Substitution, unimolecular

HVZ MPV SE1 SE1(N) Hell-V olhard-Zelinskii. Meerwein-Ponndorf-Verley reduction. Electrophilic substitution unimolecular. SEI pathway in which a nucleophile (which may be the solvent) assists in the removal of the electrofuge. 

SN1 SNlcA Nucleophilic substitution unimolecular. Nucleophilic substitution unimolecular, conjugate acid, when the leaving group departs only after protonation. Also called A1. 

SNlcB Nucleophilic substitution unimolecular conjugate base, when there is an initial deprotonation and then formation of a carbene. 

A benchmark set of barrier heights of hydrogen transfer, heavy-atom transfer (i.e. transfer of atoms other than H), nucleophilic substitution, unimolecular and association reactions, was recently assembled by Truhlar and coworkers [35, 36]. It consists of forward and reverse barrier heights for 12 reactions and will be referred to as DBH24. The best estimates of the barrier heights, as well as the geometries of all the species in this set, optimized with a correlated wave function method, are available in the supporting information of Ref. [36]. 

Method Hydrogen transfer Heavy atom transfer Nucleophilic substitution Unimolecular and association... 

Structural alerts or chemical motifs known to be associated with toxicity through either the parent compound or reactive metabolites have also been used to predict potential toxicity from chemical structures. These expert algorithms appear in commercial software programs and in onhne open access sites. For instance, there are seven chemical domains that are used to define and predict the covalent interaction between a chemical and a macromolecule (biological target) that leads to an initiating event at the beginning of an AOP [34]. These include Michael addition, acylation, Schiff base formation, aromatic nucleophilic substitution, unimolecular... 

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... 

Illustrate the stereochemistry associated with unimolecular nucleophilic substitution by con structmg molecular models of cis 4 tert butylcyclohexyl bromide its derived carbocation and the alcohols formed from it by hydrolysis under S l conditions... 

Substitution nucleophilic unimolecular(SNl) mechanism (Sec tions 4 9 and 8 8) Mechanism for nucleophilic substitution charactenzed by a two step process The first step is rate determining and is the ionization of an alkyl halide to a carbocation and a halide ion... 

The following compounds have been obtained from thiete 1,1-dioxide Substituted cycloheptatrienes, benzyl o-toluenethiosulfinate, pyrazoles, - naphthothiete 1,1-dioxides, and 3-subst1tuted thietane 1,1-dioxides.It is a dienophile in Diels-Alder reactions and undergoes cycloadditions with enamines, dienamines, and ynamines. Thiete 1,1-dioxide is a source of the novel intermediate, vinylsulfene (CH2=CHCH=SQ2). which undergoes cyclo-additions to strained olefinic double bonds, reacts with phenol to give allyl sulfonate derivatives or cyclizes unimolecularly to give an unsaturated sultene. - Platinum and iron complexes of thiete 1,1-dioxide have been reported. 

By changing from the simplest to larger aliphatic and cyclic ketones, structural factors may be introduced which favor alternative unimolecular primary photoprocesses or provide pathways to products not available to the simple model compound. In addition, both the increase in molecular size and irradiation in solution facilitate rapid vibrational relaxation of the electronically excited reactant as well as the primary products to thermally equilibrated species. In this way the course of primary and secondary reactions will also become increasingly structure-selective. In a,a -unsym-metrically substituted ketones, the more substituted bond undergoes a-cleavage preferentially. 

Section 4.9 The potential energy diagrams for separate elementary steps can be merged into a diagram for the overall process. The diagram for the reaction of a secondary or tertiary alcohol with a hydrogen halide is characterized by two intermediates and three transition states. The reaction is classified as a unimolecular- nucleophilic substitution, abbreviated as SnI. 

The proposed mechanism is outlined in Figure 8.5 and is called SnI, standing for substitution nucleophilic unimolecular. The first step, a unimolecular- dissociation of the alkyl halide to for-rn a carbocation as the key inter-mediate, is rate-deter-rnining. An energy diagrffln for the process is shown in Figure 8.6. 

The noncatalytic substitution of a 4-diazonium group by nucleophiles, which has proved widely useful in this series, is probably an example of a unimolecular nucleophilic substitution. The first example... 

Sfsjl, S)s]2, El, ElcB, E2—how can you keep it all straight and predict whal will happen in any given case Will substitution or elimination occur Wil the reaction be bimolecular or unimolecular There are no rigid answers tc... 

This possibility can be dismissed22 because it is asymmetric. Read backward, the first step would be not the second-order axial addition of Cl" but a unimolecular rearrangement, and an improbable one at that. Aside from this, the fact that the pathway is not symmetric rules it out. If it is invalid in the special case of an exchange, then it can hardly provide the general pathway for substitution at square-planar metal centers. 

The kinetic form of the decomposition in various solvents indicates competing unimolecular homolysis of the peroxide link (a) and radical induced decomposition (b). Other diacyl peroxides behave similarly, except that, in the case of acetyl peroxide, induced dceomposition is much less important. More highly branched aliphatic or a-phenyl-substituted diacyl peroxides decompose more readily, partly because induced decomposition is more important again and partly because of the occurrence of decomposition involving cleavage of more than one bond (for a mechanistic discussion of these cases, see Walling et al., 1970). 

The Sgl mechanism substitution electrophilic unimolecular) is rare, being found only in certain cases in which carbon is the leaving atom (see 11-37,11-38) or when a very strong base is present (see 11-1,11-11, and 11-42). It consists of two steps with an intermediate carbanion. The lUPAC designation is Dg + Ag. 

For aliphatic electrophilic substitution, we can distinguish at least four possible major mechanisms, which we call Sgl, 8 2 (Iront), Se2 (back), and Sgi. The Sgl is unimolecular the other three are bimolecular. 

A unimolecular ionization was shown to be the mechanism of solvolysis by means of rate studies, solvent effects, salt effects, and structural effects (179,180). The products of reaction consist of benzo [bjthiophen derivatives 209 or nucleophilic substitution products 210, depending upon the solvent system employed. By means of a series of elegant studies, Modena and co-workers have shown that the intermediate ion 208 can have either the open vinyl cation structure 208a or the cyclic thiirenium ion 208b, depending... 

The wide utility of aryl diazonium ions as synthetic intermediates results from the excellence of N2 as a leaving group. There are several general mechanisms by which substitution can occur. One involves unimolecular thermal decomposition of the diazonium ion, followed by capture of the resulting aryl cation by a nucleophile. The phenyl cation is very unstable (see Part A, Section 3.4.1.1) and therefore highly unselective.86 Either the solvent or an anion can act as the nucleophile. 

Gas-phase SN2 nucleophilic substitution reactions are particularly interesting because they have attributes of both bimolecular and unimolecular reactions.1 As discovered from experimental studies by Brauman and coworkers2 and electronic structure theory calculations,3 potential energy surfaces for gas-phase SN2 reactions of the type,... 

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