SN1

The SN1 is monomolecular nucleophilic substitution.

  • Is the general equation of a nucleophilic substitution:

GP being the group therefore, also called nucléofuge, and Nu- the nucleophilic one.

Kinetics of the SN1 reaction

The kinetics of the reaction is of type v = K. This type of reaction is known as monomolecular, because the stage kinetically limiting in the reactional mechanism utilizes only one molecule on the side of the reagents, therefore that the kinetics of the reaction, in the approximation of the stage kinetically limiting (determining), depends only on the concentration of this species.

  • represents the concentration in substrate in mol. L-1

  • v represents the reaction speed of substitution nucleophilic. It is expressed in mol.s-1
  • K is the constant speed; its unit results from that of the 2 other terms: it is thus expressed L.s-1

Mechanism of SN1

The reaction is done in two stages.

1ère stage

Group therefore GP reacts on itself. Connection R-GP breaks to give a carbocation R+ and an ion GP-. It is a very slow and determining reaction for the kinetics of the reaction.

Example with 3-chloro-3-ethylpentane: here it is the group Cl which therefore plays the part of group. The lobes in blue represents orbital p vacant.

2ème stage

Nucleophilic Nu- reacts immediately with the carbocation right after its formation. The nucleophilic one can attack the carbocation, which is of plane trigonal geometry, in lower part or with the top of the plan.

With the preceding example, the product is same whatever the attacked side. The 3-chloro-3-ethylpentane is a molecule achirale.

Stereochemistry

At the time of SN1, if the principal reagent is a chiral molecule, there is formation of a racemic mixture (50% of product of configuration S + 50% of product of configuration R).

The first stage is identical that the molecule is chiral or not. It is at the time of the second stage that the configuration of the reagent between concerned. Indeed, with a reagent chiral, the nucleophilic one can attack the carbocation formed at two different places. Statistically it there as much of chance than the nucleophilic one is placed on a side rather than another from where the formation of as much of made up R than of made up S.

Example with (S) - 3-chloro-méthylhexane:

1ère stage

Chlorine reacts on itself to form the ion chloride and a carbocation. (stage kinetically limiting)

2ème stage

  • First possibility:

The nucleophilic one played here by the ion OH- hydroxide attacks the carbocation, which forms a trigonal plan, by the top.

The compound formed is of configuration S.

  • Second possibility:

Nucleophilic OH- attacks the carbocation by lower part.

The compound formed is of configuration R.

Influence few factors

Classify R

In experiments, it is observed, at the time of a SN1 mechanism, that speed increases when one passes from the primary education compounds, with the secondaries, then with the tertiary sectors. The influence of R thus is completely opposed to that in the mechanism of SN2. One can thus schematize the reactivity:

RIII-GP > RII-GP > RI-GP (>: reacts more quickly than)
This order can be interpreted by an increasing stability of the reactional intermédaire (carbocation in the case of SN starting from a halogénoalcane), therefore of the reduction in the energy of activation , thus, an increase in the constant speed of the stage of formation of this intermédaire reactional which determines the reaction speed. (One can schematize this by the fact that the more stable the reactional intermédaire is, the more it is " facile" to form it, therefore more the reaction is fast, cf the postulate of Hammond)

Nucleophilic reagent

The " force" the nucleophilic one, its concentration or its size, does not have an influence on the reaction speed, since it does not intervene in the stage kinetically limiting. It thus has, in theory , no influence on the reaction speed.

Polarity of solvent

The reactional intermédaire is an ion (carbocation in the case of the halogénoalacane as a reagent). It is thus stabilized by a polar solvent, whereas the initial reagent is almost not it, from its total electric neutrality. Thus the reaction speed increases with the polarity of solvent for a mechanism of the SN1 type.

Nature of the nucléofuge (group therefore, GP)

Connection R-PG breaks all the more easily as it is polarizable. For that it is necessary that it is longest possible. Within the framework of SN1 on a halogénoalcane, speed thus increases RF with IH.

Compete with

This reaction is in competition with other nucleophilic substitution (SN2), but also with the reactions of eliminations, in particular the monomolecular reaction of elimination, E1, to which the conditions of realization are close.

See too

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