L- mechanism

For a proposed reaction mechanism to be valid the sum of its elementary steps must equal the equation for the overall reaction and the mechanism must be consistent with all experimental observations The S l mechanism set forth m Figure 4 6 satisfies the first criterion What about the second d... 

The rate determining step m the S l mechanism is dissociation of the alkyloxo mum ion to the carbocation... 

Rearrangements when they do occur are taken as evidence for carbocation inter mediates and point to the S l mechanism as the reaction pathway Rearrangements are never observed m 8 2 reactions... 

Solvent Effects on the Rate of Substitution by the S l Mechanism Table 8 6 lists the relative rate of solvolysis of tert butyl chloride m several media m order of increasing dielectric constant (e) Dielectric constant is a measure of the ability of a material m this case the solvent to moderate the force of attraction between oppositely charged par tides compared with that of a standard The standard dielectric is a vacuum which is assigned a value e of exactly 1 The higher the dielectric constant e the better the medium is able to support separated positively and negatively charged species 8olvents... 

Both compounds react by an S l mechanism and their relative rates reflect their acti vation energies for carbocation formation Because the allylic chloride is more reactive we reason that it ionizes more rapidly because it forms a more stable carbocation Struc turally the two carbocations differ m that the allylic carbocation has a vinyl substituent on Its positively charged carbon m place of one of the methyl groups of tert butyl cation... 

The S l mechanism, ia which the substituent leaves before the incoming nucleophile attacks, is less frequentiy encountered, although it is known to occur... 

SECTION 5.1. THE LIMITING CASES— SUBSTITUTION BY THE IONIZATION (Sf,l) MECHANISM... 

Fig. S.l. Potential eneigy diagram for nucleophilic substitution by the ionization (S l) mechanism.

The points that we have emphasized in this brief overview of the S l and 8 2 mechanisms are kinetics and stereochemistry. These features of a reaction provide important evidence for ascertaining whether a particular nucleophilic substitution follows an ionization or a direct displacement pathway. There are limitations to the generalization that reactions exhibiting first-order kinetics react by the Sj l mechanism and those exhibiting second-order kinetics react by the 8 2 mechanism. Many nucleophilic substitutions are carried out under conditions in which the nucleophile is present in large excess. When this is the case, the concentration of the nucleophile is essentially constant during die reaction and the observed kinetics become pseudo-first-order. This is true, for example, when the solvent is the nucleophile (solvolysis). In this case, the kinetics of the reaction provide no evidence as to whether the 8 1 or 8 2 mechanism operates. 

Fig. 5.5. Potential energy diagrams for substitution mechanisms. A is the S l mechanism. B is the Sjy2 mechanism with intermediate ion-pair or pentacooidi-nate species. C is the classical S).(2 mechanism. [Reproduced from T. W. Bentley and P. v. R. Schleyer, Adv.

Throne, J.L. Mechanical properties of thermoplastic structural foams, in Wendle, B.C. (ed.) Engineering Guide to Structural Foams, Technomic, Lancaster, PA, USA (1976) pp. 91 -114. 

Bender, M.L. Mechanisms of Homogeneous Catalysis from Protons to Proteins Wiley-lnterscience New York, 1971. 

In this example the S l mechanism is the route of Fig. 5-19. The 1 route is clearly much less favored (the 1 state is of higher energy) than the route for this reaction type. 

Series 7 gives a linear plot with p = —5.27, consistent with the S l mechanism. Series 8, however, shows a discontinuity, which Gassmann and Fentiman interpreted as a change in mechanism. Compounds in series 8 are capable of intramolecular assistance (neighboring group participation) by electron donation from the double bond to stabilize the cation, as in 9. 

A generally applicable reaction scheme naturally cannot be given. The reaction mechanism of one particular carbinolamine with a particular reagent can depend on the reaction conditions in nonpolar solvents, the nondissociated carbinolamine obviously reacts (Sj 2 mechanism). In polar solvents, on the other hand, the mesomeric cation reacts (S l mechanism). Formally all these reactions belong to the general class of aminomethylation. The reaction products can be considered to be Mannich bases. ... 

Reacdons of nitrothiazole derivatives v/ith anions of nitroalkanes, such as shovm in Eq 5 34, proceed via a Spj l mechanism... 

Rogers, Grover L., Mechanics of So/ids, John Wiley and Sons, New York, 1964. 

Raymond, E. L., Mechanism of Sensitisation and Stabilisation of Incoloy Nickel-lron-Chromium Alloy 825 , Corrosion, 24, 180 (1968)... 

Tertiary alcohols react with either IIC1 or HBr at 0 °C by an S l mechanism through a carbocation intermediate. Primary and secondary alcohols are much more resistant to acid, however, and are best converted into halides by treatment with either SOCI2 or PBr3 through an Sfsj2 mechanism. 

The reaction of a tertiary alcohol with HX takes place by an Sf l mechanism when acid protonates the hydroxyl oxygen atom, water is expelled to generate... 

Step 2 The second step is removal of the DMT protecting group by treatment with dichloroacetic acid in CH2CI . The reaction occurs by an S -l mechanism and proceeds rapidly because of the stability of the tertiary, benzylic dimethoxytrityl cation. 

The criterion which has been extensively employed to distinguish A-l mechanisms from A-2 or A-SE2 mechanism has been the linearity of plots of reaction rate coefficient versus the acidity function h0. The acidity function (see Volume 2, p. 358) is a measure of the proton-donating ability of a medium (as measured by its tendency to protonate a base B) which is given by equation (11), viz. 

It can be shown2 that the first-order rate coefficient for the A-l mechanism kx... 

If the activity coefficient term on the right of equation (13) stays constant with changing medium, then a linear correlation of log kx with -H0 should follow for the A-l mechanism. The above diagnosis of the A-l mechanism is known as the Zucker-Hammett hypothesis3. 

On the basis of these correlations, Gold and Satchell463 argued that the A-l mechanism must apply (see p. 4). However, a difficulty arises for the hydrogen exchange reaction because of the symmetrical reaction path which would mean that the slow step of the forward reaction [equilibrium (2) with E and X = H] would have to be a fast step [equivalent to equilibrium (1) with E and X = H] for the reverse reaction, and hence an impossible contradiction. Consequently, additional steps in the mechanism were proposed such that the initial fast equilibrium formed a 7t-complex, and that the hydrogen and deuterium atoms exchange positions in this jr-complex in two slow steps via the formation of a a-complex finally, in another fast equilibrium the deuterium atom is lost, viz. 

Other examples of linear correlations of log rate coefficients with — H0 have been reported though most of them are deficient in some respect. Satchell474 observed such a correlation in the dedeuteration of anisole and benzene in various aqueous or acetic acid solutions of sulphuric acid, and aqueous perchloric acid, media at 25 °C. First-order rate coefficients are given in Table 127 along with those for dedeuteration of [4,5-2H2]-l,3,5-trimethoxybenzene and -2-hy-droxy-l,3-dimethoxybenzene475. For the ortho- andpara-monodeuterated anisoles the slopes of the log rate versus — H0 plots were the same for aqueous sulphuric acid and for acetic acid-sulphuric acid so that it was concluded that the A-l mechanism therefore applied to non-aqueous media as well as to aqueous media. The fact that the slope of the log rate coefficient versus —H0 plot was less (1.12) for benzene in aqueous perchloric acid than that (1.36) previously found in aqueous... 

Thus it can be seen that evidence for the A-l mechanism, even if one accepted that this followed from a linear rate coefficient-acidity function correlation, was scant. On the other hand, there have been a very large number of carefully documented studies in which general acid catalysis has been observed leading to the A-Se2 mechanism for the reaction, or it has been shown that the conclusions from an acidity function dependence are not rigorous. One such study has already been described above, and Satchell478 also found that in the detritiation of [4,6-3H2]-l,2,3-trimethoxybenzene by potassium bisulphate, dichloro- and tri-fluoroacetic acids, plots of log kl versus —H0 were linear with a slope of ca. 1.0... 

Kresge and Chiang15 showed that the dependence of rate coefficient upon the Hammett acidity function was not a safe criterion of mechanism and also explained how different dependencies of rate upon acidity function for different compounds could arise. Rate coefficients for detritiation of [3H]-2,4,6-tri-methoxybenzene followed the acidity function h0 in perchloric acid up to 3 M, i.e. k = 0.484 (Ao)1 07. so that a plot of log k versus —H0 would have a slope of 1.07, which implies the A-l mechanism. (New acidity function values479 would make this slope 1.14.) However, the diagnostic value of such a correlation was... 

Finally, these results clearly show yet again that attachment of hydrogen to the aromatic is not a fast step, thereby eliminating the A-l mechanism yet the data in mixed solvents followed the prediction of the Gross-Butler cubic equation, and once again this test turns out to be valueless. 

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