{"id":574,"date":"2010-07-02T22:05:58","date_gmt":"2010-07-02T20:05:58","guid":{"rendered":"http:\/\/masterorganicchemistry.wordpress.com\/?p=574"},"modified":"2025-04-10T11:58:15","modified_gmt":"2025-04-10T16:58:15","slug":"stereoselective-stereospecific","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2010\/07\/02\/stereoselective-stereospecific\/","title":{"rendered":"Stereoselective and Stereospecific Reactions"},"content":{"rendered":"<p><strong>Stereoselective and Stereospecific Reactions<\/strong><\/p>\n<p>An ideal chemical reaction would be selective for one product only. This would mean no time spent purifying our product and no wasted materials.<\/p>\n<p>In the real world, reactions give mixtures. Not only can reactions occur with different functional groups on a given molecule, but even reactions on a single functional group can give rise to various types of isomers.<\/p>\n<p>We can classify the different types of <strong>selectivity <\/strong>in organic chemistry into several important categories.<\/p>\n<ul>\n<li><strong>Chemoselective<\/strong> reactions are selective for one functional group over another &#8211; for example, a reaction that selectively reacts with\u00a0<strong>alkynes\u00a0<\/strong>in the presence of\u00a0<strong>alkenes.\u00a0<\/strong><\/li>\n<li><strong>Regioselective<\/strong> reactions are selective for the formation of one <strong>constitutional isomer<\/strong> over another &#8211; for example, the &#8220;Markovnikov-selective&#8221; addition of strong acids to alkenes is an example of regioselectivity, as is &#8220;Zaitsev&#8217;s rule&#8221; for elimination reactions.<\/li>\n<li><strong>Stereoselective<\/strong> reactions are selective for the formation of one <strong>stereoisomer<\/strong> over another, such as the preference for partial hydrogenation of alkynes with Na\/NH<sub>3<\/sub> to give\u00a0<em>trans<\/em> alkenes.<\/li>\n<\/ul>\n<p>Whether or not a given combination of starting material and reactant is regioselective or stereoselective depends on the reaction mechanism, as well as on the specific structure of the starting material.<\/p>\n<p><strong>Selectivity<\/strong> implies some value less than perfection. A reaction which gives a 99:1 ratio of products is highly selective, but we still use the word &#8220;selective&#8221;.<\/p>\n<p>There are some reactions which are so stereoselective that they give one stereoisomer exclusively. This is often due to the existence of a concerted reaction mechanism (<span style=\"color: #993366;\"><em>like the S<sub>N<\/sub>2<\/em><\/span>).\u00a0 The the term <strong>stereospecific\u00a0<\/strong>is used to refer to reactions where starting materials differing in configuration at a single carbon (i.e. stereoisomers) result in products that are also stereoisomers.<\/p>\n<p><img fetchpriority=\"high\" decoding=\"async\" class=\"alignnone wp-image-36004\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2010\/07\/0-stereoselective-versus-stereospecific-summary-in-organic-chemistry-definition.gif\" alt=\"stereoselective versus stereospecific summary in organic chemistry definition\" width=\"640\" height=\"718\" \/><\/a><\/p>\n<p><strong>Table of Contents<\/strong><\/p>\n<ol>\n<li style=\"list-style-type: none;\">\n<ol>\n<li><a href=\"#one\">Selectivity Is A Highly Desirable Property For A Reaction To Have<\/a><\/li>\n<li><a href=\"#two\">The Different Classes of &#8220;Selectivity&#8221;<\/a><\/li>\n<li><a href=\"#three\">Chemoselectivity: Functional Group Selectivity<\/a><\/li>\n<li><a href=\"#four\">Regioselectivity<\/a><\/li>\n<li><a href=\"#five\">Stereoselective vs. Non-Stereoselective Reactions<\/a><\/li>\n<li><a href=\"#six\">Stereoselective vs. Stereospecific Reactions<\/a><\/li>\n<li><a href=\"#seven\">Enantioselectivity<\/a><\/li>\n<li><a href=\"#eight\">Diastereoselectivity<\/a><\/li>\n<li><a href=\"http:\/\/nine\">The Substrate Has A Vote On Whether A Reaction Is Selective<\/a><\/li>\n<li><a href=\"#notes\">Notes<\/a><\/li>\n<li><a href=\"#quiz\">Quiz Yourself!<\/a><\/li>\n<li><a href=\"#references\">(Advanced) References and Further Reading<\/a><\/li>\n<\/ol>\n<\/li>\n<\/ol>\n<hr \/>\n<h2><a id=\"one\"><\/a>1. Selectivity Is A Highly Desirable Property For A Reaction To Have<\/h2>\n<p>Let&#8217;s face it. We&#8217;re accustomed to things that have a single <strong>output<\/strong> for every <strong>input<\/strong>.<\/p>\n<p>I mean, when you push the &#8220;L&#8221; button on a keyboard, you&#8217;re used to seeing an &#8220;L&#8221; show up on the screen <strong>100% <\/strong>of the time.\u00a0 If that doesn&#8217;t happen, then someone hasn&#8217;t done their job.<\/p>\n<p>So imagine what it would be like to push the &#8220;L&#8221; button on your keyboard a hundred times and getting 74 L&#8217;s, 22 K&#8217;s,\u00a0 and various amounts of J, R, S, and T. That would be kind of <em>annoying<\/em>, right? Well&#8230;. welcome to life in the organic chemistry lab.<\/p>\n<p>For instance, imagine\u00a0that we run a reaction with two reactants, <strong>A<\/strong> and <strong>B<\/strong>. In an ideal world, we&#8217;d get a 100% yield of product\u00a0<strong>C<\/strong>.<\/p>\n<p>But that&#8217;s not generally how organic chemistry reactions work. Instead, in this example, we get a mixture of products <strong>C<\/strong>, <strong>D<\/strong>, and a little bit of <strong>E<\/strong>.<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-35987\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/02\/1-how-a-typical-organic-chemistry-reaction-goes-3-products-in-variable-yields.gif\" alt=\"-how a typical organic chemistry reaction goes - 3 products in variable yields\" width=\"640\" height=\"236\" \/><\/a><\/p>\n<p><span style=\"color: #993366;\"><em>And hey, there are many contexts in which a 74% yield is actually considered &#8220;good&#8221;!\u00a0<\/em><\/span><\/p>\n<p>If your reaction produces a 60% yield of C, 30% D, and 10% E, you can reasonably say that the reaction is <strong>selective<\/strong> for C. Maybe not &#8220;highly selective&#8221;, but at least &#8220;somewhat selective&#8221;.<\/p>\n<p>On the other hand,\u00a0 a reaction that gave a 33% yield each of C, D, and E can be safely called &#8220;unselective&#8221;.\u00a0 [<a href=\"#noteone\"><span style=\"color: #ff0000;\">Note 1<\/span><\/a>]<\/p>\n<p><span style=\"color: #993366;\"><em>Calling a reaction &#8220;selective&#8221; is a bit of a judgement call.\u00a0 You could make a case that a reaction that reproducibly gives a 55:45 mixture of products is selective, but you&#8217;d have to admit the selectivity is poor. On the other hand, a reaction that gives a 200:1 ratio of products (at the limits of detection for practical purposes) can clearly be called &#8220;highly selective&#8221;.<\/em><\/span><\/p>\n<p>Selectivity is a<strong> highly desirable property for a reaction to have.\u00a0<\/strong><\/p>\n<p>Organic chemists go to great lengths to maximize the selectivity of reactions, since it means less time spent on purification and less money spent on wasted reactants that give unwanted byproducts we need to dispose of.<\/p>\n<p>Numerous Nobel Prizes have been given to the discoverers of reactions that are highly selective in some way &#8211; the 2022 Nobel for &#8220;click chemistry&#8221; being the most recent.\u00a0 [<a href=\"#notetwo\"><span style=\"color: #ff0000;\">Note 2<\/span><\/a>] You might not appreciate it now, but when you take an organic chemistry course, we&#8217;re really trying to teach you some of the <em>best<\/em> and <em>most selective<\/em> reactions that are known to us!<\/p>\n<p><span style=\"color: #993366;\"><em>On the other hand, there is a large class of reactions in organic chemistry that result in the formation of thick black tar on the bottom of the reaction vessel. These reactions firmly belong in the &#8220;unselective&#8221; category.<\/em><\/span><\/p>\n<h2><a id=\"two\"><\/a>2. The Different Classes of &#8220;Selectivity&#8221;<\/h2>\n<p>It turns out that there are lots of different ways in which a reaction can be unselective, and therefore many opportunities for the formation of multiple products!<\/p>\n<ul>\n<li>The first reason is that many reagents can react with several different <strong>functional groups\u00a0<\/strong>on a molecule, giving rise to completely different products.<span style=\"color: #800080;\">\u00a0<\/span><\/li>\n<li>The second reason is that even if a given reagent reacts only with one functional group, the reaction can still result in the formation of various types of <strong>isomers<\/strong>, such as <strong>constitutional isomers<\/strong> (<span style=\"color: #993366;\"><em>also known as &#8220;regioisomers&#8221; in this context<\/em><\/span>), and <strong>stereoisomers<\/strong>. \u00a0(<span style=\"color: #800080;\"><em>See article: <a style=\"color: #800080;\" href=\"https:\/\/www.masterorganicchemistry.com\/2018\/09\/10\/types-of-isomers\/\">Types of Isomers<\/a><\/em><\/span>).<\/li>\n<\/ul>\n<p>The most important factor in ensuring a selective reaction is to simply to choose a substrate (<span style=\"color: #993366;\"><em>also known as &#8220;starting material&#8221;<\/em><\/span>) with a minimum of reactive functional groups and possibilities for isomerism. (<span style=\"color: #993366;\"><em>For more on possibilities of isomerism, including quizzes, &#8220;<span style=\"color: #993366;\"><a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2012\/05\/23\/whats-a-racemic-mixture\/\">What&#8217;s a Racemic Mixture<\/a><\/span>&#8220;)<\/em><\/span>.<\/p>\n<p>The selectivity of a reaction will also strongly depend on the <strong>reaction mechanism<\/strong>, and to a lesser extent on some external factors such as temperature and solvent.<\/p>\n<p>The word, &#8220;selectivity&#8221; in organic chemistry can be applied to several different situations. It can be helpful to start by thinking, &#8220;selective for <em>what?&#8221;.\u00a0<\/em><\/p>\n<p>For our purposes, we&#8217;ll generally discuss 4 types of selectivity:<\/p>\n<ul>\n<li>A\u00a0<strong>chemoselective<\/strong> reaction is selective for the reaction at one<strong> functional group<\/strong> over another functional group.<\/li>\n<li>A\u00a0<strong>regioselective\u00a0<\/strong>reaction is selective for the formation of one <strong>constitutional isomer<\/strong> (&#8220;regioisomer&#8221;) over another<\/li>\n<li>A <strong>stereoselective<\/strong> reaction is selective for the formation of one <strong>stereoisomer<\/strong> over another.<\/li>\n<li>An <strong>enantioselective<\/strong> reaction is selective for the formation of one <strong>enantiomer<\/strong> over another.<\/li>\n<\/ul>\n<p><img decoding=\"async\" class=\"alignnone wp-image-35988\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/02\/2-four-types-of-selectivity-in-organic-chemistry-chemoselectivity-regioselectivity-stereoselectivity-enantioselectivity.gif\" alt=\"four types of selectivity in organic chemistry - chemoselectivity regioselectivity stereoselectivity enantioselectivity\" width=\"640\" height=\"567\" \/><\/a><\/p>\n<p>Note that these types of selectivity are <strong>not<\/strong> mutually exclusive, and multiple classes of selectivity can operate at the same time.<\/p>\n<p>The E2 reaction, for example, is regioselective (for the more substituted &#8220;Zaitsev&#8221; alkene) as well as stereoselective (for <em>anti<\/em>-elimination) but <em>not<\/em> enantioselective (since no chiral centers are formed).\u00a0 (<span style=\"color: #993366;\"><em>See article: <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2012\/09\/27\/the-e2-mechanism\/\">The E2 Reaction<\/a><\/em><\/span>)<\/p>\n<h2><a id=\"three\"><\/a>3. Chemoselectivity<\/h2>\n<p>Any time a given reaction is selective for a certain functional group, that&#8217;s an example of &#8220;<strong>chemoselectivity&#8221;<\/strong>.<\/p>\n<p>The term &#8220;chemoselectivity&#8221; doesn&#8217;t come up too much in introductory organic, mostly because for simplicity&#8217;s sake you&#8217;re often only shown molecules that have a single reactive functional group (<span style=\"color: #993366;\"><em>alkanes and ethers are generally not considered &#8220;reactive&#8221; functional groups for most purposes).<\/em><\/span><\/p>\n<p>There are some examples in introductory organic where you are explicitly told that a given reagent is selective for one functional group over another. For example, in the chapter on alkynes, Lindlar&#8217;s catalyst is selective for the hydrogenation of alkynes over alkenes. (<span style=\"color: #993366;\"><em>See article: <a href=\"https:\/\/www.masterorganicchemistry.com\/2011\/08\/19\/lindlars-catalyst-partial-cis-reduction\/\">Lindlar&#8217;s Catalyst for Partial Hydrogenation of Alkyne<\/a><\/em><\/span>s). That&#8217;s a classic example of <strong>chemoselectivity<\/strong>.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35989\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/02\/3-hydrogenation-of-alkynes-with-LIndlars-catalyst-is-an-example-of-a-chemoselective-reaction.gif\" alt=\"hydrogenation of alkynes with LIndlars catalyst is an example of a chemoselective reaction\" width=\"640\" height=\"266\" \/><\/a><\/p>\n<p>Another case that comes up frequently is when a given reagent is capable of acting as both a base and a nucleophile on a given substrate, which can lead to multiple products. (<span style=\"color: #993366;\"><em>See article: <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2012\/06\/06\/nucleophilicity-vs-basicity\/\">Basicity vs Nucleophilicity<\/a><\/em><\/span>)<\/p>\n<p>Alkyl halides are a perfect example. If you&#8217;ve ever been asked to decide if an alkyl halide goes through an S<sub>N<\/sub>2 or E2 reaction with a given nucleophile\/base,\u00a0 that&#8217;s ultimately a <strong>chemoselectivity<\/strong> question.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35990\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/02\/4-reactions-that-favor-substitution-or-elimination-are-examples-of-chemoselective-reactions.gif\" alt=\"reactions that favor substitution or elimination are examples of chemoselective reactions\" width=\"640\" height=\"492\" \/><\/a><\/p>\n<p>Because in introductory organic chemistry we mostly only deal with molecules containing a single functional group, I&#8217;m not going to drill down much further on chemoselectivity, although further along in the course,\u00a0you&#8217;ll see examples of <strong>protecting groups<\/strong> as one way of trying to solve the problems of chemoselectivity. (<span style=\"color: #993366;\"><em>See article: <span style=\"color: #993366;\"><a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2015\/06\/17\/protecting-groups-for-alcohols\/\">Protecting Groups for Alcohols<\/a><\/span><\/em>)<\/span><\/p>\n<p><span style=\"color: #800080;\"><em>(Other examples of chemoselective reactions:\u00a0 selective reduction of aldehydes\/ketones over esters by NaBH<sub>4<\/sub>; selective deprotection of amine protecting groups in peptides; )\u00a0<\/em><\/span><\/p>\n<h2><a id=\"four\"><\/a>4. Regioselectivity<\/h2>\n<p>Even if a reaction is confined to a single functional group, the formation of\u00a0<strong>isomers<\/strong> is an extremely common result.<\/p>\n<p><strong>Constitutional isomers<\/strong> are molecules with the same empirical formula, but different <strong>connectivity.\u00a0<\/strong><\/p>\n<p>A classic example of a reaction that forms constitutional isomers is in the addition of strong acids to alkenes (e.g. HCl, HBr, HI, and H<sub>3<\/sub>O+).\u00a0 For example, reaction of propene with hydrochloric acid (HCl) results in a mixture of 2-chloropropane and 1-chloropropane, both with the empirical formula C<sub>3<\/sub>H<sub>7<\/sub>Cl.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35991\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/02\/5-markovnikovs-rule-is-an-example-of-a-regioselective-reaction.gif\" alt=\"markovnikovs rule is an example of a regioselective reaction\" width=\"640\" height=\"349\" \/><\/a><\/p>\n<p>In these reactions it is observed that the major product is the one where the C-Cl bond forms on the carbon of the alkene directly attached to the <strong>most<\/strong> carbon atoms (&#8220;most substituted&#8221;), and the C-H bond forms on the carbon of the alkene directly attached to the <strong>fewest<\/strong> carbon atoms (the &#8220;least substituted&#8221; carbon).\u00a0 This is known as &#8220;<strong>Markovnikov&#8217;s Rule<\/strong>&#8221; (<span style=\"color: #993366;\"><em>See article: <span style=\"color: #993366;\"><a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2013\/02\/08\/markovnikovs-rule-1\/\">Hydrohalogenation of Alkenes and Markovnikov&#8217;s Rule<\/a><\/span><\/em><\/span>).<\/p>\n<p>This type of selectivity is known as &#8220;<strong>regioselectivity<\/strong>&#8220;, (<span style=\"color: #993366;\">from <em>regio-, <\/em>Latin for &#8220;region&#8221;<\/span>) and the constitutionally isomeric products are often referred to as <strong>regioisomers<\/strong>. [<span style=\"color: #ff0000;\">Note 3<\/span>]<\/p>\n<p>In the case of addition of acids to alkenes, this regioselectivity arises due to the greater stability of carbocation intermediates on more substituted carbons (<span style=\"color: #993366;\"><em>See article: <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2011\/03\/11\/3-factors-that-stabilize-carbocations\/\">Carbocation Stability<\/a><\/em><\/span>).<\/p>\n<p>This brings us to a subtle point.<\/p>\n<p>The regioselectivity (and stereoselectivity, for that matter) of a reaction is highly dependent on the properties of the reaction mechanism itself. But the mechanism is not the only factor.<\/p>\n<p>Selectivity is also a function of the structure of the substrate (i.e. starting material).<\/p>\n<p>Markovnikov&#8217;s rule for regioselectivity <strong>only holds<\/strong> when one terminus of the alkene is bonded to more carbons than the other. If they are bonded to an equal number of carbons, then the carbocations derived from those alkenes will be roughly equal in stability, and the regioselectivity will go away.<\/p>\n<p>When each carbon of the alkene is directly connected to the same number of carbon atoms, regioselectivity is lost.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35993\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/02\/7-markovnikovs-rule-fails-when-both-ends-of-alkene-are-equally-substituted.gif\" alt=\"markovnikovs rule fails when both ends of alkene are equally substituted\" width=\"640\" height=\"337\" \/><\/a><\/p>\n<p>Another example of regioselectivity in organic chemistry is Zaitsev&#8217;s Rule in the elimination of alkenes, where formation of the\u00a0<strong>more substituted alkene\u00a0<\/strong>tends to be\u00a0<strong>more favored<\/strong>. (<span style=\"color: #993366;\"><em>See article: <span style=\"color: #993366;\"><a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2012\/08\/31\/elimination-reactions-2-zaitsevs-rule\/\">Elimination Reactions &#8211; Zaitsev&#8217;s Rule<\/a><\/span><\/em><\/span>).<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35992\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/02\/6-zaitsevs-rule-is-an-example-of-a-regioselctive-reaction.gif\" alt=\"-zaitsevs rule is an example of a regioselctive reaction\" width=\"640\" height=\"330\" \/><\/a><\/p>\n<p>This reaction tends to be regioselective due to the greater stability of alkenes as C-H bonds are replaced with C-C bonds. (<span style=\"color: #993366;\"><em>See article: <\/em><\/span><a href=\"https:\/\/www.masterorganicchemistry.com\/2020\/04\/30\/alkene-stability\/\"><span style=\"color: #993366;\"><em>Alkene Stability<\/em><\/span><\/a>)<\/p>\n<p>Furthermore, not all reactions are capable of forming constitutional isomers (regioisomers).\u00a0 So not all reactions have the possibility of being regioselective.<\/p>\n<p>For example, when two identical bonds are formed in an addition to an alkene or alkyne, there is no possibility of forming constitutional isomers. Hydrogenation and dihydroxylation are two examples of reactions for which the concept of &#8220;regioselectivity&#8221; does not apply.\u00a0 <span style=\"color: #993366;\"><em>(others include epoxidation, cyclopropanation, and halogenation).\u00a0<\/em><\/span><\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35994\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/02\/8-reactions-which-cannot-result-in-constitutional-isomers-cannot-have-regioselectivity.gif\" alt=\"reactions which cannot result in constitutional isomers cannot have regioselectivity\" width=\"640\" height=\"414\" \/><\/a><\/p>\n<p><span style=\"color: #993366;\"><em>Other examples of regioselective reactions: ortho-,para- vs meta selectivity in electrophilic aromatic substitution; Diels-Alder reactions; 1,4-additions of Gilman reagents; radical addition of HBr to alkenes; hydroboration of alkenes; free-radical substitution of tertiary C-H bonds with Br<sub>2<\/sub>)\u00a0<\/em><\/span><\/p>\n<h2><a id=\"five\"><\/a>5. Stereoselectivity<\/h2>\n<p>When a reaction has the possibility of forming\u00a0<strong>stereoisomers<\/strong>, the issue of\u00a0<strong>stereoselectivity\u00a0<\/strong>arises.<\/p>\n<p><span style=\"color: #993366;\"><em>This article assumes you are familiar with the difference between diastereomers and enantiomers. For more clarification, see <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2018\/09\/10\/types-of-isomers\/\">Types of Isomers<\/a>.\u00a0<\/em><\/span><\/p>\n<p>Reactions that proceed through carbocation or radical intermediates often, (but not always), result in poor stereoselectivity.<\/p>\n<p>As we&#8217;ve seen, the addition of acids to alkenes (we discussed above) is certainly <strong>regioselective <\/strong>with certain alkenes. However, the stereoselectivity of the reaction is poor. [<a href=\"#notefour\"><span style=\"color: #ff0000;\">Note 4<\/span><\/a>]<\/p>\n<p>When we add H-Br to the alkene below (1,2-dimethylcyclohexene), a roughly equal mixture of\u00a0<em>cis-<\/em> and\u00a0<em>trans<\/em>&#8211; <strong>stereoisomers<\/strong> are obtained.<\/p>\n<p>(<span style=\"color: #800080;\"><em>this is due to the formation of a trigonal planar carbocation intermediate, which can be attacked equally well on either face by the halide ion<\/em><\/span>).<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35995\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/02\/9-addition-of-hbr-to-alkenes-results-in-roughly-equal-mixture-of-syn-and-anti-addition.gif\" alt=\"addition of hbr to alkenes results in roughly equal mixture of syn and anti addition\" width=\"640\" height=\"279\" \/><\/a><\/p>\n<p>Another classic example of a poorly stereoselective<strong>\u00a0<\/strong>reaction is the unimolecular\u00a0substitution reaction (S<sub>N<\/sub>1) of alkyl halides, where we can start with a single pure <strong>enantiomer<\/strong> and end up with a racemic mixture of products (<span style=\"color: #993366;\"><em>a racemic mixture is a 1:1 mixture of enantiomers<\/em><\/span>).<\/p>\n<p>Like addition of H-Br to alkenes, the S<sub>N<\/sub>1 also proceeds through a carbocation intermediate that can be attacked with equal likelihood from either face, giving us a mixture of products representing <strong>retention<\/strong> and\u00a0<strong>inversion<\/strong> of stereochemistry \u00a0(<span style=\"color: #993366;\"><em>See article &#8211;<\/em><a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2012\/07\/13\/the-sn1-mechanism\/\"><em> the SN1 Mechanism<\/em><\/a><\/span>). [<a href=\"#notefour\"><span style=\"color: #ff0000;\">Note 4<\/span><\/a>]<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35996\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/02\/10-sn1-reaction-is-not-stereoselective-results-in-mixture-of-enantiomers.gif\" alt=\"sn1 reaction is not stereoselective results in mixture of enantiomers\" width=\"640\" height=\"322\" \/><\/a><\/p>\n<p>Some reactions the proceed through carbocation intermediates can still have relatively high stereoselectivity.<\/p>\n<p>In the E1 reaction, a carbocation intermediate undergoes loss of a proton to form an alkene as the final product. Elimination will generally favor formation of the alkene where the bulkiest groups are\u00a0<em>trans<\/em> to each other since this minimizes steric strain.\u00a0\u00a0(<span style=\"color: #800080;\"><em>See article &#8211; <a style=\"color: #800080;\" href=\"https:\/\/www.masterorganicchemistry.com\/2020\/04\/30\/alkene-stability\/\">Alkene Stability<\/a><\/em><\/span>).<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35997\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/02\/11-e1-reaction-is-not-stereoselective.gif\" alt=\"-e1 reaction is not stereoselective\" width=\"640\" height=\"474\" \/><\/a><\/p>\n<p>Since the (<em>E<\/em>) and (<em>Z<\/em>) alkenes are stereoisomers of each other, and the reaction favors the (<em>E<\/em>), this is an example of stereoselectivity.<\/p>\n<p><span style=\"color: #993366;\"><em>More examples of stereoselective reactions &#8211; partial hydrogenation of alkynes (both <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2011\/08\/19\/lindlars-catalyst-partial-cis-reduction\/\">Lindlar<\/a> and <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2013\/05\/08\/nanh3-partial-reduction-of-alkynes\/\">Na\/NH3<\/a>), <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2013\/03\/28\/hydroboration-of-alkenes-the-mechanism\/\">hydroboration of alkenes<\/a>, the <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2017\/08\/30\/the-diels-alder-reaction\/\">Diels-Alder reaction<\/a>, <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2019\/11\/14\/the-cope-and-claisen-rearrangements\/\">Cope rearrangement.<\/a><\/em><\/span><\/p>\n<h2><a id=\"six\"><\/a>6. Stereoselective vs. Stereospecific Reactions<\/h2>\n<p>If a reaction gives a 99:1 mixture of stereoisomers, we can feel pretty confident in calling that reaction,\u00a0 highly&#8221;stereoselective&#8221;.<\/p>\n<p>However, some reactions are so\u00a0<em>spectacularly\u00a0<\/em>stereoselective for the formation of a single type of product that the term &#8220;stereoselective&#8221; doesn&#8217;t seem to do it proper justice.<\/p>\n<p>For example, dihydroxylation of alkenes gives\u00a0<em>only<\/em> addition products where the two C-OH bonds form on the same face of the alkene. Absolutely none of the addition products are formed where the two C-OH bonds are <em>trans<\/em>.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35998\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/02\/12-oso4-reaction-dihydroxylation-of-alkenes-is-stereoselective.gif\" alt=\"oso4 reaction dihydroxylation of alkenes is stereoselective\" width=\"640\" height=\"275\" \/><\/a><\/p>\n<p>Sometimes, the term\u00a0<strong>stereospecific\u00a0<\/strong>is used to categorize reactions like this &#8211; reactions that are so super-<i>dee-<\/i>duper stereoselective that a given stereoisomer is formed\u00a0<strong>exclusively.\u00a0<\/strong><\/p>\n<p>IUPAC <span style=\"color: #800080;\">(<em>which you can think of as <\/em><em>the Ministry of Magic, but for chemistry<\/em><\/span>) frowns upon this use of the word stereospecific.<\/p>\n<p>According to IUPAC, a <strong>stereospecific <\/strong>reaction is one where starting materials differing <strong>only<\/strong> in their configuration are converted into stereoisomeric products.<\/p>\n<p>The S<sub>N<\/sub>2 reaction is an example of a stereospecific reaction. Carrying out the S<sub>N<\/sub>2 on starting materials that are enantiomers of each other results in products that are themselves enantiomers of each other! (<span style=\"color: #993366;\"><a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2012\/07\/04\/the-sn2-mechanism\/\"><em>See article &#8211; The SN2 Mechanism<\/em><\/a><\/span>).<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35999\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/02\/13-sn2-reaction-is-stereospecific.gif\" alt=\"sn2 reaction is stereospecific\" width=\"640\" height=\"534\" \/><\/a><\/p>\n<p>Compare that to the SN1 reaction, above, which is poorly stereoselective.<\/p>\n<p>A slightly preferable definition (<span style=\"color: #800080;\"><em>thx<\/em><\/span>, <a href=\"https:\/\/www.google.com\/books\/edition\/Organic_Chemistry\/kQgu2j_ber0C?hl=en&amp;gbpv=1&amp;dq=stereospecific+clayden&amp;pg=PA396&amp;printsec=frontcover\"><em>Clayden<\/em><\/a>) is that in a <strong>stereospecific reaction, the stereochemistry of the starting material determines the stereochemistry of the product.\u00a0<\/strong><\/p>\n<p>This generally points to a concerted mechanism where only one stereochemical outcome is possible. We can say that the <strong>reaction doesn&#8217;t have a choice as to which product is made<\/strong>.<\/p>\n<p>In the E1 reaction we saw above, two alkene stereoisomers were formed. If the same starting material is treated with a strong base, the E2\u00a0 mechanism (elimination, bimolecular rate-determining step) occurs instead, and we get\u00a0<strong>one<\/strong> stereoisomer\u00a0<strong>exclusively.\u00a0<\/strong><\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-36000\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/02\/14-e2-is-stereoselective-e1-is-not.gif\" alt=\"e2 is stereoselective e1 is not\" width=\"640\" height=\"408\" \/><\/a><\/p>\n<p>This is why the E2 gets far more use in modern organic chemistry than the E1. Since the E2 is stereospecific,\u00a0 you have much more control over the stereochemistry of the product and are much less likely to get mixtures.<\/p>\n<p>Halogenation of alkenes is another stereospecific reaction. The (E) and (Z) isomers of a given alkene will give rise to products that are stereoisomers of each other. Another example of the stereochemistry of the starting material determining the stereochemistry of the product.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-36001\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/02\/15-halogenation-of-alkenes-stereospecific.gif\" alt=\"halogenation of alkenes stereospecific\" width=\"640\" height=\"609\" \/><\/a><\/p>\n<p>There are many more examples of stereospecific reactions we will cover in this course.<\/p>\n<p><span style=\"color: #993366;\"><em>A non-exclusive list of other stereospecific reactions includes <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2011\/06\/17\/reagent-friday-m-cpba-meta-chloroperoxybenzoic-acid\/\">epoxidation<\/a>, <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2023\/10\/18\/cyclopropanation-of-alkenes\/\">cyclopropanation<\/a>, <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2013\/03\/15\/alkene-bromination-mechanism\/\">halogenation of alkenes<\/a>, the <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2017\/08\/30\/the-diels-alder-reaction\/\">Diels-Alder reaction<\/a>, and<a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2020\/06\/26\/electrocyclic-ring-opening-and-closure-2-six-or-eight-pi-electrons\/\"> electrocyclic reactions<\/a>.<\/em><\/span><\/p>\n<p>All stereospecific reactions are stereoselective, but not all stereoselective reactions are stereospecific.<\/p>\n<p><span style=\"color: #800080;\"><em>A final example of a reaction that is stereoselective, but not stereospecific, is the hydrogenation of alkenes with Pd\/C and H<sub>2<\/sub>.\u00a0 <\/em><\/span><em><span style=\"color: #993366;\">\u00a0<\/span><\/em><em><span style=\"color: #993366;\">See article &#8211;<\/span> <span style=\"color: #993366;\"><a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2011\/11\/25\/hydrogenation-alkenes-palladium-on-carbon-pdc\/\">Hydrogenation of Alkenes with Pd\/C<\/a><\/span><\/em>). [<a href=\"#notefive\"><span style=\"color: #ff0000;\">Note 5<\/span><\/a>]<\/p>\n<h2><a id=\"seven\"><\/a>7. Enantioselectivity<\/h2>\n<p>A subset of <strong>stereoselective<\/strong> reactions are <strong>enantioselective<\/strong> reactions, where one enantiomer is formed selectively over another.<\/p>\n<p>Enantioselectivity requires the influence of some kind of chiral material,\u00a0 such as a chiral reagent or starting material. (<span style=\"color: #800080;\"><em><span style=\"color: #800080;\">As <a style=\"color: #800080;\" href=\"https:\/\/www.masterorganicchemistry.com\/tutoring\/\">Matt<\/a> wo<\/span>uld say, &#8220;optical inactivity is preserved&#8221;<\/em><\/span>).<\/p>\n<p>Although many enantioselective reactions exist, we tend not to cover them so much in Org 1 \/ Org 2. If you take biochemistry, you will no doubt see many examples of enantioselective reactions carried out by enzymes.<\/p>\n<p>One enantioselective reaction that does see some action is the enantioselective epoxidation of allylic alcohols,\u00a0 developed by the laboratory of Barry Sharpless (<span style=\"color: #993366;\"><em>Nobel Prize in Chemistry, 2001 and 2022<\/em><\/span>) and is known as the <a href=\"https:\/\/www.masterorganicchemistry.com\/reaction-guide\/sharpless-epoxidation\/\">Sharpless asymmetric epoxidation<\/a>.<\/p>\n<p>In the Sharpless epoxidation, an allylic alcohol is treated with a catalytic amount of a titanium compound, a stoichiometric amount of oxidant (usually\u00a0<em>t<\/em>-butylhydroperoxide,\u00a0<em>t<\/em>-BHP) and a catalytic amount of an ester derived from one of the enantiomers of tartaric acid.<\/p>\n<p>The Sharpless gives very good enantioselectivity for the epoxidation of a wide variety of allylic alcohols.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-36002\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/02\/16-sharpless-epoxidation-is-example-of-enantioselective-reaction.gif\" alt=\"sharpless epoxidation is example of enantioselective reaction\" width=\"640\" height=\"397\" \/><\/a><\/p>\n<p>Note that enantioselectivity is measured by <strong>enantiomeric excess<\/strong> (e.e.) or sometimes enantiomeric ratio (e.r.) (<span style=\"color: #800080;\"><em>See article &#8211; <a style=\"color: #800080;\" href=\"https:\/\/www.masterorganicchemistry.com\/2017\/02\/24\/optical-purity-and-enantiomeric-excess\/\">Optical Activity and Enantiomeric Excess<\/a><\/em><\/span>).<\/p>\n<p>Enantioselective variants of many common reactions have been developed and you will undoubtedly encounter them if you enroll in more advanced organic chemistry courses.<\/p>\n<p><span style=\"color: #993366;\"><em>Enantiomeric excess = (mole fraction of major enantiomer) &#8211; (mole fraction of minor enantiomer) expressed as a percentage. A racemic mixture has an enantiomeric excess of 0 and a 99:1 ratio of enantiomers has an enantiomeric excess of 98. See: <span style=\"color: #993366;\"><a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2017\/02\/24\/optical-purity-and-enantiomeric-excess\/\">Optical Purity and Enantiomeric Excess<\/a>).<\/span>\u00a0<\/em><\/span><\/p>\n<h2><a id=\"eight\"><\/a>8. Diastereoselectivity<\/h2>\n<p>Diastereomers are stereoisomers that are <strong>not<\/strong> mirror images of each other (<span style=\"color: #993366;\"><em>i.e. stereoisomers that are not enantiomers<\/em><\/span>).<\/p>\n<p>A common situation where diastereomers are formed is when a molecule with a pre-existing chiral center undergoes a reaction at a functional group (e.g. an alkene).<\/p>\n<p>Remember that in order for molecules to be enantiomers, the configuration must be reversed at all chiral centers (<span style=\"color: #993366;\"><em>See article &#8211; <a href=\"https:\/\/www.masterorganicchemistry.com\/2011\/01\/24\/how-to-draw-enantiomer\/\">How To Draw The Enantiomer of a Chiral Molecule<\/a><\/em><\/span>).<\/p>\n<p>Since the configuration at the pre-existing chiral center is unaffected by the epoxidation reaction, these can&#8217;t be enantiomers. They are <strong>diastereomers<\/strong>.<\/p>\n<p>One of the most common examples of diastereoselective reactions occurs when one face of an alkene is partially\u00a0&#8220;blocked&#8221; by the presence of nearby groups that hinder the approach of a reagent. In these cases the favored product will be the one where the reagent approaches from the face where it encounters less <strong>steric hindrance.<\/strong><\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-36003\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/02\/17-diastereoselective-reaction-gives-excess-of-one-diastereomer.gif\" alt=\"diastereoselective reaction gives excess of one diastereomer\" width=\"640\" height=\"610\" \/><\/a><\/p>\n<p>The preference for one diastereomer over another is called &#8220;diastereoselectivity&#8221;.<\/p>\n<p><em>Endo<\/em> and <em>exo<\/em> products in the Diels-Alder reaction are another prominent example of diastereoselectivity. (<span style=\"color: #800080;\"><em>S<span style=\"color: #800080;\">ee article &#8211; <a style=\"color: #800080;\" href=\"https:\/\/www.masterorganicchemistry.com\/2018\/02\/09\/endo-exo-diels-alder-telling-them-apart\/\">Exo and Endo Products In the Diels-Alder<\/a><\/span><\/em><\/span>).<\/p>\n<h2><a id=\"nine\"><\/a>9. The Substrate Has A Vote On Whether A Reaction Is Selective<\/h2>\n<p>One question that comes up a lot is, &#8220;how do I know if a reaction will lead to constitutional isomers, a racemic mixture, or a mixture of diastereomers?&#8221;<\/p>\n<p>My answer to that is not very satisfying. <em>&#8220;It depends!&#8221;\u00a0<\/em><\/p>\n<p>Depends on what? Two key factors.<\/p>\n<p>The first factor is the\u00a0<strong>pattern of bonds that form and break\u00a0<\/strong>for that particular reaction, along with its pattern for\u00a0<strong>regioselectivity\u00a0<\/strong>(i.e. is it Markovnikov-selective? Zaitsev selective?) and its pattern for\u00a0<strong>stereoselectivity.\u00a0<\/strong><\/p>\n<p>Assuming you have that key pattern close at hand, the next step is to\u00a0<strong>apply it\u00a0<\/strong>to the specific starting material (&#8220;substrate&#8217;) in question.<\/p>\n<p>Whether a reaction will form enantiomers, diastereomers, or constitutional isomers will depend on the specific structure of the substrate. There is no way to memorize your way through this. You just have to\u00a0<strong>apply the patterns<\/strong> to the\u00a0<strong>reaction at hand.\u00a0<\/strong><\/p>\n<p>For lots of examples (with quizzes) of this type of thinking, see, <span style=\"color: #800080;\">&#8220;<a style=\"color: #800080;\" href=\"https:\/\/www.masterorganicchemistry.com\/2012\/05\/23\/whats-a-racemic-mixture\/\"><em>What&#8217;s A Racemic Mixture?<\/em><\/a>&#8220;.\u00a0<\/span><\/p>\n<hr \/>\n<h2><strong><a id=\"notes\"><\/a>Notes<\/strong><\/h2>\n<div class=\"related-articles\"><p><strong>Related Articles<\/strong><\/p><ul><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2018\/09\/10\/types-of-isomers\/\" class=\"\"><span>Types of Isomers: Constitutional Isomers, Stereoisomers, Enantiomers, and Diastereomers<\/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\/2013\/01\/30\/addition-reactions-stereochemistry\/\" class=\"\"><span>Stereoselectivity In Alkene Addition Reactions: Syn vs Anti Addition<\/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\/09\/27\/the-e2-mechanism\/\" class=\"\"><span>The E2 Mechanism<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2011\/07\/01\/reagent-friday-oso4-osmium-tetroxide\/\" class=\"\"><span>OsO4 (Osmium Tetroxide) for Dihydroxylation of Alkenes<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/02\/08\/markovnikovs-rule-1\/\" class=\"\"><span>Hydrohalogenation of Alkenes and Markovnikov\u2019s Rule<\/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><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/04\/02\/epoxidation-hydroxylation-cyclopropanation-alkene-mechanism\/\" class=\"\"><span>Alkene Addition Pattern #3: The \u201cConcerted\u201d Pathway<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2018\/02\/09\/endo-exo-diels-alder-telling-them-apart\/\" class=\"\"><span>Exo vs Endo Products In The Diels Alder: How To Tell Them Apart<\/span><\/a><\/li><\/ul><\/div>\n<p><strong><a id=\"noteone\"><\/a>Note 1<\/strong>. If C-H bonds are involved, statistics should be factored in when calculating selectivity.<\/p>\n<p>In the chapter on free radical reactions we covered the selectivity (or lack thereof) of free-radical chlorination reactions relative to bromination reactions. The chlorination of propane gives 55% 2-chloropropane and 45% 1-chloropropane. This is indeed not very selective, but when you consider the fact that there are 6 methyl hydrogens and 2 methylene hydrogens, a completely unselective reaction would give us a 25:75 ratio of 2-chloropropane to 1-chloropropane.\u00a0\u00a0<span style=\"color: #800080;\"><em>See article &#8211; <span style=\"color: #800080;\"><a style=\"color: #800080;\" href=\"https:\/\/www.masterorganicchemistry.com\/2013\/10\/31\/selectivity-in-free-radical-reactions-bromine-vs-chlorine\/\">Selectivity in Free Radical Reactions.\u00a0<\/a><\/span><\/em><\/span><\/p>\n<p><strong><a id=\"notetwo\"><\/a>Note 2<\/strong>.The most recent example of a Nobel Prize in Chemistry being awarded for a highly selective reaction is the <a href=\"https:\/\/www.nobelprize.org\/prizes\/chemistry\/2022\/summary\/\">2022 Prize<\/a> for &#8220;click chemistry&#8221;. One of the awardees (Bertozzi) developed a reaction that joins together two reaction partners even when they are swimming around together in a living cell.<\/p>\n<p><strong><a id=\"notethree\"><\/a>Note 3<\/strong>. Another example of regioisomer formation that we won&#8217;t discuss in detail here is when two or more of the same functional groups are present on a molecule and the reagent may react with either one.<\/p>\n<p>For example the molecule below contains two alkenes, and therefore two regioisomers may form. Which one reacts first?<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-36010\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2010\/07\/F1-another-example-of-regioselectivity-is-when-a-reagent-could-react-with-multiple-different-alkenes.gif\" alt=\"another example of regioselectivity is when a reagent could react with multiple different alkenes\" width=\"640\" height=\"435\" \/><\/a><\/p>\n<p>These types of questions usually have to be answered on a case-by-case basis since both steric and electronic factors can be at work. Organic chemistry is not math; we need to do experiments to really be sure!<\/p>\n<p><strong><a id=\"notefour\"><\/a>Note 4<\/strong>. . For the purposes of Org 1 \/ Org 2, we usually teach that alkene addition reactions with HX and the S<sub>N<\/sub>1 reaction are non-stereoselective and result in roughly equal mixtures of stereoisomers. In the real world,<a href=\"https:\/\/en.wikipedia.org\/wiki\/Intimate_ion_pair\"> <strong>intimate<\/strong> <strong>ion pairing<\/strong><\/a> effects can result in both addition reactions of H-X and S<sub>N<\/sub>1 reactions to be at least somewhat stereoselective.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-36006\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2010\/07\/F2-termolecular-addition-of-HBr-to-cycloalkenes-gives-anti-products-and-a-termolecular-transition-state.gif\" alt=\"termolecular-addition-of-HBr-to-cycloalkenes-gives-anti-products-and-a-termolecular-transition-state\" width=\"640\" height=\"325\" \/><\/a><\/p>\n<p><strong><a id=\"notefive\"><\/a>Note 5. <\/strong>The catalytic hydrogenation of alkenes with Pd\/C and H<sub>2<\/sub> is another example of a reaction that is generally stereoselective (for <em>syn<\/em> addition to alkenes) but in practice is not stereospecific. For more, see the footnotes in <span style=\"color: #993366;\"><a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2011\/11\/25\/hydrogenation-alkenes-palladium-on-carbon-pdc\/\"><em>Palladium on Carbon for Hydrogenation of Alkenes<\/em><\/a><\/span>.<\/p>\n<hr \/>\n<h2><strong><a id=\"quiz\"><\/a>Quiz Yourself!<\/strong><\/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\/3160-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\/3161-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\/3162-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\/3163-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\/3164-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><strong><a id=\"references\"><\/a>(Advanced) References and Further Reading<\/strong><\/h2>\n<ul>\n<li>Notes from Prof. Andrew Myers&#8217; course on the <a href=\"https:\/\/myers.faculty.chemistry.harvard.edu\/pages\/chem-115-handouts\">Sharpless Asymmetric Epoxidation.\u00a0<\/a><\/li>\n<li>IUPAC &#8211; <a href=\"https:\/\/goldbook.iupac.org\/terms\/view\/S05994\">Stereospecificity<\/a><\/li>\n<li>IUPAC &#8211; <a href=\"https:\/\/goldbook.iupac.org\/terms\/view\/S05991\">Stereoselectivity<\/a><\/li>\n<\/ul>\n<ol>\n<li><a href=\"https:\/\/www.nobelprize.org\/prizes\/chemistry\/2022\/summary\/\"><strong>The Nobel Prize in Chemistry 2022.<\/strong><\/a><\/li>\n<li><strong>Chemoselectivity: The Mother of Invention in Total Synthesis<\/strong><br \/>\nRyan A. Shenvi, Daniel P. O\u2019Malley, and Phil S. Baran<br \/>\n<em>Accounts of Chemical Research<\/em> <strong>2009<\/strong> 42 (4), 530-541<br \/>\n<strong>DOI<\/strong>:<a href=\"https:\/\/pubs.acs.org\/doi\/10.1021\/ar800182r\"> 10.1021\/ar800182r\u00a0<\/a><br \/>\nSome research stories from the Baran laboratory (Scripps) where chemoselective reactions have been achieved without the significant use of protecting groups.<\/li>\n<li><strong>Click Chemistry: Diverse Chemical Function from a Few Good Reactions<\/strong><br \/>\nHartmuth C. Kolb,\u00a0 M. G. Finn., K. Barry Sharpless<br \/>\n<em>Angew. Chem. Int. Ed. <\/em><strong>2001\u00a0<\/strong><em>40 (11)<\/em>, 2004-2021.<br \/>\n<strong>DOI: <a href=\"https:\/\/doi.org\/10.1002\/1521-3773(20010601)40:11%3C2004::AID-ANIE2004%3E3.0.CO;2-5\">10.1002\/1521-3773(20010601)40:11%3C2004::AID-ANIE2004%3E3.0.CO;2-5<\/a>\u00a0<\/strong><br \/>\nThe article on selective reactions that led to the 2022 Nobel Prize.<\/li>\n<\/ol>\n<p>&nbsp;<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Stereoselective and Stereospecific Reactions An ideal chemical reaction would be selective for one product only. This would mean no time spent purifying our product and <\/p>\n","protected":false},"author":1,"featured_media":36004,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1418],"tags":[554,14889,14890,431,360,375,758,757,756,273,483,274],"post_folder":[],"class_list":["post-574","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-alkene-reactions","tag-anti-markovnikoff","tag-chemoselective","tag-enantioselective","tag-isomers","tag-markovnikoff","tag-regiochemistry","tag-regioisomers","tag-regioselective","tag-specific","tag-stereochemistry","tag-stereoisomers","tag-stereoselective"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.7 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Stereoselective and Stereospecific Reactions &#8211; Master Organic Chemistry<\/title>\n<meta name=\"description\" content=\"A stereoselective reaction is one that forms one stereoisomer selectively over another. 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