Sn2 reactions inversion of configuration

Stereochemictry of the product is related to the reactant in a mechanistically defined manner no other stereochemical outcome is mechanistically possible, i.e. SN2 reaction- inversion of configuration is required... 

Nucleophilic ring opening of epoxides has many of the features of an Sn2 reaction. Inversion of configuration is observed at the carbon at which substitution occurs. 

In nucleophilic reactions at saturated carbon atoms such as Sn2 reactions, inversion of configuration occurs. Again, this can be easily rationalized by a simple frontier orbital approach (Fig. 6.3). The overlap between the HOMO of the nucleophile and the LUMO of the alkane derivative is bonding when the nucleophile approaches from the rear, but both bonding and antibonding... 

The reactions of epoxides with anionic nucleophiles have many of the characteristics of Sn2 reactions. Inversion of configuration occurs at the carbon attacked by the nucleophile ... 

A modification of GSR has been reported by Classon and co-workers.190 The idea remains the same create a covalently bound phosphorus cation and displace with a nucleophile—in this case, a halogen. Both bromine and iodine have been used.190 Three different systems were evaluated (1) chlorodiphenylphophine, iodine-bromine, and imidazole (2) p-(dimethylamino)phenyldiphenylphosphine, iodine-bromine, and imidazole or (3) polymer-bound triphenylphosphine, iodine-bromine, and imidazole. The last two were found to be very similar to just triphenylphosphine itself, and displayed reactivity inferior to the first system. The polymer-bound reagent does allow for easier removal of triphenylphosphine oxide produced in the course of the reaction. As with the original procedure, and consistent with a Sn2 mechanism, inversion of configuration occurred. Again, as with the original method vicinal diols were readily converted into alkenes.191 This... 

Reactions using SOCI2 proceed by an Sn2 mechanism = inversion of configuration. 

Nucleophilic attack on oxirane carbon usually proceeds with inversion of configuration (Scheme 44) as expected for Sn2 reactions, even under acid conditions (Scheme 45). Scheme 45 also illustrates the fact that cyclohexene oxides open in a fran5-diaxial manner this is known as the Fiirst-Plattner rule (49HCA275) and there are very few exceptions to it. 

The mechanisms by which sulfonate esters undergo nucleophilic substitution are the sfflne as those of alkyl halides. Inversion of configuration is observed in Sn2 reactions of alkyl sulfonates and predominant inversion accompanied by racernization in SnI processes. 

The reactions of alcohols with hydrogen halides to give alkyl halides (Chapter 4) are nucleophilic substitution reactions of alkyloxonium ions in which water is the leaving group. Primary alcohols react by an Sn2-like displacement of water from the alkyloxonium ion by halide. Secondary and tertiary alcohols give alkyloxonium ions which fonn caibo-cations in an SNl-like process. Reanangements are possible with secondary alcohols, and substitution takes place with predominant, but not complete, inversion of configuration. 

None of the bonds to the chirality center is broken when hydroxide attacks the carbonyl group. Had an Sn2 reaction occuned instead, inversion of configuration at the chirality center would have taken place to give (.S)-(—)-l-phenylethyl alcohol. 

A mechanism that accounts for both the inversion of configuration and the second-order kinetics that are observed with nucleophilic substitution reactions was suggested in 1937 by E. D. Hughes and Christopher Ingold, who formulated what they called the SN2 reaction—short for substitution, nucleophilic, birnolecu-lar. (Birnolecular means that two molecules, nucleophile and alkyl halide, take part in the step whose kinetics are measured.)... 

The reaction of an alkyl halide or los3 late with a nucleophiJe/base results eithe in substitution or in diminution. Nucleophilic substitutions are of two types S 2 reactions and SN1 reactions, in the SN2 reaction, the entering nucleophih approaches the halide from a direction 180° away from the leaving group, result ing in an umbrella-like inversion of configuration at the carbon atom. The reaction is kinetically second-order and is strongly inhibited by increasing stork bulk of the reactants. Thus, S 2 reactions are favored for primary and secondary substrates. 

Sn2 reactions take place with inversion of configuration, and SN1 reaction take place with racemization. The following substitution reaction, howeve occurs with complete retention of configuration. Propose a mechanism. 

One of the most important reasons for using tosylates in S j2 reactions is stereochemical. The S]s]2 reaction of an alcohol via an alkyl halide proceeds with hvo inversions of configuration—one to make the halide from the alcohol and one to substitute the halide—and yields a product with the same stereochemistry as the starting alcohol. The SN2 reaction of an alcohol via a tosylate, however, proceeds with only one inversion and yields a product of opposite stereochemistry to the starting alcohol. Figure 17.5 shows a series of reactions on the R enantiomer of 2-octanol that illustrates these stereochemical relationships. 

Walden inversion (Section 11.1) The inversion of configuration at a chirality center that accompanies an SN2 reaction. 

If it is assumed that the Mitsunobu glycosidation reaction described above proceeds through an SN2-type process with inversion of configuration at the anomeric position, then it follows that the desired / -glycoside can be formed selectively if pure a-lactol 17 is used in the reaction. Unfortunately, the /Mactol isomer of 17 is thermodynamically more stable than the a-diastereoisomer and is formed almost exclusively if the system is allowed to fully equilibrate. In the protic medium used for the Luche reduction, a signifi-... 

It is necessary to postulate the solvent cage because, if the radicals were completely free, the products would be about 50% RR, 25% RR, and 25% R R. This is generally not the case in most of these reactions RR is the predominant or exclusive product. An example where an Sn2 mechanism has been demonstrated (by the finding of inversion of configuration at R) is the reaction between allylic or benzylic... 

Markovnikov s rule is also usually followed where bromonium ions or other three-membered rings are intermediates. This means that in these cases attack by W must resemble the SnI rather than the Sn2 mechanism (see p. 461), though the overall stereospecific anti addition in these reactions means that the nucleophilic substitution step is taking place with inversion of configuration. 

For example, let s look at the stereochemistry of SnI reactions. We already saw that Sn2 reactions proceed via inversion of configuration. But SnI reactions are very different. Recall that a carbocation is sp hybridized, so its geometry is trigonal planar. When the nucleophile attacks, there is no preference as to which side it can attack, and we get both possible configurations in equal amounts. Half of the molecules would have one configuration and the other half would have the other configuration. We learned before that this is called a racemic mixture. Notice that we can explain the stereochemical outcome of this reaction by understanding the nature of the carbocation intermediate that is formed. 

These charts show the rate of reaction. If you have a 1° substrate, then the reaction will proceed via an Sn2 mechanism, with inversion of configuration. If you have a 3° substrate, then the reaction will proceed via an SnI mechanism, with racemiza-tion. What do yon do if the substrate is 2° You move on to factor 2. 

The rate of displacement and of inversion are thus identical within the limits of experimental error, and it thus follows that each act of bimolecular displacement must thus proceed with inversion of configuration. Having shown that SN2 reactions are attended by inversion of configuration, independent demonstration that a particular reaction occurs via the SN2 mode is often used to correlate the configuration of product and starting material in the reaction. 

The reaction has been shown to follow a second order rate equation, rate = fc2[ROH][SOCI2], but clearly cannot proceed by the simple Sn2 mode for this would lead to inversion of configuration (p. 87) in the product, which is not observed. 

Inversion of configuration can be observed when hydroxide ion reacts with 67,v- 1-chi oro-3-methyl cyclopentane in an SN2 reaction ... 

Inversion of configuration can be also observed when the Sn2 reaction takes place at a stereocenter (with complete inversion of stereochemistry at the chiral carbon center) ... 

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