{"id":8674,"date":"2015-01-13T16:16:37","date_gmt":"2015-01-13T21:16:37","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=8674"},"modified":"2026-05-06T19:40:57","modified_gmt":"2026-05-07T00:40:57","slug":"chiral-allenes-and-chiral-axes","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2015\/01\/13\/chiral-allenes-and-chiral-axes\/","title":{"rendered":"Chiral Allenes And Chiral Axes"},"content":{"rendered":"<p><strong>All About Allenes, Chiral and Otherwise<\/strong><\/p>\n<ul>\n<li>Allenes are examples of\u00a0<strong>cumulated dienes<\/strong>, molecules with consecutive (not conjugated) double bonds.<\/li>\n<li>Allenes are not flat. Due to the fact that the two pi-bonds of allene are at right angles to each other, the end substituents are also turned 90\u00b0 as well.<\/li>\n<li>Allene itself has two mirror planes and is <strong>achiral<\/strong>.<\/li>\n<li>However, if <strong>both ends<\/strong> of the allene are each attached to <strong>two different substituents<\/strong>, the resulting molecule lacks a plane of symmetry and is <strong>chiral<\/strong>.<\/li>\n<li>This type of chirality is referred to as &#8220;<strong>axial chirality<\/strong>&#8221; because it resembles the handedness of screws.<\/li>\n<\/ul>\n<p><img fetchpriority=\"high\" decoding=\"async\" class=\"alignnone wp-image-37423\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2015\/01\/0-summary-of-axial-chirality-and-chiral-allenes.gif\" alt=\"summary of axial chirality and chiral allenes\" width=\"640\" height=\"497\" \/><\/a><\/p>\n<p><strong>Table of Contents<\/strong><\/p>\n<ol>\n<li><a href=\"#one\">Allenes are &#8220;Cumulated&#8221; Dienes<\/a><\/li>\n<li><a href=\"#two\">The Central Carbon In Allene Is sp Hybridized<\/a><\/li>\n<li><a href=\"#three\">A 3-D &#8220;Fly-By&#8221; Of Allene Shows It Is Not Flat<\/a><\/li>\n<li><a href=\"#four\">Examples of Substituted Allenes With Mirror Planes<\/a><\/li>\n<li><a href=\"#five\">An Example of A Chiral, Di-Substituted Allene<\/a><\/li>\n<li><a href=\"#six\">Chiral Allenes Have an &#8220;Axis of Chirality&#8221;<\/a><\/li>\n<li><a href=\"#seven\">Summary: How Do I Recognize A Chiral Allene vs.\u00a0 An Achiral Allene?<\/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=\"one\"><\/a>1. Allenes are &#8220;Cumulated&#8221; Dienes<\/h2>\n<p>We&#8217;ve met alkenes before &#8211; right? Just in case you haven&#8217;t, let&#8217;s review the basics, because today&#8217;s post is going to depend on understanding a few of the most important concepts.<\/p>\n<p>Alkenes contain both <strong>sigma<\/strong> [ \u03c3] \u00a0bonding formed through\u00a0&#8220;head-on&#8221; orbital overlap [i.e. &#8220;single bonds&#8221;] and also <strong>pi<\/strong> [ \u03c0 ] bonding where orbitals have &#8220;side-on&#8221; overlap\u00a0[i.e. &#8220;multiple bonds&#8221;]<\/p>\n<p><img decoding=\"async\" class=\"aligncenter wp-image-14774\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/1-bonding-in-ethene-showing-pi-bonding-between-adjacent-p-orbitals.gif\" alt=\"bonding-in-ethene-showing-pi-bonding-between-adjacent-p-orbitals\" width=\"641\" height=\"199\" \/><\/p>\n<p>Alkenes like ethylene are <strong>flat<\/strong> [planar]. \u00a0The p orbitals are at 90 degrees to this planar structure.<\/p>\n<p><span style=\"color: #993366;\"><em>If you&#8217;ve ever used graphite lubricant, you might know it&#8217;s slippery because graphite is made up of layered sheets of flat, pi-bonded carbon, and the sheets have very little friction between each layer. <a style=\"color: #993366;\" href=\"http:\/\/en.wikipedia.org\/wiki\/Graphene\">Graphene<\/a> is the same idea, only it&#8217;s a single layer.<\/em><\/span><\/p>\n<p>Now let&#8217;s draw something weird. What do we get when we extend a second double bond directly adjacent to the first one?<\/p>\n<p><img decoding=\"async\" class=\"aligncenter wp-image-14775\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/2-allene-is-an-example-of-a-cumulated-diene-where-there-are-pi-bonds-on-adjacent-carbons-not-alternate-carbons.gif\" alt=\"allene-is-an-example-of-a-cumulated-diene-where-there-are-pi-bonds-on-adjacent-carbons-not-alternate-carbons\" width=\"638\" height=\"129\" \/><\/p>\n<p>Does this look strange to you? The first time you see this, you&#8217;ll probably think &#8211; <strong>yes!<\/strong> You might wonder &#8211; do these even exist? The answer is also &#8211; Yes! This moleule is called <strong>allene<\/strong>, and is part of a family of molecules called <strong>cumulenes<\/strong>, so named because the double bonds are cumulative (consecutive).<\/p>\n<h2><a id=\"two\"><\/a>2. The Central Carbon In Allene Is sp Hybridized<\/h2>\n<p>Now let&#8217;s ask: what might the<strong> bonding<\/strong> in this molecule look like? In other words, what does it look like in 3D?<\/p>\n<p>You might think &#8211; simple! It&#8217;s flat, \u00a0like an alkene!<\/p>\n<p>It&#8217;s actually <strong>not quite that simple!<\/strong> The key is that central carbon &#8211; it has TWO \u03c0\u00a0bonds.<\/p>\n<p>You might recall that acetylene has two \u00a0\u03c0 bonds as well, at 90 degrees to each other.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-14776\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/3-acetylene-pi-bonding-two-pi-bonds-at-90-degree-angles-sp-hybridization.gif\" alt=\"acetylene-pi-bonding-two-pi-bonds-at-90-degree-angles-sp-hybridization\" width=\"639\" height=\"135\" \/><\/p>\n<p><strong>The same is true for allene,<\/strong> except that it&#8217;s <strong>only<\/strong> the central carbon that is involved in two pi bonds.<\/p>\n<p>Here\u2019s what the orbitals of this molecule \u00a0look like.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-14777\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/4-allene-is-not-planar-the-central-carbon-is-sp-hybridized-and-pi-bonds-are-at-90-degrees-to-each-other.gif\" alt=\"allene-is-not-planar-the-central-carbon-is-sp-hybridized-and-pi-bonds-are-at-90-degrees-to-each-other\" width=\"641\" height=\"214\" \/><\/p>\n<p>The hybridization of that central carbon? It&#8217;s <strong>sp<\/strong>, just like the carbons in acetylene. The end carbons are<strong> sp<sup>2<\/sup><\/strong><\/p>\n<h2><a id=\"three\"><\/a>3. A 3-D &#8220;Fly-by&#8221; Shows That Allene Is Not Flat<\/h2>\n<p>The line diagram of allene really does not do it justice.<\/p>\n<p>So here I present a 3D &#8220;fly-by&#8221; of allene. Note how the two CH<sub>2<\/sub> groups are &#8220;offset&#8221; with respect to each other by 90 degrees.<\/p>\n<p><strong>Allene is not flat!\u00a0<\/strong>[those white balls &#8211; imagine they are hydrogen]<\/p>\n<p><iframe class=\"giphy-embed\" src=\"https:\/\/giphy.com\/embed\/26gstvBo8qCnKXWCY\" width=\"480\" height=\"480\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><\/p>\n<p><a href=\"https:\/\/giphy.com\/gifs\/26gstvBo8qCnKXWCY\">via GIPHY<\/a><\/p>\n<h2 style=\"text-align: left;\"><strong><a id=\"four\"><\/a>4. Substituted Allenes That Have Mirror Planes<\/strong><\/h2>\n<p>Like so many things in organic chemistry, this simple fact leads to consequences that aren\u2019t immediately apparent. Let\u2019s pay attention to the mirror planes in this molecule. Note that there are <strong>two<\/strong>.<\/p>\n<p>Recall that in order for a molecule to be chiral it must\u00a0<strong>not<\/strong> be superimposable on its mirror image. If the molecule has a mirror plane (plane of symmetry) then it\u00a0<strong>will\u00a0<\/strong>be superimposable on its mirror image [and therefore achiral].<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-14778\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/6-allene-two-mirror-planes-at-90-degrees-to-each-other-achiral.png\" alt=\"allene-two-mirror-planes-at-90-degrees-to-each-other-achiral\" width=\"639\" height=\"366\" srcset=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/6-allene-two-mirror-planes-at-90-degrees-to-each-other-achiral.png 655w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/6-allene-two-mirror-planes-at-90-degrees-to-each-other-achiral-300x172.png 300w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/6-allene-two-mirror-planes-at-90-degrees-to-each-other-achiral-320x183.png 320w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/6-allene-two-mirror-planes-at-90-degrees-to-each-other-achiral-640x366.png 640w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/6-allene-two-mirror-planes-at-90-degrees-to-each-other-achiral-360x206.png 360w\" sizes=\"(max-width: 639px) 100vw, 639px\" \/><\/p>\n<p>With two mirror planes\u00a0this is definitely an <strong>achiral<\/strong> molecule.<\/p>\n<p>Now let&#8217;s start playing around a bit. What happens if we replace one of the hydrogens by another group? In the drawing I&#8217;ve arbitrarily made this atom Cl but in practice it can be anything except H in this case.<\/p>\n<p>Here&#8217;s a line drawing of \u00a0the molecule and a picture of its orbitals.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-14779\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/7-monosubstituted-allene-chloroallene-has-two-pi-bonds-at-90-degrees-to-each-other.gif\" alt=\"allene-two-mirror-planes-at-90-degrees-to-each-other-achiral\" width=\"638\" height=\"175\" \/><\/p>\n<p>Now here\u2019s what it looks like in 3D. Notice that <strong>we\u2019ve lost one of the mirror planes!<\/strong> But it still has one, so it\u2019s still\u00a0<strong>achiral.<\/strong><\/p>\n<p><iframe class=\"giphy-embed\" src=\"https:\/\/giphy.com\/embed\/26gscoTcjEhoZhZmM\" width=\"480\" height=\"480\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><\/p>\n<p><a href=\"https:\/\/giphy.com\/gifs\/26gscoTcjEhoZhZmM\">via GIPHY<\/a><\/p>\n<p>Now: what happens if we add a<b>\u00a0second<\/b>\u00a0group to the same carbon?<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-25550\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2015\/01\/9-dichloroallene-where-two-chlorines-are-bonded-to-terminal-carbon-still-an-achiral-molecule-with-two-mirror-planes.gif\" alt=\"\" width=\"641\" height=\"140\" \/><\/a><\/p>\n<p>It&#8217;s <strong>still achiral!<\/strong> \u00a0&#8211; in fact, we&#8217;ve gone back to having <strong>two<\/strong> mirror planes.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-14781\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/10-model-of-dichloroallene-with-chlorines-at-terminus-two-mirror-planes.png\" alt=\"model-of-dichloroallene-with-chlorines-at-terminus-two-mirror-planes\" width=\"640\" height=\"388\" srcset=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/10-model-of-dichloroallene-with-chlorines-at-terminus-two-mirror-planes.png 643w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/10-model-of-dichloroallene-with-chlorines-at-terminus-two-mirror-planes-300x182.png 300w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/10-model-of-dichloroallene-with-chlorines-at-terminus-two-mirror-planes-320x194.png 320w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/10-model-of-dichloroallene-with-chlorines-at-terminus-two-mirror-planes-640x388.png 640w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/10-model-of-dichloroallene-with-chlorines-at-terminus-two-mirror-planes-360x218.png 360w\" sizes=\"(max-width: 640px) 100vw, 640px\" \/><\/p>\n<h2><a id=\"five\"><\/a>5. This Di-Substituted Allene Has No Mirror Plane &#8211; And is Therefore\u00a0 Chiral!<\/h2>\n<p>Now, finally, let\u2019s <strong>change the position of that second group<\/strong>. Instead of making it so that there are two identical groups on the carbon, let\u2019s change it so that there\u2019s <strong>one on each carbon<\/strong>. Like this:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-14782\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/11-a-disubstituted-allene-that-is-chiral-because-it-has-no-plane-of-symmetry.gif\" alt=\"a-disubstituted-allene-that-is-chiral-because-it-has-no-plane-of-symmetry\" width=\"641\" height=\"197\" \/><\/p>\n<p>Here&#8217;s the 3D version.<\/p>\n<p><iframe class=\"giphy-embed\" src=\"https:\/\/giphy.com\/embed\/l0ExfArtKh3aw4kbC\" width=\"480\" height=\"480\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><\/p>\n<p><a href=\"https:\/\/giphy.com\/gifs\/l0ExfArtKh3aw4kbC\">via GIPHY<\/a><\/p>\n<p>Now I ask: where are the mirror planes?<\/p>\n<p><strong>THERE\u00a0ARE NONE!<\/strong>\u00a0 If there are no mirror planes \u2014&gt; then we are looking at a chiral molecule.<\/p>\n<h2><a id=\"six\"><\/a>6. Chiral Allenes Have An &#8220;Axis Of Chirality&#8221;<\/h2>\n<p>Wait. You might ask -what trickery is this? \u00a0<strong>We have no chiral centres. Don&#8217;t we need something to have a chiral\u00a0centre to be chiral?<\/strong><\/p>\n<p>Actually you are very familiar with a chiral object that has no chiral centre.<\/p>\n<p>Have you ever screwed before?<\/p>\n<p>No, no, no! \u00a0I mean, screwed using a screwdriver. This is a chemistry blog. Don&#8217;t expect anything lascivious here.<\/p>\n<p>See, most screw threads are, to use a familiar mnemonic, \u00a0\u201crighty tighty, lefty loosey.\u201d You have to turn the screwdriver clockwise to screw it in, and counterclockwise \u00a0to loosen. <strong>Screws are chiral!<\/strong><\/p>\n<p><span style=\"color: #993366;\"><em>[By the way the enantiomer of that common screw thread would be\u2026 \u201clefty tighty, righty loosey\u201d &#8211; those types of threads exist BTW &#8211; especially in gas barbecues, so as to prevent people from trying to connect things to them such as garden hoses or other idiotic things].<\/em><\/span><\/p>\n<p>Here&#8217;s a representative picture of two chiral screws. \u00a0You can also imagine these as spiral staircases. Imagine walking up each staircase. On the staircase labelled &#8220;left- hand&#8221; below, \u00a0your right arm would be on the inside as you ascend. On the &#8220;right-hand&#8221; staircase, your left arm would be on the inside as you ascend. [see <a href=\"#notetwo\">Note 2<\/a>]<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-14783\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/13-diagram-of-left-handed-screws-and-right-handed-screws.png\" alt=\"diagram-of-left-handed-screws-and-right-handed-screws\" width=\"375\" height=\"363\" srcset=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/13-diagram-of-left-handed-screws-and-right-handed-screws.png 354w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/13-diagram-of-left-handed-screws-and-right-handed-screws-300x291.png 300w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/13-diagram-of-left-handed-screws-and-right-handed-screws-320x310.png 320w\" sizes=\"(max-width: 375px) 100vw, 375px\" \/><\/p>\n<p>Instead of having a chiral centre, screws have what we refer to as a &#8220;<strong>chiral axis<\/strong>&#8220;. Other common examples of things with chiral axes: spiral staircases, snail shells and DNA.<\/p>\n<p><strong>Our disubstituted allene has a chiral axis<\/strong> &#8211; just like a screw.<\/p>\n<p>Screws are chiral &#8211; and chiral objects\u00a0have enantiomers.<strong> And just as screws have enantiomers, so do chiral molecules with a chiral axis.<\/strong><\/p>\n<p>What would the enantiomer of the allene look like?<\/p>\n<p>Here is an enantiomeric pair of allenes. Can you see now how they are not superimposable?<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-14784\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/14-model-of-two-enantiomeric-disubstituted-allenes-not-superimposable.jpg\" alt=\"model-of-two-enantiomeric-disubstituted-allenes-not-superimposable.\" width=\"425\" height=\"420\" srcset=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/14-model-of-two-enantiomeric-disubstituted-allenes-not-superimposable.jpg 1024w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/14-model-of-two-enantiomeric-disubstituted-allenes-not-superimposable-300x296.jpg 300w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/14-model-of-two-enantiomeric-disubstituted-allenes-not-superimposable-768x759.jpg 768w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/14-model-of-two-enantiomeric-disubstituted-allenes-not-superimposable-320x316.jpg 320w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/14-model-of-two-enantiomeric-disubstituted-allenes-not-superimposable-640x633.jpg 640w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/14-model-of-two-enantiomeric-disubstituted-allenes-not-superimposable-360x356.jpg 360w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/14-model-of-two-enantiomeric-disubstituted-allenes-not-superimposable-720x712.jpg 720w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/14-model-of-two-enantiomeric-disubstituted-allenes-not-superimposable-800x791.jpg 800w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/14-model-of-two-enantiomeric-disubstituted-allenes-not-superimposable-760x751.jpg 760w\" sizes=\"(max-width: 425px) 100vw, 425px\" \/><\/p>\n<h2><a id=\"seven\"><\/a>7.\u00a0 Summary: <strong>How Do You Recognize A Chiral Allene As Opposed To An Achiral Allene?\u00a0<\/strong><\/h2>\n<p>[In other words, an allene with a chiral axis vs. an allene without a chiral axis]<\/p>\n<p>You might ask, is there some kind of short cut for recognizing a chiral allene? <strong>Sure<\/strong>.<\/p>\n<p>Examine both &#8220;ends&#8221; of the allene. If <strong>either\u00a0of those ends is attached to\u00a0two<\/strong>\u00a0<strong>identical substituents<\/strong>, it is\u00a0<b>achiral\u00a0<\/b> &#8211; because it will have a mirror plane.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-14785\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/15-examples-of-achiral-allenes-that-have-two-identical-substituents-at-at-least-one-end.gif\" alt=\"examples-of-achiral-allenes-that-have-two-identical-substituents-at-at-least-one-end\" width=\"600\" height=\"258\" \/><\/p>\n<p>If <b>neither<\/b> of the ends are attached to\u00a0<strong>two identical substituents,\u00a0<\/strong>then it is\u00a0<strong>chiral<\/strong>.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-14786\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/16-chiral-allenes-have-two-different-substitutents-at-each-end-of-the-allene.gif\" alt=\"chiral-allenes-have-two-different-substitutents-at-each-end-of-the-allen\" width=\"600\" height=\"238\" \/><\/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\/2017\/02\/24\/optical-purity-and-enantiomeric-excess\/\" class=\"\"><span>Optical Purity and Enantiomeric Excess<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2017\/02\/14\/molecular-orbital-pi-bond\/\" class=\"\"><span>Bonding And Antibonding Pi Orbitals<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2017\/01\/24\/conjugation-and-resonance\/\" class=\"\"><span>Conjugation And Resonance In Organic Chemistry<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2019\/03\/08\/enantiomers-diastereomers-or-the-same-1-using-models\/\" class=\"\"><span>Enantiomers vs Diastereomers vs The Same? Two Methods For Solving Problems<\/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\/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\/organic-chemistry-practice-problems\/stereochemistry-practice-problems-and-quizzes\/\" class=\"\"><span>Stereochemistry Practice Problems and Quizzes (MOC Membership)<\/span><\/a><\/li><\/ul><\/div>\n<p><strong><a id=\"noteone\"><\/a>Note 1. Bonus question: <\/strong>Would you expect this molecule to be chiral? Why or why not?<\/p>\n<div class=\"wq-quiz-wrapper\" data-id=\"43531\"><style type=\"text\/css\" id=\"wq-flip-custom-css\">.wq-quiz-wrapper[data-id=\"43531\"] {\n--wq-question-width: 100%;\n--wq-question-color: #009cff;\n--wq-question-height: auto;\n--wq-font-color: #444;\n}\n\n\t\t\t.wq-quiz-wrapper[data-id=\"43531\"] {\n\t\t\t\t--wq-question-width: 600px;\n\t\t\t}\n\n\t\t\t@media screen and (max-width: 600px) {\n\t\t\t\t.wq-quiz-wrapper[data-id=\"43531\"] .wq_singleQuestionWrapper { width:100% !important; height:auto !important; }\n\t\t\t}\n\t\t<\/style><!-- wp quiz -->\n<div id=\"wp-quiz-43531\" class=\"wq_quizCtr single flip_quiz wq-quiz wq-quiz-43531 wq-quiz-flip wq-layout-single wq-skin-traditional wq-should-show-correct-answer\" data-quiz-id=\"43531\">\n<div class=\"wq-questions wq_questionsCtr\">\n\t<div class=\"wq-question wq_singleQuestionWrapper wq-question-faz4r\" data-id=\"faz4r\">\n\n\t\n\t<div class=\"item_top\">\n\t\t<div class=\"title_container\">\n\t\t\t<div class=\"wq_questionTextCtr\">\n\t\t\t\t<h4 class=\"wq-question-title\"><\/h4>\n\t\t\t<\/div>\n\t\t<\/div>\n\t<\/div>\n\n\t<div class=\"card \">\n\t\t<div class=\"front\" >\n\t\n\t\t\t\t\t<img decoding=\"async\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-images\/2317-Front.gif\" \/>\n\t\t\n\t\t\n\t\n\t\n\t\t\t<span class=\"top-desc\">Click to Flip<\/span>\n\t<\/div>\n\t\t<div class=\"back\" >\n\t\n\t\t\t\t\t<img decoding=\"async\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-images\/2317-Reverse.gif\" \/>\n\t\t\n\t\t\n\t\n\t<\/div>\n\t<\/div>\n\n\t\n<\/div>\n<\/div>\n<\/div>\n<!-- \/\/ wp quiz-->\n<\/div><!-- End .wq-quiz-wrapper -->\n<p><strong><a id=\"notetwo\"><\/a>Note 2. Nerdy note about staircases, possibly a myth<\/strong>. This may or may not be true, but I recall reading a book in my childhood about castles where it was said that castles were built with staircases that forced someone ascending them [an attacker, presumably] to have their right (sword) hand on the inside and a descending defender to have their sword hand on the outside. The idea being that there is much more freedom of motion if your sword hand is on the outside because you have to swing it across your body.<\/p>\n<p><strong>Note 3.\u00a0<\/strong>Recall that free rotation about the C-C pi bond in alkenes is not possible without breaking the C-C pi bond. This requires about 62 kcal\/mol in energy (for trans-2-butene). What about allenes? Considerably less energy is required &#8211; about 46 kcal\/mol for the allene below. Still, this requires heating the molecule at about 260\u00b0C to get any meaningful rate of isomerization.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-40875\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2025\/06\/F1-Rotational-barrier-in-allenes-is-approximately-46-kcal-per-mol-relative-to-62-kcal-per-mol-for-trans-2-butene.gif\" alt=\"Rotational barrier in allenes is approximately 46 kcal per mol relative to 62 kcal per mol for trans 2 butene\" width=\"640\" height=\"321\" \/><\/a><\/p>\n<hr \/>\n<h2><a id=\"quizzes\"><\/a>Quiz Yourself!<\/h2>\n<p><br \/>\n<img loading=\"lazy\" decoding=\"async\" class=\"wp-image-36214 aligncenter\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/2317-Front-Image-Only.png\" alt=\"\" width=\"600\" height=\"450\" \/><\/a><\/p>\n<p style=\"text-align: center;\"><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a MOC member<\/strong><\/a> to see the clickable quiz with answers on the back.<\/p>\n<p><br \/>\n<br \/>\n<img loading=\"lazy\" decoding=\"async\" class=\"wp-image-36214 aligncenter\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/0921-Front-Image-Only.png\" alt=\"\" width=\"600\" height=\"450\" \/><\/a><\/p>\n<p style=\"text-align: center;\"><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a MOC member<\/strong><\/a> to see the clickable quiz with answers on the back.<\/p>\n<p><br \/>\n<br \/>\n<img loading=\"lazy\" decoding=\"async\" class=\"wp-image-36214 aligncenter\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/0910-Front-Image-Only.png\" alt=\"\" width=\"600\" height=\"450\" \/><\/a><\/p>\n<p style=\"text-align: center;\"><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a MOC member<\/strong><\/a> to see the clickable quiz with answers on the back.<\/p>\n<p><br \/>\n<br \/>\n<img loading=\"lazy\" decoding=\"async\" class=\"wp-image-36214 aligncenter\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/0909-Front-Image-Only.png\" alt=\"\" width=\"600\" height=\"450\" \/><\/a><\/p>\n<p style=\"text-align: center;\"><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a MOC member<\/strong><\/a> to see the clickable quiz with answers on the back.<\/p>\n<p><br \/>\n<br \/>\n<img loading=\"lazy\" decoding=\"async\" class=\"wp-image-36214 aligncenter\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/0907-Front-Image-Only.png\" alt=\"\" width=\"600\" height=\"450\" \/><\/a><\/p>\n<p style=\"text-align: center;\"><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a MOC member<\/strong><\/a> to see the clickable quiz with answers on the back.<\/p>\n<p><br \/>\n<br \/>\n<img loading=\"lazy\" decoding=\"async\" class=\"wp-image-36214 aligncenter\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/0903-Front-Image-Only.png\" alt=\"\" width=\"600\" height=\"450\" \/><\/a><\/p>\n<p style=\"text-align: center;\"><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a MOC member<\/strong><\/a> to see the clickable quiz with answers on the back.<\/p>\n<p><br \/>\n<br \/>\n<img loading=\"lazy\" decoding=\"async\" class=\"wp-image-36214 aligncenter\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/0901-Front-Image-Only.png\" alt=\"\" width=\"600\" height=\"450\" \/><\/a><\/p>\n<p style=\"text-align: center;\"><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a MOC member<\/strong><\/a> to see the clickable quiz with answers on the back.<\/p>\n<p><\/p>\n","protected":false},"excerpt":{"rendered":"<p>All About Allenes, Chiral and Otherwise Allenes are examples of\u00a0cumulated dienes, molecules with consecutive (not conjugated) double bonds. Allenes are not flat. Due to the <\/p>\n","protected":false},"author":1,"featured_media":37423,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1413],"tags":[1037,1039,1040,1038,654,344],"post_folder":[],"class_list":["post-8674","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-stereochemistry-chirality","tag-allenes","tag-asymmetry","tag-chiral-allenes","tag-chiral-axis","tag-chirality","tag-pi-bonds"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.7 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Chiral Allenes And Chiral Axes &#8211; Master Organic Chemistry<\/title>\n<meta name=\"description\" content=\"Allenes are &quot;cumulated&quot; dienes where the central carbon is sp-hybridized. 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