{"id":6660,"date":"2012-11-08T11:51:02","date_gmt":"2012-11-08T16:51:02","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=6660"},"modified":"2026-04-18T06:26:16","modified_gmt":"2026-04-18T11:26:16","slug":"comparing-the-e1-and-sn1-reactions","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2012\/11\/08\/comparing-the-e1-and-sn1-reactions\/","title":{"rendered":"Comparing the E1 vs SN1 Reactions"},"content":{"rendered":"<p><strong>The Important Role of The Counter-Ion In Determining E1 vs S<sub>N<\/sub>1<\/strong><\/p>\n<ul>\n<li>With tertiary alkyl halides, E1 will generally be favored over S<sub>N<\/sub>1 when <strong>heat<\/strong> is applied<\/li>\n<li>Secondly, in E1 reactions of alcohols where acid is added,\u00a0 the E1 is favored when the counter-ion of the acid is a <strong>poor nucleophile<\/strong> (e.g. H<sub>2<\/sub>SO<sub>4<\/sub>, H<sub>3<\/sub>PO<sub>4<\/sub>, TsOH)<\/li>\n<\/ul>\n<p><img fetchpriority=\"high\" decoding=\"async\" class=\"alignnone wp-image-37399\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/09\/0-summary-of-E1-vs-SN1-reactions-importance-of-heat-with-alkyl-halides-and-counter-ion-with-alcohols.gif\" alt=\"summary of E1 vs SN1 reactions importance of heat with alkyl halides and counter ion with alcohols\" width=\"640\" height=\"511\" \/><\/a><\/p>\n<ol>\n<li><a href=\"#one\">Alcohols Don&#8217;t Undergo Elimination Reactions Until OH Is Converted To A Better Leaving Group<\/a><\/li>\n<li><a href=\"#two\">Adding Acid To Alcohols Results In A Better Leaving Group&#8217;<\/a><\/li>\n<li><a href=\"#three\">When Elimination And Substitution Compete: E1 vs SN1<\/a><\/li>\n<li><a href=\"#four\">In Order To Get E1 To Predominate vs S<sub>N<\/sub>1 In The Reaction Of Alcohols Use H<sub>2<\/sub>SO<sub>4<\/sub>, TsOH, or H<sub>3<\/sub>PO<sub>4<\/sub> (And Heat)<\/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. Alcohols Don&#8217;t Undergo Elimination Reactions Until OH Is Converted To A Better Leaving Group<\/h2>\n<p>Imagine you&#8217;re starting with the alcohol on the left and you&#8217;d like to get to the alkene on the right.<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-14885\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/1-elimination-of-alcohols-does-not-generally-happen-without-acid-because-hydroxide-leaving-group-is-strong-base.gif\" alt=\"elimination of alcohols does not generally happen without acid because hydroxide leaving group is strong base\" width=\"600\" height=\"221\" \/><\/p>\n<p>What bonds are formed and broken here? We&#8217;re forming C-C (\u03c0), we&#8217;re breaking C-H, and we&#8217;re breaking C-OH. It&#8217;s an elimination reaction.<\/p>\n<p>Notice a problem here? We need to have HO(-) as a leaving group. If you&#8217;ll recall, strong bases [like HO(-) ] are terrible leaving groups &#8211; which makes the E1 pathway unlikely. (<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>So what if we tried to use a strong base, maybe trying to promote an E2 reaction? Well, that would be even worse &#8211; we&#8217;d likely deprotonate OH before the C-H, and you can imagine that we&#8217;d have to have O(<sup>2-<\/sup>) as a leaving group here. Not good!<\/p>\n<p>That means that the reaction, as written, is very unlikely to happen.<\/p>\n<h2><a id=\"two\"><\/a>2. Adding Acid To Alcohols Results In A Better Leaving Group<\/h2>\n<p>Yet, there is something very simple that we can do to make this reaction work. We&#8217;d need to have a better leaving group (a weaker base). How can we do this?<\/p>\n<p>Add acid!<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-14886\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/2-acid-makes-alcohols-much-better-leaving-group-h2o-is-much-weaker-base-than-ho-.gif\" alt=\"acid makes alcohols much better leaving group h2o is much weaker base than ho-\" width=\"630\" height=\"351\" \/><\/p>\n<p>If we add a strong acid, we turn OH into H<sub>2<\/sub>O<sup>+<\/sup>, <a href=\"https:\/\/www.masterorganicchemistry.com\/2012\/08\/07\/the-conjugate-acid-is-a-better-leaving-group\/\">the conjugate acid is a better leaving group.<\/a> Now, water can leave, forming a carbocation; and then a base can break the C-H bond, forming the alkene.<\/p>\n<p>Notice that this is now a classic E1 reaction. The rate is going to be dependent on the stability of the carbocation. This one is tertiary, so it should proceed at a reasonably high rate.<\/p>\n<p>A question arises here. What&#8217;s going to act as the base? As it stands, a C-H bond adjacent to a carbocation has an extremely high acidity (at least below -2, if you follow pK<sub>a<\/sub>). That means that just about any weak base (water, or the conjugate base of the acid) is sufficient to deprotonate the carbon.<\/p>\n<p>It&#8217;s possible that more than one species can act as a base here. I&#8217;ve shown water removing the proton, but it&#8217;s not unreasonable to show the conjugate base of the acid removing the proton in most circumstances.<\/p>\n<h2><a id=\"three\"><\/a>3. When Elimination And Substitution Compete: E1 vs SN1<\/h2>\n<p>Now comes one of the things about organic chemistry that often causes trouble for students. For one of the first times in our discussions here, we&#8217;re dealing with a situation where we can have\u00a0<strong>competing<\/strong> reactions.<\/p>\n<p>Let&#8217;s back up. The E1 reaction goes through a carbocation, correct? Well, if you&#8217;ll recall, so does the S<sub>N<\/sub>1 reaction.<\/p>\n<p><a href=\"https:\/\/www.masterorganicchemistry.com\/2012\/08\/07\/the-conjugate-acid-is-a-better-leaving-group\/\">We&#8217;ve already seen examples<\/a> where a carbocation was formed from an alcohol by adding a strong acid like HCl, HBr, or HI, and we ended up with the alkyl halide.<\/p>\n<p>Why?\u00a0\u00a0The halide ions (Cl- , Br-, I- ) are decent nucleophiles under the reaction conditions.<\/p>\n<p>So how can we stack the deck in favor of the E1 process?<\/p>\n<p>Use a strong acid with a conjugate base that is a<strong> poor nucleophile.<\/strong><\/p>\n<p><strong>\u00a0<\/strong>The usual choice is H<sub>2<\/sub>SO<sub>4<\/sub>. The HSO<sub>4<\/sub>(-) ion is a relatively poor nucleophile due to the negative charge of the oxygen being distributed throughout the molecule (resonance). Two other acids you might see for this purpose are p-toluenesulfonic acid (p-TsOH), which is essentially a cousin of H<sub>2<\/sub>SO<sub>4<\/sub>, and phosphoric acid (H<sub>3<\/sub>PO<sub>4<\/sub>).<\/p>\n<p>Also, don&#8217;t forget that elimination reactions are favored by heat (<em>See article: <a href=\"https:\/\/www.masterorganicchemistry.com\/2012\/09\/10\/elimination-reactions-are-favored-by-heat\/\">Elimination Reactions Are Favored By Heat<\/a><\/em>)<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-14887\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/3-competition-between-sn1-and-e1-arises-with-acid-if-hx-is-used-get-alkyl-halides-if-h2so4-or-h3po4-is-used-get-elimination.gif\" alt=\"competition between sn1 and e1 arises with acid if hx is used get alkyl halides if h2so4 or h3po4 is used get elimination\" width=\"600\" height=\"325\" \/><\/p>\n<h2><a id=\"four\"><\/a>4. In Order To Get E1 To Predominate vs S<sub>N<\/sub>1 In The Reaction Of Alcohols Use H<sub>2<\/sub>SO<sub>4<\/sub>, TsOH, or H<sub>3<\/sub>PO<sub>4<\/sub> (And Heat)<\/h2>\n<p>In summary, if you&#8217;d like E1 to predominate over S<sub>N<\/sub>1: choose an acid with a weakly nucleophilic counterion [H<sub>2<\/sub>SO<sub>4<\/sub>, TsOH, or H<sub>3<\/sub>PO<sub>4<\/sub>], and heat.<\/p>\n<p>If you&#8217;d like S<sub>N<\/sub>1 to predominate over E1, choose an acid like HCl, HBr, or HI.<\/p>\n<p>We&#8217;re almost done talking about elimination reactions. Next post &#8211; we&#8217;ll talk about rearrangements.<\/p>\n<p><a href=\"https:\/\/www.masterorganicchemistry.com\/2012\/11\/09\/elimination-reactions-with-rearrangments\/\"><strong>Next Post: The E1 Reaction With Rearrangements<\/strong><\/a><\/p>\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\/09\/19\/the-e1-reaction\/\" class=\"\"><span>The E1 Reaction<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2011\/03\/11\/3-factors-that-stabilize-carbocations\/\" class=\"\"><span>3 Factors That Stabilize Carbocations<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2020\/04\/30\/alkene-stability\/\" class=\"\"><span>Alkene Stability<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2012\/09\/10\/elimination-reactions-are-favored-by-heat\/\" class=\"\"><span>Elimination Reactions Are Favored By Heat<\/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\/2012\/08\/07\/the-conjugate-acid-is-a-better-leaving-group\/\" class=\"\"><span>The Conjugate Acid Is A Better Leaving Group<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2012\/08\/31\/elimination-reactions-2-zaitsevs-rule\/\" class=\"\"><span>Elimination Reactions (2): The Zaitsev Rule<\/span><\/a><\/li><\/ul><\/div>\n<hr \/>\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\/2478-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\/3610-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\/0247-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\/0230-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\/0245-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\/3578-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>Since the E1 and S<sub>N<\/sub>1 reactions both proceed through a common carbocation intermediate, these pathways can and do compete with each other under a lot of reaction conditions.<\/p>\n<ol>\n<li><strong>Ion Pairs in Elimination<br \/>\n<\/strong>M. Cocivera and S. Winstein<strong><br \/>\n<\/strong><em>Journal of the American Chemical Society<\/em> <strong>1963,<\/strong> <em>85<\/em> (11), 1702-1703<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/pubs.acs.org\/doi\/abs\/10.1021\/ja00894a046\">10.1021\/ja00894a046<\/a><br \/>\nProf. Saul Winstein (UCLA) was a highly influential physical organic chemist in the early 20<sup>th<\/sup> century, and introduced several concepts that are now fundamental to organic mechanisms, such as anchimeric assistance, ion-pairing, internal return, and many others. In this paper, Prof. Winstein shows that the E1 reaction has a strong solvent dependence \u2013 as the dielectric constant (or polarity) of the solvent decreases, the amount of olefin obtained also decreases, since carbocations cannot be formed easily. In anhydrous ethanol, t-butyl chloride and bromide give 44% and 36% yield of alkene respectively, whereas in glacial acetic acid, 73% and 69% of olefin, respectively, is obtained.<\/li>\n<li><strong> Mechanism of elimination reactions. Part XI. Kinetics of olefin elimination from tert.-butyl and tert.-amyl bromides in acidic and alkaline alcoholic media<br \/>\n<\/strong>M. L. Dhar, E. D. Hughes, and C. K. Ingold<strong><br \/>\n<\/strong><em>J. Chem. Soc.<\/em> <strong>1948<\/strong>, 2065-2072<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/pubs.rsc.org\/en\/content\/articlelanding\/1948\/jr\/jr9480002065#!divAbstract\">10.1039\/JR9480002065<\/a><br \/>\nThe E1 reaction is not very useful synthetically for olefin synthesis, because the ratio of elimination to substitution products is substantially lower than in the E2 reaction. For example, solvolysis of t-butyl bromide in dry ethanol only yields 19% isobutylene, whereas 93% yield of the alkene is obtained with 2M ethoxide.<\/li>\n<li><strong>Chemical Effects of Steric Strains. II. The Effect of Structure on Olefin Formation in the Hydrolysis of Tertiary Aliphatic Chlorides<br \/>\n<\/strong>Herbert C. Brown and Roslyn Silber Fletcher<strong><br \/>\n<\/strong><em>Journal of the American Chemical Society<\/em> <strong>1950,<\/strong> <em>72<\/em> (3), 1223-1226<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/pubs.acs.org\/doi\/pdfplus\/10.1021\/ja01159a043\">1021\/ja01159a043<\/a><br \/>\nThis paper by Nobel Laureate Prof. H. C. Brown provides a perfect study of how the structure of the substrate can influence the E1-S<sub>N<\/sub>1 competition. With a series of <em>t<\/em>-alkyl chlorides, the proportion of olefin obtained on solvolysis in 80% ethanol rises as the alkyl groups become more highly branched.<\/li>\n<li><strong> Mechanism of elimination reactions. Part VIII. Temperature effects on rates and product-proportions in uni- and bi-molecular substitution and elimination reactions of alkyl halides and sulphonium salts in hydroxylic solvents<br \/>\n<\/strong>K. A. Cooper, E. D. Hughes, C. K. Ingold, and B. J. MacNulty<strong><br \/>\n<\/strong><em>J. Chem. Soc.<\/em><strong> 1948, <\/strong>2049-2054<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/pubs.rsc.org\/en\/content\/articlelanding\/1948\/jr\/jr9480002049#!divAbstract\">10.1039\/JR9480002049<\/a><br \/>\nProf. Ingold states: \u201c[..] <em>for any given pair of simultaneous bimolecular processes, the elimination has, in each of the investigated cases, an Arrhenius energy of activation which lies higher than that of the accompanying substitution by 1-2 kcal\/g.-mol. The elimination thus has always the larger temperature coefficient, so that a rise of temperature increases the proportions in which olefin is formed<\/em>\u201d. An increase in temperature produces a modest increase in the proportion of olefin in E1-S<sub>N<\/sub>1 reactions.<\/li>\n<li><strong> Mechanism of elimination reactions. Part VII. Solvent effects on rates and product-proportions in uni- and bi-molecular substitution and elimination reactions of alkyl halides and sulphonium salts in hydroxylic solvents<br \/>\n<\/strong>K. A. Cooper, M. L. Dhar, E. D. Hughes, C. K. Ingold, B. J. MacNulty and L. I. Woolf<strong><br \/>\n<\/strong><em>J. Chem. Soc.<\/em> <strong>1948<\/strong>, 2043-2049<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/pubs.rsc.org\/en\/content\/articlelanding\/1948\/jr\/jr9480002043#!divAbstract\">10.1039\/JR9480002043<\/a><br \/>\nAnother modest increase in the yield of olefin in E1-S<sub>N<\/sub>1 reactions is found on going from aqueous ethanol to anhydrous ethanol.<\/li>\n<li><strong> The mechanism of elimination reactions. Part II. Unimolecular olefin formation from sec.-octyl halides in aqueous alcohol. A new criterion of mechanism<br \/>\n<\/strong>Edward D. Hughes, Christopher K. Ingold, and Uriel G. Shapiro<strong><br \/>\n<\/strong><em>J. Chem. Soc.<\/em> <strong>1937<\/strong>, 1277-1280<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/pubs.rsc.org\/en\/content\/articlelanding\/1937\/jr\/jr9370001277#!divAbstract\">10.1039\/JR9370001277<\/a><br \/>\nThis paper demonstrates how the E1 and SN1 reactions compete when secondary substrates are used. 2-bromo and 2-chlorooctane each give around 14% of olefin when heated to 100 \u00b0C in 60% aqueous ethanol.<\/li>\n<\/ol>\n","protected":false},"excerpt":{"rendered":"<p>The Important Role of The Counter-Ion In Determining E1 vs SN1 With tertiary alkyl halides, E1 will generally be favored over SN1 when heat is <\/p>\n","protected":false},"author":1,"featured_media":37399,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1415],"tags":[471,688,472,473,201,226,825],"post_folder":[],"class_list":["post-6660","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-elimination-reactions","tag-base","tag-conjugate-acid","tag-e1","tag-e2","tag-elimination","tag-leaving-groups","tag-nucleophile"],"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 E1 vs SN1 Reactions &#8211; 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