Bimolecular intermediates

Toluene disproportionation (TDP) is a well-known acid reaction, occurring through the same mechanism as xylene disproportionation (Figure 9.4). Like this latter reaction, toluene disproportionation requires most likely two protonic sites for it catalysis, hence the density of protonic sites has a very positive effect on the catalyst activity. Furthermore, the bimolecular intermediates (methyldiphenyl-... 

Many carbene complexes, especially those bearing one or two hydrogen substituents, are thermally unstable with respect to the formation of alkenes or their complexes (Figure 5.13), usually via bimolecular intermediates. For this reason the thermal stability of carbene complexes can normally be enhanced by inclusion of sterically demanding co-ligands. Often within a triad the stability of carbene complexes increases in the order 4d < 3d < 5d when analogous compounds can be obtained and compared. 

Hydrocarbon nitro derivatives can also be reduced by this reagent Allen70 obtained up to 100% of aniline from nitrobenzene. Bimolecular intermediates are rarely obtained in spite of the alkalinity of the reaction medium. The proportions of reactants used are in accord with the reaction ... 

While it was found by means of isotopic studies than on amorphous silica-alumina the reaction proceed by an intramolecular mechanism (194), in zeolite Y, the distribution of isomers in the trimethylbenzene fraction indicates that some of the isomers could be obtained by a bimolecular mechanism (172,175). In a very recent work (196,197) it has been demonstrated by means of isotopic studies, that on some 12 MR zeolites such as Y, and mordenite, xylenes are isomerized by both uni and bimolecular transalkylation mechanism. The ratio of the uni to bimolecular increases when increasing the Si/Al ratio, and decreases when increasing the reaction temperature, the partial pressure of the feed, and the contact time. Another 12 MR, Beta zeolite, while being able to disproportionate xylene, does not isomerize via the bimolecular mechanism. This was explained by space constraints to accommodate a xylene and a trimethylbenzene as a bimolecular intermediate in the channels of the zeolite. A medium pore zeolite (ZSM-5) does isomerize only through a unimolecular 1,2 methyl-shift mechanism. 

Iridium.—The thermal decomposition of the octyl iridium complex (3) gives predominantly octene. Octane, however, is also produced in substantial amounts which are dependent on the concentration of triphenylphosphine (L). It is proposed that octane is lost from the bimolecular intermediate (4) (Scheme 8). ... 

Nickel.— Phosphine and phosphite complexes of nickel(0) react with strong acids to produce complexes [NiHL4]+. Reaction with weak acids may proceed further, with attack of the anion at the nickel. Thus the complex Ni(LL)a, where LL = l,4-bis(diphenylphosphino)butane, reacts with hydrogen cyanide to form firstly a hydride, which reacts quickly with cyanide to give the bimolecular intermediate Ni2(CN)2(LL)a. The ultimate products are the nickel(n) monomer Ni(CN)2(LL)2 and dimer [Ni(CN)2-(LL)]2. Ni(PPh3)4 undergoes normal oxidative elimination reactions with aryl halides to produce the new nickel(n) complexes Ni(aryl)(X)-(PPh3)2. ... 

It is generally believed that the base-catalyzed condensation mechanism involves penta- or hexacoordinated silicon intermediates or transition states [79,81,90-92]. For silicic acid polymerization, Okkerse [90] proposed a bimolecular intermediate involving one hexacoordinated silicon ... 

Primary alcohols do not react with hydrogen halides by way of carbo cation intermediates The nucleophilic species (Br for example) attacks the alkyloxonium ion and pushes off a water molecule from carbon m a bimolecular step This step is rate determining and the mechanism is Sn2... 

Dehydration of alcohols (Sections 5 9-5 13) Dehydra tion requires an acid catalyst the order of reactivity of alcohols IS tertiary > secondary > primary Elimi nation is regioselective and proceeds in the direction that produces the most highly substituted double bond When stereoisomeric alkenes are possible the more stable one is formed in greater amounts An El (elimination unimolecular) mechanism via a carbo cation intermediate is followed with secondary and tertiary alcohols Primary alcohols react by an E2 (elimination bimolecular) mechanism Sometimes elimination is accompanied by rearrangement... 

Partial but not complete loss of optical activity m S l reactions probably results from the carbocation not being completely free when it is attacked by the nucleophile Ionization of the alkyl halide gives a carbocation-hahde ion pair as depicted m Figure 8 8 The halide ion shields one side of the carbocation and the nucleophile captures the carbocation faster from the opposite side More product of inverted configuration is formed than product of retained configuration In spite of the observation that the products of S l reactions are only partially racemic the fact that these reactions are not stereospecific is more consistent with a carbocation intermediate than a concerted bimolecular mechanism... 

Reaction 21 is the decarbonylation of the intermediate acyl radical and is especially important at higher temperatures it is the source of much of the carbon monoxide produced in hydrocarbon oxidations. Reaction 22 is a bimolecular radical reaction analogous to reaction 13. In this case, acyloxy radicals are generated they are unstable and decarboxylate readily, providing much of the carbon dioxide produced in hydrocarbon oxidations. An in-depth article on aldehyde oxidation has been pubHshed (43). 

Studies of the stereochemical course of rmcleophilic substitution reactions are a powerful tool for investigation of the mechanisms of these reactions. Bimolecular direct displacement reactions by the limSj.j2 meohanism are expected to result in 100% inversion of configuration. The stereochemical outcome of the lirnSj l ionization mechanism is less predictable because it depends on whether reaction occurs via one of the ion-pair intermediates or through a completely dissociated ion. Borderline mechanisms may also show variable stereochemistry, depending upon the lifetime of the intermediates and the extent of internal return. It is important to dissect the overall stereochemical outcome into the various steps of such reactions. 

The preceding Sections illustrate several experimental features of heteroaromatic substitutions. It is now intended to comment on some of these features which are most significant in terms of reaction mechanism. As stated in the Introduction, a possible mechanism of nucleophilic bimolecular aromatic substitution reactions is that represented by Eq. (14), where an intermediate of some stability... 

It is quite reasonable to expect the bimolecular two-stage mechanism Sj Ar ) to predominate in most aromatic nucleophilic substitutions of activated substrates. However, only in rare instances is there adequate evidence to rule out the simultaneous occurrence or predominance of other mechanisms. The true significance of the alternative mechanisms in azines needs to be determined by trapping the intermediates or by applying modem separation and characterization methods to the identification of at least the major portion of the products, especially in kinetic studies. 

It should be pointed out that the existence of stable structures of the intermediate-complex type (also known as a-complexes or Wheland complexes) is not of itself evidence for their being obligate intermediates in aromatic nucleophilic substitution. The lack of an element effect is suggested, but not established as in benzene derivatives (see Sections I,D,2 and II, D). The activated order of halogen reactivity F > Cl Br I has been observed in quantita-tivei36a,i37 Tables II, VII-XIII) and in many qualitative studies (see Section II, D). The reverse sequence applies to some less-activated compounds such as 3-halopyridines, but not in general.Bimolecular kinetics has been established by Chapman and others (Sections III, A and IV, A) for various reactions. 

The k2 term suggests a simple bimolecular process in which nucleophilic attack by Y leads to a SN2 reaction. Associative paths will involve a 5-coordinate (sp or tbp) intermediate, and the relative rarity of isolable 5-coordinate plati-num(II) species - compared with 4-coordinate - is not inconsistent with their involvement as reactive intermediates (Figure 3.81). 

Baltrop and Bunce (Ref 20) employed a variety of radiation wavelengths, nitrocompds and solvents. For wavelengths less than 2900A, aniline was the main product, while above 2900A, bimolecular species such as azobenzene predominated. Since oxygen had little effect on aniline production, expts were performed in the presence of oxygen. For nitrobenzene in isopropyl alcohol, no azoxybenzene was produced as with Hurley and Testa (See above Ref 17). They concluded that the excited state abstracts H-atoms, and suggest that the nitrobenzene triplet is in tt, ti, and that nitrosobenzene is an unobserved intermediate... 

The experiments with 2-(3-butenyloxy)benzenediazonium ions (10.55, Z = 0, n = 2, R=H) and benzenethiolate showed a significant shift of the product ratio in favor of the uncyclized product 10.57. They also indicated that the covalent adduct Ar — N2 — SC6H5 is formed as an intermediate, which then undergoes homolytic dissociation to produce the aryl radical (Scheme 10-83). Following the bimolecular addition of the aryl radical to a thiolate ion (Scheme 10-84), the chain propagation reaction (Scheme 10-85) yielding the arylphenylsulfide is in competition with an alternative route leading to the uncyclized product 10.57. 

A careful distinction must be drawn between transition states and intermediates. As noted in Chapter 4, an intermediate occupies a potential energy minimum along the reaction coordinate. Additional activation, whether by an intramolecular process (distortion, rearrangement, dissociation) or by a bimolecular reaction with another component, is needed to enable the intermediate to react further it may then return to the starting materials or advance to product. One can divert an intermediate from its normal course by the addition of another reagent. This substance, referred to as a trap or scavenger, can be added prior to the start of the reaction or (if the lifetime allows) once the first-formed intermediate has built up. Such experiments are the trapping experiments referred to in Chapters 4 and 5. 

Note that two H+ and one CU ions are formed. The rate-controlling step may be a bimolecular reaction of the intermediates so formed note that the composition of its transition state does, indeed, conform to the data ... 

Sn2 stands for substitution nucleophilic bimolecular. The lUPAC designation (p. 384) is AnDn- In this mechanism there is backside attack The nucleophile approaches the substrate from a position 180° away from the leaving group. The reaction is a one-step process with no intermediate (see, however, pp. 392-393 and 400). The C—Y bond is formed as the C—X bond is broken ... 

This intermediate is similar to those encountered in the neighboring-group mechanism of nucleophilic substitution (see p. 404). The attack of W on an intermediate like 2 is an Sn2 step. Whether the intermediate is 1 or 2, the mechanism is called AdE2 (electrophilic addition, bimolecular). 

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