Two step mechanism

Despite this overwhelming body of evidence, two-step mechanisms have been suggested for the Diels-Alder reaction, probably inspired by special cases, where highly substituted dienes and/or dienophiles have been found to react through zwitterionic or biradicalintermediates (Scheme 1.2). 

The cases of pentamethylbenzene and anthracene reacting with nitronium tetrafluoroborate in sulpholan were mentioned above. Each compound forms a stable intermediate very rapidly, and the intermediate then decomposes slowly. It seems that here we have cases where the first stage of the two-step process is very rapid (reaction may even be occurring upon encounter), but the second stages are slow either because of steric factors or because of the feeble basicity of the solvent. The course of the subsequent slow decomposition of the intermediate from pentamethylbenzene is not yet fully understood, but it gives only a poor yield of pentamethylnitrobenzene. The intermediate from anthracene decomposes at a measurable speed to 9-nitroanthracene and the observations are compatible with a two-step mechanism in which k i k E and i[N02" ] > / i. There is a kinetic isotope effect (table 6.1), its value for the reaction in acetonitrile being near to the... 

One possibility is the two step mechanism of Figure 5 12 m which the carbon-halogen bond breaks first to give a carbocation intermediate followed by depro tonation of the carbocation m a second step... 

The dissolution of soluble sihcates is of considerable commercial importance. Its rate depends on the glass ratio, sohds concentration, temperature, pressure, and glass particle size. Commercially, glasses are dissolved in either batch atmospheric or pressure dissolvers or continuous atmospheric processes. Dissolution of sodium sihcate glass proceeds through a two-step mechanism that involves ion exchange (qv) and network breakdown (18). 

The above stm study also discovered a facile transport of surface gold atoms in the presence of the Hquid phase, suggesting that the two-step mechanism does not provide a complete picture of the surface reactions, and that adsorption/desorption processes may have an important role in the formation of the final equiHbrium stmcture of the monolayer. Support for the importance of a desorption process comes from atomic absorption studies showing the existence of gold in the alkanethiol solution. The stm studies suggest that this gold comes from terraces, where single-a tomic deep pits are formed (281—283). 

Although the precise mechanism of plasminogen activation is unknown, three principal theories have developed based on studies of the in vitro activation of native human plasminogen. Activation of native Glu-plasminogen in the absence of any plasmin inhibitor yields Lys —plasmin plus the so-called pre-activation peptides (PAP) formed by cleavage at LySg2 S E3 Activation takes place by a two-step mechanism in... 

Formation of mixtures of (E)- and (Z)-oxaziridines from sterically defined Schiff bases fits a two step mechanism through (244) (70CC745). 

Cement mortar will be attacked by waters that have an excess of free carbon dioxide compared with that of waters that are in a lime-carbonic acid equilibrium. There is a two-step mechanism with a carbonization process according to... 

The reaction of 4,4-bis(tnfluoromethyl)-I,3-diaza-1,3-butadienes with certain a,P-unsaturated ketones yields pyrimidine derivatives A two-step mechanism, metathesis-electrocyclic ring closure and metathesis-intramolecular ene reaction, is a plausible explanation for the experimental results (pathway 4, equa-bon 25) [259]... 

Electrophilic addition of HX to an alkene involves a two-step mechanism, the overall rate being given by the rate of the initial protonation step. Differences in protonation energies are usually explained by considering differences in carbocation stability, but the relief or buildup of strain can also be a factor. One of the following alkenes protonates much more easily than the other. 

Bromination usually follows a two-step mechanism, the rate-limiting step involving formation of an adduct with Br. Calculate energies for Br addition to phenylacetylene and styrene, leading to phenylacetylene+Br+ and styrene+Br+, respectively. (The energy of Br+ is given at right.) Which reaction is more favorable Is this the same preference as seen for Br2 addition ... 

Benzene and substituted benzenes reaet with electrophiles, leading to new functionality. The two-step mechanism involves initial attack by an electrophile to form an intermediate (benzenium ion), followed by elimination of a proton to generate the substituted benzene. 

The latter mechanism is characterized by a transition-state structure of type 64 and by its being (in part) analogous to the mechanism at a saturated carbon. The preference of a two-step mechanism to the apparently simpler one-step mechanism is suggested by the isolation of Meisenheimer complexes and by the kinetics of their formation. The experimental evidence on these... 

Pasinszki and Westwood investigated the dimerization of chloronitrile oxide CICNO to 3,4-dichloro-l,2,5-oxadiazole-2-oxide 78 (Scheme 48) [98JPC(A) 4939]. From B3-LYP/6-31G calculations, they conclude that the reaction path can be characterized as a typical Firestone-type cycloaddition, a two-step mechanism with a C—C bond forming characterizing the first reaction step. The activation... 

Honk et al. concluded that this FMO model imply increased asynchronicity in the bond-making processes, and if first-order effects (electrostatic interactions) were also considered, a two-step mechanisms, with cationic intermediates become possible in some cases. It was stated that the model proposed here shows that the phenomena generally observed on catalysis can be explained by the concerted mechanism, and allows predictions of the effect of Lewis acid on the rates, regioselectivity, and stereoselectivity of all concerted cycloadditions, including those of ketenes, 1,3-dipoles, and Diels-Alder reactions with inverse electron-demand [2],... 

To illustrate this process, consider the two-step mechanism for the low-temperature reaction between CO and N02. 

This analysis explains why the rate expression for the two-step mechanism is different from that for the direct, one-step reaction. 

To illustrate this situation, consider the reaction between nitric oxide and chlorine, which is believed to proceed by a two-step mechanism ... 

There are two general methods within this subcategory, involving one- or two-step mechanisms. Nitrenes and metalonitrenes thus add to alkenes by a direct azir-idination reaction, whereas nonmetallic nitrenoids usually react through an addition-elimination process (Scheme 4.6). 

Klotz and associates120 found that PEI-D-Im-type models catalyzed hydrolyses of 4-nitrocatechol sulfate by a two step mechanism resembling that of a natural enzyme. The polymer was reported to accelerate the rate by a factor of 1012 arm-pared to imidazole itself, and of 102 compared to a type-11A arylsulfatase enzyme. 

Finally, it should be of interest that the rearrangements of several cyclic benzylic sulfinates have also been described in the literature by Durst46 and Hogeveen47, and seem to proceed by a special two-step mechanism retro-Diels-Alder extrusion of S02, followed by its chelotropic addition to the unstable quinodimethane intermediate (e.g. equation 10). 

In order to introduce an S03 group into long-chain fatty acids, direct sulfonation with sulfur trioxide had been suggested in 1962 [29], similar to the Hell-Volhard-Zelinskii reaction of the a-bromation studied by Watson [34], Ten years previously de Boer had reviewed this mechanism among others for lower aliphatic sulfocarboxylic acids [35]. The sulfonation process was proposed as a two-step mechanism. The first step is the rapid addition of the S03 into... 

Schmid et al. studied in detail the sulfonation reaction of fatty acid methyl esters with sulfur trioxide [37]. They measured the time dependency of the products formed during ester sulfonation. These measurements together with a mass balance confirmed the existence of an intermediate with two S03 groups in the molecule. To decide the way in which the intermediate is formed the measured time dependency of the products was compared with the complex kinetics of different mechanisms. Only the following two-step mechanism allowed a calculation of the measured data with a variation of the velocity constants in the kinetic differential equations. 

FIGURE 13.17 An alternative, two-step mechanism for the decomposition of ozone. In the first step, an energized ozone molecule shakes off an oxygen atom. In the second step, that oxygen atom attacks another ozone molecule. 

Two-step mechanism In the first step, an C)3 molecule is energized by solar radiation and dissociates into an O atom and an 02 molecule. In the second step, the O atom attacks another 03 molecule to produce two more 02 molecules (Fig. 13.17) ... 

Any forward reaction that can take place is also accompanied, in principle at least, by the corresponding reverse reaction. Therefore, the unimolecular decay of 03 in step 1 of the two-step mechanism is accompanied by the formation reaction... 

Self-Test 13.1 IB The proposed two-step mechanism for a reaction is H2A + B — ... 

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