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{{Short description|Substitution reaction with a carbocation intermediate}} {{Redirect|SN1|the Canadian sports channel|Sportsnet One}} {{DISPLAYTITLE:S<sub>N</sub>1 reaction}} [[File:SN1 general reaction.svg|thumb|400px|General reaction scheme for the S<sub>N</sub>1 reaction. The leaving group is denoted "X", and the nucleophile is denoted "Nu–H".]] The '''unimolecular nucleophilic substitution''' ('''S<sub>N</sub>1''') reaction is a [[substitution reaction]] in [[organic chemistry]]. The [[Hughes-Ingold symbol]] of the mechanism expresses two properties—"S<sub>N</sub>" stands for "[[nucleophilic substitution]]", and the "1" says 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 the substrate and zero-order dependence on the [[nucleophile]]. This relationship holds for situations where the amount of nucleophile is much greater than that of the 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 (chemistry)|secondary or tertiary alcohols]]. With primary and secondary 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 substitution]]''. This dissociation pathway is well-described by the [[cis effect]]. A [[reaction mechanism]] was first introduced by [[Christopher Ingold]] et al. in 1940.<ref>{{Cite journal | vauthors =Bateman LC, Church MG, Hughes ED, Ingold CK, Taher NA | 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 depend much on the strength of the nucleophile, unlike the S<sub>N</sub>2 mechanism. This type of mechanism involves two steps. The first step is the ionization of alkyl halide in the presence of aqueous acetone or ethyl alcohol. This step provides a carbocation as an intermediate. In the first step of S<sub>N</sub>1 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. This is because the nucleophilic species attacks the carbocation even before the departing halides ion has moved sufficiently away from the carbocation. 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. Thus the actual product no doubt consists of a mixture of enantiomers but the enantiomers with inverted configuration would predominate and complete racemization does not occur.<ref>Dr Hussain | Class Lecture | Archeived 17/08/2024</ref> ==Mechanism== An example of a reaction taking place with an S<sub>N</sub>1 [[reaction mechanism]] is the [[hydrolysis]] of [[tert-butyl bromide]] forming [[tert-butanol|''tert''-butanol]]: :[[File:ReakcjaSn1hydrolizabromkutertbutylowego.svg|reaction tert-butylbromide water overall]] This S<sub>N</sub>1 reaction takes place in three steps: * Formation of a [[Butyl|''tert''-butyl]] carbocation by separation of a [[leaving group]] (a [[bromide]] anion) from the carbon atom: this step is slow.<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> :[[File:Sn1pierwszyetapreakcjipowstaniekarbokationu.svg|S<sub>N</sub>1 mechanism: dissociation to carbocation]] :[[File:Nucleophilic attack of oxonium ion.gif|thumb|Recombination of carbocation with 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. :[[File:NS1 reaction part2 recombination carbocation nucleophile.svg|Recombination of carbocation with a nucleophile]] * [[Deprotonation]]: Removal of a proton on the [[protonation|protonated]] nucleophile by water acting as a base forming the [[Alcohol (chemistry)|alcohol]] and a [[hydronium ion]]. This reaction step is fast. [[File:NS1 reaction part3 proton transfer forming alcohol.svg|Proton transfer forming the alcohol]] == Rate law == Although the rate law of the S<sub>N</sub>1 reaction is often regarded as being first order in alkyl halide and zero order in nucleophile, this is a simplification that holds true only under certain conditions. While it, too, is an approximation, the rate law derived from the steady state approximation (SSA) provides more insight into the kinetic behavior of the S<sub>N</sub>1 reaction. Consider the following reaction scheme for the mechanism shown above: [[File:SN1-steady-state-approximation.png|center|frameless|520x520px]]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: <math>\frac{d[\text{tBu}^+]}{dt} = 0 = k_1[\text{tBuBr}] - k_{-1}[\text{tBu}^+][\text{Br}^-] - k_2[\text{tBu}^+][\text{H}_2\text{O}]</math> (2) Concentration of t-butyl cation, based on steady state assumption: <math>[\text{tBu}^+] = \frac{k_1[\text{tBuBr}]}{k_{-1}[\text{Br}^-] + k_2[\text{H}_2\text{O}]}</math> (3) Overall reaction rate, assuming rapid final step: <math>\frac{d[\text{tBuOH}]}{dt} = k_2[\text{tBu}^+][\text{H}_2\text{O}]</math> (4) Steady state rate law, by plugging (2) into (3): <math>\frac{d[\text{tBuOH}]}{dt} = \frac{k_1 k_2 [\text{tBuBr}][\text{H}_2\text{O}]}{k_{-1}[\text{Br}^-] + k_2[\text{H}_2\text{O}]}</math> Under normal synthetic conditions, the entering nucleophile is more nucleophilic than the leaving group and is present in excess. Moreover, kinetic experiments are often conducted under initial rate conditions (5 to 10% conversion) and without the addition of bromide, so <math>[\text{Br}^-]</math> is negligible. For these reasons, <math>k_{-1}[\text{Br}^-] \ll k_2[\text{H}_2\text{O}]</math> often holds. Under these conditions, the SSA rate law reduces to: <math>\text{rate} = \frac{d[\text{tBuOH}]}{dt} = \frac{k_1 k_2 [\text{tBuBr}][\text{H}_2\text{O}]}{k_2[\text{H}_2\text{O}]} = k_1[\text{tBuBr}]</math> 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 H<sub>2</sub>O 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. However, under certain conditions, non-first-order reaction kinetics can be observed. In particular, when a large concentration of bromide is present while the concentration of water is limited, the reverse of the first step becomes important kinetically. As the SSA rate law indicates, under these conditions there is a fractional (between zeroth and first order) dependence on [H<sub>2</sub>O], while there is a negative fractional order dependence on [Br<sup>–</sup>]. Thus, S<sub>N</sub>1 reactions are often observed to slow down when an exogenous source of the leaving group (in this case, bromide) is added to the reaction mixture. This is known as the ''[[Common-ion effect|common ion effect]]'' and the observation of this effect is evidence for an S<sub>N</sub>1 mechanism (although the absence of a common ion effect does not rule it out).<ref>{{Cite book|last=Anslyn, Eric V., 1960-|url=https://www.worldcat.org/oclc/55600610|title=Modern physical organic chemistry|date=2006|publisher=University Science Books|others=Dougherty, Dennis A., 1952-|isbn=1-891389-31-9|location=Mill Valley, California|pages=638–639|oclc=55600610}}</ref><ref>{{Cite book|last=Lowry, Thomas H.|url=https://www.worldcat.org/oclc/14214254|title=Mechanism and theory in organic chemistry|date=1987|publisher=Harper & Row|others=Richardson, Kathleen Schueller.|isbn=0-06-044084-8|edition=3rd |location=New York|pages=330–331|oclc=14214254}}</ref> == Scope == 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. 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>{{cite journal | last1 = Wagner | first1 = Carl E. | last2 = Marshall | first2 = Pamela A. | year = 2010 | title = Synthesis of 2,5-Dichloro-2,5-dimethylhexane by an SN1 Reaction| journal = [[J. Chem. Educ.]] | volume = 87 | issue = 1| pages = 81–83 | doi = 10.1021/ed8000057 | bibcode = 2010JChEd..87...81W}}</ref> :[[File:SN1reactionWagner2009.svg|Synthesis of 2,5-Dichloro-2,5-dimethylhexane by an S<sub>N</sub>1 reaction]] 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. ==Stereochemistry== The carbocation intermediate formed in the reaction's rate determining step (RDS) is an ''sp<sup>2</sup>'' 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.<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: [[File:SN1 stereochemistry.svg|center|600px|A typical S<sub>N</sub>1 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 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. ==Side reactions== 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. ==Solvent effects== {{See also|Solvent effects}} Since the S<sub>N</sub>1 reaction involves formation of an unstable carbocation intermediate in the rate-determining step (RDS), anything that can facilitate this process will speed up the reaction. The normal solvents of choice are both ''[[Polar aprotic solvents|polar]]'' (to stabilize ionic intermediates in general) and ''[[protic solvent]]s'' (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. 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 :<math> \log { \left ( \frac{k}{k_0} \right ) } = mY \,</math> 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 |author1=Ernest Grunwald |author2=S. Winstein |name-list-style=amp | 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 |author1=Arnold H. Fainberg |author2=S. Winstein |name-list-style=amp | journal = J. Am. Chem. Soc. | year = 1956 | pages = 2770 | volume = 78 | issue = 12 | doi = 10.1021/ja01593a033}}</ref> == See also == *[[Arrow pushing]] *[[Nucleophilic acyl substitution]] *[[Neighbouring group participation]] *[[SN2 reaction|S<sub>N</sub>2 reaction]] == References == {{Reflist}} == External links == * [http://www.chemhelper.com/sn1.html Diagrams]: [[Frostburg State University]] * [https://web.archive.org/web/20030708024541/http://www.usm.maine.edu/~newton/Chy251_253/Lectures/Sn1/Sn1FS.html Exercise]: the University of Maine {{Reaction mechanisms}} {{Authority control}} {{DEFAULTSORT:Sn1 Reaction}} [[Category:Nucleophilic substitution reactions]] [[Category:Reaction mechanisms]]
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