The S_N1 reaction is a substitution reaction in organic chemistry. "S_N" stands for substitution nucleophilic and the "1" represents the fact that the rate-determining step is unimolecular. It involves a carbocation intermediate, and it is commonly seen in reactions of secondary or tertiary alkyl halides, or (under strongly acidic conditions) with secondary or tertiary alcohols. With primary alkyl halides, the alternative mechanism of the S_N2 reaction occurs instead. Among inorganic chemists, S_N1 is referred to perhaps more accessibly as a dissociative mechanism.

Mechanism

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The S_N1 reaction between a molecule A and a nucleophile B takes place in two steps:

1. Formation of a carbocation from molecule A by separation of leaving group from the carbon.
2. Nucleophilic attack: a nucleophile (molecule B) joins onto the carbon from A. If the nucleophile B is a neutral molecule (very often it is a solvent molecule such as water), a third step is required to complete the reaction.
3. Deprotonation: Removal of a proton on the protonated nucleophile by a nearby ion or molecule.

An example reaction:

(CH₃)₃CBr + H₂O → (CH₃)₃COH + HBr

This goes via the three step reaction mechanism described above:

1. (CH₃)₃CBr → (CH₃)₃C⁺ + Br⁻
2. (CH₃)₃C⁺ + H₂O → (CH₃)₃C-OH₂⁺
3. (CH₃)₃C-OH₂⁺ + H₂O → (CH₃)₃COH + H₃O⁺

Kinetics

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In contrast to S_N2, S_N1 reactions take place in two steps (excluding any protonation or deprotonation). The rate determining step is the first step, so the rate of the overall reaction is essentially equal to that of carbocation formation, and does not involve the attacking nucleophile. Thus nucleophilicity is irrelevant and the overall reaction rate depends on the concentration of the reactant RX only.

rate = k^*

In some cases the S_N1 reaction will occur at an abnormally high rate due to neighbouring group participation, NGP often lowers the energy barrier required for the formation of the carbocation intermediat.

Scope of the reaction

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The S_N1 mechanism tends to predominate when the central carbon atom is surrounded by bulky groups because such groups interfere sterically with the S_N2 reaction. In addition 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 stabilisation and hyperconjugation from attached alkyl groups, and the Hammond-Leffler postulate suggests that this too will increase the rate of carbocation formation. The S_N1 mechanism therefore dominates in reactions at tertiary alkyl centres, and it is also observed at secondary alkyl centres when weak nucleophiles are used.

Stereochemistry

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Because the intermediate carbocation, R⁺, is planar, the central carbon is not a stereocenter, even if it was a stereocenter in the original reactant, so the original configuration at that atom is lost. Nucleophilic attack can occur from either side of the plane, so the product may consist of a mixture of two stereoisomers in the event of the molecule chirality. In fact, if the central carbon is the only stereocenter in the reaction, racemization may occur. However, an excess of inversion is usually observed, as the leaving group can remain in proximity to the carbocation intermediate for a short time and block nucleophilic attack. For example, in the reaction of 3S-chloro-3-methylhexane with iodide ion, if the carbocation intermediate is free of the leaving group then it is achiral and stands an equal chance of attack on either side. This leads to a mixture of 3R-iodo-3-methylhexane and 3S-iodo-3-methylhexane:

Side reactions

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Two common side reactions are elimination reactions and carbocation rearrangement. 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. Even if the reaction is performed cold, some alkene may be formed. If an attempt is made to perform an S_N1 reaction using a strongly basic nucleophile such as hydroxide or methoxide ion, the alkene will again be formed, this time via an E2 elimination. This will be especially true if the reaction is heated. Finally, if the carbocation intermediate can rearrange to a more stable carbocation, it will give a product derived from the more stable carbocation rather than the simple substitution product.

Solvent effects

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Since the S_N1 reaction involves formation of an unstable carbocation intermediate in the rate-determining step, anything that can facilitate this will speed up the reaction. The normal solvents of choice are both polar (to stabilise ionic intermediates in general) and protic (to solvate the leaving group in particular). Typical polar protic solvents include water and alcohols, which will also act as nucleophiles.

See also

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• Substitution reaction
• Nucleophilic substitution
• SN2 reaction
• SNi reaction
• Nucleophilic acyl substitution
• Neighbouring group participation

External links

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• Diagrams: Frostburg State University
• Exercise: the University of Maine

References

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1. L. G. Wade, Jr., Organic Chemistry, 6th ed., Pearson/Prentice Hall, Upper Saddle River, New Jersey, USA, 2005.
2. J. March, Advanced Organic Chemistry, 4th ed., Wiley, New York, 1992.

polymerspolymer chemistryOrganic reactions

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