{"id":11950,"date":"2019-03-08T17:05:47","date_gmt":"2019-03-08T23:05:47","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=11950"},"modified":"2026-04-18T05:03:33","modified_gmt":"2026-04-18T10:03:33","slug":"enantiomers-diastereomers-or-the-same-1-using-models","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2019\/03\/08\/enantiomers-diastereomers-or-the-same-1-using-models\/","title":{"rendered":"Enantiomers vs Diastereomers vs The Same? Two Methods For Solving Problems"},"content":{"rendered":"<p><strong>Are These Molecules Enantiomers, Diastereomers or The Same?\u00a0<\/strong><\/p>\n<ul>\n<li>A classic exam question is to determine whether two molecules are enantiomers, diastereomers or the same.<\/li>\n<li>This is important not only in the chapter on stereochemistry, but will also become important once you learn the reactions of alkenes.<\/li>\n<li>Recall that <strong>isomers<\/strong> are two molecules that have the same\u00a0<strong>molecular formula.\u00a0<\/strong>\u00a0<strong>Constitutional isomers\u00a0<\/strong>have the same molecular formula, but different connectivity.\u00a0<strong>Stereoisomers\u00a0<\/strong>have the same connectivity, but different\u00a0<strong>arrangement in space<\/strong>.<\/li>\n<li><strong>Enantiomers\u00a0<\/strong>are stereoisomers that are non-superimposable mirror images. <strong>Diastereomers<\/strong> are stereoisomers that are not non-superimposable mirror images. Also, in organic chemistry,\u00a0two molecules that are <strong>superimposable <\/strong>upon each other are considered to be <strong>identical<\/strong>.<\/li>\n<li>One common method of determining whether molecules are\u00a0&#8220;enantiomers, diastereomers, or the same&#8221; is to build the two molecules using a model kit and compare them. This can be helpful in the beginning, especially if the molecules are drawn in different representations (e.g. Fischer vs. Newman)<\/li>\n<li>In the longer term, it can be\u00a0<strong>extremely helpful<\/strong> to get good at identifying the chiral centers in each molecule and determining their <strong>absolute configuration\u00a0<\/strong>(i.e.\u00a0<em>R<\/em> or\u00a0<em>S<\/em>).<\/li>\n<li>This takes slightly longer to learn in the short term but pays\u00a0<strong>big benefits<\/strong> iin the longer term, because once you know the designation of each chiral center, it&#8217;s easy to determine their relationships.<\/li>\n<li><strong>Enantiomers\u00a0<\/strong>always have the\u00a0<strong>same connectivity<\/strong> but\u00a0<strong>opposite\u00a0<\/strong><em>R, S\u00a0<\/em>designations (e.g. the enantiomer of (<em>2<span style=\"color: #0000ff;\">R<\/span>, 3<span style=\"color: #0000ff;\">R<\/span><\/em>)-tartaric acid is (<em>2<span style=\"color: #ff0000;\">S<\/span>, 3<span style=\"color: #ff0000;\">S<\/span><\/em>)-tartaric acid.<\/li>\n<li><strong>Diastereomers\u00a0<\/strong>also have the same connectivity but share the same configuration on at\u00a0<strong>at least one<\/strong> but not all chiral centers (e.g. a diastereomer of (<em>2<span style=\"color: #0000ff;\">R<\/span>, 3<span style=\"color: #0000ff;\">R<\/span><\/em>) tartaric acid is (<em>2<span style=\"color: #0000ff;\">R<\/span>, 3<span style=\"color: #ff0000;\">S<\/span><\/em>) tartaric acid.<\/li>\n<li>If the connectivity of the two molecules is the same, and the configurations of all the chiral centers are the same, the two molecules are <strong>identical<\/strong> .<\/li>\n<\/ul>\n<p><img fetchpriority=\"high\" decoding=\"async\" class=\"alignnone wp-image-36064\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/03\/0-summary-enantiomers-diastereomers-or-the-same-two-methods-for-solving-problems-drake.gif\" alt=\"summary enantiomers diastereomers or the same two methods for solving problems-drake\" width=\"800\" height=\"641\" \/><\/a><\/p>\n<p><strong>Table of Contents<\/strong><\/p>\n<ol>\n<li style=\"text-align: left;\"><a href=\"#section1\">The key distinctions:\u00a0 isomers vs. non-isomers, constitutional isomers vs. stereoisomers, and enantiomers vs. diastereomers.\u00a0<\/a><\/li>\n<li><a href=\"#section2\">How to distinguish isomers from non-isomers<\/a><\/li>\n<li><a href=\"#section3\">How to distinguish constitutional isomers from stereoisomers<\/a><\/li>\n<li><a href=\"#section4\">Enantiomers, Diastereomers or The Same? Ground rules &amp; easy examples<\/a><\/li>\n<li><a href=\"#section5\">Examples with multiple chiral centers: Technique #1 &#8211; <strong>Using a model kit<\/strong><\/a><\/li>\n<li><a href=\"#section6\">Examples with multiple chiral centers: Technique #2 &#8211; <strong>The &#8220;R\/S&#8221; Method<\/strong><\/a><\/li>\n<li><a href=\"#seven\">Summary: Enantiomers vs Diastereomers vs The Same<\/a><\/li>\n<li><a href=\"#notes\">Notes<\/a><\/li>\n<li><a href=\"#quizzes\">Quiz Yourself!<\/a><\/li>\n<\/ol>\n<hr \/>\n<h2><a id=\"section1\"><\/a>1. The Three Key Distinctions Between Isomers<\/h2>\n<p><em><strong>[Note: This\u00a0post was co-authored with Matthew Pierce of\u00a0<a href=\"http:\/\/organicchemistrysolutions.com\">Organic Chemistry Solutions<\/a>.\u00a0 Ask Matt about scheduling an online tutoring session\u00a0<a href=\"https:\/\/masterorganic.wufoo.com\/forms\/q1yg3qx8076h7gx\/\">here<\/a>. ]<\/strong><\/em><\/p>\n<p>Last post we described the most important classes of isomers. <em>See post: <a href=\"https:\/\/www.masterorganicchemistry.com\/2018\/09\/10\/classification-of-isomers\/\">Classification of Isomers<\/a><\/em><\/p>\n<p>The three important distinctions are:<\/p>\n<ul>\n<li><strong>isomers<\/strong><em> (same molecular formula)\u00a0<\/em>versus <strong>non-isomers<\/strong> <em>(different molecular formula)<\/em><\/li>\n<li><strong>constitutional isomers<\/strong> <em>(different connectivity)<\/em> versus <strong>stereoisomers<\/strong> <em>(different arrangement in space)<\/em><\/li>\n<li><strong>enantiomers<\/strong> <em>(stereoisomers that are non-superimposable mirror images)<\/em> versus <strong>diastereomers<\/strong> (<em>stereoisomers that are <span style=\"text-decoration: underline;\">not<\/span> non-superimposable mirror images<\/em>)<\/li>\n<\/ul>\n<p>Among these distinctions, it&#8217;s the third (enantiomers versus diastereomers) that gives students the most headaches, and we&#8217;re going to focus on it nearly exclusively.<\/p>\n<p>There&#8217;s also an important &#8220;non-distinction&#8221;:<\/p>\n<blockquote><p><strong>two m<\/strong><strong>olecules that are superimposable on each other, through rotation of bonds or of the whole molecule, are considered to be &#8220;identical molecules&#8221;.<\/strong><\/p><\/blockquote>\n<p>So in practice, questions that might otherwise be asking if two molecules are &#8220;enantiomers\u00a0or<em>\u00a0<\/em>diastereomers&#8221; often becomes, are these two molecules &#8220;enantiomers, diastereomers, or the same&#8221; ?<\/p>\n<p>But before we get there, let&#8217;s quickly discuss the first two distinctions.<\/p>\n<h2><strong><a id=\"section2\"><\/a>2. Isomers or Non-Isomers?\u00a0<\/strong><\/h2>\n<p>In theory, identifying isomers is simple. It&#8217;s like being able to tell that the words &#8220;SLIME&#8221; , &#8220;MILES&#8221;,\u00a0 &#8220;SMILE&#8221; are made up of the exact same letters but differ in their arrangement.<\/p>\n<p>What makes it trickier in organic chemistry is that you need to be able to interpret line diagrams, and interpret them quickly.<\/p>\n<p>Here&#8217;s an exercise: find the three pairs of constitutional isomers below (and one decoy). Shouldn&#8217;t take more than a minute.<\/p>\n<p><img decoding=\"async\" class=\"aligncenter wp-image-14702\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/1-collection-of-molecules-containing-three-pairs-of-constitutional-isomers-and-one-decoy.gif\" alt=\"collection-of-molecules-containing-three-pairs-of-constitutional-isomers-and-one-decoy\" width=\"545\" height=\"98\" \/><\/p>\n<p><span style=\"color: #993366;\"><em>If this is tough for you, I don&#8217;t have any short cuts, but I do have a prescription. Hunt down some exercises that will teach you these skills: look for a chapter in a textbook entitled &#8220;alkanes&#8221; , usually chapter 3 or so, and start working the exercises.<\/em><\/span><\/p>\n<p>Going into an exam without being able to quickly and accurately interpret line diagrams will be fatal to your grade.<\/p>\n<h2><strong><a id=\"section3\"><\/a>3. Constitutional Isomers or Stereoisomers?<\/strong><\/h2>\n<p>If two molecules have the<strong> same molecular formula<\/strong>, the next question is whether or not they have the <strong>same <em>connectivity<\/em><\/strong> or not.<\/p>\n<ul>\n<li><strong>Constitutional isomers<\/strong> have the <strong>same<\/strong> molecular formula and <strong>different<\/strong> connectivity<\/li>\n<li><strong>Stereoisomers<\/strong> have the <strong>same<\/strong> molecular formula but the <strong>same<\/strong> connectivity<\/li>\n<\/ul>\n<p>How can we tell if molecules have the same connectivity?<\/p>\n<p>We said in the last post that <strong>molecules with the same connectivity will have the same IUPAC names<\/strong> (not counting the stereochemical descriptors like <em>R<\/em>\/<em>S,\u00a0<\/em> <em>cis<\/em>\/<em>trans<\/em>\u00a0, or <em>E\/Z<\/em>).<\/p>\n<p><strong>So if you can quickly tell if two given molecules have the same IUPAC name<\/strong>\u00a0(or not), you&#8217;re all set for recognizing if they&#8217;re constitutional isomers.<\/p>\n<p>Darnit. Does this mean that you have to do a full IUPAC name before you can answer an exam question?<\/p>\n<p><strong>Usually not.<\/strong> If naming molecules takes a long time for you, take heart.\u00a0 A few questions will usually get you at least 80% of the way:<\/p>\n<ul>\n<li>are the same functional groups present?<\/li>\n<li>is the length of the main chain the same in both molecules?<\/li>\n<li>what substituents are present in both molecules? are they identical?<\/li>\n<li>are the substituents located in the same place relative to the main chain?<\/li>\n<\/ul>\n<p>If they are still identical at this point, then do a nitpicky double-check (with a pen or pencil, ideally) to make sure the connectivity is identical. <strong>If<\/strong> it is, and<strong> if <\/strong>the molecules only differ in their <em>R\/S<\/em>, <em>cis\/trans<\/em>, or <em>E\/Z<\/em> designations,<strong> then<\/strong> they are stereoisomers.<\/p>\n<p>Of course, if they have identical connectivity, <strong>and<\/strong> all\u00a0 <em>R\/S,<\/em> <em>cis\/trans<\/em> and <em>E\/Z<\/em> designations are identical, you&#8217;re dealing with the <strong>same molecule<\/strong>!<\/p>\n<p>Here&#8217;s a few (simple) examples:<\/p>\n<p><img decoding=\"async\" class=\"aligncenter wp-image-14703\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/2-four-pairs-of-molecules-constitutional-isomers-or-stereoisomers.gif\" alt=\"four-pairs-of-molecules-constitutional-isomers-or-stereoisomers\" width=\"615\" height=\"285\" \/><\/p>\n<p>If you found this easy, don&#8217;t pat yourself on the back just yet.\u00a0 When drawn in line\/wedge form, this question is a little too easy to be exam material.\u00a0 Expect to see some curveballs.<\/p>\n<p>The classic way to make this type of question more challenging is to change the type of depiction, using Fischer, Newman, or a mixture of multiple projections:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-14704\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/3-quiz-constitutional-isomers-or-stereoisomers-drawn-in-newman-projection-fischer.gif\" alt=\"quiz-constitutional-isomers-or-stereoisomers-drawn-in-newman-projection-fischer\" width=\"615\" height=\"298\" \/><\/p>\n<p>When preparing for an exam on stereochemistry, I advise trying to do pretty much every &#8220;classify these isomers&#8221; question you can until it gets boring.<\/p>\n<p><span style=\"color: #993366;\"><em>The <a href=\"https:\/\/www.masterorganicchemistry.com\/organic-chemistry-practice-problems\/stereochemistry-practice-problems-and-quizzes\/\">Stereochemistry Practice Quizzes<\/a> have a lot of these kinds of problems (must be an MOC Member for full access)<\/em><\/span>. The point is to get used to solving problems for molecules in different renderings.<\/p>\n<h2><strong><a id=\"section4\"><\/a>4. &#8220;Enantiomers vs Diastereomers vs the Same?&#8221; Some Ground Rules<\/strong><\/h2>\n<p>Now we get to the third, and most challenging case, where we will spend the bulk of our time.\u00a0 &#8220;Are these molecules enantiomers, diastereomers, or the same?&#8221;.<\/p>\n<p>Let&#8217;s cross off <strong>three<\/strong><strong>\u00a0quick wins<\/strong> to start:<\/p>\n<ul>\n<li>\n<h3><strong>First: Superimposable Molecules Are Identical<br \/>\n<\/strong><\/h3>\n<p>Molecules that are superimposable are considered to be <strong>identical molecules\u00a0<\/strong>(i.e. &#8220;the same&#8221;).<\/p>\n<p>Note that &#8220;superimposable&#8221;\u00a0 <em>includes<\/em>\u00a0molecules that are<br \/>\n1. &#8220;superimposable through bond-rotations&#8221; (conformational isomers),\u00a0<strong>as well as<\/strong><\/p>\n<p>2.molecules that are superimposable through rotation of the whole molecule.<\/li>\n<\/ul>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-14705\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/4-examples-of-superimposable-molecules-drawn-differently-conformations.gif\" alt=\"examples-of-superimposable-molecules-drawn-differently-conformations\" width=\"615\" height=\"247\" \/><\/p>\n<blockquote><p>It also includes some molecules that are\u00a0<em>drawn<\/em> as having stereochemistry&#8230; but don&#8217;t.\u00a0<strong><em>Look closely\u00a0\u2193<\/em><\/strong><\/p><\/blockquote>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-14706\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/5-molecules-that-are-identical-except-trick-question-flip-achiral-center-from-wedge-to-dash.gif\" alt=\"molecules-that-are-identical-except-trick-question-flip-achiral-center-from-wedge-to-dash\" width=\"615\" height=\"157\" \/><\/p>\n<blockquote>\n<h3><strong>Second: Recall The Key Differences Between Enantiomers and Diastereomers:<\/strong><\/h3>\n<p>Stereoisomers\u00a0<em>always<\/em> have the same connectivity. Among molecules with the same connectivity:<\/p><\/blockquote>\n<ul>\n<li>Molecules that are mirror images but\u00a0<em>non-<\/em>superimposable are <strong>enantiomers<\/strong>.<\/li>\n<li>If they aren&#8217;t superimposable, and they aren&#8217;t mirror images, then they&#8217;re <strong>diastereomers<\/strong>.<\/li>\n<\/ul>\n<blockquote>\n<h3><strong>Third: <em>cis-trans<\/em> isomers and double-bond isomers are always diastereomers<\/strong><\/h3>\n<p>One quick question you can always answer:<strong> double-bond isomers or <em>cis-trans<\/em> isomers will always be diastereomers. <\/strong><\/p>\n<p><em>No exceptions.<\/em><\/p><\/blockquote>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-14707\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/6-double-bond-isomers-and-cis-trans-geometric-isomers-are-always-diastereomers.gif\" alt=\"summary enantiomers diastereomers or the same two methods for solving problems 2\" width=\"545\" height=\"277\" \/><\/p>\n<p>Now for the not-so-easy cases.<\/p>\n<h2><strong><a id=\"section5\"><\/a>5. &#8220;Enantiomers vs Diastereomers vs The Same&#8221; On Molecules With Multiple Chiral Centers: Using A Model Kit<\/strong><\/h2>\n<p>What about molecules with two or more chiral centers? Like these two examples?<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-14708\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/7-are-these-molecules-enantiomers-diastereomers-or-the-same-fischer-and-newman-projections.gif\" alt=\"are-these-molecules-enantiomers-diastereomers-or-the-same-fischer-and-newman-projections\" width=\"615\" height=\"195\" \/><\/p>\n<p>Here, I have good news and bad news.<\/p>\n<p>The bad news is that there is no feature on any of these molecules (like different arrangement on a double bond or ring) that allows for quick and easy determination of the relationship in &lt; 5 seconds.<\/p>\n<p>Therefore, the only way to solve these problems is to compare the configurations on each of the chiral centers and see how they relate to each other.<\/p>\n<p>The good news is that these problems can be readily solved using one of two key techniques.<\/p>\n<ul>\n<li>One method is easy to learn but tends to require a long time to solve problems.<\/li>\n<li>The second method takes some time to get good at, but allows problems to be solved extremely quickly.<\/li>\n<\/ul>\n<h2><strong>Technique #1 :\u00a0 Use A Model Kit<\/strong><\/h2>\n<p>The traditional method to solve &#8220;enantiomers, diastereomers, or the same&#8221; is to build models, and see if the two molecules are superimposable or not.<\/p>\n<p><strong>By all means, start with this!\u00a0<\/strong>Especially in a non-exam situation.<\/p>\n<p>I recommend using this method the first few times you do the exercise <strong>because you have to start somewhere<\/strong>, and the model kit will serve as a set of training wheels to help you build confidence.<\/p>\n<p>Models also help you get used to the fact that <strong>molecules are 3-dimensional objects.\u00a0<\/strong>In this respect they are fundamentally\u00a0no different from cars, cats, or any other commonplace object you care to name.<\/p>\n<p>So let&#8217;s go to our &#8220;enantiomers, diastereomers, or the same&#8221; question, and try to solve each of them using models.<\/p>\n<p><strong>The first set: 2,3-dibromobutanes.\u00a0<\/strong><\/p>\n<p>Let&#8217;s build the model of the 2,3-dibromobutane isomer using the Fischer projection. (Remember that &#8220;the arms come out to hug you&#8221;). That gives us the following model:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-14709\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/8-example-of-building-a-model-for-fischer-projection-determining-r-and-s.gif\" alt=\"example-of-building-a-model-for-fischer-projection-determining-r-and-s\" width=\"545\" height=\"255\" \/><\/p>\n<p>Next, we build the second model, based on the Newman projection. Here&#8217;s what the model would look like:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-14710\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/9-example-of-building-a-model-of-newman-projection-for-r-and-s.gif\" alt=\"example-of-building-a-model-of-newman-projection-for-r-and-s\" width=\"545\" height=\"241\" \/><\/p>\n<p>Finally, once we&#8217;ve built these two models, we can then try and move them around to see how they relate to each other.<\/p>\n<p>Here, we take the model on the right hand side, and see that if we perform a rotation about the C2-C3 bond, the two molecules are actually completely<strong> superimposable!\u00a0<\/strong><\/p>\n<p><iframe class=\"giphy-embed\" src=\"https:\/\/giphy.com\/embed\/KyCMti3JqbWVkjDdFR\" width=\"480\" height=\"270\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><\/p>\n<p><a href=\"https:\/\/giphy.com\/gifs\/newman-projection-master-organic-chemistry-fischer-KyCMti3JqbWVkjDdFR\">via GIPHY<\/a><\/p>\n<p><strong>Being superimposable, they are therefore are the same!<\/strong><\/p>\n<p>Let&#8217;s look at the second set.<\/p>\n<p><strong>The second set: pentan-2,3,4-triol<\/strong><\/p>\n<p>When we build the model for the first molecule of the set, we get the following:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-14711\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/11-enantiomer-diastereomer-build-model-line-diagram.gif\" alt=\"enantiomer-diastereomer-build-model-line-diagram\" width=\"545\" height=\"221\" \/><\/p>\n<p>When we build the model for the second molecule of the set as a Fischer projection (remembering again that the &#8220;arms come out to hug you&#8221;) we get this:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-14712\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/12-enantiomer-diastereomer-build-model-fischer-projection.gif\" alt=\"enantiomer-diastereomer-build-model-fischer-projection\" width=\"545\" height=\"236\" \/><\/p>\n<p>Now, let&#8217;s try to move around the molecule on the right so that we can compare it to the one on the left.<\/p>\n<p><iframe class=\"giphy-embed\" src=\"https:\/\/giphy.com\/embed\/82ukpMLKDxA1p1CUvf\" width=\"480\" height=\"300\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><\/p>\n<p><a href=\"https:\/\/giphy.com\/gifs\/ochem-bondrotations-masterorganicchemistry-82ukpMLKDxA1p1CUvf\">via GIPHY<\/a><\/p>\n<p>Again, these two models turn out to be<strong> the same<\/strong>!<\/p>\n<h2><strong><a id=\"section6\"><\/a>6. An Easier, Faster Way (IMO) : The R\/S Method<\/strong><\/h2>\n<p>While there&#8217;s nothing <em>wrong<\/em> with making models, there is a significant drawback.<\/p>\n<p>It takes time to build the darn things, and then, it takes even more time to compare the two models. While this might be OK for molecules with a stereocenter or two, model-building becomes a huge bottleneck as molecular complexity increases.<\/p>\n<p>For instance, say you were asked to compare the following two molecules:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-14713\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/14-using-the-r-s-method-to-answer-relationship-ribose-fischer-versus-line-diagram.gif\" alt=\"using-the-r-s-method-to-answer-relationship-ribose-fischer-versus-line-diagram\" width=\"615\" height=\"197\" \/><\/p>\n<p>Are you <em>really<\/em> going to take ~ 5 minutes to make the models, rotate each of them around, and then compare them? Probably not.<\/p>\n<p>Thankfully there is an easier way. <strong>And when you get good, you can answer these questions in less than 60 seconds.\u00a0<\/strong><\/p>\n<h3><strong>Step 1: Start by identifying all of the chiral centers in each molecule, and determining each of their configurations as\u00a0<em>R<\/em> or\u00a0<em>S<\/em><\/strong>.<\/h3>\n<p>Why is this helpful?<\/p>\n<p>Take the question above. Say that you were given the<strong> full names<\/strong> of the two molecules, without their structures.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-14714\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/15-r-s-method-for-enantiomer-diastereomer-same-use-the-name.gif\" alt=\"r-s-method-for-enantiomer-diastereomer-same-use-the-name\" width=\"615\" height=\"319\" \/><\/p>\n<p>Could you figure out how these molecules are related based on the names alone?<\/p>\n<p>Of course!<\/p>\n<p>Because once you know\u00a0the (<em>R,S)\u00a0<\/em>configurations of a molecule, <strong>you can easily figure out what the configurations of its enantiomer and its diastereomers should be!<\/strong><\/p>\n<h3><strong>Step 2: Compare The Configurations of The Chiral Centers To Obtain Their Stereochemical Relationship<\/strong><\/h3>\n<p>For two molecules with the same connectivity:<\/p>\n<ul>\n<li><strong>The enantiomer of a molecule will always have an opposite\u00a0<em>R<\/em>\/<em>S<\/em> configuration<\/strong>. So to get the enantiomer of\u00a0(2<em>R<\/em>, 3<em>R<\/em>, 4<em>R<\/em>)-2,3,4,5-tetrahydroxypentanal\u00a0all we need to do is flip all the stereocenters: (2<em>S<\/em>, 3<em>S<\/em>, 4<em>S<\/em>)<\/li>\n<li><strong>Diastereomers<\/strong> arise when at least two molecules share at least\u00a0<strong>one<\/strong> (but not all) chiral center(s) with identical (R\/S) configuration. So to find the diastereomers of\u00a0(2<em>R<\/em>,3<em>R<\/em>,4<em>R<\/em>)-2,3,4,5-tetrahydroxypentanal\u00a0, all we need to do is keep at least\u00a0<strong>one<\/strong> stereocenter the same, and flip any or all of the rest.<\/li>\n<li><span style=\"color: #993366;\"><em>(obviously, if the two molecules have the same connectivity with all the R\/S designations are the same, the two molecules are also the same)<\/em><\/span><\/li>\n<\/ul>\n<p>Our molecule (2<em>R<\/em>, 3<em>R<\/em>, 4<em>R<\/em>)-2,3,4,5-tetrahydroxypentanal goes by a more pronounceable name: D-ribose.<\/p>\n<ul>\n<li>The enantiomer is (2<em>S<\/em>, 3<em>S<\/em>, 4<em>S)<\/em><\/li>\n<li>There are six possible diastereomers (see below) which possess at least one (but not all) identical configuration(s) at their chiral centers.<\/li>\n<\/ul>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-14715\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/16-determining-relationships-from-r-s-based-on-name-alone-for-ribose.gif\" alt=\"determining-relationships-from-r-s-based-on-name-alone-for-ribose\" width=\"545\" height=\"454\" \/><\/p>\n<p>The other bonus of this method is that since it doesn&#8217;t matter what conformation (or what projection) a molecule is drawn in. Since all you&#8217;re really doing is comparing 1) connectivity and 2) configuration of stereocenters, you don&#8217;t need to worry about figuring out if two models are non-superimposable mirror images.<\/p>\n<p>Let&#8217;s do a few more examples just based on the name alone.<\/p>\n<ul>\n<li>(<em>S<\/em>)-2-butanol and (<em>R<\/em>)-2-butanol.\u00a0<strong>enantiomers<\/strong>, since they have opposite\u00a0<em>R,S<\/em>.<\/li>\n<li>(<em>S,R)-<\/em>cyclopentane-1,2-diol and (<em>R,R<\/em>)-cyclopentane-1,2-diol.\u00a0<strong>diastereomers\u00a0<\/strong>since they share the same configuration of one of the stereocenters<\/li>\n<li>To be really ambitious, let&#8217;s do:\u00a0methyl <strong>(1R,2R,3S,5S<\/strong>)-3- (benzoyloxy)-8-methyl-8-azabicyclo[3.2.1] octane-2-carboxylate and\u00a0methyl <strong>(1S,2S,3R,5R<\/strong>)-3- (benzoyloxy)-8-methyl-8-azabicyclo[3.2.1] octane-2-carboxylate.<br \/>\nSame IUPAC descriptor, opposite (R,S)-descriptors:\u00a0<strong>enantiomers.\u00a0<\/strong><\/li>\n<\/ul>\n<h3><strong>Step 3: Double Check That They&#8217;re Not Meso<\/strong><\/h3>\n<p>What about (<em>S<\/em>,<em>R<\/em>)-2,3-dibromobutane and (<em>R,S<\/em>)-2,3-dibromobutane? <em>Opposite R\/S, therefore<\/em>\u00a0<em>e<\/em><em>nan<\/em>&#8230;\u00a0oh, snap<strong>.\u00a0They&#8217;re actually the same.\u00a0<\/strong><\/p>\n<p>We can&#8217;t forget to mention the one little fly that sometimes lands in the ointment: <strong>meso compounds<\/strong>.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-14716\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/17-r-s-method-for-enantiomer-diastereomer-same-double-check-compounds-are-not-meso.gif\" alt=\"r-s-method-for-enantiomer-diastereomer-same-double-check-compounds-are-not-meso\" width=\"615\" height=\"536\" \/><\/p>\n<p>If a molecule with chiral centers has a plane of symmetry, then it can be written two equivalent ways that have the <em>appearance<\/em> of being enantiomers, but are in fact <strong>the same<\/strong>:<\/p>\n<ul>\n<li>(<em>S<\/em>,<em>R<\/em>)-2,3-dibromobutane and (<em>R,S<\/em>)-2,3-dibromobutane<\/li>\n<li><span style=\"text-decoration: underline;\"><em>(<\/em><\/span><em>R,S<\/em>)-tartaric acid and (<em>S,R<\/em>)-tartaric acid<\/li>\n<li>(<em>R,S<\/em>)-cyclohexane-1,2-diol and (<em>S,R<\/em>)-cyclohexane-1,2-diol<\/li>\n<\/ul>\n<p>So before declaring that two molecules with opposite (R,S) designations are enantiomers, <strong>double check that the molecule doesn&#8217;t have a plane of symmetry.<\/strong><\/p>\n<p>One way to do this is to <strong>try to name the molecule in two directions.<\/strong> If you obtain the same IUPAC name going left-to-right as you do going right-to-left (or clockwise \/ counterclockwise in the case of cyclic molecules) then you are looking at a meso compound.<\/p>\n<h2><a id=\"seven\"><\/a>7. Summary: Enantiomers vs Diastereomers vs The Same<\/h2>\n<p>The drawback of this final method is that it takes <em>lots<\/em> of practice to get good at quickly assigning\u00a0<em>R<\/em> and\u00a0<em>S.<\/em><\/p>\n<p>To that end,<strong> I recommend doing lots of exercises<\/strong> where you work on molecules drawn as line diagrams, Newman projections, Fischer projections, Sawhorse projections, and so on.<\/p>\n<p>That&#8217;s probably enough for now. In the next post I&#8217;ll provide several examples and applications of using the R\/S method to determine if molecules are enantiomers, diastereomers, or the same.<\/p>\n<p><strong>Next pos<\/strong>t: <a href=\"https:\/\/www.masterorganicchemistry.com\/2019\/05\/21\/how-to-determine-r-and-s-configurations-on-a-fischer-projection\/\">Determining R\/S Configuration On A Fischer Projection<\/a><\/p>\n<p>In the meantime, I recommend the following videos, since they apply these principles:<\/p>\n<p><a href=\"https:\/\/www.youtube.com\/watch?v=dpgq-cvZiAo&amp;list=PLXJNb-oxhvtqzPk4Fec2ey1s_OrziN2wS\"><strong>5-Video Playlist of &#8220;Enantiomers, Diastereomers, Or The Same&#8221; Problems<\/strong><\/a><\/p>\n<p><em><strong>[Note: Thanks again to Matthew Pierce of\u00a0<a href=\"http:\/\/organicchemistrysolutions.com\">Organic Chemistry Solutions\u00a0<\/a>\u00a0for co-authoring.\u00a0 Ask Matt about scheduling an online tutoring session\u00a0<a href=\"https:\/\/masterorganic.wufoo.com\/forms\/q1yg3qx8076h7gx\/\">here<\/a>. ]<\/strong><\/em><\/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\/2016\/10\/20\/introduction-to-assigning-r-and-s-the-cahn-ingold-prelog-rules\/\" class=\"\"><span>Introduction to Assigning (R) and (S): The Cahn-Ingold-Prelog Rules<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2017\/01\/17\/determining-rs-2-the-method-of-dots\/\" class=\"\"><span>Assigning Cahn-Ingold-Prelog (CIP) Priorities (2) \u2013 The Method of Dots<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2019\/05\/21\/how-to-determine-r-and-s-configurations-on-a-fischer-projection\/\" class=\"\"><span>How To Determine R and S Configurations On A Fischer Projection<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2017\/02\/01\/assigning-rs-to-newman-projections-and-converting-newman-to-line-diagrams\/\" class=\"\"><span>Assigning R\/S To Newman Projections (And Converting Newman To Line Diagrams)<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2011\/01\/24\/the-single-swap-rule\/\" class=\"\"><span>The Single Swap Rule<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2011\/01\/12\/the-meso-trap\/\" class=\"\"><span>The Meso Trap<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2012\/05\/23\/whats-a-racemic-mixture\/\" class=\"\"><span>What\u2019s a Racemic Mixture?<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2023\/11\/06\/bond-rotation-how-to-draw\/\" class=\"\"><span>How To Draw A Bond Rotation<\/span><\/a><\/li><\/ul><\/div>\n<p>Thanks to Chl\u00f6e and the staff at IndustriousHQ for lending their steady hands towards filming the GIFs used in this post.<\/p>\n<h2><strong><a id=\"quizzes\"><\/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\/0859-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\/0852-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\/0855-Front-Image-Only.png\" alt=\"\" width=\"640\" height=\"616\" \/><\/p>\n<p><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a\u00a0 MOC member<\/strong><\/a> to see the clickable quiz with answers on the back. <\/p>\n<p>Many more &#8220;enantiomers, diastereomers, or the same&#8221; questions on the <a href=\"https:\/\/www.masterorganicchemistry.com\/organic-chemistry-practice-problems\/stereochemistry-practice-problems-and-quizzes\/\">Stereochemistry Practice Quizzes<\/a> page (MOC Membership required for full access).<\/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\/0856-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\/2613-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","protected":false},"excerpt":{"rendered":"<p>Are These Molecules Enantiomers, Diastereomers or The Same?\u00a0 A classic exam question is to determine whether two molecules are enantiomers, diastereomers or the same. This <\/p>\n","protected":false},"author":1,"featured_media":36056,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1413],"tags":[879,1154,663,652,197,202,1404,483],"post_folder":[],"class_list":["post-11950","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-stereochemistry-chirality","tag-cahn-ingold-prelog","tag-cip","tag-configuration","tag-constitutional-isomers","tag-diastereomers","tag-enantiomers","tag-meso-compounds","tag-stereoisomers"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.7 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Enantiomers vs Diastereomers vs The Same? 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