Olefins relative rates

Olefin Relative Rate Olefin Relative Rate... 

Olefins Relative rate constant Olefins Relative rate constant... 

No absolute rate data exist for fluorine atom additions to olefins. This is not at all surprising in view of the tremendous experimental difficulties in carrying out reactions between fluorine and organic compounds of almost any sort. Rowland and co-workers [45] have been studying the interaction of fluorine atoms with a variety of compounds, however, and these include olefins. Relative rate data are given in Table 11 in a form such that the inter- and intramolecular reactivity of various unsaturated carbons is normalized against ethylene. The total reactivity is, of course, the summation of the positional reactivities of the individual carbons for any given compound. Rowland and co-workers [45a] have also measured the relative reactivity of the terminal versus the central carbon of propene towards fluorine atoms, and find the terminal position to be 1.35 times more reactive. 

These singlet and triplet state species exhibit the important differences in chemical behavior to be expected. The former species, with their analogy to carbonium ions, are powerful electrophiles and the relative rates of their reaction with a series of substrates increases with the availability of electrons at the reaction center their addition reactions with olefins are stereospecific. Triplet state species are expected to show the characteristics of radicals i.e., the relative rates of additions to olefins do not follow the same pattern as those of electrophilic species and the additions are not stereospecific. 

Acetylenes are, in general, much more strongly adsorbed than the corresponding olefin, and as long as the acetylene is present, it will occupy most of the catalyst sites and be reduced preferentially, regardless of the relative rates of the acetylene and olefin hydrogenation when not in competition for sites. The reduction... 

M is an unsaturated hydrocarbon or an organic compound such as CH3OH, CH3I, CH3N02, (CH3)2CO, CH3NH2, etc. When M is an olefin, Reaction 27 or 28 will compete with a hydride transfer process (see earlier discussion) and a condensation process. For instance, in the radiolysis of C3D8-CH3CHCH2 mixtures (9), the relative rates of Reactions 29, 30, 31, and 32... 

Some years ago, we tackled (ref. 7) the particular question of bromine bridging, related mainly to stereochemistry, postulating that bromonium ions and bromo-carbocations are formed in separate pathways as shown in Scheme 3. The relative rates of reaction by these pathways depend on the olefin structure. As demonstrated later... 

It is also difficult to determine exactly the relative stabilities of vinyl cations and the analogous saturated carbonium ions. The relative rates of solvolysis of vinyl substrates and their analogous saturated derivatives have been estimated to be 10 to 10 (131, 134, 140, 154) in favor of the saturated substrates. These rate differences, however, do not accurately reflect the inherent differences in stability between vinyl cations and the analogous carbonium ions, for they include effects that result from the differences in ground states between reactants, as well as possible differences between the intermediate ions resulting from differences in solvation, counter-ion effects, etc. The same difficulties apply in the attempt to estimate relative ion stabilities from relative rates of electrophilic additions to acetylenes and olefins, (218), or from relative rates of homopropargylic and homoallylic solvolysis. 

By analogy with hydroformylation, dicobalt octacarbonyl has been examined as a hydrosilylation catalyst. Various silanes and a-olefins react, often exothermically. Thermal deactivation occurs above 60° C hence, large exotherms and high temperatures must be avoided (56, 57,130). Isomerization is more pronounced than for the bridged olefin complexes of Pt(II) and Rh(I) (see below) it even occurs with trialkoxysilanes (57). Though isomerization is faster than hydrosilylation, little variation in the relative rates of these two processes with the nature of the silane is observed this is in marked contrast to the bridged systems (55). 

The relative rates of the olefins studied decreases in the order... 

The quantum yields for oxetane formation have not been determined in every case, and only a few relative rate constants are known. The reactivities of singlet and triplet states of alkyl ketones are very nearly equal in attack on electron rich olefins. 72> However, acetone singlets are about an order of magnitude more reactive in nucleophilic attack on electron-deficient olefins. 61 > Oxetane formation is competitive with a-cleavage, hydrogen abstraction and energy-transfer reactions 60 64> so the absolute rates must be reasonably high. Aryl aldehydes and ketones add to olefins with lower quantum yields, 66> and 3n-n states are particularly unreactive. 76>... 

Information published from several sources about 1970 presented details on both the halide-containing RhCl(CO)(PPh3)2- and the hydride-containing HRh(CO)(PPh3)3-catalyzed reactions. Brown and Wilkinson (25) reported the relative rates of gas uptake for a number of different olefinic substrates, including both a- and internal olefins. These relative rates are listed in Table XV. 1-Alkenes and nonconjugated dienes such as 1,5-hexadiene reacted rapidly, whereas internal olefins such as 2-pentene or 2-heptene reacted more slowly by a factor of about 25. It should also be noted that substitution on the 2 carbon of 1-alkene (2-methyl-l-pentene) drastically lowered the rate of reaction. Steric considerations are very important in phosphine-modified rhodium catalysis. 

The nature and distribution of the products of an olefin oligomerization reaction will depend, inter alia, on the relative rate constants of the insertion step (ki)vs. the displacement step (fcd) [Eq. (11)] ... 

Olefin metathesis is a catalytic process whose key step is a reaction between an olefin and a transition metal alkylidene complex, usually M=CHR (Eq. 1) or M= CH2, in a 2+2 fashion to give an unstable intermediate metalacyclobutane ring [1]. All possible reactions of this general type are reversible, possibly nonproductive, and in competition with one another, so the overall result depends heavily on relative rates, and in the case of formation of volatile or insoluble products, displacement of equilibria as those products form. 

Initially, one carbon atom (a methyl group) is attached to the metal of the catalyst (A in Figure 1). In the first step, it will capture and insert a propylene molecule via either 1,2- or 2,1-insertion route. Thus, one of these insertion events is stochastically chosen this choice, however, is not totally random but weighted by the probabilities of the two reactions. Here the relative probabilities are proportional to the relative rates. Now, if one assumes that the 1,2-insertion has happened in the first step, i.e. the iso-butyl group is attached to the catalyst (B) after insertion. At this stage four different elementary events are possible two alternative insertion routes (1,2- and 2,1-) proceeded by the capture of olefin, the termination reaction,... 

Already, at an early stage of the studies on the captodative effect, Viehe s group (Lahousse et ai, 1984) measured relative rates for the addition of t-butoxyl radicals to 4,4 -disubstituted 1,1-diphenylethylenes and to substituted styrenes. This study did not reveal a special character of captodative-substituted olefins in such reactions. It might be that the stability of the radical to be formed does not influence the early transition state of the addition step. The rationalization of the kinetic studies mentioned above in terms of the FMO model indicates, indeed, an early transition state for these reactions, with the consequence that product properties should not influence the reactivity noticeably. 

Table 11. Relative rates of addition of singlet and triplet bis(methoxy carbonyl) carbene to olefins...

Although Eq. (3) indicates that CO absorption is required for aldehyde formation, it has been shown by Karapinka and Orchin 18) that at 25° and with a moderate excess of olefin the rate of reaction and the yield of aldehyde are similar when either 1 atm of CO or 1 atm of Nj is present. Obviously CO is not essential for the reaction and a CO-deficient intermediate, probably an acylcobalt tricarbonyl, can be formed under these conditions. The relative rates of HCo(CO)4 cleavage of tricarbonyl and tetracarbonyl are not known, and thus the stage at which CO is absorbed in the stoichiometric hydroformylation of olefins under CO is not known with certainty. Heck (19) has shown conclusively that acylcobalt tetracarbonyls are in equilibrium with the acylcobalt tricarbonyl ... 

If the addition of the second hydrogen atom is the rate-controlling surface reaction, then the preceding steps would tend to be reversed, the degree of reversibility being a function of the relative rates of the several reactions. Two effects are expected (1) the isomerization of the initial olefin is pronounced and (2) the proportion of saturated products should tend towards the equilibrium distribution. Indeed, such effects are commonly observed when palladium catalysts are employed (5, 65, 66) (Fig. 9). 

Vrbaski, T., and R. I. Cvetanovic. Relative rates of reaction of ozone with olefins in the vapor phase. Can. J. Chem. 38 1053-1062, 1960. 

In competitive ethylations of aromatics studied by Pines and Schaap 23), a large excess of aromatic over olefin was used so that relative rates o... 

Two extreme epoxidation modes, spiro and planar, are shown in Fig. 9 [33, 34, 53, 54, 76-85]. Baumstark and coworkers had observed that the epoxidation of cis-hexene of dimethyldioxirane was seven to nine times faster than the corresponding epoxidation of tran.y-hexene [79, 80]. The relative rates of the epoxidation of cisitrans olefins suggest that spiro transition state is favored over planar. In spiro transition states, the steric interaction for cw-olefm is smaller than the steric interaction for fran -olefm. In planar transition states, similar steric interactions would be expected for both cis- and trans-olefms. Computational studies also showed that the spiro transition state is the optimal transition state for oxygen atom transfer from dimethyldioxirane to ethylene, presumably due to the stabilizing interactions... 

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