SN1 reaction

The Hughes-Ingold symbol of the mechanism expresses two properties—"SN" stands for "nucleophilic substitution", and the "1" says that the rate-determining step is unimolecular.

This relationship holds for situations where the amount of nucleophile is much greater than that of the intermediate.

The reaction involves a carbocation intermediate and is commonly seen in reactions of secondary or tertiary alkyl halides under strongly basic conditions or, under strongly acidic conditions, with secondary or tertiary alcohols.

With primary and secondary alkyl halides, the alternative SN2 reaction occurs.

A reaction mechanism was first introduced by Christopher Ingold et al. in 1940.

[3] This reaction does not depend much on the strength of the nucleophile, unlike the SN2 mechanism.

The first step is the ionization of alkyl halide in the presence of aqueous acetone or ethyl alcohol.

In the first step of SN1 mechanism, a carbocation is formed which is planar and hence attack of nucleophile (second step) may occur from either side to give a racemic product, but actually complete racemization does not take place.

The negatively charged halide ion shields the carbocation from being attacked on the front side, and backside attack, which leads to inversion of configuration, is preferred.

Consider the following reaction scheme for the mechanism shown above: Though a relatively stable tertiary carbocation, tert-butyl cation is a high-energy species that is present only at very low concentration and cannot be directly observed under normal conditions.

Thus, the SSA can be applied to this species: (1) Steady state assumption:

(2) Concentration of t-butyl cation, based on steady state assumption:

(3) Overall reaction rate, assuming rapid final step:

Moreover, kinetic experiments are often conducted under initial rate conditions (5 to 10% conversion) and without the addition of bromide, so

the simple first-order rate law described in introductory textbooks.

Under these conditions, the concentration of the nucleophile does not affect the rate of the reaction, and changing the nucleophile (e.g. from H2O to MeOH) does not affect the reaction rate, though the product is, of course, different.

In this regime, the first step (ionization of the alkyl bromide) is slow, rate-determining, and irreversible, while the second step (nucleophilic addition) is fast and kinetically invisible.

As the SSA rate law indicates, under these conditions there is a fractional (between zeroth and first order) dependence on [H2O], while there is a negative fractional order dependence on [Br–].

Additionally, bulky substituents on the central carbon increase the rate of carbocation formation because of the relief of steric strain that occurs.

The Hammond–Leffler postulate suggests that this, too, will increase the rate of carbocation formation.

The SN1 mechanism therefore dominates in reactions at tertiary alkyl centers.

An example of a reaction proceeding in a SN1 fashion is the synthesis of 2,5-dichloro-2,5-dimethylhexane from the corresponding diol with concentrated hydrochloric acid:[8] As the alpha and beta substitutions increase with respect to leaving groups, the reaction is diverted from SN2 to SN1.

The carbocation intermediate formed in the reaction's rate determining step (RDS) is an sp2 hybridized carbon with trigonal planar molecular geometry.

This allows two different ways for the nucleophilic attack, one on either side of the planar molecule.

If neither approach is favored, then these two ways occur equally, yielding a racemic mixture of enantiomers if the reaction takes place at a stereocenter.

[9] This is illustrated below in the SN1 reaction of S-3-chloro-3-methylhexane with an iodide ion, which yields a racemic mixture of 3-iodo-3-methylhexane: However, an excess of one stereoisomer can be observed, as the leaving group can remain in proximity to the carbocation intermediate for a short time and block nucleophilic attack.

If the reaction is performed under warm or hot conditions (which favor an increase in entropy), E1 elimination is likely to predominate, leading to formation of an alkene.

If an attempt is made to perform an SN1 reaction using a strongly basic nucleophile such as hydroxide or methoxide ion, the alkene will again be formed, this time via an E2 elimination.

The normal solvents of choice are both polar (to stabilize ionic intermediates in general) and protic solvents (to solvate the leaving group in particular).

Typical polar protic solvents include water and alcohols, which will also act as nucleophiles, and the process is known as solvolysis.