{"id":5454,"date":"2012-08-08T09:00:44","date_gmt":"2012-08-08T09:00:44","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=5454"},"modified":"2026-04-18T05:56:35","modified_gmt":"2026-04-18T10:56:35","slug":"comparing-the-sn1-and-sn2-reactions","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2012\/08\/08\/comparing-the-sn1-and-sn2-reactions\/","title":{"rendered":"Comparing the SN1 and SN2 Reactions"},"content":{"rendered":"<p><strong>Comparing the S<sub>N<\/sub>1 and S<sub>N<\/sub>2 Reactions<\/strong><\/p>\n<p>In <strong>nucleophilic substitution reactions<\/strong>, a bond between carbon and a leaving group (C\u2013LG) is broken, and a new bond between carbon and a nucleophile (C\u2013Nu) is formed.<\/p>\n<p>Nucleophilic substitution reactions of alkyl halides occur through two main pathways. The key difference lies in the <strong>timing<\/strong> of the bond-forming and bond-breaking steps.<\/p>\n<ul>\n<li>The <a href=\"https:\/\/www.masterorganicchemistry.com\/2012\/07\/13\/the-sn1-mechanism\/\">S<sub>N<\/sub>1 mechanism<\/a> (<strong>S<\/strong>ubstitution, <strong>N<\/strong>ucleophilic, <strong>UNI<\/strong>molecular rate determining step) generally passes through <strong>two<\/strong> steps; first, a (slow, rate-determining) breaking of\u00a0 the C\u2013LG bond on the substrate to form an intermediate <strong>carbocation<\/strong>, followed by (fast) addition of a nucleophile to the carbocation (form C\u2013Nu) to give the substitution product\u00a0 (<span style=\"color: #993366;\"><em>there is often a third acid-base step which follows the substitution reaction when neutral nucleophiles like H<sub>2<\/sub>O or ROH are used<\/em><\/span>)<\/li>\n<li>The <a href=\"https:\/\/www.masterorganicchemistry.com\/2012\/07\/04\/the-sn2-mechanism\/\">S<sub>N<\/sub>2 mechanism<\/a> (<strong>S<\/strong>ubstitution, <strong>N<\/strong>ucleophilic, <strong>Bi<\/strong>molecular rate determining step) occurs in a <strong>single<\/strong>, concerted step: attack of the nucleophile on the backside of the C\u2013LG bond, passing through a transient five-membered transition state en route to a tetrahedral product where configuration at the carbon has been inverted.<\/li>\n<\/ul>\n<p>Since the rate-determining step in the S<sub>N<\/sub>1 is formation of a carbocation, it can be helpful to think of the &#8220;big barrier&#8221; to the S<sub>N<\/sub>1 reaction as being <strong>carbocation stability<\/strong>, That is, any factor which leads to the increased stability of a carbocation intermediate will increase the rate of the S<sub>N<\/sub>1. That&#8217;s why this pathway tends to be favored by tertiary alkyl halides, since the order of carbocation stability generally proceeds tertiary &gt; secondary &gt; primary (<span style=\"color: #993366;\"><em>See article: <a href=\"https:\/\/www.masterorganicchemistry.com\/2011\/03\/11\/3-factors-that-stabilize-carbocations\/\">Carbocation Stability<\/a><\/em><\/span>)<\/p>\n<p>Similarly, since the rate-determining step of the S<sub>N<\/sub>2 is the backside attack of a nucleophile on carbon,\u00a0 it can be helpful to think of the &#8220;big barrier&#8221; to the S<sub>N<\/sub>2 reaction as being <strong>steric hindrance <\/strong>(<span style=\"color: #993366;\"><em>See article: <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2011\/07\/18\/steric-hindrance-is-like-a-fat-goalie\/\">Steric Hindrance<\/a><\/em><\/span>). Any factor which increases the difficulty with which the nucleophile can access the sigma* orbital of the C\u2013LG bond will result in a slower reaction, which helps us to rationalize why the S<sub>N<\/sub>2 is faster with methyl and primary alkyl halides than for secondary and tertiary alkyl halides.<\/p>\n<p>Owing to these two different mechanisms, the S<sub>N<\/sub>1 and S<sub>N<\/sub>2 reactions exhibit key differences in<\/p>\n<ul>\n<li>observed <strong>rate laws<\/strong> (unimolecular for S<sub>N<\/sub>1, bimolecular for S<sub>N<\/sub>2)<\/li>\n<li>patterns of <strong>stereochemistry<\/strong> (retention + inversion for S<sub>N<\/sub>1, inversion for S<sub>N<\/sub>2)<\/li>\n<li><strong>relative rates<\/strong> of reaction for primary, secondary, and tertiary alkyl halides (tertiary &gt; secondary &gt; primary for S<sub>N<\/sub>1, primary &gt; secondary &gt; tertiary for S<sub>N<\/sub>2<\/li>\n<\/ul>\n<p>Additionally, the S<sub>N<\/sub>1 and S<sub>N<\/sub>2 reactions are sensitive to the identity of the solvent and of the strength of the participating nucleophile. More detail below!<\/p>\n<p><img fetchpriority=\"high\" decoding=\"async\" class=\"alignnone wp-image-35026\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/06\/0-table-comparing-sn1-and-sn2-reactions-solvent-rate-law-nucleophile-stereochemistry-alkyl-halide.gif\" alt=\"table comparing sn1 and sn2 reactions solvent rate law nucleophile stereochemistry alkyl halide\" width=\"640\" height=\"631\" \/><\/a><\/p>\n<p><strong>Table of Contents<\/strong><\/p>\n<ol>\n<li><a href=\"#one\">But First: The Story Of The Cats And The Comfy Chair<\/a><\/li>\n<li><a href=\"#two\">The SN1 Proceeds Through A Stepwise Mechanism. The SN2 Proceeds Through a Concerted Mechanism<\/a><\/li>\n<li><a href=\"#three\">Reaction Coordinate Diagrams of the SN1 and SN2 Reactions<\/a><\/li>\n<li><a href=\"#four\">The Rate Laws of the SN1 and SN2 Reactions<\/a><\/li>\n<li><a href=\"#five\">Primary, Secondary, and Tertiary Alkyl Halides in SN1 and SN2 Reactions<\/a><\/li>\n<li><a href=\"#six\">Comparing the Stereochemistry of SN1 and SN2 Reactions<\/a><\/li>\n<li><a href=\"#seven\">Solvents and Nucleophiles &#8211; SN1 Reaction<\/a><\/li>\n<li><a href=\"#eight\">Solvents and Nucleophiles &#8211; SN2 Reaction<\/a><\/li>\n<li><a href=\"#nine\">Back To The Cats<\/a><\/li>\n<li><a href=\"#notes\">Notes<\/a><\/li>\n<li><a href=\"#quizzes\">Quiz Yourself!<\/a><\/li>\n<li><a href=\"#references\">(Advanced) References and Further Reading<\/a><\/li>\n<\/ol>\n<hr \/>\n<h2><a id=\"one\"><\/a>1. But First: The Cats And The Comfy Chair<\/h2>\n<p>But first &#8211; have you ever heard the story of the cats and the comfy chair?<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-35027\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/06\/1-using-cats-to-compare-sn1-and-sn2-reactions-master-organic-chemistry.png\" alt=\"-using cats to compare sn1 and sn2 reactions master organic chemistry\" width=\"640\" height=\"407\" srcset=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/06\/1-using-cats-to-compare-sn1-and-sn2-reactions-master-organic-chemistry.png 1200w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/06\/1-using-cats-to-compare-sn1-and-sn2-reactions-master-organic-chemistry-300x191.png 300w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/06\/1-using-cats-to-compare-sn1-and-sn2-reactions-master-organic-chemistry-1024x652.png 1024w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/06\/1-using-cats-to-compare-sn1-and-sn2-reactions-master-organic-chemistry-768x489.png 768w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/06\/1-using-cats-to-compare-sn1-and-sn2-reactions-master-organic-chemistry-760x484.png 760w\" sizes=\"(max-width: 640px) 100vw, 640px\" \/><\/a><\/p>\n<p>Cat #1 finds Cat #2 on his\u00a0comfy chair and wants to sit. He has\u00a0two options.<\/p>\n<ol>\n<li>He can wait for Cat #2 to leave, and then sit in the comfy chair.<\/li>\n<li>He can kick the Cat #2 out of his comfy chair.<\/li>\n<\/ol>\n<div>Think about that for a second. In the meantime, let&#8217;s compare the S<sub>N<\/sub>1 and the S<sub>N<\/sub>2.<\/div>\n<h2><strong><a id=\"two\"><\/a>2. <\/strong><strong>The S<sub>N<\/sub>1 Proceeds Through A Stepwise Mechanism. The S<sub>N<\/sub>2 Proceeds Through a Concerted Mechanism<\/strong><\/h2>\n<p>Let&#8217;s compare the mechanisms of the S<sub>N<\/sub>1 and S<sub>N<\/sub>2 reactions first, since every other difference we will observe is a consequence of their different mechanisms. <span style=\"color: #993366;\"><em>(Generally, proposing a mechanism only comes <strong>after<\/strong> collecting a lot of experimental evidence (e.g. rate laws, stereochemistry, relative rates, etc.) , but since this isn&#8217;t an Agatha Christie novel, we&#8217;re giving away the ending first).\u00a0<\/em><\/span><\/p>\n<p>The S<sub>N<\/sub>1 generally passes through a two-step &#8220;<strong>stepwise<\/strong>&#8221; mechanism where<\/p>\n<ul>\n<li>The leaving group leaves (break C\u2013LG) to give a carbocation intermediate (slow, rate-determining step)<\/li>\n<li>The resulting carbocation intermediate is attacked by a nucleophile to give a new product (form C\u2013Nu)\u00a0 (fast step)<\/li>\n<\/ul>\n<p><span style=\"color: #993366;\"><em>(In many cases, there is often a third step involving deprotonation of the nucleophile to give a neutral product, especially if the nucleophile is neutral)<\/em><\/span><\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-35028\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/06\/2-mechanism-of-the-sn1-reaction-showing-mixture-of-retention-and-inversion.gif\" alt=\"mechanism of the sn1 reaction showing mixture of retention and inversion\" width=\"640\" height=\"628\" \/><\/a><\/p>\n<p>Note that the carbocation intermediate has a trigonal planar geometry at carbon, and its empty p-orbital can undergo addition by a nucleophile equally well at <strong>either<\/strong> face. In cases where substitution occurs on a stereogenic carbon (aka &#8220;chiral carbon&#8221;) this can result in a mixture of <strong>retention<\/strong> and <strong>inversion<\/strong> of stereochemistry, relative to the configuration of the original alkyl halide. [<a href=\"#noteone\"><span style=\"color: #ff0000;\">Note 1<\/span><\/a>]<\/p>\n<p>In contrast, the S<sub>N<\/sub>2 reaction passes through a one-step <strong>concerted<\/strong> mechanism where one equivalent of nucleophile adds to one equivalent of substrate, resulting in formation of a new bond to the nucleophile and the loss of a leaving group from carbon.<\/p>\n<p>This is achieved through donation of a pair of electrons from the nucleophile into the empty, antibonding sigma-star (\u03c3<sup>*<\/sup>) orbital on the backside of the C\u2013LG bond. The carbon-nucleophile bond (C-Nu) forms at the same time that the carbon-leaving group bond (C-LG) breaks.<\/p>\n<p>Since carbon cannot comfortably accommodate more than four bonding partners at one time these two bonds are considered to have <em>partial<\/em> bonding character in the transition state (<span style=\"color: #993366;\"><em>note the dashed lines, below<\/em><\/span>) where carbon adopts a &#8220;trigonal bipyramidal&#8221; geometry.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35030\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/06\/3-mechanism-of-the-sn2-reaction-showing-concerted-transition-state-and-inversion-of-configuration.gif\" alt=\"mechanism of the sn2 reaction showing concerted transition state and inversion of configuration\" width=\"640\" height=\"393\" \/><\/a><\/p>\n<p>As the C-Nu bond forms and the C-LG bond breaks, the three &#8220;trigonal&#8221; substituents on the &#8220;equator&#8221; of the carbon relax to a tetrahedral geometry, with the difference that they are completely <strong>inverted<\/strong> from their original position (<span style=\"color: #993366;\"><em>like the often-invoked metaphor of the &#8220;umbrella turning inside-out in a strong wind&#8221;<\/em><\/span>) .<\/p>\n<h2><a id=\"three\"><\/a>3. Reaction Coordinate Diagrams Of The S<sub>N<\/sub>1 and S<sub>N<\/sub>2 Reactions<\/h2>\n<p>One way to visualize the differences between these two mechanisms is to sketch out their <strong>reaction coordinate diagrams<\/strong>, where we graph changes in potential energy (vertical axis) the starting materials pass along the &#8220;reaction coordinate&#8221; toward their conversion into products (horizontal axis) (<em><span style=\"color: #993366;\">These diagrams resemble a graph of changes in altitude (also potential energy!) experienced by a hiker navigating a mountain pass between two destinations.<\/span>)<\/em><\/p>\n<p>In these diagrams the &#8220;peaks&#8221; (local maxima) represent<strong> transition states<\/strong> whereas &#8220;valleys&#8221; (local minima) represent <strong>intermediates<\/strong>. (<span style=\"color: #993366;\"><em>A transition state is a transient species with partial bonds. An intermediate is a potentially isolable species. )<\/em><\/span><\/p>\n<p>The reaction coordinate diagram of the S<sub>N<\/sub>1 reaction has a two peaks, representing the two transition states (Step 1 and Step 2, respectively) flanking a single &#8220;valley&#8221; representing the carbocation intermediate.<\/p>\n<p>Each step of the process has an\u00a0<strong>activation energy\u00a0<\/strong>represented by the difference in energy between the reactant and the transition state.<\/p>\n<p>The <strong>rate-determining step <\/strong>of a reaction is the step requiring the highest <strong>activation energy<\/strong>, that is, the largest change in potential energy from reactant to transition state. In the S<sub>N<\/sub>1 reaction, the rate determining step\u00a0 is (<span style=\"color: #ff00ff;\">illustrated in pink<\/span>) loss of the leaving group from the alkyl halide to give the carbocation.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35031\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/06\/4-reaction-coordinate-diagram-of-the-sn1-reaction.gif\" alt=\"reaction coordinate diagram of the sn1 reaction\" width=\"640\" height=\"483\" \/><\/a><\/p>\n<p>The reaction coordinate diagram of the S<sub>N<\/sub>2 reaction shows only a single transition state (one &#8220;peak&#8221;) corresponding to the <strong>concerted<\/strong> formation of C-Nu and breakage of C-LG without any intermediate. (<a href=\"#notetwo\"><span style=\"color: #ff0000;\">Note<\/span> <span style=\"color: #ff0000;\">2<\/span><\/a>)<\/p>\n<p>One note &#8211; the difference in energy between the starting material and the product reflects the fact that the leaving group is generally a weaker base than the nucleophile, as well as the difference in bond strengths to carbon. (<span style=\"color: #993366;\"><em>See article: <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2011\/04\/12\/what-makes-a-good-leaving-group\/\">What Makes A Good Leaving Group<\/a><\/em><\/span>)<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35032\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/06\/5-reaction-coordinate-diagram-of-the-sn2-reaction-showing-one-transition-state.gif\" alt=\"reaction coordinate diagram of the sn2 reaction showing one transition state\" width=\"640\" height=\"513\" \/><\/a><\/p>\n<h2><a id=\"four\"><\/a>4. The Rate Laws Of The S<sub>N<\/sub>1 and S<sub>N<\/sub>2 Reactions<\/h2>\n<p>One way of probing the mechanism of a given substitution reaction is to measure the changes in <strong>reaction rates<\/strong> when the concentration of both nucleophile and substrate are varied.<\/p>\n<p>Since the rate-determining step of the S<sub>N<\/sub>1 reaction is loss of a leaving group from the substrate &#8211; a <strong>unimolecular<\/strong> reaction &#8211; the rate of product formation in the S<sub>N<\/sub>1 should depend only on the concentration of substrate.<\/p>\n<p>Rate =\u00a0<em>k<\/em> [Concentration of alkyl halide]<\/p>\n<p><strong>Doubling<\/strong>, <strong>tripling<\/strong>, or <strong>quadrupling<\/strong> the concentration of <strong>substrate<\/strong> should thus result in a <strong>doubling<\/strong>, <strong>tripling<\/strong>, or <strong>quadrupling<\/strong> of the rate of product formation, respectively.<\/p>\n<p><strong>Doubling<\/strong>, <strong>tripling<\/strong>, or <strong>quadrupling<\/strong> the concentration of <strong>nucleophile<\/strong> on the other hand should have <strong>no effect<\/strong> on the reaction rate since the nucleophile is <strong>not<\/strong> involved in the rate-determining step.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35033\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/06\/6-rate-law-of-the-sn1-reaction-showing-dependence-of-rate-on-concentration-of-substrate.gif\" alt=\"rate law of the sn1 reaction showing dependence of rate on concentration of substrate\" width=\"640\" height=\"497\" \/><\/a><\/p>\n<p>The S<sub>N<\/sub>2 reaction, by contrast, has a <strong>bimolecular\u00a0<\/strong>rate-determining step where one equivalent of nucleophile combines with one equivalent of substrate.<\/p>\n<p>The overall rate law of the S<sub>N<\/sub>2 is thus dependent on both the concentration of substrate and the concentration of nucleophile, taking the form:<\/p>\n<p>Rate = <em>k<\/em> [Concentration of alkyl halide] [Concentration of nucleophile]<\/p>\n<p>We say this is &#8220;first-order&#8221; in nucleophile <strong>and<\/strong> &#8220;first-order&#8221; in substrate, or &#8220;second order&#8221; overall.<\/p>\n<ul>\n<li><strong>Doubling<\/strong> the concentration of <strong>substrate<\/strong> will <strong>double<\/strong> the rate of formation of product.<\/li>\n<li>Likewise, <strong>doubling<\/strong> the concentration of <strong>nucleophile<\/strong> will also <strong>double<\/strong> the rate of product formation.<\/li>\n<li><strong>Doubling<\/strong> the concentration of substrate <strong>and<\/strong> nucleophile will result in a <strong>quadrupling<\/strong> of the rate of product formation.<\/li>\n<\/ul>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35034\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/06\/7-illustration-of-the-rate-law-of-the-sn1-reaction-showing-dependence-of-rate-on-concentration-of-substrate-but-no-dependence-on-nucleophile.gif\" alt=\"illustration of the rate law of the sn1 reaction showing dependence of rate on concentration of substrate but no dependence on nucleophile\" width=\"640\" height=\"574\" \/><\/a><\/p>\n<h2><a id=\"five\"><\/a>5. The Effect of Substrate On The Rate of S<sub>N<\/sub>1 and S<sub>N<\/sub>2 Reactions<\/h2>\n<p>The dependence of the rate of the S<sub>N<\/sub>1 on <strong>carbocation stability\u00a0<\/strong>and the rate of the S<sub>N<\/sub>2 on <strong>steric hindrance<\/strong> means that the trends of their reaction rates with primary, secondary, and tertiary alkyl halides proceeds in opposite directions.<\/p>\n<p>The rate-determining step of the S<sub>N<\/sub>1 reaction is formation of a carbocation. Since tertiary carbocations are more stable than secondary carbocations which are in turn far more stable than primary (and methyl) carbocations, we should observe that the rate of S<sub>N<\/sub>1 reactions is fastest with <strong>tertiary<\/strong> alkyl halides.<\/p>\n<p>This is indeed the case, as illustrated when various alkyl halides are subjected to typical S<sub>N<\/sub>1 conditions (a poorly nucleophilic, polar protic solvent such as H<sub>2<\/sub>O) [<span style=\"color: #ff0000;\">Ref<\/span>]<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35035\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/06\/8-the-role-of-tertiary-secondary-primary-substrates-in-the-sn1-reaction.gif\" alt=\"the role of tertiary secondary primary substrates in the sn1 reaction\" width=\"640\" height=\"446\" \/><\/a><\/p>\n<p><span style=\"color: #993366;\"><em>(Note that these are relative rates, where the rate of substitution at t-butyl bromide 1.2 \u00d7 10<sup>6<\/sup> is measured relative to the rate of substitution at ethyl bromide (1))\u00a0<\/em><\/span><\/p>\n<p>The rate determining step of the S<sub>N<\/sub>2 reaction is backside attack of a nucleophile on an alkyl halide. Since hydrogen atoms are smaller than carbon atoms, we should expect that the rate of S<sub>N<\/sub>2 reactions is fastest with methyl and primary alkyl halides and slowest with tertiary alkyl halides.<\/p>\n<p>This also agrees with experiment, as shown below when a variety of alkyl halides are treated with the strong nucleophile NaCN.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35036\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/06\/9-the-role-of-primary-secondary-tertiary-substrates-in-the-sn2-reaction.gif\" alt=\"the role of primary secondary tertiary substrates in the sn2 reaction\" width=\"640\" height=\"547\" \/><\/a><\/p>\n<h2><a id=\"six\"><\/a>6. Comparing The Stereochemistry of S<sub>N<\/sub>1 and S<sub>N<\/sub>2 Reactions<\/h2>\n<p>The\u00a0<strong>stereochemistry\u00a0<\/strong>of the products relative to those of the starting material are also a useful probe of S<sub>N<\/sub>1 versus S<sub>N<\/sub>2 pathway.<\/p>\n<p>When a stereogenic center loses a leaving group to become a trigonal planar carbocation, it loses chirality.<\/p>\n<p>Since the resulting carbocation can be attacked on either face by a nucleophile, the resulting product will be a mixture of retention and inversion of stereochemistry.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35037\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/06\/10-the-stereochemistry-of-the-sn1-reaction-gives-a-mixture-of-retention-and-inversion.gif\" alt=\"the stereochemistry of the sn1 reaction gives a mixture of retention and inversion\" width=\"640\" height=\"412\" \/><\/a><\/p>\n<p>An S<sub>N<\/sub>2 reaction that occurs on a stereogenic carbon will result in\u00a0<strong>inversion\u00a0<\/strong>of configuration, but will\u00a0<strong>retain<\/strong> optical purity.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35038\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/06\/11-the-stereochemistry-of-the-sn2-reaction-gives-only-inversion.gif\" alt=\"the stereochemistry of the sn2 reaction gives only inversion\" width=\"640\" height=\"335\" \/><\/a><\/p>\n<p><span style=\"color: #993366;\"><em>Note that inversion still happens whether or not the starting material is chiral or not; it just won&#8217;t be observable.\u00a0<\/em><\/span><\/p>\n<h2><a id=\"seven\"><\/a>7. Solvents And Nucleophiles &#8211; S<sub>N<\/sub>1<\/h2>\n<p>The SN1 reaction tends to occur when alkyl halides capable of forming reasonably stable carbocations are dissolved in polar protic solvents that are capable of acting as nucleophiles.<\/p>\n<p>Loss of a leaving group to give a carbocation results in the formation of a transient <strong>ion pair<\/strong> (<span style=\"color: #993366;\"><em>i.e. the carbocation and the leaving group<\/em><\/span>) from a neutral species (<span style=\"color: #993366;\"><em>i.e. the alkyl halide<\/em><\/span>)<\/p>\n<p>Just as salts like NaCl are unlikely to dissolve in non-polar solvents like hexane (<em><span style=\"color: #993366;\">dielectric constant<\/span>, <span style=\"color: #993366;\">\u03b5 = 2<\/span><\/em>), carbocation formation is much more favorable in <strong>polar<\/strong> solvents like water (<span style=\"color: #993366;\"><em>\u03b5 = 78<\/em><\/span>), alcohols (<span style=\"color: #993366;\"><em>\u03b5 ~ 20-40<\/em><\/span>), or carboxylic acids (<em><span style=\"color: #993366;\">e.g.\u00a0 formic acid, \u03b5 = 51<\/span><\/em>) , which are able to <strong>stabilize<\/strong> charges through hydrogen bonding or other dipolar interactions.<\/p>\n<p>Carbocations resemble neutral compounds of boron (e.g. BF<sub>3<\/sub>), in that they are excellent Lewis acids containing 6 valence electrons and an empty p-orbital.\u00a0 Once formed, carbocations readily undergo addition even with poor Lewis bases to give products with a full octet around carbon.<\/p>\n<p>So in practice, this generally means the <strong>solvent<\/strong> is the <strong>nucleophile<\/strong>, since it is present in much higher concentrations relative to anything else. [<a href=\"#notethree\"><span style=\"color: #ff0000;\">Note 3<\/span><\/a>]<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35039\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/06\/12-considerations-of-solvent-and-nucleophile-in-the-sn1-reaction.gif\" alt=\"considerations of solvent and nucleophile in the sn1 reaction\" width=\"640\" height=\"324\" \/><\/a><\/p>\n<h2><a id=\"eight\"><\/a>8. Solvents and Nucleophiles &#8211; S<sub>N<\/sub>2<\/h2>\n<p>S<sub>N<\/sub>2 reactions can certainly be carried out in polar protic solvents like alcohols, especially with primary alkyl halides.<\/p>\n<p>But since the rate of the S<sub>N<\/sub>2 isn&#8217;t dependent on carbocation formation, a much wider variety of (less polar) solvents may be used.<\/p>\n<p>Furthermore, alkyl halides are poorer electrophiles than carbocations. This is actually a good thing, since they are much less likely to undergo side reactions like rearrangement or elimination, or react with the first Lewis base they see.<\/p>\n<p>For practical purposes, S<sub>N<\/sub>2 reactions tend to be carried out with stronger (i.e. charged) nucleophiles. A wide variety of nucleophilic partners can be used, which makes the S<sub>N<\/sub>2 extremely versatile,\u00a0 especially with primary alkyl halides. (<span style=\"color: #993366;\"><em>See article &#8211; <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2012\/07\/11\/why-the-sn2-reaction-is-powerful\/\">Why The S<sub>N<\/sub>2 Reaction Is Powerful<\/a><\/em><\/span>)<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35040\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/06\/13-considerations-of-solvent-and-nucleophile-in-the-sn2-reaction.gif\" alt=\"considerations of solvent and nucleophile in the sn2 reaction\" width=\"640\" height=\"334\" \/><\/a><\/p>\n<p>Polar <strong>aprotic<\/strong> solvents such as DMSO, acetone, DMF and acetonitrile are often chosen for S<sub>N<\/sub>2 reactions since they are polar enough to dissolve the reaction partners, but cannot form hydrogen bonds to the nucleophile. This has the practical effect of making the nucleophile less bulky, since the nucleophile isn&#8217;t surrounded by a shell of solvent molecules everywhere it goes.<\/p>\n<div>\n<div>\n<h2><a id=\"nine\"><\/a>9. Back To The Cats<\/h2>\n<div>So does the story about the cats and the comfy chair make more sense now?<\/div>\n<div>\n<ul>\n<li>In the S<sub>N<\/sub>2, the nucleophile (Cat #1) forms a bond to the substrate (comfy\u00a0chair) at the same time the leaving group (Cat #2) leaves.<\/li>\n<li>In the S<sub>N<\/sub>1, the leaving group (Cat #2) leaves the substrate (comfy chair), and then the nucleophile (Cat #1) forms a bond.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<div>If this makes sense, you might be ready for the <strong><a href=\"https:\/\/www.masterorganicchemistry.com\/2012\/11\/21\/deciding-sn1sn2e1e2-1-the-substrate\/\">Quick N&#8217; Dirty Guide to S<sub>N<\/sub>1\/S<sub>N<\/sub>2\/E1\/E2 reactions<\/a><\/strong>. Otherwise, join us for our next post when we discuss\u00a0<strong><a href=\"https:\/\/www.masterorganicchemistry.com\/2011\/10\/17\/introduction-to-rearrangement-reactions\/\">rearrangement reactions.\u00a0<\/a><\/strong><\/div>\n<div>\n<div><\/div>\n<div>\n<hr \/>\n<p><strong>Don&#8217;t forget &#8211; you can download a free 1-page Summary Sheet of S<sub>N<\/sub>1 vs S<sub>N<\/sub>2 reactions containing all the material on this blog post here:\u00a0<a href=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2012\/08\/SN1-vs-SN2.pdf\">Download SN1 vs SN2 Summary Sheet PDF<\/a><\/strong><\/p>\n<\/div>\n<\/div>\n<\/div>\n<hr \/>\n<h2><a id=\"notes\"><\/a>Notes<\/h2>\n<div class=\"related-articles\"><p><strong>Related Articles<\/strong><\/p><ul><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2012\/07\/13\/the-sn1-mechanism\/\" class=\"\"><span>The SN1 Mechanism<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2012\/07\/04\/the-sn2-mechanism\/\" class=\"\"><span>The SN2 Mechanism<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2023\/01\/18\/where-will-substitution-elimination-reactions-occur\/\" class=\"\"><span>Identifying Where Substitution and Elimination Reactions Happen<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2012\/04\/27\/polar-protic-polar-aprotic-nonpolar-all-about-solvents\/\" class=\"\"><span>Polar Protic? Polar Aprotic? Nonpolar? All About Solvents<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2011\/07\/18\/steric-hindrance-is-like-a-fat-goalie\/\" class=\"\"><span>Steric Hindrance is Like a Fat Goalie<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2011\/04\/12\/what-makes-a-good-leaving-group\/\" class=\"\"><span>What makes a good leaving group?<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2010\/06\/16\/1-2-3-4\/\" class=\"\"><span>Primary, Secondary, Tertiary, Quaternary In Organic Chemistry<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2012\/11\/21\/deciding-sn1sn2e1e2-1-the-substrate\/\" class=\"\"><span>Deciding SN1\/SN2\/E1\/E2 (1) \u2013 The Substrate<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2011\/10\/17\/introduction-to-rearrangement-reactions\/\" class=\"\"><span>Introduction to Rearrangement Reactions<\/span><\/a><\/li><\/ul><\/div>\n<div>\n<p><em>Cat Illustration by my talented cousin, political cartoonist <a href=\"http:\/\/mackaycartoons.net\">Graeme MacKay<\/a>\u00a0<\/em><\/p>\n<p><strong>Note 1.\u00a0<\/strong>[Intimate ion pairs can affect this].<\/p>\n<p><strong>Note 2.\u00a0<\/strong>[If the nucleophile is neutral, e.g. H2O, then there will be a second, lower-energy transition state corresponding to the deprotonation step]<\/p>\n<p><strong>Note 3. [<\/strong>It is possible to trap carbocations with external nucleophiles.]<\/p>\n<p><strong><br \/>\nUPDATE<\/strong> . The most perfect cat video ever. Thanks to Alex Roche (Rutgers U.) for sending.<\/p>\n<blockquote class=\"imgur-embed-pub\" lang=\"en\" data-id=\"a\/uuwVUNB\"><p><a href=\"\/\/imgur.com\/uuwVUNB\">There Can Be Only One<\/a><\/p><\/blockquote>\n<p><script async src=\"\/\/s.imgur.com\/min\/embed.js\" charset=\"utf-8\"><\/script><\/p>\n<h2><a id=\"quizzes\"><\/a>Quiz Yourself<\/h2>\n\n<p class=\"p1\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-26714\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/2690-Front-Image-Only.png\" alt=\"\" width=\"640\" height=\"616\" \/><\/a><\/p>\n<p><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a\u00a0 MOC member<\/strong><\/a> to see the clickable quiz with answers on the back.<\/p>\n\n<p class=\"p1\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-26714\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/2687-Front-Image-Only.png\" alt=\"\" width=\"640\" height=\"616\" \/><\/p>\n<p><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a\u00a0 MOC member<\/strong><\/a> to see the clickable quiz with answers on the back.<\/p>\n<p><br \/>\n<\/p>\n<p class=\"p1\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-26714\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/2693-Front-Image-Only.png\" alt=\"\" width=\"640\" height=\"616\" \/><\/p>\n<p><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a\u00a0 MOC member<\/strong><\/a> to see the clickable quiz with answers on the back. <\/p>\n\n<p class=\"p1\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-26714\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/2694-Front-Image-Only.png\" alt=\"\" width=\"640\" height=\"616\" \/><\/p>\n<p><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a\u00a0 MOC member<\/strong><\/a> to see the clickable quiz with answers on the back. <\/p>\n<hr \/>\n<h2><a id=\"references\"><\/a>(Advanced) References and Further Reading<\/h2>\n<p><a href=\"https:\/\/archive.org\/details\/solvolyticdispla00stre\">Solvolytic Displacement Reactions<\/a> by Andrew Streitweiser is an oldie but a goodie in terms of compiling a lot of information (particularly reaction rates) of S<sub>N<\/sub>1 and S<sub>N<\/sub>2 reactions. Available on the Internet Archive for 1-hour loans <a href=\"https:\/\/archive.org\/details\/solvolyticdispla00stre\">here<\/a>.<\/p>\n<ol>\n<li><strong>Reaction kinetics and the Walden inversion. Part VI. Relation of steric orientation to mechanism in substitutions involving halogen atoms and simple or substituted hydroxyl groups<br \/>\n<\/strong>W. A. Cowdrey, E. D. Hughes, C. K. Ingold, S. Masterman, and A. D. Scott<br \/>\n<em>J. Chem. Soc.<\/em> <strong>1937<\/strong>, 1252-1271<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/pubs.rsc.org\/en\/content\/articlelanding\/1937\/jr\/jr9370001252#!divAbstract\">10.1039\/JR9370001252<\/a><strong><br \/>\n<\/strong>The points listed in the summary are worth reading for understanding what influences the S<sub>N<\/sub>1 and S<sub>N<\/sub>2 pathways.<\/li>\n<li><strong> Mechanism of substitution at a saturated carbon atom. Part XXVI. The r\u00f4le of steric hindrance. (Section A) introductory remarks, and a kinetic study of the reactions of methyl, ethyl, n-propyl, isobutyl, and neopentyl bromides with sodium ethoxide in dry ethyl alcohol<br \/>\n<\/strong>I. Dostrovsky and E. D. Hughes<strong><br \/>\n<\/strong><em>J. Chem. Soc.<\/em> <strong>1946<\/strong>, 157-161<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/pubs.rsc.org\/EN\/content\/articlelanding\/1946\/jr\/jr9460000157#!divAbstract\">10.1039\/JR9460000157<\/a><br \/>\nTable I in this paper shows the reduction in reaction rate for the S<sub>N<\/sub>2 reaction of R-Br with OEt- when R goes from methyl -&gt; ethyl -&gt; <em>n<\/em>-propyl -&gt; isobutyl -&gt; <em>t<\/em>-amyl. This can be attributed to steric hindrance, as backside attack of the substituted carbon becomes increasingly challenging.<\/li>\n<li><strong> Mechanism of substitution at a saturated carbon atom. Part III. Kinetics of the degradations of sulphonium compounds<br \/>\n<\/strong>John L. Gleave, Edward D. Hughes and Christopher K. Ingold<strong><br \/>\n<\/strong><em>J. Chem. Soc<\/em>. <strong>1935<\/strong>, 234-244<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/pubs.rsc.org\/en\/content\/articlelanding\/1935\/JR\/JR9350000236#!divAbstract\">10.1039\/JR9350000236<\/a><br \/>\nThis is a useful paper \u2013 in the beginning the terms \u201cS<sub>N<\/sub>1\u201d and \u201cS<sub>N<\/sub>2\u201d are introduced and defined, and Figs. 1 and 2 depict how the two mechanisms can compete depending on the structure of the substrate.<\/li>\n<li><strong> Influence of poles and polar linkings on the course pursued by elimination reactions. Part XVI. Mechanism of the thermal decomposition of quaternary ammonium compounds<br \/>\n<\/strong>E. D. Hughes, C. K. Ingold, and C. S. Patel<br \/>\n<em>J. Chem. Soc<\/em>. <strong>1933<\/strong>, 526-530<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/pubs.rsc.org\/en\/content\/articlelanding\/1933\/JR\/JR9330000526#!divAbstract\">10.1039\/JR9330000526<\/a><br \/>\nAt the end of this paper, the authors make an important point: \u201c<em>When the various series can be more fully filled in, what has been described as a \u201c point \u201d of mechanistic change will probably appear as a region, and thus, just as with reaction (A), we now generalise the original conception of reaction (B) by the contemplation of a range of mechanisms, (Bl)-(B2), both extremes of which have been experimentally exemplified<\/em>\u201d. Basically, the S<sub>N<\/sub>1 and S<sub>N<\/sub>2 mechanisms as taught are two extremes of a continuum, and in practice most reactions lie somewhere in between.<\/li>\n<li><strong> Mechanism of substitution at a saturated carbon atom. Part IX. The r\u00f4le of the solvent in the first-order hydrolysis of alkyl halides<br \/>\n<\/strong>Leslie C. Bateman and Edward D. Hughes<strong><br \/>\n<\/strong><em>J. Chem. Soc.<\/em> <strong>1937<\/strong>, 1187-1192<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/pubs.rsc.org\/en\/content\/articlelanding\/1937\/jr\/jr9370001187#!divAbstract\">10.1039\/JR9370001187<\/a><\/li>\n<li><strong>The Common Basis of Intramolecular Rearrangements. VI.1 Reactions of Neopentyl Iodide<br \/>\n<\/strong>Frank C. Whitmore, E. L. Wittle, and A. H. Popkin<strong><br \/>\n<\/strong><em>Journal of the American Chemical Society<\/em> <strong>1939,<\/strong> <em>61<\/em> (6), 1586-1590<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/pubs.acs.org\/doi\/pdf\/10.1021\/ja01875a073\">1021\/ja01875a073<\/a><br \/>\nAn early paper demonstrating that S<sub>N<\/sub>1 reactions can be induced by reaction of an alkyl halide with silver salts. In this case, the neopentyl cation quickly rearranges to the significantly more stable <em>t<\/em>-amyl cation, and those products are obtained.<\/li>\n<li><strong> Reaction kinetics and the Walden inversion. Part I. Homogeneous hydrolysis and alcoholysis of \u03b2-n-octyl halides<br \/>\n<\/strong>Edward D. Hughes, Christopher K. Ingold and Standish Masterman<strong><br \/>\n<\/strong><em>J. Chem. Soc.<\/em> <strong>1937<\/strong>, 1196-1201<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/pubs.rsc.org\/en\/content\/articlelanding\/1937\/JR\/jr9370001196#!divAbstract\">10.1039\/JR9370001196<\/a><\/li>\n<li><strong> Reaction kinetics and the Walden inversion. Part IV. Action of silver salts in hydroxylic solvents on \u03b2-n-octyl bromide and \u03b1-phenylethyl chloride<br \/>\n<\/strong>Edward D. Hughes, Christopher K. Ingold and Standish Masterman<br \/>\n<em>J. Chem. Soc.,<\/em> <strong>1937<\/strong>, 1236-1243<br \/>\n<strong>DOI<\/strong>: <a href=\"https:\/\/pubs.rsc.org\/en\/content\/articlelanding\/1937\/jr\/jr9370001236#!divAbstract\">10.1039\/JR9370001236<\/a><br \/>\nThese two papers examine reactions of 2-octyl halides in an attempt to see if pure S<sub>N<\/sub>1 or S<sub>N<\/sub>2 pathways on the same substrate can be favored simply by varying the reaction conditions.<\/li>\n<\/ol>\n<\/div>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Comparing the SN1 and SN2 Reactions In nucleophilic substitution reactions, a bond between carbon and a leaving group (C\u2013LG) is broken, and a new bond <\/p>\n","protected":false},"author":1,"featured_media":14839,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1414],"tags":[848,332,226,243,502,271,275,279],"post_folder":[],"class_list":["post-5454","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-substitution-reactions","tag-backside-attack","tag-carbocation-stability","tag-leaving-groups","tag-nucleophiles","tag-sn1","tag-sn2","tag-steric-hindrance","tag-substitution"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.7 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Comparing The SN1 vs Sn2 Reactions &#8211; Master Organic Chemistry<\/title>\n<meta name=\"description\" content=\"SN1 vs SN2 : how are they different? 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