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###Criterea for a stereospecific reaction

###The empirical evidence

###The orbital explanation

[![Orbital interactions in an SE2 reaction][1]][1]

[![Orbital interactions in an SN2 reaction][2]][2]

###Conclusion

###References and Notes:

^{[4]: IUPAC Recommendations for the Representation of Reaction Mechanisms: Acc. Chem. Res. 1989, 22, 343} [1]: https://i.sstatic.net/FbLGL.png [2]: https://i.sstatic.net/SFJfS.png

Criterea for a stereospecific reaction

The empirical evidence

The orbital explanation

Conclusion

References and Notes:

^{[4]: IUPAC Recommendations for the Representation of Reaction Mechanisms: Acc. Chem. Res. 1989, 22, 343}

The S_E2 mechanism is most commonly found using organometallic reagents (R-Li, R-MgBr etc.) in which the metal is the electropositive element react with electrophiles.

Unlike with S_N2, there are multiple stereochemical outcomes, which can make it difficult to determine whether the reaction is indeed stereospecific. This is described in Modern Physical Organic Chemistry (emphasis mine):

What this means in practice is that if one enantiomer of a starting material gives one enantiomer of product, then the opposite enantiomer of starting material must give the opposite enantiomer of product. What stereospecific does not mean is high selectivity, many reactions are highly stereoselective but in no way mechanistically stereospecific (and equally stereospecific reactions are not always 100% stereoselective).

Unlike the S_N2 reaction, where many examples have been studied, it has been considerably more challenging to examine the stereochemical cause of the S_E2 reaction. In general, chiral alkyl-metal species are prone to racemisation (chiral Grignards, as an example, are barely known), and those chiral species that are stable and usually such for steric reasons (such as a constrained ring system).

If we consider the orbitals involved during an S_E2 reaction, it becomes clearer why there are multiple stereochemical outcomes, consider first S_E2 at a saturated centre, such as MeLi attacking an electrophile :

[![enter image description here][1]][1]

In this reaction, the electrophile is quite able to interact in such a way as to give rise to overall inversion or overall retention, in order to distinguish, these two electrophilic substitutions are labelled S_E2_back and S_E2_front respectively, though both are formally designated D_EA_E if the IUPAC system is followed.^[4]

This differs from the S_N2 reaction in which inversion is almost always observed due to an anti-bonding interaction in the transition state if the nucleophile approaches in such a way as to give retention:

[![enter image description here][2]][2]

The $\mathrm{S_E2}$ mechanism is most commonly found using organometallic reagents ($\ce{R-Li, R-MgBr}$ etc.) in which the metal is the electropositive element react with electrophiles.

Unlike with $\mathrm{S_N2}$, there are multiple stereochemical outcomes, which can make it difficult to determine whether the reaction is indeed stereospecific. This is described in Modern Physical Organic Chemistry (emphasis mine):

What this means in practice is that if one enantiomer of a starting material gives one enantiomer of product, then the opposite enantiomer of starting material must give the opposite enantiomer of product. What stereospecific does not mean is high selectivity, many reactions are highly stereoselective but in no way mechanistically stereospecific (and equally stereospecific reactions are not always $100~\%$ stereoselective).

Unlike the $\mathrm{S_E2}$ reaction, where many examples have been studied, it has been considerably more challenging to examine the stereochemical cause of the $\mathrm{S_E2}$ reaction. In general, chiral alkyl-metal species are prone to racemisation (chiral Grignards, as an example, are barely known), and those chiral species that are stable and usually such for steric reasons (such as a constrained ring system).

If we consider the orbitals involved during an $\mathrm{S_E2}$ reaction, it becomes clearer why there are multiple stereochemical outcomes, consider first $\mathrm{S_E2}$ at a saturated centre, such as $\ce{MeLi}$ attacking an electrophile :

[![Orbital interactions in an SE2 reaction][1]][1]

In this reaction, the electrophile is quite able to interact in such a way as to give rise to overall inversion or overall retention, in order to distinguish, these two electrophilic substitutions are labelled $\mathrm{S_E2}_\text{back}$ and $\mathrm{S_E2}_\text{front}$ respectively, though both are formally designated D_EA_E if the IUPAC system is followed.^[4]

This differs from the $\mathrm{S_N2}$ reaction in which inversion is almost always observed due to an anti-bonding interaction in the transition state if the nucleophile approaches in such a way as to give retention:

[![Orbital interactions in an SN2 reaction][2]][2]

The S_E2 mechanism is most commonly found when organometallic reagents (R-Li, R-MgBr etc.) in which the metal is the electropositive element react with electrophiles.

One of the most interesting aspects of these reactions is stereochemistry. Varying results are obtained depending upon the metal, the solvent, and the R-group.

_{Source: Modern Physical Organic Chemistry, Ansyln and Dougherty} ^[1]

What this means in practice is that if one enantiomer of a starting material gives one enantiomer of product, then the opposite enantiomer of starting material must give the opposite enantiomer of product.

Many of the experiments that have been conducted are described in March^[3], and the overwhelming majority of them go with retention (note that this doesn't necessarily imply a stereospecific process in itself).

The tl;dr answer is that it doesn't appear that the reaction is stereospecific. Orbitally speaking, there are two pathways leading to two different products (front or back attack), which may be observed in poor selectivity. It appears, with the reactions studied so far that the outcome is far more likely to be down to sterics/electronics as to whether the electrophile approaches from one side or the other

^{[1]: E. V. Ansyln and D. A. Dougherty Modern Physical Organic Chemistry; University Science Books, 2004}

^{[2]: I. Fleming Molecular Orbitals and Organic Chemical Reactions (Reference Edition); Wiley, 2012}

^{[3]: J. March and M. B. Smith Advanced Organic Chemistry; Wiley, 2013}

The S_E2 mechanism is most commonly found using organometallic reagents (R-Li, R-MgBr etc.) in which the metal is the electropositive element react with electrophiles.

One of the most interesting aspects of these reactions is stereochemistry. Varying results are obtained depending upon the metal, the solvent, and the R-group.

_{Source: Modern Physical Organic Chemistry, Ansyln and Dougherty} ^[1]

What this means in practice is that if one enantiomer of a starting material gives one enantiomer of product, then the opposite enantiomer of starting material must give the opposite enantiomer of product. What stereospecific does not mean is high selectivity, many reactions are highly stereoselective but in no way mechanistically stereospecific (and equally stereospecific reactions are not always 100% stereoselective).

Many of the experiments that have been conducted are described in March^[3], and the overwhelming majority of them go with retention (note that this doesn't necessarily imply a stereospecific process in itself and it has often been hard to study both enantiomers, for various reasons).

The short answer is that it doesn't appear that the reaction is stereospecific. Orbitally speaking, there are two pathways leading to two different products (front or back attack), which may be observed in poor selectivity. 

It appears, with the reactions studied so far, that the outcome is far more likely to be down to sterics/electronics as to whether the electrophile approaches from one side or the other

^{[1]: Ansyln, E.V.; Dougherty, D.A. Modern Physical Organic Chemistry; University Science Books:California, 2004}

^{[2]: I. Fleming Molecular Orbitals and Organic Chemical Reactions (Reference Edition); Wiley:Chichester, 2012}

^{[3]: March, J.; Smith, M.B. Advanced Organic Chemistry; Wiley:New Jersey, 2013}