{"id":7943,"date":"2014-02-10T14:35:47","date_gmt":"2014-02-10T19:35:47","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=7943"},"modified":"2026-04-11T07:21:01","modified_gmt":"2026-04-11T12:21:01","slug":"socl2-and-the-sni-mechanism","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2014\/02\/10\/socl2-and-the-sni-mechanism\/","title":{"rendered":"SOCl2 Mechanism For Alcohols To Alkyl Halides: SN2 versus SNi"},"content":{"rendered":"<p><strong>SOCl<sub>2<\/sub> Mechanism With Alcohols, With And Without Pyridine: Nucleophilic Substitution (S<sub>N<\/sub>2) Versus Nucleophilic Substitution With Internal Return (S<sub>N<\/sub>i)<\/strong><\/p>\n<ul>\n<li>Most of the time, the reaction of alcohols with thionyl chloride is taught as an S<sub>N<\/sub>2 reaction. And indeed, on primary alcohols this is definitely the case.<\/li>\n<li>The problem arises with secondary alcohols, where the reaction can be taught either as a classical S<sub>N<\/sub>2 with inversion, or&#8230; as a reaction with\u00a0<em>retention!?\u00a0<\/em>via the S<sub>N<\/sub>i mechanism. That latter part is what this post is about.<\/li>\n<\/ul>\n<p><strong>Table of Contents<\/strong><\/p>\n<ol>\n<li><a href=\"#one\">&#8220;I&#8217;m Sorry But Who Taught You That Mechanism&#8221; ?<\/a><\/li>\n<li><a href=\"#two\"><span style=\"font-size: medium;\">What Really Happens In The Reaction Of SOCl<sub>2<\/sub> With Secondary Alcohols: The S<sub>N<\/sub>i Mechanism<\/span><\/a><\/li>\n<li><a href=\"#three\">Nucleophilic Substitution With Internal Return: S<sub>N<\/sub>i<\/a><\/li>\n<li><a href=\"#four\">Adding SOCl<sub>2<\/sub> AND Pyridine Leads To Inversion (via S<sub>N<\/sub>2)<\/a><\/li>\n<li><a href=\"#five\">Pyridine Shuts Down The S<sub>N<\/sub>i Mechanism<\/a><\/li>\n<li><a href=\"#six\">Summary: SOCl<sub>2<\/sub> And Alcohols &#8211; S<sub>N<\/sub>2 versus S<sub>N<\/sub>i<\/a><\/li>\n<li><a href=\"#notes\">Notes: How Do Schools In North America Deal With This Dichotomy?<\/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. Conversion Of Alcohols To Chlorides With SOCl<sub>2<\/sub> Proceeds With Inversion&#8230;Right? Well, Maybe Not Always<\/h2>\n<p>Some time ago I published <a href=\"https:\/\/www.masterorganicchemistry.com\/2011\/12\/03\/reagent-friday-thionyl-chloride-socl2\/\">this post about SOCl<sub>2<\/sub><\/a> discussing the mechanism of SOCl<sub>2<\/sub> converting secondary alcohols to alkyl chlorides with secondary \u00a0through an S<sub>N<\/sub>2 pathway:<br \/>\n<img fetchpriority=\"high\" decoding=\"async\" class=\"alignnone wp-image-15288\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/1-conversion-of-alcohols-to-alkyl-chlorides-with-inversion-using-socl2-no-pyridine-so-is-this-right-or-not.png\" alt=\"conversion of alcohols to alkyl chlorides with inversion using socl2 no pyridine so is this right or not\" width=\"450\" height=\"385\" srcset=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/1-conversion-of-alcohols-to-alkyl-chlorides-with-inversion-using-socl2-no-pyridine-so-is-this-right-or-not.png 451w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/1-conversion-of-alcohols-to-alkyl-chlorides-with-inversion-using-socl2-no-pyridine-so-is-this-right-or-not-300x257.png 300w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/1-conversion-of-alcohols-to-alkyl-chlorides-with-inversion-using-socl2-no-pyridine-so-is-this-right-or-not-320x274.png 320w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/1-conversion-of-alcohols-to-alkyl-chlorides-with-inversion-using-socl2-no-pyridine-so-is-this-right-or-not-360x308.png 360w\" sizes=\"(max-width: 450px) 100vw, 450px\" \/><a href=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2014\/02\/SOCl2-mech.png\"><br \/>\n<\/a>About six months ago this post arrived in the comments:<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-15289\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/2-moc-blog-comment-in-2013-saying-secondary-alcohols-are-not-converted-to-alkyl-chlorides-with-inversion-but-retention-due-to-sni-internal-return.png\" alt=\"moc blog comment in 2013 saying secondary alcohols are not converted to alkyl chlorides with inversion but retention due to sni internal return\" width=\"450\" height=\"147\" srcset=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/2-moc-blog-comment-in-2013-saying-secondary-alcohols-are-not-converted-to-alkyl-chlorides-with-inversion-but-retention-due-to-sni-internal-return.png 642w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/2-moc-blog-comment-in-2013-saying-secondary-alcohols-are-not-converted-to-alkyl-chlorides-with-inversion-but-retention-due-to-sni-internal-return-300x98.png 300w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/2-moc-blog-comment-in-2013-saying-secondary-alcohols-are-not-converted-to-alkyl-chlorides-with-inversion-but-retention-due-to-sni-internal-return-320x105.png 320w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/2-moc-blog-comment-in-2013-saying-secondary-alcohols-are-not-converted-to-alkyl-chlorides-with-inversion-but-retention-due-to-sni-internal-return-640x209.png 640w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/2-moc-blog-comment-in-2013-saying-secondary-alcohols-are-not-converted-to-alkyl-chlorides-with-inversion-but-retention-due-to-sni-internal-return-360x118.png 360w\" sizes=\"(max-width: 450px) 100vw, 450px\" \/><\/p>\n<p><span style=\"line-height: 1.5em;\">Rico is correct that the mechanism showing inversion with SOCl<sub>2<\/sub> is not what happens experimentally. When a secondary alcohol is treated with SOCl<sub>2<\/sub> (and nothing else)<strong> the usual pathway is<\/strong> <strong>retention<\/strong>.\u00a0<\/span><\/p>\n<p>The record should be set straight about this, so this post will cover:<\/p>\n<ol>\n<li>What really happens in the reaction of SOCl<sub>2<\/sub> with secondary alcohols (the S<sub>N<\/sub>i mechanism) and why it gives retention<\/li>\n<li>Why adding pyridine to SOCl<sub>2<\/sub> results in inversion (via S<sub>N<\/sub>2) and not retention<\/li>\n<li>How do most textbooks and schools across North America deal with this mechanistic dichotomy (hint: most don&#8217;t)<\/li>\n<li>What&#8217;s an instructor to do?<\/li>\n<\/ol>\n<h2><a id=\"two\"><\/a>2. What Really Happens In The Reaction of SOCl<sub>2<\/sub> With Secondary Alcohols: The S<sub>N<\/sub>i Mechanism<\/h2>\n<p><span style=\"line-height: 1.5em;\">In the late 19th century, Paul Walden performed a series of fundamental experiments on the stereochemistry of various reactions of sugars (and sugar derivatives). Walden noted that when (+)-malic acid\u00a0 treated with PCl<\/span><sub style=\"line-height: 1.5em;\">5<\/sub><span style=\"line-height: 1.5em;\">, the product was (\u2013) chlorosuccinic acid &#8211; a process that proceeded with <\/span><strong style=\"line-height: 1.5em;\">inversion<\/strong><span style=\"line-height: 1.5em;\"> of stereochemistry. When (+) malic acid was treated with thionyl chloride (SOCl<\/span><sub style=\"line-height: 1.5em;\">2<\/sub><span style=\"line-height: 1.5em;\">), however the product was (+)-chlorosuccinic acid. This proceeds with <\/span><strong style=\"line-height: 1.5em;\">retention<\/strong><span style=\"line-height: 1.5em;\"> of stereochemistry.\u00a0<\/span><\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-15290\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/3-walden-observation-treatment-of-malic-acid-with-socl2-to-give-chlorosuccinic-acid-with-retention-versus-pcl4-gives-inversion-why-is-this-sn2.gif\" alt=\"walden observation treatment of malic acid with socl2 to give chlorosuccinic acid with retention versus pcl4 gives inversion why is this sn2\" width=\"630\" height=\"183\" \/><\/p>\n<p>How can we understand this?<br \/>\nThe reaction of malic acid with PCl<sub>5<\/sub> leading to inversion of stereochemistry is an example of what we now call the <strong>S<sub>N<\/sub>2 reaction<\/strong>, and Walden was the first to make the observation that the stereochemistry is inverted. In fact the process of stereochemical inversion observed during the S<sub>N<\/sub>2 reaction is sometimes called <strong><a href=\"http:\/\/en.wikipedia.org\/wiki\/Walden_inversion\">Walden inversion<\/a><\/strong> in his honor. By the time most students encounter SOCl<sub>2<\/sub> in their courses, the S<sub>N<\/sub>2 is a familiar reaction.<\/p>\n<p>What is much more curious is the \u00a0observation that malic acid treated with SOCl<sub>2<\/sub> leads to <strong>substitution with retention<\/strong>. \u00a0Sharp readers may recall that &#8220;retention&#8221; of stereochemistry can be obtained if two successive S<sub>N<\/sub>2 reactions occur [double inversion = retention]. Perhaps that is what is going on here? Maybe the carboxylic acid of malice acid can act as a nucleophile in a first (intramolecular) S<sub>N<\/sub>2, and then Cl- coming in for the second?<\/p>\n<h2><a id=\"three\"><\/a>3. Nucleophilic Substitution With Internal Return: S<sub>N<\/sub>i<\/h2>\n<p>Good idea \u2013 but this retention of configuration <strong>occurs even in cases where no group can possibly do an intramolecular S<sub>N<\/sub>2<\/strong>. There must be something else going on. And after a lot of experimental work, this is the best proposal we have:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15291\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/4-why-does-socl2-give-retention-showing-full-mechanism-of-sni-or-substitution-with-internal-return-via-chlorosulfite-intimate-ion-pair-no-pyridine.gif\" alt=\"why does socl2 give retention showing full mechanism of sni or substitution with internal return via chlorosulfite intimate ion pair no pyridine\" width=\"630\" height=\"555\" \/><\/p>\n<p>This is called, <strong><a href=\"http:\/\/en.wikipedia.org\/wiki\/SNi\">S<sub>N<\/sub>i (nucleophilic substitution with internal return)<\/a><\/strong>: what happens here is that SOCl<sub>2<\/sub> corrdinates to the alcohol, with loss of HCl and formation of a good leaving group (&#8220;chlorosulfite&#8221;). The chlorosulfite leaving group can spontaneously depart, forming a carbocation, and when it does so, \u00a0an &#8220;intimate ion pair&#8221; is formed, where the carbocation and negatively charged leaving group are held tightly together in space. From here, the chlorine can act as a nucleophile &#8211; attacking the carbocation on the same face from which it was expelled &#8211; and after expulsion of SO<sub>2<\/sub>, we have formation of an alkyl chloride with retention of configuration.<\/p>\n<p>So the chlorosulfite leaving group (SO<sub>2<\/sub>Cl) is quite special in that it can deliver a nucleophile (chlorine) to the same face it departs from, with simultaneous loss of SO<sub>2<\/sub>.<\/p>\n<p>If it ended there, life might be simpler. But less interesting! [<span style=\"color: #993366;\"><em>That is the sound of a can of worms being opened]<\/em><\/span>.<\/p>\n<h2><a id=\"four\"><\/a>4.Why Adding SOCl<sub>2<\/sub> AND Pyridine Leads To Inversion via The S<sub>N<\/sub>2 Mechanism<\/h2>\n<p>Here&#8217;s the twist. \u00a0As it turns out, the stereochemistry of this reaction can change to <strong>inversion<\/strong> if we add a mild base &#8211; such as pyridine.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15292\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/5-use-of-chiral-secondary-alcohol-with-socl2-and-pyridine-gives-inversion-of-configuration-why-is-this.gif\" alt=\"use of chiral secondary alcohol with socl2 and pyridine gives inversion of configuration why is this\" width=\"600\" height=\"224\" \/><\/p>\n<p><span style=\"line-height: 1.5em;\">Retention of stereochemistry with SOCl<\/span><sub style=\"line-height: 1.5em;\">2<\/sub><span style=\"line-height: 1.5em;\"> alone, inversion with SOCl<\/span><sub style=\"line-height: 1.5em;\">2<\/sub><span style=\"line-height: 1.5em;\"><strong> and pyridine<\/strong>. What&#8217;s happening here? How does pyridine affect the course of this reaction?\u00a0<\/span><\/p>\n<p>Both reactions form the &#8220;chlorosulfite&#8221; intermediate. But when pyridine (a decent nucleophile) is present, it can attack the chlorosulfite, displacing\u00a0chloride ion and forming a charged intermediate. Now, if the leaving group departs, forming a carbocation,\u00a0<strong>there&#8217;s no lone pair nearby on the same face that can attack<\/strong>. <strong><br \/>\n<\/strong><\/p>\n<p>In other words, by displacing chloride ion, <strong>pyridine shuts down the S<sub>N<\/sub>i mechanism<\/strong>.<\/p>\n<h2><a id=\"five\"><\/a>5. Adding Pyridine To SOCl<sub>2<\/sub> Shuts Down The S<sub>N<\/sub>i Mechanism<\/h2>\n<p>Even though the S<sub>N<\/sub>i can&#8217;t occur here, we still have a very good leaving group, and a decent nucleophile &#8211; chloride ion &#8211; and so <strong>chloride attacks the carbon from the backside, leading to inversion of configuration and formation of a C-Cl bond<\/strong>. This, of course, the S<sub>N<\/sub>2 reaction.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15293\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/6-mechanism-for-inversion-of-configuration-of-chiral-secondary-alcohol-with-socl2-and-pyridine-where-pyridine-attacks-thiosulfite-and-prevents-internal-return.gif\" alt=\"mechanism for inversion of configuration of chiral secondary alcohol with socl2 and pyridine where pyridine attacks thiosulfite and prevents internal return\" width=\"630\" height=\"525\" \/><\/p>\n<h2><a id=\"six\"><\/a>6. Summary: SOCl<sub>2<\/sub> And Alcohols, With Or Without Pyridine &#8211; S<sub>N<\/sub>2 Versus S<sub>N<\/sub>i<\/h2>\n<p>The bottom line is this:<\/p>\n<p><strong>SOCl<sub>2<\/sub> plus alcohol<\/strong> gives<strong> retention<\/strong> of configuration, <strong>SOCl<sub>2<\/sub> plus alcohol plus pyridine<\/strong> gives <strong>inversion<\/strong> of configuration (S<sub>N<\/sub>2)<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15294\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/7-bottom-line-for-socl2-with-chiral-alcohols-socl2-wtih-no-pyridine-gives-retention-whereas-with-pyridine-gives-inversion.gif\" alt=\"bottom line for socl2 with chiral alcohols socl2 wtih no pyridine gives retention whereas with pyridine gives inversion\" width=\"630\" height=\"222\" \/><\/p>\n<p>You might be asking, &#8220;how common is this S<sub>N<\/sub>i mechanism? Is it something which occurs in a large number of other reactions we commonly encounter in introductory organic chemistry?&#8221;<\/p>\n<p>To be frank, not really. There are some cases where species called <a href=\"http:\/\/onlinelibrary.wiley.com\/doi\/10.1002\/poc.1121\/abstract\">chloroformates<\/a> can also undergo the S<sub>N<\/sub>i with loss of CO<sub>2<\/sub> but this isn&#8217;t seen very often at all in your typical first year course.<\/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\/2011\/12\/03\/reagent-friday-thionyl-chloride-socl2\/\" class=\"\"><span>Reagent Friday: Thionyl Chloride (SOCl2)<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2015\/07\/27\/synthesis-6-reactions-of-alcohols\/\" class=\"\"><span>Alcohol Reactions Roadmap (PDF)<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2015\/03\/20\/pbr3-and-socl2\/\" class=\"\"><span>PBr3 and SOCl2<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2015\/03\/10\/tosylates-and-mesylates\/\" class=\"\"><span>Tosylates And Mesylates<\/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\/2012\/07\/11\/why-the-sn2-reaction-is-powerful\/\" class=\"\"><span>Why the SN2 Reaction Is Powerful<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2015\/02\/27\/making-alkyl-halides-from-alcohols\/\" class=\"\"><span>Making Alkyl Halides From Alcohols<\/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\/1343-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. <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\/1344-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. <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\/1364-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. <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\/1345-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. <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\/1361-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<p>This might not interest everybody so I&#8217;m putting it in a note.<\/p>\n<h2>How Do Most Textbooks And Schools Across North America Deal With This Mechanistic Dichotomy?<\/h2>\n<p>Conversion of alcohols to alkyl halides is a useful transformation because alcohols are poor leaving groups by themselves, whereas alkyl chlorides will readily participate in substitution and elimination reactions.\u00a0In many introductory organic chemistry courses, SOCl<sub>2<\/sub>\u00a0has traditionally been used as an example of a reagent that will convert alcohols to alkyl chlorides.<\/p>\n<p>When I consulted my textbook collection for how the mechanism is covered, here&#8217;s what I found:<\/p>\n<ul>\n<li>Wade (5th ed. p 463) <strong>Shows conversion of secondary alcohol to secondary alkyl chloride via S<sub>N<\/sub>i<\/strong>\u00a0(with dioxane solvent)<\/li>\n<li>Solomons (8th ed p. 506-507) Shows conversion of primary alcohol to primary alkyl chloride via S<sub>N<\/sub>2. No mention of S<sub>N<\/sub>i or stereochemistry.<\/li>\n<li>McMurry (6th ed p. 608) Shows conversion of primary alcohol to primary alkyl chloride (S<sub>N<\/sub>2) No stereochemistry shown.<\/li>\n<li>Vollhardt (2nd ed p. 288) Shows mechanism (S<sub>N<\/sub>2) for primary alcohol; no discussion of S<sub>N<\/sub>2.<\/li>\n<li>Jones (2nd ed p. 830) Shows S<sub>N<\/sub>2 of Cl on &#8220;R&#8221; ; no mention of stereochem<\/li>\n<li>Clayden, Klein &#8211; no mention of SOCl<sub>2<\/sub> as a reagent for converting alcohols to alkyl chlorides<\/li>\n<\/ul>\n<p>Only\u00a0<strong>one<\/strong> textbook (in this admittedly incomplete sample) mentions the S<sub>N<\/sub>i mechanism at all. <strong>In four textbooks where SOCl<sub>2<\/sub> is mentioned, the reaction is shown as proceeding\u00a0through an S<sub>N<\/sub>2 mechanism.\u00a0<\/strong>There&#8217;s no warning sign saying, &#8220;wait! the S<sub>N<\/sub>2 doesn&#8217;t happen for secondary alcohols&#8221;. If it&#8217;s not in the textbook, chances are it won&#8217;t be in the course.\u00a0\u00a0So it&#8217;s not surprising that the most common interpretation of this is that\u00a0<strong>inversion will occur for secondary alcohols:\u00a0<\/strong><br \/>\nThis leads to situations like the following. Here is a part of an exam key from a very non-obscure R1 university:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15295\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F1-example-of-secondary-alcohol-reaction-with-socl2-from-unnamed-university-goes-with-inversion.png\" alt=\"example of secondary alcohol reaction with socl2 from unnamed university goes with inversion\" width=\"450\" height=\"152\" srcset=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F1-example-of-secondary-alcohol-reaction-with-socl2-from-unnamed-university-goes-with-inversion.png 493w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F1-example-of-secondary-alcohol-reaction-with-socl2-from-unnamed-university-goes-with-inversion-300x102.png 300w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F1-example-of-secondary-alcohol-reaction-with-socl2-from-unnamed-university-goes-with-inversion-320x108.png 320w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F1-example-of-secondary-alcohol-reaction-with-socl2-from-unnamed-university-goes-with-inversion-360x122.png 360w\" sizes=\"(max-width: 450px) 100vw, 450px\" \/><\/p>\n<p>This is a question that tests stereochemistry, and students are expected to write that the SOCl<sub>2<\/sub> proceeds with inversion at a secondary carbon, proceeding through an S<sub>N<\/sub>2 mechanism.<\/p>\n<p>There are exceptions. Another school *of similar reputation) tests this reaction as an S<sub>N<\/sub>i.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15296\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F2-example-of-secondary-alcohol-with-socl2-in-abnsence-of-pyridine-giving-retention-of-configuration.png\" alt=\"example of secondary alcohol with socl2 in abnsence of pyridine giving retention of configuration\" width=\"600\" height=\"414\" srcset=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F2-example-of-secondary-alcohol-with-socl2-in-abnsence-of-pyridine-giving-retention-of-configuration.png 790w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F2-example-of-secondary-alcohol-with-socl2-in-abnsence-of-pyridine-giving-retention-of-configuration-300x207.png 300w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F2-example-of-secondary-alcohol-with-socl2-in-abnsence-of-pyridine-giving-retention-of-configuration-768x530.png 768w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F2-example-of-secondary-alcohol-with-socl2-in-abnsence-of-pyridine-giving-retention-of-configuration-320x221.png 320w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F2-example-of-secondary-alcohol-with-socl2-in-abnsence-of-pyridine-giving-retention-of-configuration-640x442.png 640w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F2-example-of-secondary-alcohol-with-socl2-in-abnsence-of-pyridine-giving-retention-of-configuration-360x248.png 360w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F2-example-of-secondary-alcohol-with-socl2-in-abnsence-of-pyridine-giving-retention-of-configuration-720x497.png 720w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F2-example-of-secondary-alcohol-with-socl2-in-abnsence-of-pyridine-giving-retention-of-configuration-760x524.png 760w\" sizes=\"(max-width: 600px) 100vw, 600px\" \/><\/p>\n<p><strong>In summary, across North America at least, the discussion of the stereochemistry of SOCl<sub>2<\/sub> reactions with secondary alcohols is a huge mess.\u00a0<\/strong>I don&#8217;t have any data to back this up, but in all my hours of tutoring I have encountered the S<sub>N<\/sub>i reaction of SOCl<sub>2<\/sub> being taught\u2026 once.<\/p>\n<h2>So What&#8217;s An Instructor To Do?<\/h2>\n<p>First of all, a mea culpa. I drew the SOCl<sub>2<\/sub> as proceeding through inversion and an S<sub>N<\/sub>2 process because I&#8217;ve aimed the Reagent Guide at the broadest sub-section of students, and <strong>it&#8217;s most often taught as giving inversion<\/strong>. I should have been more clear that it was more complicated and there was so much confusion on the topic &#8211; so I&#8217;m grateful to commenters like Rico and others who have brought this to my attention.<\/p>\n<p>Organic chemistry is so wonderfully rich and deep. With the luxury of having already learned all this stuff, I can look back and find it fascinating that just by switching from a primary to a secondary carbon, or from switching to a SO<sub>2<\/sub>Cl leaving group, one can change the mechanism from S<sub>N<\/sub>2 to S<sub>N<\/sub>i. The leaving group can provide its own nucleophile! How cool!<\/p>\n<p>If I was in an introductory class with a full course load and a lot of other lab courses however, my attitude might be different: more like, &#8220;Jeezus, YHGTBFKM, is this ever obscure.&#8221;<\/p>\n<p>I&#8217;ve asked other instructors what they do when they encounter this topic. Here&#8217;s what one has to say:<\/p>\n<blockquote><p>At the second yr \/ intro level, we keep it very simple. We only talk about it being an S<sub>N<\/sub>2 and going with inversion and thus complementary to the HX reactions. We ignore solvent effects for the thionyl chloride reactions.<\/p><\/blockquote>\n<p>Here&#8217;s another:<\/p>\n<blockquote><p>I teach it as inversion. Oxygen attacks sulfur, kicks out chloride. Pyridine deprotonates oxygen. Chloride attacks carbon, C-O bond breaks to form 2nd pi bond of SO<sub>2<\/sub>, kicks out chloride. Inversion of stereochemistry as chloride attack is S<sub>N<\/sub>2-like.<\/p><\/blockquote>\n<p>It&#8217;s an instructors&#8217; prerogative to pick their battles. I can completely understand how time and attention are limiting factors, and instructors inevitably have to make compromises about what gets included, what gets skipped, and how much detail they choose to include. \u00a0The fundamental lesson here &#8211; to pay attention to stereochemistry of chiral alcohols when converting to alkyl chlorides &#8211; is ultimately more important than whether the reaction goes S<sub>N<\/sub>2 or S<sub>N<\/sub>i in certain situations. However, \u00a0it would be really nice to see more consistency on this reaction from the textbook writers so that everyone is singing from the same hymnal.<\/p>\n<p>This instructor said it best:<\/p>\n<blockquote><p>Some of my colleagues just use PCl<sub>5<\/sub> and move on with their lives : &#8211; )<\/p><\/blockquote>\n<hr \/>\n<h2><a id=\"references\"><\/a>(Advanced) References and Further Reading<\/h2>\n<ol>\n<li><strong>Ueber die gegenseitige Umwandlung optischer Antipoden<br \/>\n<\/strong>Walden<strong><br \/>\n<\/strong><em>Chem. Ber.<\/em><strong> 1896, <\/strong><em>29<\/em> (1): 133\u2013138<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/chemistry-europe.onlinelibrary.wiley.com\/doi\/abs\/10.1002\/cber.18960290127\">10.1002\/cber.18960290127<\/a><br \/>\nOriginal publication on <em>Walden inversion<\/em>.It is interesting to trace the development of this reaction mechanism through the literature. Early papers were in disagreement regarding the mechanism reconciling the observations that <em>inversion<\/em> of configuration was observed with base (e.g. pyridine), and <em>retention<\/em> of configuration without base.<\/li>\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<\/strong><br \/>\nW. 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<br \/>\n<strong>DOI<\/strong>: <a href=\"https:\/\/pubs.rsc.org\/en\/content\/articlelanding\/1937\/JR\/jr9370001252#!divAbstract\">10.1039\/JR9370001252<\/a><br \/>\nProf. C. K. Ingold and Hughes developed the \u2018S<sub>N<\/sub>\/E\u2019 nomenclature used to describe reaction mechanisms, now known as the Hughes-Ingold nomenclature. In part C of this paper, they do note the observation that SOCl<sub>2<\/sub> alone reacts with secondary alcohols with retention of configuration, whereas SOCl<sub>2<\/sub>+pyridine goes through inversion. However, no mechanism is proposed as they try to fit these observations into their limiting paradigm of S<sub>N<\/sub>1 vs. S<sub>N<\/sub>2.<\/li>\n<li><strong> The decomposition of chlorosulphinic esters<br \/>\n<\/strong>Michael P. Balfe and Joseph Kenyon<br \/>\n<em>J. Chem. Soc.<\/em> <strong>1940<\/strong>, 463-464<br \/>\n<strong>DOI<\/strong>: <a href=\"https:\/\/pubs.rsc.org\/en\/content\/articlelanding\/1940\/jr\/jr9400000463#!divAbstract\">10.1039\/JR9400000463<\/a><br \/>\nThis early paper also features an attempt to rationalize the observed stereochemistries. Retention of configuration is due to \u201c<em>a molecular rearrangement the steric course of which is controlled by the dimensions of the chlorosulphinate molecule<\/em>\u201d, while inversion is caused by pyridine binding to sulfur. The yield of inverted product can be increased by using an excess of pyridine.<\/li>\n<li><strong>A Study of the Reaction of Alcohols with Thionyl Chloride<\/strong><br \/>\nWilliam E. Bissinger and Frederick E. Kung<br \/>\n<em>Journal of the American Chemical Society<\/em> <strong>1947,<\/strong> <em>69<\/em> (9), 2158-2163<br \/>\n<strong>DOI<\/strong>: <a href=\"https:\/\/pubs.acs.org\/doi\/abs\/10.1021\/ja01201a030?src=recsys\">10.1021\/ja01201a030<\/a><br \/>\nA nice study on the reaction of alcohols with SOCl2, useful if one is looking for a place to start optimization of this reaction (with regards to stoichiometry).<\/li>\n<li><strong>The Kinetics and Stereochemistry of the Decomposition of Secondary Alkyl Chlorosulfites<\/strong><br \/>\nEdward S. Lewis and Charles E. Boozer<br \/>\n<em>Journal of the American Chemical Society<\/em> <strong>1952,<\/strong> <em>74<\/em> (2), 308-311<br \/>\n<strong>DOI<\/strong>: <a href=\"https:\/\/pubs.acs.org\/doi\/abs\/10.1021\/ja01122a005\">10.1021\/ja01122a005<\/a><\/li>\n<li><strong>The Decomposition of Secondary Alkyl Chlorosulfites. II. Solvent Effects and Mechanisms<\/strong><br \/>\nE. Boozer and E. S. Lewis<br \/>\n<em>Journal of the American Chemical Society<\/em> <strong>1953,<\/strong> <em>75<\/em> (13), 3182-3186<br \/>\n<strong>DOI<\/strong>: <a href=\"https:\/\/pubs.acs.org\/doi\/abs\/10.1021\/ja01109a042\">10.1021\/ja01109a042<\/a><br \/>\nRef. <strong>4<\/strong> describes mechanisms for the decomposition of secondary alkyl chlorosulfites. Apparently different mechanisms are in effect when these are decomposed in dioxane or toluene. In dioxane, retention of configuration is observed, while in toluene inverted chlorides are obtained. This is ascribed to the ability of dioxane to coordinate to the carbon and assist with C-S bond cleavage.<\/li>\n<li><strong>Studies in Stereochemistry. XVI. Ionic Intermediates in the Decomposition of Certain Alkyl Chlorosulfites<\/strong><br \/>\nDonald J. Cram<br \/>\n<em>Journal of the American Chemical Society<\/em> <strong>1953,<\/strong> <em>75<\/em> (2), 332-338<br \/>\n<strong>DOI<\/strong>: <a href=\"https:\/\/pubs.acs.org\/doi\/abs\/10.1021\/ja01098a024\">10.1021\/ja01098a024<\/a><br \/>\nAn early paper by Prof. D. J. Cram (UCLA), who was a contemporary of Prof. Saul Winstein (who came up with the concepts of \u2018internal return\u2019 and \u2018intimate ion pair\u2019 used to describe this S<sub>N<\/sub>i mechanism). Prof. Cram would later on receive the Nobel Prize in Chemistry in 1987 for his work on molecular host-guest chemistry. This is <u>the<\/u> paper rationalizing the differing stereochemistries of the reaction of alcohols with SOCl<sub>2<\/sub> in the presence\/absence of base (e.g. pyridine) and is the first paper in the literature describing the reaction of alcohols + SOCl<sub>2<\/sub> as an S<sub>N<\/sub>i process.<\/li>\n<li>The textbook <em>March\u2019s Advanced Organic Chemistry <\/em>(7<sup>th<\/sup>) mentions:<br \/>\n<em>\u201c<\/em>[\u2026]<em> the reaction of alcohols with thionyl chloride to give alkyl halides usually proceeds in this way, with the first step in this case being ROH + SOCl<sub>2<\/sub> <\/em><em>\u00e0<\/em><em> ROSOCl (these alkyl chlorosulfites can be isolated).<br \/>\nEvidence for this mechanism is as follows: The addition of pyridine to the mixture of alcohol and thionyl chloride results in the formation of alkyl halide with <\/em><strong><em>inverted<\/em><\/strong><em> configuration. Inversion results because the pyridine reacts with ROSOCl to give ROSONC<sub>5<\/sub>H<sub>5<\/sub> before anything further can take place. The Cl<sup>&#8211;<\/sup> freed in this process now attacks from the rear. The reaction between alcohols and thionyl chloride is second order, which is predicted by this mechanism, but the decomposition by simple heating of ROSOCl is first order<\/em>\u201d.<br \/>\nUnfortunately, no references are provided.Prof. Jih Ru Hwu (now in Taiwan) attempted to popularize reagents that would react via internal return (such as SOCl<sub>2<\/sub>) as \u2018 <em>Counterattack Reagents<\/em>\u2019 early in his career:<\/li>\n<li><strong>Counterattack reagents in organic reactions and in syntheses<br \/>\n<\/strong>Jih Ru Hwu, Bryant A. Gilbert<br \/>\n<em>Tetrahedron<\/em> <strong>1989<\/strong>, <em>45<\/em> (5), 1233-1261<br \/>\n<strong>DOI<\/strong>: <a href=\"https:\/\/www.sciencedirect.com\/science\/article\/abs\/pii\/0040402089801231\">10.1016\/0040-4020(89)80123-1<\/a><\/li>\n<li><strong>Silicon reagents in chemical transformations: the concept of \u2018counterattack reagent\u2019<br \/>\n<\/strong> R. Hwu, S.-C. Tsay, K. Y. King and D.-N. Horng<strong><br \/>\n<\/strong><em>Pure Appl. Chem.<\/em><strong> 1999, <\/strong><em>71<\/em> (3), 445-451<strong><br \/>\nDOI: <\/strong><a href=\"http:\/\/publications.iupac.org\/pac\/71\/3\/0445\/index.html\">10.1351\/pac199971030445<\/a><\/li>\n<\/ol>\n","protected":false},"excerpt":{"rendered":"<p>SOCl2 Mechanism With Alcohols, With And Without Pyridine: Nucleophilic Substitution (SN2) Versus Nucleophilic Substitution With Internal Return (SNi) Most of the time, the reaction of <\/p>\n","protected":false},"author":1,"featured_media":15288,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1420],"tags":[964,522,301,236,962,271,961,273,963,593],"post_folder":[],"class_list":["post-7943","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-alcohols-epoxides-ethers","tag-corrections","tag-instructors","tag-inversion","tag-mistakes","tag-retention","tag-sn2","tag-sni","tag-stereochemistry","tag-textbooks","tag-thionyl-chloride"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.7 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>SOCl2 Mechanism For Alcohols To Alkyl Halides: SN2 versus SNi<\/title>\n<meta name=\"description\" content=\"SOCl2 added to alcohols results in an alkyl chloride. 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