SN1 mechanism

Aryl cations are highly reactive intermediates with two possible electronic configurations, the singlet and triplet states. The former is a localized cation with a vacant a orbital at the dicoordinated carbon atom, whereas the latter has a diradical character with single occupancy of the g orbital and the charge being delocalized in the 

Singlet phenyl cations are unselective electrophiles, and reaction via such intermediacy mostly results in solvolysis, giving new C—O or C—N bonds. Triplet phenyl cations react selectively with n nucleophiles such as electron-rich olefins, alkynes and aromatics, forming new Ar—C bonds. 

The thermal methods available for the formation of aryl cation in solution are limited to the solvolysis of suitable perfluorophenylalkyl esters and aromatic diazo-nium salts [6]. Furthermore, these methods lead to unselective chemistry due to the singlet nature of the aryl cation formed. Under photostimulation, and according to the substituents, aromatic halides give either the singlet or triplet phenyl cation. The mediation of triplet aryl cations is essential for achieving the chemoselectivity desired in these reactions [12]. In order to ensure the formation of such intermediates, certain conditions should be met  

In addition to the synthetic capability of the SN1 arylation reactions, their mechanism has been studied in depth, and their intermediates detected using 

Carbanions from Ketones, Esters, Acids, Amides, and Imides as the Nucleophiles 

When a vinyl carbonium ion is formed it is linear, and so the stereochemistry of the final product after the addition of the electrophile will be randomised. 

In the case of an alkynyl halide, SN1 substitution is even more disfavoured than in the case of a vinyl halide, because it would result in a positive charge on an sp hybridised carbon, after having broken a bond that is stronger and less easily polarised than is the case of the vinyl halide. 

We will now consider the possibility of the SN2 reaction mechanism in unsaturated systems. First, draw a diagram of bromoethene, showing the n orbitals of the double bond. Then, consider the arrangement of the electrons in space and how they may interact with an incoming nucleophile. 

In the case of an alkene, there is a large electron cloud that is close to the line of approach of any potential nucleophile. This will exert a disfavourable Coulombic interaction and so inhibit the approach of the nucleophile. 

Finally, consider the possibility of inversion at such an unsaturated carbon atom. 

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POTENTIAL ENERGY DIAGRAMS FOR MULTISTEP REACTIONS THE Sn1 mechanism... 

One possibility is a limiting SN1 mechanism with a five-coordinate cobalt complex as an intermediate,... 

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, ArCMe2Cl54. This reaction follows an SN1 mechanism, with intermediate formation of the cation ArCMe2 +. 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 + 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. 

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 SN1 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. 

Increasing the temperature of the reaction favors reaction by the El mechanism at the expense of the SN1 mechanism. 

Several interesting observations have been made on this reaction. First, the rate of isomerization was found to be the same as the rate of dehydration. All attempts to dehydrate the starting complex by conventional techniques were found to lead to isomerization. On the basis of this and other evidence, the mechanism proposed involves the aquation in the complex followed by anation. In this process, water first displaces Cl- in the coordination sphere and then is displaced by the Cl-, possibly by an SN1 mechanism. A trigonal bipyramid transition state could account for the Cl- reentering the coordination sphere to give an cis product. The rate law for this reaction is of the form... 

The propensity of S-S dications to undergo dealkylation was found to decrease in the order of methyl > ethyl > benzyl. This order of reactivity parallels the increase in the stability of the corresponding carbocations.94 Dealkylation of dication 77 affords thiosulfonium salt 78 in quantitative yield.95 Kinetic studies suggest SN1 mechanism of dealkylation. In addition, reaction of sulfoxide 79 with a substituent chiral at the a-carbon results in racemic amide 80 after hydrolysis. 

The mechanism of phosphate ester hydrolysis by hydroxide is shown in Figure 1 for a phosphodiester substrate. A SN2 mechanism with a trigonal-bipyramidal transition state is generally accepted for the uncatalyzed cleavage of phosphodiesters and phosphotriesters by nucleophilic attack at phosphorus. In uncatalyzed phosphate monoester hydrolysis, a SN1 mechanism with formation of a (POj) intermediate competes with the SN2 mechanism. For alkyl phosphates, nucleophilic attack at the carbon atom is also relevant. In contrast, all enzymatic cleavage reactions of mono-, di-, and triesters seem to follow an SN2... 

Figure 8.17 Reaction of an alkyl halide with hydroxide ion. (a) A primary halide reacts by an SN2 mechanism, causing Walden inversion about the central, chiral carbon, (b) A tertiary halide reacts by an SN1 mechanism (the rate-determining step of which is unimolecular dissociation, minimizing the extent of Walden inversion and maximizing the extent of racemization). Secondary alcohols often react with both Sn 1 and SN2 mechanistic pathways proceeding concurrently...

The basic ideas presented above correspond to an analysis of a typical unimolecular process, as for instance, SN1 mechanism where the solvent may have achieved the stabilization of the di-ionic quantum state and has favored ionic dissociation as opposed to homolytic dissociation. The chemical interconversion appears here to be a quantum mechanical change of state where the solvent fluctuations would play the role of... 

Many theories have been put forward to explain the mechanism of inversion. According to the accepted Hugles, Ingold theory aliphatic nucleophilic substitution reactions occur eigher by SN2 or SN1 mechanism. In the SN2 mechanism the backside attack reduces electrostatic repulsion in the transition state to a minimum when the leaving meleophile leaves the asymmetric carbon, naturally an inversion of configuration occurs at the central carbon atom. 

We have seen that substitution in secondary and tertiary alkyl halides proceeds by an SN1 mechanism in which there is first slow ionisation resulting in the formation of a flat carbocation and hence the attack by the nucleophilic reagent can take place equally well from either side, i.e., equal amounts of (+) and (-)- forms are likely to be produced giving a racemic product ... 

The kinetics of the esterification of potassium p-nitrobenzoate by benzyl bromide in dichloromethane-water catalysed by dicyclohexyl-18-crown-6 ([20] + [21]) has been studied by Wong (1978). At low catalyst concentrations (e.g. 5.0 x 10 3 M) he found that 22% of the product was formed by the SN1 and 78% by the SN2 mechanism. Higher catalyst concentrations increased the salt concentration in the organic phase, and the contribution of the SN1 mechanism becomes negligibly small. 

Acid-catalyzed glycerol etherification probably occurs via an SN1 mechanism, consisting of the consecutive formation of an oxonium and carbenium ion, and its electrophilic attack on a glycerol O atom (Scheme 11.3). 

Hughes, Ingold, and their co-workers also attempted to generalize about the mechanism by which elimination of substituents, rather than substitution for substituents, occurs in the aliphatic molecule, resulting in the formation of an olefin in the course of what initially was predicted to be a SN1 mechanism. 

It is worth noting that Murr and Donnelly (1970a,b) have demonstrated that the secondary a-deuterium KIE is only approximately 75% of the theoretical maximum kinetic isotope effect when the ionization (ki) step of the reaction (Scheme 1) is fully rate determining, i.e. when the reaction occurs via a limiting SN1 mechanism (Shiner, 1970b Westaway, 1987c). 

Fig. 7.1. a) Specific acid catalysis (proton catalysis) with acyl cleavage in ester hydrolysis. Pathway a is the common mechanism involving a tetrahedral intermediate. Pathway b is SN1 mechanism observed in the presence of concentrated inorganic acids. Not shown here is a mechanism of alkyl cleavage, which can also be observed in the presence of concentrated inorganic acids, b) Schematic mechanism of general acid catalysis in ester hydrolysis. 

The prominent role of alkyl halides in formation of carbon-carbon bonds by nucleophilic substitution was evident in Chapter 1. The most common precursors for alkyl halides are the corresponding alcohols, and a variety of procedures have been developed for this transformation. The choice of an appropriate reagent is usually dictated by the sensitivity of the alcohol and any other functional groups present in the molecule. Unsubstituted primary alcohols can be converted to bromides with hot concentrated hydrobromic acid.4 Alkyl chlorides can be prepared by reaction of primary alcohols with hydrochloric acid-zinc chloride.5 These reactions proceed by an SN2 mechanism, and elimination and rearrangements are not a problem for primary alcohols. Reactions with tertiary alcohols proceed by an SN1 mechanism so these reactions are preparatively useful only when the carbocation intermediate is unlikely to give rise to rearranged product.6 Because of the harsh conditions, these procedures are only applicable to very acid-stable molecules. 

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