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| {{other uses|SN1 (disambiguation)}}
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| {{DISPLAYTITLE:S<sub>N</sub>1 reaction}}
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| The '''S<sub>N</sub>1 reaction''' is a [[substitution reaction]] in [[organic chemistry]]. "S<sub>N</sub>" stands for [[nucleophilic substitution]] and the "1" represents the fact that the [[rate-determining step]] is [[molecularity|unimolecular]].<ref>L. G. Wade, Jr., ''Organic Chemistry'', 6th ed., Pearson/Prentice Hall, Upper Saddle River, New Jersey, USA, 2005</ref><ref>{{cite book |first=J. |last=March |title=Advanced Organic Chemistry |edition=4th |publisher=Wiley |location=New York |year=1992 |isbn=0-471-60180-2 }}</ref> 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 (chemistry)|steady-state kinetics]]. The reaction involves a [[carbocation]] intermediate and is commonly seen in reactions of secondary or tertiary [[alkyl halide]]s under strongly basic conditions or, under strongly acidic conditions, with [[Alcohol#Primary.2C_secondary.2C_and_tertiary_alcohols|secondary or tertiary alcohols]]. With primary alkyl halides, the alternative [[SN2 reaction|S<sub>N</sub>2 reaction]] occurs. In [[inorganic chemistry]], the S<sub>N</sub>1 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.<ref>{{Cite journal | author = Leslie C. Bateman, Mervyn G. Church, Edward D. Hughes, Christopher K. Ingold and Nazeer Ahmed Taher | doi = 10.1039/JR9400000979 | title = 188. Mechanism of substitution at a saturated carbon atom. Part XXIII. A kinetic demonstration of the unimolecular solvolysis of alkyl halides. (Section E) a general discussion | year = 1940 | journal = Journal of the Chemical Society (Resumed) | pages = 979}}</ref> This reaction does not take account much on the strength of the nucleophile unlike the S<sub>N</sub>2 mechanism.
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| ==Mechanism==
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| An example of a reaction taking place with an S<sub>N</sub>1 [[reaction mechanism]] is the [[hydrolysis]] of [[tert-butyl bromide]] with water forming [[Tert-Butanol|''tert''-butanol]]:
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| :[[Image:ReakcjaSn1hydrolizabromkutertbutylowego.svg|reaction tert-butylbromide water overall]]
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| This S<sub>N</sub>1 reaction takes place in three steps:
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| * Formation of a [[Butyl|''tert''-butyl]] [[carbocation]] by separation of a [[leaving group]] (a [[bromide]] anion) from the carbon atom: this step is slow and [[reversible reaction|reversible]].<ref>{{Cite journal | title = Nature of Dynamic Processes Associated with the SN1 Reaction Mechanism | author = Peters, K. S. | journal = [[Chem. Rev.]] | year = 2007 | volume = 107 | issue = 3 | pages = 859–873 | doi = 10.1021/cr068021k | pmid = 17319730}}</ref>
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| :[[Image:Sn1pierwszyetapreakcjipowstaniekarbokationu.svg|S<sub>N</sub>1 mechanism: dissociation to carbocation]]
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| * [[nucleophile|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.
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| :[[Image:NS1 reaction part2 recombination carbocation nucleophile.svg|Recombination of carbocation with nucleophile]]
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| * [[Deprotonation]]: Removal of a proton on the [[protonation|protonated]] nucleophile by water acting as a base forming the [[alcohol]] and a [[hydronium ion]]. This reaction step is fast.
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| [[Image:NS1 reaction part3 proton transfer forming alcohol.svg|Proton transfer forming the alcohol]]
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| == Scope ==
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| The S<sub>N</sub>1 mechanism tends to dominate when the central carbon atom is surrounded by bulky groups because such groups [[steric hindrance|sterically hinder]] the S<sub>N</sub>2 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 effect|inductive]] stabilization and [[hyperconjugation]] from attached [[alkyl]] groups. The [[Hammond-Leffler postulate]] suggests that this too will increase the rate of carbocation formation. The S<sub>N</sub>1 mechanism therefore dominates in reactions at [[tertiary alkyl]] centers and is further observed at [[secondary alkyl]] centers in the presence of weak [[nucleophile]]s.
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| An example of a reaction proceeding in a S<sub>N</sub>1 fashion is the synthesis of ''2,5-dichloro-2,5-dimethylhexane'' from the corresponding diol with concentrated [[hydrochloric acid]]:<ref>''Synthesis of 2,5-Dichloro-2,5-dimethylhexane by an SN1 Reaction'' Carl E. Wagner and Pamela A. Marshall , [[J. Chem. Educ.]], '''2010''', 87 (1), pp 81–83 {{DOI|10.1021/ed8000057}}</ref>
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| :[[File:SN1reactionWagner2009.svg|Synthesis of 2,5-Dichloro-2,5-dimethylhexane by an S<sub>N</sub>1 reaction]]
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| As the alpha and beta substitutions increase with respect to leaving groups the reaction is diverted from S<sub>N</sub>2 to S<sub>N</sub>1.
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| ==Stereochemistry==
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| The carbocation intermediate formed in the reaction's rate limiting step is an ''sp<sup>2</sup>'' 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.<ref>Sorrell, Thomas N. "Organic Chemistry, 2nd Edition" University Science Books, 2006</ref> This is illustrated below in the S<sub>N</sub>1 reaction of S-3-chloro-3-methylhexane with an iodide ion, which yields a racemic mixture of 3-iodo-3-methylhexane:
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| [[Image:SN1Stereochemistry.png|center|600px|A typical S<sub>N</sub>1 reaction, showing how racemisation occurs]]
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| 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 S<sub>N</sub>2 mechanism, which is a stereospecific mechanism where stereochemistry is always inverted as the nucleophile comes in from the rear side of the leaving group.
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| ==Side reactions==
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| Two common side reactions are [[elimination reaction]]s and [[Rearrangement reaction|carbocation rearrangement]]. If the reaction is performed under warm or hot conditions (which favor an increase in entropy), [[Elimination_reaction#E1_mechanism|E1 elimination]] is likely to predominate, leading to formation of an [[alkene]]. At lower temperatures, S<sub>N</sub>1 and E1 reactions are competitive reactions and it becomes difficult to favor one over the other. Even if the reaction is performed cold, some alkene may be formed. If an attempt is made to perform an S<sub>N</sub>1 reaction using a strongly basic nucleophile such as [[hydroxide]] or [[methoxide]] ion, the alkene will again be formed, this time via an [[Elimination_reaction#E2_mechanism|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.
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| ==Solvent effects==
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| {{see also|Solvent effects}}
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| Since the S<sub>N</sub>1 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 ''[[Chemical polarity|polar]]'' (to stabilize ionic intermediates in general) and ''[[protic solvent|protic]]'' (to [[solvation|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.
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| The '''Y scale''' correlates [[solvolysis]] reaction rates of any solvent ('''k''') with that of a standard solvent (80% v/v [[ethanol]]/[[water]]) ('''k<sub>0</sub>''') through
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| :<math> \log { \left ( \frac{k}{k_0} \right ) } = mY \,</math> | |
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| with '''m''' a reactant constant (m = 1 for [[Tert-Butyl chloride|''tert''-butyl chloride]]) and '''Y''' a solvent parameter.<ref>{{cite journal | title = The Correlation of Solvolysis Rates | author = Ernest Grunwald and S. Winstein | journal = [[J. Am. Chem. Soc.]] | year = 1948 | volume = 70 | issue = 2 | pages = 846 | doi = 10.1021/ja01182a117 }}</ref> For example 100% ethanol gives Y = −2.3, 50% ethanol in water Y = +1.65 and 15% concentration Y = +3.2.<ref>{{cite journal | title = Correlation of Solvolysis Rates. III.1 t-Butyl Chloride in a Wide Range of Solvent Mixtures | author = Arnold H. Fainberg and S. Winstein | journal = [[J. Am. Chem. Soc.]] | year = 1956 | pages = 2770 | volume = 78 | issue = 12 | doi = 10.1021/ja01593a033 }}</ref>
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| == See also ==
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| *[[Arrow pushing]]
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| *[[Nucleophilic acyl substitution]]
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| *[[Neighbouring group participation]]
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| *[[SN2 reaction|S<sub>N</sub>2 reaction]]
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| == References ==
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| {{reflist}}
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| ==Further reading==
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| *Electrophilic Bimolecular Substitution as an Alternative to Nucleophilic Monomolecular Substitution in Inorganic and Organic Chemistry / N.S.Imyanitov. J. Gen. Chem. USSR (Engl. Transl.) '''1990'''; 60 (3); 417-419.
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| *Unimolecular Nucleophilic Substitution does not Exist! / N.S.Imyanitov. [http://sciteclibrary.ru/eng/catalog/pages/9330.html SciTecLibrary]
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| == External links ==
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| * [http://www.chemhelper.com/sn1.html Diagrams]: [[Frostburg State University]]
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| * [http://www.usm.maine.edu/~newton/Chy251_253/Lectures/Sn1/Sn1FS.html Exercise]: the University of Maine
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| {{Reaction mechanisms}}
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| {{DEFAULTSORT:Sn1 Reaction}}
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| [[Category:Substitution reactions]]
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| [[Category:Reaction mechanisms]]
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Jerrie is what you can also call me but Which i don't like when people use my full determine. My husband's comments and I chose to measure in Massachusetts. What I love doing is to play croquet and furthermore now I have opportunity to take on new things. The job I've been occupying for years is an sequence clerk. See what's new on your website here: http://prometeu.net
Check out my web site: clash of Clans hacks