Alkenes reaction rates, 243, Table

The notion that carbocation formation is rate determining follows from our previous experience and by observing how the reaction rate is affected by the shucture of the aUcene Table 6 2 gives some data showing that alkenes that yield relatively stable carbocations react faster than those that yield less stable carbocations Protonation of ethylene the least reactive aUcene m the table yields a primary carbocation protonation of 2 methylpropene the most reactive m the table yields a tertiary carbocation As we have seen on other occa sions the more stable the carbocation the faster is its rate of formation... 

Table 6 3 shows that the effect of substituents on the rate of addition of bromine to alkenes is substantial and consistent with a rate determining step m which electrons flow from the alkene to the halogen Alkyl groups on the carbon-carbon double bond release electrons stabilize the transition state for bromonium ion formation and increase the reaction rate... 

Chlorination generally exhibits second-order kinetics, first-order in both alkene and chlorine. The reaction rate also increases with alkyl substitution, as would be expected for an electtophilic process. The magnitude of the rate increase is quite large, as shown in Table 6.3. 

Part B of Table 12.2 gives some addition reaction rates. Comparison of entries 19 and 20 shows that the phenyl radical is much more reactive toward addition than the benzy 1 radical. Comparison of entries 22 and 23 shows that methyl radicals are less reactive than phenyl radicals in additions to an aromatic ring. Note that additions to aromatic rings are much slower than additions to alkenes. 

Simple alkyl radicals such as methyl are considered to be nonnucleophilic. Methyl radicals are somewhat more reactive toward alkenes bearing electron-withdrawing substituents than towards those with electron-releasing substituents. However, much of this effect can be attributed to the stabilizing effect that these substiments have on the product radical. There is a strong correlation of reaction rate with the overall exothermicity of the reaction. Hydroxymethyl and 2-hydroxy-2-propyl radicals show nucleophilic character. The hydroxymethyl radical shows a slightly enhanced reactivity toward acrylonitrile and acrolein, but a sharply decreased reactivity toward ethyl vinyl ether. Table 12.9 gives some of the reactivity data. 

Representative data illustrating the influence of Lewis base functional groups in the ADMET reaction are shown in Table 1. When molybdenum catalysts are used to polymerize ether or thioether dienes, little change in reaction rate is observed as compared with the standard, 1,9-decadiene, which possesses no heteroatoms in its structure. When a sulfur atom is three carbons atoms away from the alkene site, the reaction rate is reduced approximately one order of magnitude otherwise, the kinetics are all essentially unaffected [20a]. 

The hydrogenation of unfunctionalized alkenes is readily performed by Group III and lanthanide cyclopentadienyl hydride derivatives, one key feature being the high TOFs of these systems (up to 120000 IT1 for hydrogenations catalyzed by Lu, Tables 6.8 and 6.9) [119, 120]. The reaction rate depends heavily on the metal and the ligands. It is inversely proportional to the metal radius (Lu>Sm>Nd>La), and it is faster for the Cp M derivatives than for the ansa di-... 

Transition metals can display selectivities for either carbonyls or olefins (Table 20.3). RuCl2(PPh3)3 (24) catalyzes reduction of the C-C double bond function in the presence of a ketone function (Table 20.3, entries 1-3). With this catalyst, reaction rates of the reduction of alkenes are usually higher than for ketones. This is also the case with various iridium catalysts (entries 6-14) and a ruthenium catalyst (entry 15). One of the few transition-metal catalysts that shows good selectivity towards the ketone or aldehyde function is the nickel catalyst (entries 4 and 5). Many other catalysts have never been tested for their selectivity for one particular functional group. 

The reactivity of the dienophile depends on the electron demand of the reaction. Electron-withdrawing groups in the dienophile will accelerate the reaction rate. Simple alkenes and alkynes such as ethylene and acetylene are poor dienophiles. In the following table some representative dienophiles have been tabulated along with their reaction with cyclopentadiene. 

Hydroformylation results in Table 8.3 show that, with the exception of ligands 28 and 30, the rate of the reaction increases with decreasing phosphine basicity. An explanation for the deviant behaviour of 28 and 29 can be that catalyst formation is incomplete or deactivation of the catalyst occurs. Decreasing phosphine basicity facilitates CO dissociation from the (diphosphine)Rh(CO)2H complex and enhances alkene coordination to form the (diphosphine)Rh(CO)H(alkene) complex, and therefore, the reaction rate increases. 

Dimerization is the characteristic reaction of radical-anions from activated alkenes. The rate constants for dimerization are high and the conjugate acids from such alkene radical-anions in many cases have low pKa values and. The data in Table 3.4 were obtained by following the changes in uv-absorbance after pule-radiolysis of the substrate in an aqueous buffer. Attachment of a solvated electron leads to the radical-anion. Changes in the initial absorbance with pH lead to determination of the pKg value, while the dimerization rate can be determined from changes in absorbance over a longer time scale. Radical-anions from esters and amides are pro-... 

As for other organics in the atmosphere, the OH radical is a major oxidant for alkenes. Table 6.8 gives the rate constants for some OH-alkene reactions as well as their temperature dependence in Arrhenius form. Several points are noteworthy (1) the reactions are very fast, approaching 10-l() cm3 molecule-1 s-1 for the larger alkenes (2) the rate constants have a pressure dependence (3) the apparent Arrhenius activation energies are negative. ... 

Ethene and ethane account for 80% of the mass of the hydrocarbons identified as products. Trace amounts of methane and acetylene are also produced (Orth and Gillham, 1996). The reduction of PCE forms cis-1,2-dichloroethylene (DCE), frans-l,2-DCE, 1,1-DCE, vinyl chloride, ethylene, dichloroacetylene, acetylene, ethene, ethane, chloroacetylene, methane, and several alkenes ranging from C3 to C6. The trace amounts of dichloro-ethylene and vinyl chloride formed during the reduction of PCE and TCE are further reduced (Burris et al., 1995). Reaction rates vary with substrate, chemical, and microbiological conditions. Selected f1/2 values are provided in Table 13.3. 

As expected for an electrophilic addition, the reaction rate increases as alkyl groups are substituted on the double bond. The electron-donating alkyl groups make the alkene more nucleophilic. Table 11.1 lists the relative rates of bromination of a series of alkenes. As can be seen from this table, replacing all four of the hydrogens of ethene with methyl groups results in an increase in the rate of the reaction by a factor of 2 million. 

For electrophilic additions of halogens to alkenes, not only is the reaction rate strongly solvent-dependent [79-81] [cf. Eq. (5-29) in Section 5.3.2), but the stereochemical course may also be affected by the polarity of the medium [79, 386-388], For example, the stereoselectivity of bromine addition to cis- and trans -stilbene according to Eq. (5-140) has been found to be solvent-dependent, as shown in Table 5-23 [79, 386],... 

In contrast, bimolecular additions of allylmagnesium halides (5) to nonstrained alkenes (6) (Scheme 4 Table 1) are notoriously inefficient due to low reaction rates and the formation of regio- and stereoisomer mixtures, as well as uncontrolled consecutive reactions of the metallated addition products (7) and (8). Only trimethylsilylethylene showed a useful reactivity towards allylic Grignard reagents ... 

The addition of a C-2 (equation 1 R = H > alkyl, aryl > OMe NR2), C-3, or C-4 electron-donating substituent to a 1 -oxa-1,3-butadiene electronically decreases its rate of 4ir participation in a LUMOdiene-controlled Diels-Alder reaction (c/. Table 5). Nonetheless, a useful set of C-3 substituted l-oxa-l,3-buta-dienes have proven to be effective dienes ° and have been employed in the preparation of carbohydrates (Table 6). The productive use of such dienes may be attributed to the relative increased stability of the cisoid versus transoid diene conformation that in turn may be responsible for the Diels-Alder reactivity of the dienes. Clear demonstrations of the anticipated [4 + 2] cycloaddition rate deceleration of 1-oxa-1,3-butadienes bearing a C-4 electron-donating substituent have been detailed (Table 6 entry 4). >> "3 In selected instances, the addition of a strong electron-donating substituent (OR, NR2) to the C-4 position provides sufficient nucleophilic character to the 1-oxa-1,3-butadiene to permit the observation of [4 + 2] cycloaddition reactions with reactive, electrophilic alkenes including ketenes and sul-fenes, often in competition with [2 + 2] cycloaddition reactions. ... 

However, other results indicate that the P/Al catalyst has very low activity for C —N or C —C bond breaking 74, 81). A detailed kinetics study by Jian and Prins 81) showed that phosphorus addition decreases the C—N bond cleavage (rate constants k[ and k) in the reaction networks Figs. 33a and 33b) and the subsequent alkene hydrogenation (rate constant k() reactions in piperidine and DHQ HDN (see Table XIV). On the other hand, the presence of phosphorus increases aromatic ring hydrogenation of OPA... 

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