Alkene theory

This theory was proposed by Knox [39] following a series of careful kinetic and analytical studies of the oxidation of ethane [40], propane [41], and isobutane [42] in the early stages of reaction ( 1% fuel consumption). For these lower alkanes, he found that at this stage of the reaction 75—80 % of the alkane consumed appeared in the products as the conjugate alkene both at low temperature (ca. 300 °C) and at high temperature (ca. 450 °C). He concluded, therefore, that the primary oxidation process for alkyl radicals is the same at both temperatures, viz. 

As the fuel consumption increased, the alkene concentration eventually reached a kinetically controlled stationary value [43]. Since the major product at all stages in the oxidation of isobutene is acetone [42] (Fig, 4), it was possible to follow the consumption of isobutene during the oxidation of isobutane by observing the formation of acetone. It can be seen from Fig. 5 that a marked increase in the yield of acetone in the later 

Knox further concluded, therefore, that the conjugate alkenes must also play an important role in the intermediate and later stages of alkane oxidation. 

The 20 % minor products were a complex mixture of oxygenates and propene, the composition of which varied with the nature of the reaction vessel surface as shown in Fig. 6. In contrast, however, the ratio of major/minor products was little affected by surface conditions. It was 

The alkylperoxy radicals then form an unknown intermediate Z which is sufficiently stable to diffuse to the walls where it decomposes to yield the minor products [44] 

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More recently, Lucquin and co-workers [63, 64] have shown from studies of the oxidation of n-butane and isobutane that the alkene theory is in fact at variance with experiment. Thus, on this theory the negative temperature coefficient is seen as a direct consequence of the increasing instability with temperature of the hydroperoxyalkyl radical, viz. 

Whilst this is acceptable kinetically, it fails to explain the analytical observations. Thus, carbonyl compounds and consequently carbon oxides are necessarily formed in the brEmching reaction (11), but the yields of these compounds increase in the negative temperature region whore the branching is suppressed. Furthermore, little further reaction of the intermediate conjugate alkene occurs, hence the carbonyl compounds must be formed by a different route from that proposed in the alkene theory under these conditions. 

In contrast to the alkene theory the predominant mode of oxidation of the alkyl radicals is by oxygen addition and the alkylperoxy radical so formed then undergoes homogeneous intramolecular rearrangement (reaction (14)). Decomposition of the rearranged radical (reaction (16)) usually leads to a hydroxyl radical and stable products which include O-heterocycles, carbonyl compounds and alcohols with rearranged carbon skeletons relative to the fuel and alkenes. The chain-cycle is then completed by unselective attack on the fuel by the hydroxyl radical (reaction (12)). 

On the basis of the alkene theory the hydroperoxalkyl radical initially formed must necessarily be the a-hydroperoxyalkyl radical, e.g. for the oxidation of n-butane... 

It has been known for more than a century that hydrocarbons containing double bonds are more reactive than their counterparts that do not contain double bonds. Alkenes are, in general, more reactive than alkanes. We call electrons in double bonds 71 electrons and those in the much less reactive C—C or CH bonds Huckel theory, we assume that the chemistry of unsaturated hydrocarbons is so dominated by the chemistry of their double bonds that we may separate the Schroedinger equation yet again, into an equation for potential energy. We now have an equation of the same fomi as Eq. (6-8), but one in which the Hamiltonian for all elections is replaced by the Hamiltonian for Ji electrons only... 

Catalytic hydrogenation is mostly used to convert C—C triple bonds into C C double bonds and alkenes into alkanes or to replace allylic or benzylic hetero atoms by hydrogen (H. Kropf, 1980). Simple theory postulates cis- or syn-addition of hydrogen to the C—C triple or double bond with heterogeneous (R. L. Augustine, 1965, 1968, 1976 P. N. Rylander, 1979) and homogeneous (A. J. Birch, 1976) catalysts. Sulfur functions can be removed with reducing metals, e. g. with Raney nickel (G. R. Pettit, 1962 A). Heteroaromatic systems may be reduced with the aid of ruthenium on carbon. 

It IS worth remembering that a theory can never be proven correct It can only be proven incor rect incomplete or inadequate Thus theories are always being tested and refined As important as anything else in the scientific method is the testable hypothesis Once a theory is proposed experiments are designed to test its validity If the results are con sistent with the theory our belief in its soundness is strengthened If the results conflict with it the theory IS flawed and must be modified Section 6 7 describes some observations that support the theory that car bocations are intermediates in the addition of hydro gen halides to alkenes... 

We saw in Chapter 12 that aromaticity reveals itself in various ways Qualitatively aro matic compounds are more stable and less reactive than alkenes Quantitatively their heats of hydrogenation are smaller than expected Theory especially Huckels rule furnishes a structural basis for aromaticity Now lets examine some novel fea tures of their NMR spectra... 

In theory, three isoxazolines are capable of existence 2-isoxazoline (2), 3-isoxazoline and 4-isoxazoline. The position of the double bond may also be designated by the use of the prefix A with an appropriate numerical superscript. Of these only the 2-isoxazolines have been investigated in any detail. The preparation of the first isoxazoline, 3,5-diphenyl-2-isoxazoline, from the reaction of )3-chloro-)3-phenylpropiophenone with hydroxylamine was reported in 1895 (1895CB957). Two major syntheses of 2-isoxazolines are the cycloaddition of nitrile A-oxides to alkenes and the reaction of a,/3-unsaturated ketones with hydroxylamine. Since 2-isoxazolines are readily oxidized to isoxazoles and possess some of the unique properties of isoxazoles, they also serve as key intermediates for the synthesis of other heterocycles and natural products. 

The condition defined by equation (8) is met by adjustment of (Qg(3)) nd (T(3)). The pressures at the second stripping flow inlet and that of the outlet for solute (C) must be made equal, or close to equal, to prevent cross-flow. Scott and Maggs [7] designed a three stage moving bed system, similar to that described above, to extract pure benzene from coal gas. Coal gas contains a range of saturated aliphatic hydrocarbons, alkenes, naphthenes and aromatics. In the above theory the saturated aliphatic hydrocarbons, alkenes and naphthenes are represented by solute (A). 

Thermal dimerization of ethylene to cyclobutane is forbidden by orbital symmetry (Sect 3.5 in Chapter Elements of a Chemical Orbital Theory by Inagaki in this volume). The activation barrier is high E =44 kcal mof ) [9]. Cyclobutane cannot be prepared on a preparative scale by the dimerization of ethylenes despite a favorable reaction enthalpy (AH = -19 kcal mol" ). Thermal reactions between alkenes usually proceed via diradical intermediates [10-12]. The process of the diradical formation is the most favored by the HOMO-LUMO interaction (Scheme 25b in chapter Elements of a Chemical Orbital Theory ). The intervention of the diradical intermediates impfies loss of stereochemical integrity. This is a characteric feature of the thermal reactions between alkenes in the delocalization band of the mechanistic spectrum. 

The orbital phase theory was applied to the conformations of alkenes (a- and P-substituted enamines and vinyl ethers) [31] and alkynes [32], The conformational stabilities of acetylenic molecules are described here. 

Apart from type 62, which is only slowly convergent to the optimised geometry, the other centres are well described by the ROHF method. Polyhedral views of the three type a structures are shown in Fig. 6. These all illustrate the change of hybridisation at the point of muonium attachment and at the adjacent carbon atom where the unpaired electron is effectively localised as expected from addition to an alkene. The bi and c defects (Fig. 7) are quite different. The expected hybridisation change to sp is clearly present for the atom bonded to muonium, but other significant distortions are not obvious. This is consistent with the prediction from resonance theory (Fig. 8) that the unpaired electron for these structures is delocalised over a large number of centres. 

The [2 + 2] photodimerization of a, j8-unsaturated sulfones is correctly viewed as a photoreaction of alkenes, rather than the sulfone group, and this aspect has been reviewed recently by Reid, as part of a wider survey of the photoreaction of O- and S-heterocycles. The topic continues to attract considerable interest and a few recent examples, as well as some synthetic applications, will be discussed here. Much of the photodimerization work has been carried out on the benzo[fc]thiophene (thianaphthene) 1,1-dioxide system. For example. Porter and coworkers have shown that both 3-carboxybenzo[i]thiophene 1,1-dioxide (65) and its methyl ester give only the head-to-head (hth), anti dimer (66) on irradiation in ethanol. In a rather unusual finding for such systems, the same dimer was obtained on thermal dimerization of 65. Similar findings for a much wider variety of 3-substituted benzo[fi]thiophene 1,1-dioxides have been reported more recently by Geneste and coworkers . In the 2-substituted analogs, the hth dimer is accompanied by some of the head-to-tail (htt), anti dimer. The formation of the major dimer appears to proceed by way of an excited triplet and the regiochemistry observed is in accord with frontier MO theory. 

Frontier orbital theory can also explain the regioselectivity observed when both the diene and alkene are unsymmetrically substituted.4 Generally, there is a preference... 

The first experiments on chemoautotrophic theory were carried out by Stetter at the University of Regensburg. It was found that synergy in the FeS/H2S system determined the reductive effect, for example, in the conversion of nitrate to ammonia or of alkynes to alkenes. The conditions used corresponded to those present in hydrothermal systems aqueous phase, 373 K, almost neutral pH and anaerobic conditions (Blochl et al 1992). Two years later, the formation of an amide bond without the use of a condensation agent was successfully demonstrated in the same laboratory (Keller et al 1994). 

Some years later, Kochi et al (Fukuzumi and Kochi, 1981) applied their theory on electron and charge transfers to electrophilic alkene bromination (Kochi, 1988) by comparing the reactivities of various alkenes in bromination and in mercuration. Although the substituent effect trends in the two reactions are totally different, a linear relationship (7) is observed when the reactivities... 

The regioselectivity in radical addition reactions to alkenes in general has successfully been interpreted by a combination of steric and electronic effects1815,47. In the absence of steric effects, regiochemical preferences can readily be explained with FMO theory. The most relevant polyene orbital for the addition of nucleophilic radicals to polyenes will be the LUMO for the addition of electrophilic orbitals it will be the HOMO. Table 10 lists the HOMO and LUMO coefficients (without the phase sign) for the first three members of the polyene family together with those for ethylene as calculated from Hiickel theory and with the AMI semiempirical method48. 

The protocol developed by Jacobsen and Katsuki for the salen-Mn catalyzed asymmetric epoxidation of unfunctionalized alkenes continues to dominate the field. The mechanism of the oxygen transfer has not yet been fully elucidated, although recent molecular orbital calculations based on density functional theory suggest a radical intermediate (2), whose stability and lifetime dictate the degree of cis/trans isomerization during the epoxidation <00AG(E)589>. 

Natural resonance theory (Section 1.6) provides a quantitative gauge of the contributions of various resonance structures to the total electronic density. The results are shown in Table 4.41 demonstrating the remarkable intermediacy in the nature of metal-alkene interaction relating metallacycle, nonbonded, and carbanion-type resonance forms. 

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