1,6-addition rate-determining step

Both steps m this general mechanism are based on precedent It is called elec trophilic addition because the reaction is triggered by the attack of an acid acting as an electrophile on the rr electrons of the double bond Using the two rr electrons to form a bond to an electrophile generates a carbocation as a reactive intermediate normally this IS the rate determining step... 

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... 

The rate-determining step involves addition or subtraction of the proton (118) which is donated by the catalyst. 

Generally, the reactions of halopyrazines and haloquinoxalines with nucleophiles are believed to proceed by way of addition/elimination sequences, although there are clear-cut examples where this is not the case (see Section 2.14.2.2) and, consistent with a mechanism which involves bond forming, rather than bond breaking, reactions in the rate-determining step, fluoro derivatives are considerably more reactive ca. xlO ) than the corresponding chloro derivatives. 

Kinetic studies of the addition of hydrogen chloride to styrene support the conclusion that an ion-pair mechanism operates because aromatic conjugation is involved. The reaction is first-order in hydrogen chloride, indicating that only one molecule of hydrogen chloride participates in the rate-determining step. ... 

An alternative view of these addition reactions is that the rate-determining step is halide-assisted proton transfer, followed by capture of the carbocation, with or without rearrangement Bromide ion accelerates addition of HBr to 1-, 2-, and 4-octene in 20% trifluoroacetic acid in CH2CI2. In the same system, 3,3-dimethyl-1-butene shows substantial rearrangement Even 1- and 2-octene show some evidence of rearrangement, as detected by hydride shifts. These results can all be accoimted for by a halide-assisted protonation. The key intermediate in this mechanism is an ion sandwich. An estimation of the fate of the 2-octyl cation under these conditions has been made ... 

Reaction of 4-octyne with trifiuoroacetic acid in CH2CI2 containing 0.1-1.0A/Br leads mainly to Z-4-bromo-4-octene by an anti addition. The presence of Br greatly accelerates the reaction as compared to reaction with trifiuoroacetic acid alone, indicating the involvement of the Br in the rate-determining step. ... 

For alkyl-substituted alkynes, there is a difference in stereochemistry between mono-and disubstituted derivatives. The former give syn addition whereas the latter react by anti addition. The disubstituted (internal) compounds are considerably ( 100 times) more reactive than the monosubstituted (terminal) ones. This result suggests that the transition state of the rate-determining step is stabilized by both of the alkyl substituents and points to a bridged intermediate. This would be consistent with the overall stereochemistry of the reaction for internal alkynes. 

I > Br > Cl > F. In nucleophilic aromatic substitution, the formation of the addition intermediate is usually the rate-determining step so the ease of C—X bond breaking does not affeet the rate. When this is the ease, the order of reactivity is often F > Cl > Br > I. This order is the result of the polar effeet of the halogen. The stronger bond dipoles assoeiated with the more eleetronegative halogens favor the addition step and thus inerease the overall rate of reaetion. 

The ratio of products 15 and 16 is dependent on the structures, base, and the solvent. The kinetics of the reaction is likewise dependant on the structures and conditions of the reaction. Thus addition or cyclization can be the rate-determining step. In a particularly noteworthy study by Zimmerman and Ahramjian, it was reported that when both diastereomers of 20 were treated individually with potassium r-butoxide only as-epoxy propionate 21 was isolated. It is postulated that the cyclization is the rate-limiting step. Thus, for these substrates, the retro-aldolization/aldolization step reversible. ... 

The most widely accepted mechanism of reaction is shown in the catalytic cycle (Scheme 1.4.3). The overall reaction can be broken down into three elementary steps the oxidation step (Step A), the first C-O bond forming step (Step B), and the second C-O bond forming step (Step C). Step A is the rate-determining step kinetic studies show that the reaction is first order in both catalyst and oxidant, and zero order in olefin. The rate of reaction is directly affected by choice of oxidant, catalyst loadings, and the presence of additives such as A -oxides. Under certain conditions, A -oxides have been shown to increase the rate of reaction by acting as phase transfer catalysts. ... 

A great deal of kinetie information on the AE reaetion has been obtained. The rate of reaetion is first order in allylie aleohol, Ti(0-iPr)2(tartrate), and TBHP. In addition, the rate is inversely-square dependent on isopropoxide. This refleets the required replaeement of two isopropoxide ligands on Ti(0-iPr)2(tartrate) with TBHP and the allylie aleohol. The rate-determining step is oxygen transfer from the peroxide to the olefin. 

Once formed, 7 and 8 undergo a Michael reaction that gives rise to ketoenamine 9. Ring closure, to form 10, and loss of water then afforded 1,4-dihydropyridine 11. The presence of 9 and 10 could not be detected thus ring closure and dehydration were deduced to proceed faster than the Michael addition. This has the result of making the Michael addition the rate-determining step in this sequence. Conversely, if the reaction is run in the presence of a small amount of diethylamine, compounds related to 10 could be isolated. Diol 20 has been isolated in an unique case (R = CFb). Attempts to dehydrate this compound under a variety of conditions were unsuccessful. Stereoelectronic effects related to the dehydration may be the cause. In related heterocyclic ring formations, it has been determined that dehydration (20 —> 10) is about 10 times slower than diol formation (19 —> 20). Therefore, one would expect 20 to... 

The results are consistent with the rate-determining step being addition of the aryl radical to the aromatic ring, Eq. (9). Support for this mechanism is derived from the results of three other studies (a) When A -nitrosoacetanilide is decomposed in pyridine, the benzene formed by abstraction of hydrogen from pyridine by phenyl radical accounts for only 1 part in 120 of the reaction leading to phenyl-pyridines. (b) 9,9, 10,lCK-Tetrahydro-10,10 -diphenyl-9,9 -bianthryl is formed in the reaction between phenyl radicals and anthracene, probably by the addition mechanism in Eq. (11). Adducts are also formed in the reactions of benzyl radicals with anthracene- and acridine. ... 

Water plays a crucial role in the inclusion process. Although cyclodextrin does form inclusion complexes in such nonaqueous solvents as dimethyl sulfoxide, the binding is very weak compared with that in water 13 Recently, it has been shown that the thermodynamic stabilities of some inclusion complexes in aqueous solutions decrease markedly with the addition of dimethyl sulfoxide to the solutions 14,15>. Kinetic parameters determined for inclusion reactions also revealed that the rate-determining step of the reactions is the breakdown of the water structure around a substrate molecule and/or within the cyclodextrin cavity 16,17). 

The rate-determining step in the formation of the x-lithio ethers is the formation of a carbon radical as a precursor to the anion. The intermediate radical in the tetrahydropyranyl system is expected to be nonplanar, to be capable of rapid equilibration between the quasiequatorial and quasiaxial epimers, and to exist largely or entirely in the axial configuration at — 78 °C. However, treatment of the a-phenylthio ether 4 with LDMAN at higher temperature in the presence of A, A, lV, ./V -tetramethylethylenediamine leads to the more stable equatorial epimer of the lithio ether 5 and, after addition to benzaldehyde, the axial- and equatorial-substituted products were obtained in a ratio of 13 87. 

Such a complex, cw-Rh(CO)2I2, is the active species in the Monsanto process for low-pressure carbonylation of methanol to ethanoic acid. The reaction is first order in iodomethane and in the rhodium catalyst the rate-determining step is oxidative addition between these followed by... 

---