S 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... 

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

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.

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. ... 

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. 

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. 

Substitution of the free epoxide, which generally occurs under basic or neutral conditions, usually involves an Sn2 mechanism. Since primary substrates undergo Sn2 attack more readily than secondary, unsymmetrical epoxides are attacked in neutral or basic solution at the less highly substituted carbon, and stereospecifically, with inversion at that carbon. Under acidic conditions, it is the protonated epoxide that undergoes the reaction. Under these conditions the mechanism can be either SnI or Sn2. In S l mechanisms, which favor tertiary carbons, we might expect that attack would be at the more highly substituted carbon, and this is indeed the case. However, even when protonated epoxides react by the 8 2 mechanism, attack is... 

An example of a reaction series in which large deviations are shown hy — R para-substituents is provided by the rate constants for the solvolysis of substituted t-cumyl chlorides, ArCMejCl ". This reaction follows an S l mechanism, with intermediate formation of the cation ArCMej. A. — R para-substituent such as OMe may stabilize the activated complex, which resembles the carbocation-chloride ion pair, through delocalization involving structure 21. Such delocalization will clearly be more pronounced than in the species involved in the ionization of p-methoxybenzoic acid, which has a reaction center of feeble -t- R type (22). The effective a value for p-OMe in the solvolysis of t-cumyl chloride is thus — 0.78, compared with the value of — 0.27 based on the ionization of benzoic acids. 

Reactions of nitrothiazole derivatives with anions of nitroalkanes, such as shown in Eq. 5.34, proceed via a S l mechanism.52... 

Haloarenes have been found to undergo nucleopilic substitution when irradiated with the triphenyl stannyl anion46, reacting via a radical S l mechanism. In many cases the reaction will only occur under photochemical conditions. The reaction is found to proceed with chloro- and bromo-substituted arenes, but not iodo-compounds. The anion is produced either by treatment of triphenyltin chloride or hexaphenylditin with sodium metal in liquid ammonia, and will react with a wide variety of arenes (reaction 30). 

An example of a reaction series in which large deviations are shown by —R para-substituents is provided by the rate constants for the solvolysis of substituted t-cumyl chlorides. ArCMe2Cl82. This reaction follows an S l mechanism, with intermediate formation of the cation ArCMe2+. A —R para-substituent such as OMe may stabilize the... 

The reaction of 2-chloroquinoline with benzenethiolate ions in liquid ammonia (Amatore et al., 1979a) is a good example for testing the possible occurrence of an Shn2, instead of S l, mechanism, in which the formation of RNu from RX-" and Nu would involve concerted, rather than stepwise, bond breaking and bond formation (Scheme 11). This mechanism... 

For any other substrate acting by an S l mechanism, it might be expected that... 

Reference has already been made in the last chapter to the generation of carbocations, in ion pairs, as intermediates in some displacement reactions at a saturated carbon atom, e.g. the solvolysis of an alkyl halide via the S l mechanism. Carbocations are, however, fairly widespread in occurrence and, although their existence is often only transient, they are of considerable importance in a wide variety of chemical reactions. 

The reaction develops according to typical S l mechanism (see Scheme 7.43) and proceeds without photoirradiation. 

Similarly, interphase catalysis permits carbonylation of haloaryls and halovinyls by NaCo(CO)4 with CO in water. The reaction proceeds according to S l mechanism and leads to high enough yields of carboxylic acids. Because C r-Cl bond is inert in these conditions, the direct and selective synthesis of chlorocarboxylic acids becomes possible. Thus, l-bromo-4-chlorobenzene gives 4-chlorobenzoic acid only (Brunet et al. 1983 Scheme 7.67). 

Instability of phenyl cation In case of haloarenes, the phenyl cation formed as a result of self-lonlsatlon will not be stabilised by resonance and therefore, S l mechanism Is ruled out. Because of the possible repulsion. It Is less likely for the electron rich nucleophile to approach electron rich arenes. Replacement by hydroxyl group... 

Nalimov, S.R, Todes, O.M. and Radin, S.L, Mechanism of thermal subdivision of granules during drying of solutions in a fluidized bed, /. Appl. Chem. USSR, 48 (1975) 2Q73-2Q77. 

In the paper, we explore the possibility of interpreting these results in terms of a limiting type of S l mechanism with an intermediate which has a proposed lifetime sufficient to discriminate between various nucleophiles present in the system. 

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