Unimolecular mechanism

The unimolecular mechanism involves formation of a protonated cyclopropane ring first, which avoids the formahon of a primary carbenium ion until after skeletal rearrangement has taken place. Such reachon intermediates were first 

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While monomolecular collision-free predissociation excludes the preparation process from explicit consideration, themial imimolecular reactions involve collisional excitation as part of the unimolecular mechanism. The simple mechanism for a themial chemical reaction may be fomially decomposed into tliree (possibly reversible) steps (with rovibronically excited (CH NC) ) ... 

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... 

A number of possible unimolecular mechanisms can be envisaged for decarboxylation ranging from one involving a free carbanion intermediate [Eq. (4)], to one involving a free carbonium ion intermediate [Eq. (5)]. In... 

Thermochemical parameters estimated by semiempirical AMI calculations have been found to support the proposal that isobutene formation on gas-phase thermolysis of iV-methyl-A-phenyl-fert -butylsulfenamide and morpholinyl-ferf -butylsulfenamide occurs by a unimolecular mechanism involving a four-centre cyclic transition state and co-formation of the corresponding thiohydroxylamines." ... 

The reactions on Rh/Ir usually proceed via oxidative addition to give hydrido (alkynyl) complexes, which then undergo 1,3-H shifts to form the vinylidene complexes. In general, a unimolecular mechanism has been considered to be operative. Recent studies of RhCl(PPr 2R)2 (R = C=NCBu =CHNMe) complexes have shown a remarkable acceleration of the isomerization, with the =C=CHBu complex being formed within seconds [32]. Suitable cross-over experiments showed that a bimolecular mechanism, earlier suggested by some experimental and computational results [33], did not operate. 

It is easy to argue that the behavior of the [Co en2 Cl2]+ isomers is not surprising. The correlation of aquation rates of complexes of the type, [Co en2 A Cl]n+ with the electron displacement properties of the nonparticipating ligand, A, has led to the belief that ligands able to donate a second pair of electrons to the metal can thereby stabilize the 5-coordinate intermediate and hence promote a unimolecular reaction (2, 18, 24). Chlorine is such a ligand, Cl—Co- -Cl, and the essentially first-order kinetic form could be used as evidence for a unimolecular mechanism, once the ion association pre-equilibrium effects for the displacement of chloride under the electron-displacing influence of the other chlorine atom have been taken into account. 

Activation parameters have been measured for these reactions, and are also in agreement with the unimolecular mechanism, since the entropies of activation are all positive. For lsO- exchange from solvent water into mesitoic acid, under the conditions described above, Bunton et al.49 found a AS of... 

The two unimolecular mechanisms for acid-catalyzed ester hycrolysis described above represent exceptional behaviour. They are generally observed only with compounds with narrowly defined structural characteristics, or under extreme conditions. The vast majority of esters are hydrolyzed under the vast majority of acidic conditions by the Aac2 mechanism, and since ester hydrolysis has been a traditional proving ground for theories relating structure and reactivity, a wealth of data is available. Yet the detailed mechanism of the reaction is still in dispute. 

The detailed mechanism, or mechanisms, of the solvolysis of acid chlorides is still a matter of dispute. There are at least four possible mechanisms, (a)-(d) below, all of which have been proposed either to act separately or in various combinations, and there is a unified mechanism, that of Minato93 which will be discussed later. The bimolecular mechanisms (a) and (b) differ in that (a) includes a tetrahedral intermediate whereas (b) does not. The former is commonly accepted as the most likely for the bimolecular mechanism and the arguments against (b) have been stated in the introduction. There is, however, good evidence for (A), at least in the case of the hydrolysis of chloracetyl chloride94. The acylium ion mechanism (c) and the hydrated carbonium ion mechanism (d) are both unimolecular mechanisms. Whereas the acylium ion XXVII has never been directly observed in hydrolysis or alcoholysis reactions, it is favoured as an intermediate by many workers, although it is kinetically indistinguishable from XXVIII. 

Note, however, that the comparison is with measurements for the benzoyl halides in a more aqueous medium than the acetyl halides and that this would favour a greater contribution from the unimolecular mechanism in the benzoyl halides. 

Now, Sn2 mechanisms such as (a) and (b), p. 227, would be favoured by electron withdrawal, making bond making more important and the attack of a nucleophile more likely. Unimolecular mechanisms will be favoured by electron donation and a very powerful electron-donating substituent should favour a rate-determining ionisation of type (c) rather than an initial pre-equilibrium of type (d). The shift of mechanism with increased polarity of the medium reflects the more polar transition state favoured by such a change. 

Since benzoyl chloride reacts by a mechanism which is either a combination of Sn1/Sn2 (unimolecular/bimolecular) or some intermediate mechanism, a great deal of work has been directed towards elucidation of the factors which control its reactivity. In the context of the 3 and 4-substituted benzoyl halides, it is seen from the evidence of Hammett plots to react predominantly by a bimolecular mechanism in solvents of low polarity and by a unimolecular mechanism in solvents of high polarity. 

Another important factor determining the bimoleeular course of simple homogeneous gaseous decompositions is that the unimolecular mechanism would result, in many instances, in the production of free atoms, thus... 

In the bimolecular reaction (eqn. 3.2-47), bond-breaking and bond-forming take place simultaneously and therefore a negative activation volume would be expected. In the unimolecular mechanism, bond-breaking during the first, rate-determining slow reaction (eqn. 

A concerted unimolecular mechanism has been proposed to account for the formation of elimination products on solvolysis of the tertiary l-(4-methoxyphenyl)-3-methy 1-3-butyl derivatives (38-X) in aqueous solvents.19 Thus, in 50 50 (v/v) CF3CH20H-H20, (38a) and (38b) give 39% and 56%, respectively, of the alkene... 

It has been suggested that reaction 276 proceeds according to the A-2 mechanism shown in equation 277 at low acidities and according to the A-l unimolecular mechanism at high acidities (equation 278). The protonation behaviour of 469 and the problem whether the hydrolysis proceeds via N- or O-protonated conjugate acid is currently being studied536-541. 

Now the rate of carbanion formation will be proportional to [RMX ][B], and the mechanism denoted as SE1-B. For instance Ingold3 has described a unimolecular mechanism catalysed by two bromide ions as SEl-2Br . 

Nucleophilic catalysis in the unimolecular mechanism is straightforward, since there is but one reactant that can bring the nucleophile into the transition state. If, however, a bimolecular substitution is found to be subject to nucleophilic catalysis and if it is concluded that, say, one molecule of the nucleophile, B, is involved in the transition state, then two main possibilities exist. The nucleophile may be brought into the transition state either by the organometallic substrate as in process (22) or by the electrophile as in process (23), viz. 

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