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nu main menu move to sidebar hide navigation main page contents current events random article about wikipedia contact us contribute help learn to edit community portal recent changes upload file special pages search search appearance donate create account log in personal tools donate create account log in contents move to sidebar hide top 1 mechanism 2 rate law 3 scope 4 stereochemistry 5 side reactions 6 solvent effects 7 see also 8 references 9 external links toggle the table of contents s n 1 reaction 25 languages العربية català čeština dansk español eesti فارسی français हिन्दी magyar bahasa indonesia italiano 日本語 한국어 bahasa melayu nederlands norsk bokmål polski português română simple english svenska ไทย 粵語 中文 edit links article talk english read edit view history tools tools move to sidebar hide actions read edit view history general what links here related changes upload file permanent link page information cite this page get shortened url switch to legacy parser print export download as pdf printable version in other projects wikimedia commons wikidata item appearance move to sidebar hide from wikipedia the free encyclopedia substitution reaction with a carbocation intermediate sn1 redirects here for the canadian sports channel see sportsnet one general reaction scheme for the s n 1 reaction the leaving group is denoted x and the nucleophile is denoted nu h the unimolecular nucleophilic substitution s n 1 reaction is a substitution reaction in organic chemistry the hughes ingold symbol of the mechanism expresses two properties s n stands for nucleophilic substitution and the 1 says that the rate determining step is unimolecular 1 2 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 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 and secondary alkyl halides the alternative s n 2 reaction occurs in inorganic chemistry the s n 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 3 this reaction does not depend much on the strength of the nucleophile unlike the s n 2 mechanism which involves two steps the first step of the s n 1 reaction is the ionization of alkyl halide in the presence of aqueous acetone or ethyl alcohol this step provides a carbocation as an intermediate which is planar in later steps attack of nucleophile 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 4 mechanism edit an example of a reaction taking place with an s n 1 reaction mechanism is the hydrolysis of tert butyl bromide forming tert butanol this s n 1 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 5 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 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 rate law edit although the rate law of the s n 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 n 1 reaction 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 d tbu d t 0 k 1 tbubr k 1 tbu br k 2 tbu h 2 o displaystyle 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 2 concentration of t butyl cation based on steady state assumption tbu k 1 tbubr k 1 br k 2 h 2 o displaystyle text tbu frac k_ 1 text tbubr k_ 1 text br k_ 2 text h _ 2 text o 3 overall reaction rate assuming rapid final step d tbuoh d t k 2 tbu h 2 o displaystyle frac d text tbuoh dt k_ 2 text tbu text h _ 2 text o 4 steady state rate law by plugging 2 into 3 d tbuoh d t k 1 k 2 tbubr h 2 o k 1 br k 2 h 2 o displaystyle 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 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 br displaystyle text br is negligible for these reasons k 1 br k 2 h 2 o displaystyle k_ 1 text br ll k_ 2 text h _ 2 text o often holds under these conditions the ssa rate law reduces to rate d tbuoh d t k 1 k 2 tbubr h 2 o k 2 h 2 o k 1 tbubr displaystyle 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 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 2 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 2 o while there is a negative fractional order dependence on br thus s n 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 and the observation of this effect is evidence for an s n 1 mechanism although the absence of a common ion effect does not rule it out 6 7 scope edit the s n 1 mechanism tends to dominate when the central carbon atom is surrounded by bulky groups because such groups sterically hinder the s n 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 stabilization and hyperconjugation from attached alkyl groups the hammond leffler postulate suggests that this too will increase the rate of carbocation formation the s n 1 mechanism therefore dominates in reactions at tertiary alkyl centers an example of a reaction proceeding in a s n 1 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 s n 2 to s n 1 stereochemistry edit the carbocation intermediate formed in the reaction s rate determining step rds is an sp 2 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 s n 1 reaction of s 3 chloro 3 methylhexane with an iodide ion which yields a racemic mixture of 3 iodo 3 methylhexane a typical s n 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 n 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 edit 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 at lower temperatures s n 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 n 1 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 edit see also solvent effects since the s n 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 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 the y scale correlates solvolysis reaction rates of any solvent k with that of a standard solvent 80 v v ethanol water k 0 through log k k 0 m y displaystyle log left frac k k_ 0 right my with m a reactant constant m 1 for tert butyl chloride and y a solvent parameter 10 for example 100 ethanol gives y 2 3 50 ethanol in water y 1 65 and 15 concentration y 3 2 11 see also edit arrow pushing nucleophilic acyl substitution neighbouring group participation s n 2 reaction references edit l g wade jr organic chemistry 6th ed pearson prentice hall upper saddle river new jersey usa 2005 march j 1992 advanced organic chemistry 4th ed new york wiley isbn 0 471 60180 2 bateman lc church mg hughes ed ingold ck taher na 1940 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 journal of the chemical society resumed 979 doi 10 1039 jr9400000979 dr hussain class lecture archeived 17 08 2024 peters k s 2007 nature of dynamic processes associated with the s n 1 reaction mechanism chem rev 107 3 859 873 doi 10 1021 cr068021k pmid 17319730 anslyn eric v 1960 2006 modern physical organic chemistry dougherty dennis a 1952 mill valley california university science books pp 638 639 isbn 1 891389 31 9 oclc 55600610 cite book cs1 maint multiple names authors list link cs1 maint numeric names authors list link lowry thomas h 1987 mechanism and theory in organic chemistry richardson kathleen schueller 3rd ed new york harper row pp 330 331 isbn 0 06 044084 8 oclc 14214254 wagner carl e marshall pamela a 2010 synthesis of 2 5 dichloro 2 5 dimethylhexane by an sn1 reaction j chem educ 87 1 81 83 bibcode 2010jched 87 81w doi 10 1021 ed8000057 sorrell thomas n organic chemistry 2nd edition university science books 2006 ernest grunwald s winstein 1948 the correlation of solvolysis rates j am chem soc 70 2 846 bibcode 1948jachs 70 846g doi 10 1021 ja01182a117 arnold h fainberg s winstein 1956 correlation of solvolysis rates iii 1 t butyl chloride in a wide range of solvent mixtures j am chem soc 78 12 2770 bibcode 1956jachs 78 2770f doi 10 1021 ja01593a033 external links edit diagrams frostburg state university exercise the university of maine v t e basic reaction mechanisms nucleophilic substitutions unimolecular nucleophilic substitution s n 1 bimolecular nucleophilic substitution s n 2 nucleophilic internal substitution s n i nucleophilic acyl substitution s n acyl electrophilic substitutions electrophilic aromatic substitution s e ar elimination reactions e1cb elimination e i elimination addition reactions electrophilic addition a e nucleophilic addition a n free radical addition cycloaddition oxidative addition unimolecular reactions intramolecular reaction isomerization photodissociation lindemann mechanism rrkm theory electron proton transfer reactions redox harpoon reaction grotthuss mechanism marcus theory inner sphere electron transfer outer sphere electron transfer medium effects solvent effects cage effect matrix isolation related topics elementary reaction reaction dynamics reactive intermediate radical chemistry molecularity stereochemistry catalysis collision theory arrow pushing potential energy surface more o ferrall jencks plot chemical kinetics rate equation equilibrium constant rate determining step reaction coordinate energy profile chemistry transition state theory activation energy activated complex arrhenius equation eyring equation michaelis menten kinetics diffusion controlled reaction retrieved from https en wikipedia org w index php title sn1_reaction oldid 1306727995 categories nucleophilic substitution reactions reaction mechanisms hidden categories articles with short description short description is different from wikidata cs1 maint multiple names authors list cs1 maint numeric names authors list this page was last edited on 19 august 2025 at 10 42 utc page was rendered with parsoid text is available under the creative commons attribution sharealike 4 0 license additional terms may apply by 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