Second Step of the Mechanism

It has even been suggested that the ester 4 is formed directly from the zwit-terion 2 In our opinion this does not seem very likely. We will discuss successively the second step of this mechanism in juxtacyclict linear series, as well as in cyclic series. In linear substrates the transformation of 1 via the cyclopropanone intermediate 3, or via the zwitterion 2, will depend on the base strength and the solvation. In cyclic substrates, structural features will have to be considered in addition. 

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In the second step of the mechanism described m Figure 4 6 the alkyloxonium ion dissociates to a molecule of water and a carbocation, an ion that contains a positively charged carbon... 

Coming back to the mechanism of dediazoniation, mechnism B in Scheme 9-2 is consistent with all experimental data known in 1973. Mechanism B was, indeed, mentioned in that paper (Zollinger, 1973 a) as an explanation, but not proposed as the explanation because it violated the common knowledge mentioned above. If that reverse reaction of the phenyl cation is faster than the forward reaction with water or metal halides, the rate is dependent on the concentrations of compounds involved only in the second step of the mechanism, even if that step is much faster than the first (forward) step. 

Carbocations can stabilize themselves in various ways (see p. 227), but for this type of ion the most likely way is by loss of either or . The aromatic sextet is then restored, and in fact this is the second step of the mechanism ... 

In the second step of the mechanism, we also have two arrows from a lone pair to form a bond, and then from a bond to form a lone pair ... 

NO2 NO -I- O. Oxygen atoms are known to be highly reactive, so it is reasonable to predict that this intermediate reacts rapidly with NO2 molecules. Compared with the fast second step of this mechanism, the step that forms the oxygen atoms is e.xpected to be slow and rate-determining. Similarly, the first step of Mechanism It, NO2 NO3 + NO, produces NO3. This species is known to be unstable, so it will decompose in the second step of Mechanism II almost as soon as it forms. Again, the second step of the mechanism is expected to be fast, so the step that forms the reactive intermediate is slow and rate-determining. Later in this chapter, we discuss experiments that make it possible to distinguish between Mechanisms I and II. 

The two-step process, depicted by path b, involves initial addition of the carbene carbon to an adjacent it bond to form bicyclo[4.1,0]hepta-2,4,6-triene (2a). This process has precedent in the analogous rearrangement of vinylcar-bene to cyclopropene (Scheme 6),lc18 and is supported by Gaspar s work on 1-cyclohexenylcarbene.17 In the second step of the mechanism in Scheme 5, subsequent six-electron electrocyclic ring opening of 2a yields the cyclic allene 3a. 

In the dehydrogenation of C2H6 to produce C2H4, CH is a minor coproduct this is also reflected in the second step of the mechanism hence, both the overall reaction and the proposed mechanism do not strictly represent a simple system. 

It has been recently reported that, contrary to previous assumptions, primary steric effects due to a branching in the amine do not produce a large decrease in the reaction rate when the first step is rate-determining82. In Sat Ar reactions of amines with fluoronitroben-zene, it is generally accepted that the second step of the mechanism depicted in Scheme 1 is rate-determining base catalysis is frequently found and the observed rate constants obey equation 2. Nevertheless, the reaction of o-fluoronitrobenzene with n- and iso-propylamine in toluene and in DMSO are only slightly sensitive to the nucleophile concentration. The... 

In the second step of the mechanism (shown in Figure 4-13), the bromide ion from the HBr attacks the allylic carbocation at one or the other of the partially positive carbon atoms. Attack on the second carbon gives 1,2-addition, while attack on the fourth carbon gives 1,4-addition. 

Note that H2 is consumed only in the second step of the mechanism ... 

OH is the base typically used in an aldol reaction. Recall from Section 23.3B that only a small amount of enolate forms with OH. In this case, that s appropriate because the starting aldehyde is needed to react with the enolate in the second step of the mechanism. 

The second kind of thermally allowed [2 + 2] cycloaddition occurs when one of the atoms involved is a second-row or heavier element, as in the Wittig reaction. Whether the first step of the Wittig reaction actually proceeds in a concerted fashion is a matter of debate, but the point here is that a concerted mechanism is a reasonable possibility. Moreover, there is no controversy over whether the second step of the mechanism is a concerted [2 + 2] retro-cycloaddition. 

The second step of the mechanism is deacylation, which involves an attack by water on the carbonyl of the newly formed ester. This step is rate-determining with ester substrates but is faster than acylation with peptides. 

In Eq 1, sulfur compounds are combusted to sulfur monoxide (SO) and other products. In Eq 2, the second step of the mechanism, light energy (hv) in the blue region of the spectrum is emitted from the excited species resulting from the ozone reaction. The basic mechanism of the DP-SCD is the same as that described above, but two plasmas (flames) instead of one are provided to improve selectivity and the ability to measure lower sulfur levels without hydrocarbon interferences. A conceptual drawing of the flow dynamics used in the Dual Plasma burner is shown (Fig. 1). 

The second step of the mechanism is the same rapid carbocation-anion combination that we saw in Section 4.8 as the last step in the mechanism of the reaction of alcohols with hydrogen halides. 

Kinetic measurements have shown that the reaction of acylcobalt tetracarbonyls with triphenylphosphine is first order in the cobalt compound and zero order in the triphenylphosphine down to at least 0.02 M (12). A first-order dissociation of the acylcobalt tetracarbonyl into an acylcobalt tricarbonyl and carbon monoxide has been proposed as the rate-determining step to explain the kinetics. The fast second step of the mechanism is then the reaction of the acylcobalt tricarbonyl with the triphenylphosphine, forming the acyl(triphenylphosphine)cobalt tricarbonyl. 

When the leaving group departs, its negative charge is attracted to the positive metal center, and it rapidly adds to the metal in the second step of the mechanism. The rate is enhanced by small, highly polarizable nucleophiles and soft, unsaturated... 

Acid catalysis occurs in the second step of the mechanism. This causes the reaction rate to increase proportionately with rising hydrogen ion concentration. The release of hydrogen ions in the third step acidifies the solution and promotes the reaction, but this effect is almost counterbalanced by a decrease in HS03 concentration caused by the shift in the equilibrium HSO -P H+ SO2 -P H2O forcing SO2 to return to the gas phase. 

An S l reaction is illustrated by the solvolysis reaction of 2-bromo-2-methylpropane (fert-butyl bromide) in methanol to form 2-methoxy-2-methylpropane terthutyl methyl ether). You may notice that the second step of the mechanism is identical to the second step of the mechanism for the addition of hydrogen hahdes (H—X) to alkenes (Section 5.3A) and the acid-catalyzed hydration of alkenes (Section 5.3B). 

Scheme 7.1 displays the commonly accepted mechanism of an S Ar reaction that proceeds through an addition-elimination route [8, 72, 75]. In the first step, the nucleophilic attack to an activated aromatic ring leads to the formation of an anionic a-adduct (MC). When the nucleophile is neutral, the second step of the mechanism may occur through two routes. The first one is the detachment of the LG from MC in Scheme 7.1), followed by a fast proton transfer, yielding the substitution product. The other pathway is that the proton transfer from the MC to the LG be catalyzed by a second molecule of the neutral nucleophile (NuH), followed by a fast departure of the LG (l j[NuH] in Scheme 7.1). It has been suggested that other channels may be operative, but in the experimental conditions considered in this chapter, they are normally discarded [72, 76]. Applying the steady-state condition to the MC intermediate in Scheme 7.1 leads to the rate law...

The second step of the mechanism requires one curved arrow showing the nucleophilic attack in which the carbocation intermediate is captured by the nucleophile (chloride) ... 

In the first step, the it bond of the alkene is protonated, generating a carbocation intermediate. In the second step, this intermediate is attacked by a bromide ion. Figure 9.1 shows an energy diagram for this two-step process. The observed r ioselectivity for this process can be attributed to the first step of the mechanism (proton transfer), which is the rate-determining step because it exhibits a higher transition state energy than the second step of the mechanism. 

When drawing the second step of the mechanism (nucleophilic attack of chloride), only one curved arrow is needed. The chloride ion, formed in the previous step, functions as a nucleophile and attacks the carbocation ... 

One curved arrow is drawn with its tail on the it bond and its head on the proton, while the second curved arrow is drawn with its tail on an O—H bond and its head on the oxygen atom of water. When drawing the second step of the mechanism (nucleophilic attack of water), only one curved arrow is required. The tail should be placed on a lone pair of water, and the head should be placed on the carbocation ... 

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