SN2

bimolecular nucleophilic Substitution, SN2

Kinetics of bimolecular nucleophilic substitution

The reaction of nucleophilic substitution bimolecular is called thus because it is about a reaction into 1 stage, where 2 reagents react; it thus has kinetics of second order. The expression its reaction speed is thus the following one:

v = K \ left \ mathrm {X-ray} \ right \ Naked left \ right
This expression speed remains right as long as the nucleophilic compound remains in reasonable concentrations. If this last is the solvent, one observes a behavior of pseudo first kinetic order, i.e. a in the following way expressed speed:
v^ {\ premium} = k^ {\ premium} \ left \ mathrm {X-ray} \ right

Reactional mechanism

Figure: the three connections around central carbon are coplanar in ET≠.

Bimolecular nucleophilic substitution, or SN2 is a reaction being done in only one stage, without reactional intermediary. Atom X is more electronegative than the carbon on which it is dependant. It collects the electrons of the connection and in the same time, a nucleophilic group - noted Nu - binds to carbon, other with dimensions of central carbon. One can then observe an inversion of the relative stereochemistry of central carbon, but a conservation of the absolute configuration. It is this characteristic which makes SN2 reaction a stereospecific reaction. The inversion quoted previously is named “Inversion of Walden”. The Postulate of Hammond makes it possible to define a state of transition - ET≠.

Stereochemistry in SN2

The reaction of nucleophilic substitution bimolecular, a contrario of SN1, is at the same time stéréosélective, stéréospécifique and even enantierospecific . Since the absolute configuration is preserved by the reaction, a compound of configuration rectus or sinister will respectively give a compound sinister or rectus by the reaction of SN2. The same applies to the couples of compounds, such as the enantiomers, or the diastereoisomers, for example:

\ begin {matrix} R \ mathrm {X} + Nu^ {-} & \ rightarrow & RNu + X^ {-} \ \ R^ {\ premium} \ mathrm {X} + Nu^ {-} & \ rightarrow & Naked R^ {\ premium} + X^ {-} \ end {matrix}
If the compounds R \ mathrm {X} and R^ {\ premium} \ mathrm {X} have a particular stereochemistry, their products by SN2 have same stereochemistry.

Influence certain factors

Influence nucléofuge

Two factors must be taken into account, the nucleophilicity and the polarizability of the nucléofuge (to be noted that these two factors are typically in opposition). The nucleophilicity of the nucléofuge is important insofar as it determines partially the polar character of chemical bond X-ray, which returns the " molecule cible" vulnerable to the nucleophilic attacks. Thus, one will find best the nucléofuges on the right periodic table (halogens).

This known as, more the nucléofuge will be polarizable, more the reaction will be fast. When nucleophilic anion approaches the " molecule cible" , it will induce a dipole there more easily if the nucléofuge is polarizable. Typically, the polarizability of an atom is corrélatée with the ray of its electronic cloud. The size of an atom increases as one goes down in a column from the periodic table.

Among the halogens, one can classify the nucleofugacity like this:

\ mathrm {I} > \ mathrm {Br} > \ mathrm {Cl} > \ mathrm {F}
Thus, in spite of a substantially less nucleophilicity, iodine is better a nucléofuge than chlorine thanks to its greater polarizability.

Influence the nucleophilic one

In a simple and obvious way, plus the nucleophilic one - Nu - will be polarizable, and more SN2 fast and will be supported, kinetically speaking.

Influence solvent

In a way similar to the influence of nucleophilic, a polar solvent would stabilize the nucleophilic compound, this is why SN2 will be favoured in a not very polar and aprotic solvent - solvent not creating hydrogen bonds.

Influence grouping alkyl

One observes reactivities and speeds different according to the class from the reagent. Indeed, the possibility of the inversion of Walden, developed previously, tributary of steric gene is caused by the chains related to central carbon. One thus introduces the following classes of made up:

SN2 will be favoured for a primary education reagent. And strongly underprivileged for a tertiary compound: indeed the chains R1, R2 and R3 propose an important embarrassment on the side of attack of nucleophilic, the state of transition are high in energy and can be impossible to reach, the products cannot then be formed. One cannot rule has priori on the fate of the secondary compound, the reaction being able or not to occur according to the experimental conditions.

Example of SN2

This example shows that the product of a chiral molecule by SN2 is a chiral molecule.

See too

References

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