings its electron pair ... 

For reason of symmetry conservation, a thermal concerted [2-1-2]-cycloaddition is forbidden as far as D2h symmetry can be assumed for the activated complex. This is not necessarily the case for the concerted dimerization of iminoboranes, in which the symmetry requirements seem to be essentially lowered. Nevertheless, a two-step mechanism according to Eq. (21) must be taken into account. The assumed intermediate in Eq. (21) contains a sextet boron atom with a linear... 

Species such as 5 and 6 are called benzynes (sometimes dehydrobenzenes), or more generally, arynes, and the mechanism is known as the benzyne mechanism. Benzynes are very reactive. Neither benzyne nor any other aryne has yet been isolated under ordinary conditions,34 but benzyne has been isolated in an argon matrix at 8 K,35 where its ir spectrum could be observed. In addition, benzynes can be trapped e.g., they undergo the Diels-Alder reaction (see 5-47). It should be noted that the extra pair of electrons does not affect the aromaticity. The original sextet still functions as a closed ring, and the two additional electrons are merely located in a tt orbital that covers only two carbons. Benzynes do not have a formal triple bond, since two canonical forms (A and B) contribute to the hybrid. 

Paraffins with more than eight carbon atoms can dehydrocyclize to form bicyclic products. According to Shuikin and Bekauri, bicyclic products can be formed from paraffins by either successive dehydrocyclization or by simultaneous closure of several carbon-carbon bonds (35). The second possibility follows Balandin s sextet model (56). A large number of hydrocarbons follow the consecutive mechanism (27). Thus far there is no evidence for simultaneous closure. 

The relation observed between regioselectivity and number of Clar sextets of reactants and products is not unexpected for a reaction that is controlled by thermodynamics i.e. by the difference between free enthalpy of formation of reactants and products. Conversely, however, no conclusions regarding the mechanism of the reaction can be derived from the relation. 

Fig. 14.18. Mechanism-based diastereoselectivity in the semipinacol rearrangement of an epoxide. This rearrangement is stereoselective, since there is only one H atom in the position next to the sextet center and the H atom undergoes the [l,2]-migration on the same face of the five-mem bered ring.

Figure 15.20 shows the multistep mechanism of the [4+2]-cycloaddition between 1-(dimethylamino)-l,3-butadiene and cis-dicyanoethenedicarboxylic acid diester. The reaction proceeds via an intermediate, which must be zwitterion conformer B. The anionic moiety of this zwitterion is well stabilized because it represents the conjugate base of a carbon-acidic compound (Section 13.1.2). The cationic moiety of zwitterion B also is well stabilized. It is an iminium ion (i.e., a species with valence electron octet) rather than a carbenium ion (which is a species with valence electron sextet). Moreover, the iminium ion is stabilized by conjugation to a C=C double bond. 

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