First Step of the Mechanism

According to the above discussion, one should observe the epimerization of substrates with an axial halogen (Table 2, step C). Furthermore, under iden- 

TABLE 2. Epimerization of a-Bromocyclohexanone under Basic Conditions 

When the cyclic substrate has an equatorial halogen (Table 2, step A), the experiment shows that the enolization at the a position does not lead to epimerization. This again is in perfect agreement with the stereoelectronic control which favors fixation of the deuterium by the enolate in the axial direction. This enolization does not seem to have any further influence on the Favorskii rearrangement.  

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We can extend the general principles of electrophilic addition to acid catalyzed hydration In the first step of the mechanism shown m Figure 6 9 proton transfer to 2 methylpropene forms tert butyl cation This is followed m step 2 by reaction of the car bocation with a molecule of water acting as a nucleophile The aUcyloxomum ion formed m this step is simply the conjugate acid of tert butyl alcohol Deprotonation of the alkyl oxonium ion m step 3 yields the alcohol and regenerates the acid catalyst... 

The possible mechanism for the reactions involving stoichiometric amount of preformed Ni(0) complexes is shown in Fig. 9.8. The first step of the mechanism involves the oxidative addition of aryl halides to Ni(0) to form aryl Ni(II) halides. Disproportion of two aryl Ni(II) species leads to a diaryl Ni(II) species and a Ni(II) halide. This diaryl Ni(II) species undergoes rapid reductive elimination to form the biaryl product. The generated Ni(0) species can reenter the catalytic cycle. 

Although alkyl groups in general increase the rates of electrophilic addition, we have already mentioned (p. 974) that there is a different pattern depending on whether the intermediate is a bridged ion or an open carbocation. For brominations and other electrophilic additions in which the first step of the mechanism is rate determining, the rates for substituted alkenes correlate well with the ionization potentials of the alkenes, which means that steric effects are not important. Where the second step is rate determining [e.g., oxymercuration (15-3), hydroboration (15-17)], steric effects are important. ... 

The first step of the mechanism is formation of a complex (30) (ions like form complexes with alkynes, p. 102). Water then attacks in an 8 2 type process to give the intermediate 31,... 

Epoxides can be converted to alkenes by treatment with triphenylphosphine or triethyl phosphite P(OEt)3. The first step of the mechanism is nucleophilic substitution (10-50), followed by a four-center elimination. Since inversion accompanies the substitution, the overall elimination is anti, that is, if two groups A and C are cis in the epoxide, they will be trans in the alkene ... 

In the first step of the mechanism, the OH group is converted by the reagent to a better leaving group (e.g., proton acids convert it to OHj). After that, the mecha-nism follows a course analogous to that for the Schmidt reaction of ketones (18-16) from the formation of 71 on ... 

In other words, we must determine whether the reaction is a Markovnikov addition or an anii-Markovnikov addition. As promised, the answer to this question is contained in the mechanism. In the first step of the mechanism, a proton was transferred to the alkene, to form a carbocation. When starting with an unsymmetrical alkene, we are confronted with two possible carbocations that can form (depending on where we place the proton) ... 

At first glance, the product is not what we might have expected. Once again, we turn to the mechanism for an explanation. The first step of the mechanism is identical to what we have seen so far—we protonate the double bond to produce the more stable carbocation (secondary, rather than primary) ... 

In this example, the OH groups clearly added in a syn addition. In order to explain why this reaction proceeds via a syn addition, we must look at the first step of the mechanism ... 

The predicted rate law is first order for a reaction whose first step is unimolecular and rate-determining. The predicted rate law is second order overall for a reaction whose first step is bimolecular and rate-determining. For example, the first step of the mechanism for the C5 Hi 1 Br reaction is unimolecular and slow, so the rate law... 

Example treats a reaction of this kind. The experimental rate law for the reaction of H2 gas with Br2 gas depends on the square root of the Bf2 concentration, and the reaction also is first order in H2 H2+Br2 2HBr Rate =. "[H2] [Br2] Despite the simple 1 1 stoichiometry of the overall reaction, this experimental rate law cannot be explained by a simple mechanism. For the first step of the mechanism for this reaction to be rate-determining, it would have to include a half-molecule of Bf2. There is no... 

C15-0095. Oxygen reacts with NO to form NO2 as the oniy product. Here is a proposed first step of the mechanism O2 +N0 NO2 + O... 

The adsorption of at least one reactant is the first step of the mechanism of any catalytic reaction. This step is followed by surface interactions between adsorbed species or between a gaseous reactant and adsorbed species. In many cases, these interactions may be detected by the successive adsorptions of the reactants in different sequences. Heat-flow microcalorimetry can be used with profit for such studies (19). 

The calculated thermodynamic parameters in Table X are for 150 °C, a value close to typical operating temperatures, and without pressure corrections, there is no H2 and both alkane and alkene could be at unit concentration. The bottom part of the table is the same as Table VI. The free-energy profile of the transfer reaction mechanism is shown in Fig. 4. In the first step of the mechanism (IVR), sacrificial alkene (in this case ethylene) was bound to the (L)Ir(H)2 complex 4 to form an olefin complex 7. As mentioned above, the forward reaction of this step has no... 

Notice that in the reaction, the catalyst, Fe3+, reduced to Fe2+, in the first step of the mechanism. In the second step, oxidation of Fe2+ back to Fe3+ occurred. Overall then, the catalyst remains unchanged. Notice also that although the catalyzed reaction is a two-step reaction, it is significantly faster than the original uncatalyzed one-step reaction. 

The details of the first step of the mechanism were studied for diphenylselenide... 

Rate-determining steps can occur anywhere in a reaction mechanism. If a slow step is not the first step of the mechanism, however, the overall rate law is more complicated to determine. One reason is that fast reactions occurring early in a mechanism are often reversible—both the forward and reverse reactions take place. (You will learn more about these reactions in Unit 4.)... 

They proposed a mechanism as shown in Scheme 10, because they found close contact between the oxygen atom of the C=0 group and the carbon atom of the C=S group in a range of 2.77-2.86 A. These contacts are well below the sum of the van der Waals radii for oxygen and carbon (3.22 A). Thus, in the crystalline state, the oxygen atom is ideally positioned for excited-state nucleophilic attack on the C=S double bond, and this process may be the first step of the mechanism by which thiobenzanilides are formed in this medium. This would lead reversibly to the 1,3-oxazetidinium ion 18, which could react with water present in the medium and then break down, via species 19, to the enol of thiobenzanilide 16 and 3-phenylpropanoic acid 17. 

Secondary bromides and tosylates react with inversion of stereochemistry, as in the classical SN2 substitution reaction.21 Alkyl iodides, however, lead to racemized product. Aryl and alkenyl halides are reactive, even though the direct displacement mechanism is not feasible. With there halides the mechanism probably consists of two steps. The addition of halides to transition-metal species with low oxidation states is a common reaction in transition-metal chemistry and is called oxidative addition. An oxidative addition to the copper occurs in the first step of the mechanism, and the formal oxidation... 

The first step of the mechanism leading The electrochemical study of the seven-to the formation of 8 and free nitrite coordinate complex [Mo(N2RR )(dtc)3]+ from the reaction of 7 with O2 probably 9+ (R, R = alkyl or aryl, dtc = 5 2CNMe2) involved a single electron transfer. Sub- provided an example of electrode-induced sequent radical-radical coupling of the activation of a hydrazido(2—) ligand. Corn-products, to afford a molybdenum-bound plex 9+ was shown to reduce in two nitrate, followed by N—O bond cleavage separate diffusion-controlled one-electron would eventually lead to the observed steps, with the first one reversible on the products (Sch. 8) [27]. CV timescale at room temperature and... 

The driving forces for electron transfer are a high-energy level of the highest occupied molecular orbital or a steric strain of the starting molecule. Complexation of oxygen by the electron-rich organic molecule has often been indicated as the first step of the mechanism. 

Methanol is the nucleophile and the nucleophilic centre is oxygen. However, methanol is a relatively weak nucleophile, so, the carbonyl group has to be activated by adding an acid catalyst if a reaction is to occur. The first step of the mechanism involves the oxygen of the carbonyl group using a lone pair of electrons to form a bond to a proton. This results in a charged intermediate... 

When the chloride ion leaves in the first step of the mechanism, a reactive intermediate is formed. This reactive intermediate is a high-energy, reactive species. Under most conditions it has a very short lifetime. However, it differs from a transition state in that it is located at a minimum on the energy curve. It has an activation barrier, although small, that must be surmounted for reaction in either the forward or reverse direction. Although its lifetime is short, it is significantly longer than that of a transition state. It may be possible, under certain circumstances, to obtain experimental observations of a reactive intermediate. 

Q In the presence of acid the double bond of the enol gets proton-ated. This step of the mechanism is identical to the first step of the mechanism for addition of the hydrogen halides and the acid-catalyzed addition of water to alkenes. The addition occurs so that the positive charge is located on the carbon that is bonded to the hydroxy group because this carbocation is stabilized by resonance. 

This process follows a radical chain mechanism and is catalyzed by light. (This is one reason why compounds are often sold and stored in brown glass bottles.) In the first step of the mechanism an initiating radical (In-) is generated by light or some other means. This radical abstracts a hydrogen from the substrate to produce a carbon radical (R ) ... 

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