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SN1 reaction

The SN1 reaction is a substitution reaction in organic chemistry. "SN" stands for nucleophilic substitution and the "1" represents the fact that the rate -determining step is unimolecular.[1][2] Thus, the rate equation is often shown as having first-order dependence on electrophile and zero-order dependence on nucleophile. This relationship holds for situations where the amount of nucleophile is much greater than that of the carbocation intermediate. Instead, the rate equation may be more accurately described using steady-state kinetics . 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 alkyl halides, the alternative SN2 reaction occurs. In inorganic chemistry, the SN1 reaction is often known as the dissociative mechanism. This dissociation pathway is well-described by the cis effect. A reaction mechanism was first proposed by Christopher Ingold et al. in 1940.[3] This reaction does not take account much on the strength of the nucleophile unlike the SN2 mechanism.

1 Mechanism
2 Scope
3 Stereochemistry
4 Side reactions
5 Solvent effects
6 See also
7 References
8 Further reading
9 External links


An example of a reaction taking place with an SN1 reaction mechanism is the hydrolysis of tert-butyl bromide with water forming tert-butanol:

reaction tert-butylbromide water overall

This SN1 reaction takes place in three steps:

Formation of a tert-butyl carbocation by separation of a leaving group (a bromide anion) from the carbon atom: this step is slow and reversible.[4]

SN1 mechanism: dissociation to carbocation

Nucleophilic attack : the carbocation reacts with the nucleophile. If the nucleophile is a neutral molecule (i.e. a solvent) a third step is required to complete the reaction. When the solvent is water, the intermediate is an oxonium ion. This reaction step is fast.

Recombination of carbocation with nucleophile

Deprotonation: Removal of a proton on the protonated nucleophile by water acting as a base forming the alcohol and a hydronium ion. This reaction step is fast.

Proton transfer forming the alcohol

The SN1 mechanism tends to dominate when the central carbon atom is surrounded by bulky groups because such groups sterically hinder the SN2 reaction. Additionally, bulky substituents on the central carbon increase the rate of carbocation formation because of the relief of steric strain that occurs. The resultant carbocation is also stabilized by both inductive stabilization and hyperconjugation from attached alkyl groups. 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 and is further observed at secondary alkyl centers in the presence of weak nucleophiles.

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 aci[5]

Synthesis of 2,5-Dichloro-2,5-dimethylhexane by an SN1 reaction

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 limiting step is an sp2 hybridized carbon with trigonal planar molecular geometry. This allows two different avenues for the nucleophilic attack , one on either side of the planar molecule. If neither avenue is preferentially favored, these two avenues occur equally, yielding a racemic mix of enantiomers if the reaction takes place at a stereocenter.[6] 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:
A typical SN1 reaction, showing how racemisation occurs

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 . This stands in contrast to the SN2 mechanism, which is a stereospecific mechanism where stereochemistry is always inverted as the nucleophile comes in from the rear side of the leaving group.
Side reactions