{"id":10495,"date":"2017-02-16T16:46:10","date_gmt":"2017-02-16T22:46:10","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=10495"},"modified":"2025-11-07T08:27:44","modified_gmt":"2025-11-07T14:27:44","slug":"molecular-orbitals-of-the-allyl-cation-allyl-radical-and-allyl-anion","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2017\/02\/16\/molecular-orbitals-of-the-allyl-cation-allyl-radical-and-allyl-anion\/","title":{"rendered":"Molecular Orbitals of The Allyl Cation, Allyl Radical, and Allyl Anion"},"content":{"rendered":"<p><strong>How To Draw The Molecular Orbitals of The Allyl Cation, Allyl Radical And Allyl Anion<\/strong><\/p>\n<p>Drawing the molecular orbitals of a pi system like allyl (3 conjugated p-orbitals) is a bit like construction: build the house (orbitals) first, and fill it with people (electrons) second.<\/p>\n<ul>\n<li>When the two p-orbitals of a pi bond can overlap with a p-orbital on an adjacent carbon, the resulting &#8220;pi-system&#8221; containing 3 p-orbitals is referred to as the\u00a0<strong>allyl\u00a0<\/strong>pi system<\/li>\n<li>The\u00a0<strong>allyl cation\u00a0<\/strong>has 3 p-orbitals and\u00a0<strong>two\u00a0<\/strong>pi-electrons<\/li>\n<li>The\u00a0<strong>allyl radical\u00a0<\/strong>has 3 p-orbitals and\u00a0<strong>three\u00a0<\/strong>pi-electrons<\/li>\n<li>The\u00a0<strong>allyl anion\u00a0<\/strong>has 3 p-orbitals and\u00a0<strong>four\u00a0<\/strong>pi-electrons<\/li>\n<li>Drawing out the molecular orbitals of each of these systems can be done in a stepwise manner.<\/li>\n<li>The\u00a0<strong>lowest-energy\u00a0<\/strong>molecular orbital (\u03c0<sub>1\u00a0<\/sub>) will have all three p-orbitals aligned with the same phase and zero <strong>nodes<\/strong><\/li>\n<li>The\u00a0<strong>highest-energy\u00a0<\/strong>molecular orbital (\u03c0<sub>3<\/sub>) will have all three p-orbitals with\u00a0<strong>opposite\u00a0<\/strong>phases and <strong>two\u00a0<\/strong>nodes.<\/li>\n<li>The intermediate energy molecular orbital has\u00a0<strong>one<\/strong> node that is on the central atom.<\/li>\n<li>Once the molecular orbital diagram is drawn, the next step is to fill it with pi-electrons:\u00a0<strong>two\u00a0<\/strong>for the allyl cation,\u00a0<strong>three\u00a0<\/strong>for the allyl radical, and\u00a0<strong>four\u00a0<\/strong>for the allyl anion.<\/li>\n<\/ul>\n<p><img fetchpriority=\"high\" decoding=\"async\" class=\"alignnone wp-image-33851\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/01\/0-summary-of-molecular-orbitals-of-the-allyl-cation-allyl-radical-and-allyl-anion.gif\" alt=\"summary of molecular orbitals of the allyl cation allyl radical and allyl anion\" width=\"640\" height=\"574\" \/><\/a><\/p>\n<p><strong>Table of Contents<\/strong><\/p>\n<ol>\n<li class=\"p1\"><a href=\"#one\"><span class=\"s1\">Quick Review: Molecular Orbitals For A Simple Pi Bond<\/span><\/a><\/li>\n<li class=\"p1\"><a href=\"#two\"><span class=\"s1\"><span class=\"s1\">The Rules For Building Up Pi Molecular Orbital<\/span><\/span><\/a><\/li>\n<li class=\"p1\"><a href=\"#three\">The Allyl Cation, Allyl Radical, and Allyl Anion<\/a><\/li>\n<li class=\"p1\"><a href=\"#four\">The Molecular Orbitals For The Allyl System (N=3)<\/a><\/li>\n<li class=\"p1\"><a href=\"#five\">The Tricky Middle Orbital Of The Allyl System<\/a><\/li>\n<li class=\"p1\"><a href=\"#six\">Molecular Orbitals of The Allyl Cation (2 pi electrons)<\/a><\/li>\n<li class=\"p1\"><a href=\"#seven\">Molecular Orbitals of the Allyl Radical (3 pi electrons)<\/a><\/li>\n<li class=\"p1\"><a href=\"#eight\">Molecular Orbitals of the Allyl Anion (4 pi electrons)<\/a><\/li>\n<li class=\"p1\"><a href=\"#nine\">Conclusion: The Allyl System<\/a><\/li>\n<li><a href=\"#notes\">Notes<\/a><\/li>\n<li><a href=\"#quizzes\">Quiz Yourself!<\/a><\/li>\n<li><a href=\"#references\">(Advanced) References and Further Reading<\/a><\/li>\n<\/ol>\n<hr \/>\n<h2><a id=\"one\"><\/a>1. Quick Review: Molecular Orbitals For A Simple Pi Bond<\/h2>\n<p>In the last post,\u00a0we showed <a href=\"https:\/\/www.masterorganicchemistry.com\/2017\/02\/14\/molecular-orbital-pi-bond\/\">how to build\u00a0a molecular orbital (MO) diagram for a typical C-C pi bond.<\/a><\/p>\n<p>We saw that:<\/p>\n<ul>\n<li>The number of molecular orbitals equalled the number of contributing atomic orbitals. The overlap of <strong>two<\/strong> atomic (p) orbitals gave rise to <strong>two<\/strong> molecular (pi, or\u00a0\u03c0 ) orbitals<\/li>\n<li>The lowest-energy molecular orbital had all the phases in the contributing p-orbitals aligned the same way. In other words, there were no\u00a0<strong>nodes<\/strong> between the p-orbitals.<\/li>\n<li>Energy increases with the number of nodes (just like for standing waves). The highest-energy molecular orbital had a single node, where the p orbitals changed phase between each other.<\/li>\n<\/ul>\n<p>The molecular orbitals for a pi bond look like this:<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-15570\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/1-molecular-orbitals-for-a-pi-bond-with-two-molecular-orbitals-and-two-pi-electrons-lowest-energy-has-zero-nodes-highest-energy-has-one-node.gif\" alt=\"molecular orbitals for a pi bond with two molecular orbitals and two pi electrons lowest energy has zero nodes highest energy has one node\" width=\"600\" height=\"333\" \/><\/p>\n<p>Drawing out the molecular orbitals for a single pi bond seems simple enough. But what happens to the molecular orbital diagram if we add a <strong>third<\/strong> contributing p-orbital? Or if we have an adjacent double bond,\u00a0contributing 2 further p-orbitals, giving us 4 conjugated p orbitals in total? Understanding what happens to the molecular orbitals will allow us to understand their energies, and (as we&#8217;ll see later) their reactivity.<\/p>\n<h2><a id=\"two\"><\/a>2. Generalizing The Rules For Building Up Pi Molecular Orbitals<\/h2>\n<p>We can use the lessons learned during building up the molecular orbitals of the pi bond to generalize toward building up larger (linear) pi systems containing N molecular orbitals. [<a href=\"#noteone\">Note 1<\/a>]<\/p>\n<ul>\n<li>For a system of N atomic obitals we will have N molecular orbitals<\/li>\n<li>the lowest-energy orbital will always have <strong>zero<\/strong> nodes (all phase-aligned)<\/li>\n<li>The next-highest energy level up will have <strong>one<\/strong> node<\/li>\n<li>the level after that, <strong>two<\/strong> nodes,<\/li>\n<li>and so on, up to the highest energy level, which will have <strong>N-1<\/strong> nodes (all of alternate phase)<\/li>\n<\/ul>\n<p>Two key lessons,\u00a0really. First, the number of p orbitals tells you the number of pi orbitals. Second, the lowest and highest energy orbitals are the easiest to draw: the lowest energy one will have phases all aligned, and the highest energy one will have phases all alternating. We&#8217;ll be using this generalization a lot.<\/p>\n<p>So let&#8217;s take this to the next level. Three molecular orbitals. We call this the <strong>allyl<\/strong> system. Let&#8217;s go!<\/p>\n<h2><a id=\"three\"><\/a>3. The Allyl Cation, Allyl Radical, and Allyl Anion<\/h2>\n<p>You&#8217;ve heard of methyl, ethyl, propyl, and butyl; our name for the\u00a0three carbon unit H<sub>2<\/sub>C=CH-CH<sub>2 <\/sub>\u00a0is\u00a0&#8220;<strong>allyl<\/strong>&#8220;. <a href=\"https:\/\/en.wikipedia.org\/wiki\/Allyl_alcohol\">Allyl alcohol,<\/a> for example, is H<sub>2<\/sub>C=CH-CH<sub>2<\/sub>OH .<\/p>\n<p>By now you&#8217;ve likely encountered the allyl cation, the allyl radical, or the allyl anion? \u00a0Maybe you&#8217;ve learned that the allyl cation is a &#8220;resonance-stabilized&#8221; carbocation, or the allyl radical is a &#8220;resonance stabilized&#8221; radical?<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-15571\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/2-pi-electrons-in-the-allyl-cation-allyl-radical-and-allyl-anion-allyl-cation-has-2-pi-electrons-allyl-radical-has-3-pi-electrons-allyl-anion-has-4-pi-electrons.gif\" alt=\"pi electrons in the allyl cation allyl radical and allyl anion allyl cation has 2 pi electrons allyl radical has 3 pi electrons allyl anion has 4 pi electrons\" width=\"600\" height=\"255\" \/><\/p>\n<p>In each of these &#8220;pi systems&#8221; the allyl carbon has an available p-orbital that is <strong>conjugated<\/strong> with the adjacent pi bond. Another way of saying the same thing is that the allyl carbon is in resonance with the pi bond, such that we can draw resonance structures of each of these molecules. [We won&#8217;t do that again here: we covered it two posts ago, in this post on \u00a0<a href=\"https:\/\/www.masterorganicchemistry.com\/2017\/01\/24\/conjugation-and-resonance\/\"><strong>conjugation and resonance<\/strong><\/a> &#8211; if you need a refresher, \u00a0go back and read it to re-familiarize yourself with conjugation, resonance, and orbital overlap. ]<\/p>\n<p>Since the allyl system has three p-orbitals that are in conjugation, <strong>how many Pi molecular orbitals will it have?<\/strong><\/p>\n<p>Three. (N=3)<\/p>\n<p>Next question. What will those three pi molecular orbitals look like? Say you&#8217;re asked the following question:<\/p>\n<p><strong>&#8220;Draw the molecular orbitals of the allyl cation, the allyl radical, and the allyl anion.&#8221;<\/strong><\/p>\n<p>(a common enough exam question, believe it or not!).<\/p>\n<p>How would you approach that?<\/p>\n<p>The allyl cation, radical, and anion <strong>all utilize the same framework of molecular orbitals<\/strong>. They only differ in the number of pi electrons they possess (2, 3, and 4, respectively).<\/p>\n<p>Knowing that, we can divide this problem into two stages.<\/p>\n<ul>\n<li><strong>First<\/strong>, draw the orbitals themselves.<\/li>\n<li><strong>Second,<\/strong> populate the orbitals with the appropriate number of pi electrons.<\/li>\n<\/ul>\n<p>Let&#8217;s do this.<\/p>\n<h2><a id=\"four\"><\/a>4. The Molecular Orbitals For The Allyl System (N=3)<\/h2>\n<p><strong>Draw the lowest-energy and highest-energy orbitals first<\/strong><\/p>\n<p>We&#8217;ll start by drawing the orbitals. The two easiest\u00a0to draw are the lowest-energy and highest-energy orbitals, so let&#8217;s do those first.<\/p>\n<ul>\n<li>The <strong>lowest-energy<\/strong> molecular orbital will have complete alignment of the phases in the p-orbitals, and zero nodes between them. Let&#8217;s call this &#8220;\u03c0<sub>1<\/sub>&#8220;, the lowest-energy pi molecular orbital. [We could alternatively have drawn the &#8220;shaded&#8221; phases all on top: it amounts to the same thing].<\/li>\n<\/ul>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15572\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/3-molecular-orbitals-of-the-allyl-system-lowest-energy-orbital-has-three-p-orbitals-all-same-phase.gif\" alt=\"molecular orbitals of the allyl system lowest energy orbital has three p orbitals all same phase\" width=\"600\" height=\"150\" \/><\/p>\n<ul>\n<li>The <strong>highest-energy<\/strong> molecular orbital is also easy to draw: all the phases will alternate, giving it (N-1) = 2 nodes. We can call this\u00a0&#8220;\u03c0<span style=\"font-size: small;\"><span style=\"line-height: 20px;\">3<\/span><\/span>&#8220;, the highest-energy pi orbital of the three. The nodes are marked with red lines.<\/li>\n<\/ul>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15573\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/4-highest-energy-orbital-of-the-allyl-system-has-all-three-p-orbitals-alternating-two-nodes-between-orbitals-marked-with-red-line.gif\" alt=\"highest energy orbital of the allyl system has all three p orbitals alternating two nodes between orbitals marked with red line\" width=\"630\" height=\"145\" \/><\/p>\n<h2><strong><a id=\"five\"><\/a>5. The Tricky Middle Orbital Of The Allyl System<\/strong><\/h2>\n<p>What about the middle orbital, \u03c0<sub>2<\/sub> ? It should have <strong>one<\/strong> node, and be intermediate in energy between\u00a0\u03c0<sub>1<\/sub><sub>\u00a0<\/sub>and\u00a0\u03c0<sub>3<\/sub><\/p>\n<p>Where to put that single node? If you&#8217;re like I was, your first temptation for drawing the orbitals will likely be to put a node between one of the carbons, giving orbitals that look something like this:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15574\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/5-drawing-the-molecular-orbital-of-the-allyl-system-with-one-node-the-wrong-way-is-uneven-dont-draw-middle-pi-2-orbital-this-way.gif\" alt=\"drawing the molecular orbital of the allyl system with one node the wrong way is uneven dont draw middle pi 2 orbital this way\" width=\"600\" height=\"276\" \/><\/p>\n<p>It&#8217;s actually incorrect!<\/p>\n<p>Why? That&#8217;s a good question, and the answer isn&#8217;t obvious. It turns out that because of the way the math works in the Schrodinger\u00a0equation, the nodes are always &#8220;balanced&#8221; with respect to the centre of the orbital. The node can&#8217;t just be placed anywhere; when there is one node, it <strong>must<\/strong> be in the centre. [By the way, this is a general feature of MO&#8217;s where\u00a0the number of contributing orbitals is odd, e.g. N=3, N=5, N=7&#8230; the first node will be\u00a0on the central carbon.]<\/p>\n<p><strong>The Node Goes In The Middle!\u00a0<\/strong><\/p>\n<p>That means that the node is <em>directly<\/em> on that middle carbon.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15575\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/6-in-the-allyl-system-the-middle-molecular-orbital-the-node-goes-in-the-middle-carbon-zero-electron-density-on-this-carbon.gif\" alt=\"in the allyl system the middle molecular orbital the node goes in the middle carbon zero electron density on this carbon\" width=\"600\" height=\"229\" \/><\/p>\n<p>Weird, right? What does that actually mean? It means that in that molecular orbital there is zero electron density on that central carbon. We don&#8217;t need to go into further detail for our purposes, but I&#8217;m including a note where it gives us some clues about reactivity. [<a href=\"#notetwo\"><strong>Note 2<\/strong><\/a>]<\/p>\n<p>Now, if we arrange these orbitals together by energy (most stable at bottom) we get the following orbital diagram for the allyl system.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15576\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/7-diagrams-for-molecular-orbitals-of-the-allyl-system-no-electrons-but-showing-lowest-energy-middle-energy-and-highest-energy.gif\" alt=\"diagrams for molecular orbitals of the allyl system no electrons but showing lowest energy middle energy and highest energy\" width=\"630\" height=\"428\" \/><\/p>\n<p>This is just the framework for the orbitals in the allyl system. It&#8217;s not\u00a0<em>specific<\/em> to the allyl cation, radical, or anion yet, because those three species differ in the number of Pi electrons they possess.<\/p>\n<p>Now, let&#8217;s populate the orbitals with electrons.<\/p>\n<h2><strong><a id=\"six\"><\/a>6. Molecular Orbitals of The Allyl Cation (2 pi electrons)<\/strong><\/h2>\n<p>In the allyl cation we have 2 pi electrons in total: in the resonance form below, we see two electrons in the pi bond, and zero on the allyl carbon.<\/p>\n<p>When we populate our allyl orbitals with two electrons, building from the ground floor up, we get the following:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15577\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/8-molecular-orbitals-for-allyl-cation-system-three-orbitals-two-electrons-in-the-lowest-energy-level-which-is-the-homo-and-middle-orbital-is-lumo.gif\" alt=\"molecular orbitals for allyl cation system three orbitals two electrons in the lowest energy level which is the homo and middle orbital is lumo\" width=\"630\" height=\"466\" \/><\/p>\n<p>Note that the &#8220;highest occupied&#8221; molecular orbital of the allyl cation is\u00a0\u03c0<sub>1<\/sub> &#8211; the lowest-energy orbital in the system. The &#8220;lowest unoccupied&#8221; molecular orbital is\u00a0\u03c0<sub>2\u00a0<\/sub>.<\/p>\n<p>We generally abbreviate the terms &#8220;highest occupied molecular orbital&#8221; \u00a0as HOMO and &#8220;lowest-unoccupied molecular orbital &#8221; as LUMO. They are often called the &#8220;frontier&#8221; molecular orbitals \u00a0and are where most of the action happens in reactions, as we&#8217;ll see in future posts.<\/p>\n<h2><a id=\"seven\"><\/a>7. Molecular Orbitals of the Allyl Radical (3 pi electrons)<\/h2>\n<p>What about the allyl radical? Here we have 3 pi electrons: two in the pi bond, and a third one in that half-filled p orbital.<\/p>\n<p>Where do we put that third electron? In the next-highest orbital, of course. Into\u00a0\u03c0<sub>2<\/sub> it goes!<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15578\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/9-molecular-orbitals-for-allyl-radical-showing-three-pi-electrons-homo-is-pi-2-with-a-single-electron-and-the-pi3-is-the-lumo.gif\" alt=\"molecular orbitals for allyl radical showing three pi electrons homo is pi 2 with a single electron and the pi3 is the lumo\" width=\"630\" height=\"495\" \/><\/p>\n<p>That gives us a half-filled\u00a0\u03c0<sub>2<\/sub>, which is our new &#8220;HOMO&#8221;, and\u00a0\u03c0<sub>3\u00a0<\/sub>now becomes our &#8220;LUMO&#8221;.<em> [<span style=\"color: #993366;\">I don&#8217;t want to add to the confusion, but sometimes half-filled molecular orbitals like this are referred to as a &#8220;singly occupied molecular orbital&#8221;, or SOMO. Not super-important at this stage<\/span>].\u00a0<\/em><\/p>\n<h2><strong><a id=\"eight\"><\/a>8. Molecular Orbitals of the Allyl Anion (4 pi electrons)\u00a0<\/strong><\/h2>\n<p>What about the allyl anion?\u00a0Here we have 4 pi\u00a0electrons: two in the pi bond, and two more as a lone pair on the terminal carbon. \u00a0This fills up\u00a0\u03c0<sub>2\u00a0<\/sub>.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15579\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/10-molecluar-orbital-diagram-for-the-allyl-anion-with-4-pi-electrons-showing-middle-orbital-is-the-homo-with-two-electrons-top-orbital-is-lumo-4-electrons-total.gif\" alt=\"molecluar orbital diagram for the allyl anion with 4 pi electrons showing middle orbital is the homo with two electrons top orbital is lumo 4 electrons total\" width=\"600\" height=\"410\" \/><\/p>\n<p>Our HOMO remains\u00a0\u03c0<sub>2\u00a0<\/sub>and our LUMO remains\u00a0\u03c0<sub>3<\/sub> .<\/p>\n<p>This satisfactorily shows the molecular orbitals for the allyl cation, radical, and anion.<\/p>\n<h2><a id=\"nine\"><\/a>9. Conclusion: The Allyl System<\/h2>\n<p>Here, we&#8217;ve shown how to extend the general principles of molecular orbitals we learned in building up the molecular orbitals of a pi bond (N=2) \u00a0toward the allyl system (N=3). The key points are as follows:<\/p>\n<ul>\n<li><strong>N<\/strong> atomic p orbitals give rise to <strong>N<\/strong>\u00a0pi molecular orbitals<\/li>\n<li>The N<sup>th<\/sup> molecular orbital will have N-1 nodes. (e.g. the 3rd MO in the allyl system has (3\u20131 = 2) nodes.<\/li>\n<li>When asked to draw molecular orbitals, a good rule of thumb is to <strong>start with the lowest-energy and highest-energy molecular orbitals<\/strong>. The lowest-energy molecular orbital has zero nodes (all p orbitals in phase) and the highest-energy molecular orbital will have (N-1) nodes (all p orbitals out of phase).<\/li>\n<li>Don&#8217;t add electrons to the system until you&#8217;ve drawn out all the molecular orbitals first.<\/li>\n<\/ul>\n<p>In the next post, let&#8217;s look at the molecular orbitals for the butadienyl system (N=4).<\/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\/2017\/02\/28\/pi-molecular-orbitals-of-butadiene\/\" class=\"\"><span>Pi Molecular Orbitals of Butadiene<\/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\/2017\/02\/14\/molecular-orbital-pi-bond\/\" class=\"\"><span>Bonding And Antibonding Pi Orbitals<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2011\/03\/08\/are-these-alkenes-conjugated\/\" class=\"\"><span>Are these molecules conjugated?<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2011\/03\/11\/3-factors-that-stabilize-carbocations\/\" class=\"\"><span>3 Factors That Stabilize Carbocations<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/11\/25\/allylic-bromination\/\" class=\"\"><span>Allylic Bromination<\/span><\/a><\/li><\/ul><\/div>\n<p><strong><a id=\"noteone\"><\/a>Note 1<\/strong>. The rule of thumb that for &#8220;the N<sup>th<\/sup> molecular orbital will have N-1 nodes&#8221; applies best to linear molecules. Cyclic molecules (such as benzene) follow the same general principles, but\u00a0due to symmetry will have &#8220;nodal planes&#8221;. For example the lowest energy MO of benzene has zero nodes, but the next-highest energy level of benzene is &#8220;doubly degenerate&#8221; meaning that there are two ways to draw a single nodal plane.<a href=\"https:\/\/www.masterorganicchemistry.com\/2017\/05\/05\/the-pi-molecular-orbitals-of-benzene\/\"> More here<\/a>. We&#8217;ll cover this in a separate post.<\/p>\n<p><strong><a id=\"notetwo\"><\/a>Note 2. <\/strong>[An advanced note: one consequence of the node being in the centre of \u03c0<sub>2<\/sub>\u00a0is that \u00a0the radical has zero electron density in the middle carbon (C-2). This is consistent with everything we&#8217;ve ever observed about the reactivity of allyl radicals &#8211; they react at the termini (C-1 or C-3), and never the middle carbon (C-2). ]<\/p>\n<p><strong><a id=\"notethree\"><\/a>Note 3. <\/strong>Here&#8217;s a final (advanced) footnote, that only the extremely curious should look at. It&#8217;s showing how the 3 molecular orbitals of the allyl system are formed through interaction of two Pi molecular orbitals with a p orbital. This results in lowering of the energy of the \u00a0\u03c0 orbital (resulting in a stabilization of energy E) and raising of the energy of the \u00a0\u03c0* orbital. The energy of the p orbital remains unchanged; intermediate in energy between bonding and antibonding, it is referred to as a &#8220;nonbonding&#8221; molecular orbital.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15580\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F1-full-molecular-orbital-diagram-for-the-allyl-system-formed-through-interaction-of-a-pi-bond-with-a-p-orbital.gif\" alt=\"full molecular orbital diagram for the allyl system formed through interaction of a pi bond with a p orbital\" width=\"630\" height=\"644\" \/><\/p>\n<p>Adapted from Ian Fleming&#8217;s excellent &#8220;Frontier Orbitals and Organic Chemical Reactions&#8221;.<\/p>\n<p><strong>Note 4.\u00a0<\/strong>Calculations on the allyl cation, radical, and anion courtesy of <a href=\"https:\/\/rowansci.com\/\">Rowan<\/a>:<\/p>\n<p>Allyl cation (<a href=\"https:\/\/www.rowansci.com\/blog\/electronegativity-and-conjugation-in-bonding\">source<\/a>)<br \/>\nCalculated C\u2013C bond length: 1.376 \u00c5<br \/>\nCalculated C\u2013C\u2013C bond angle: 119.2\u00b0.<\/p>\n<p><iframe title=\"allyl cation\" src=\"https:\/\/labs.rowansci.com\/public\/orbitals\/0cc97a8b-1fd5-42a9-bb5e-2ca9e0aa468a\" width=\"640\" height=\"640\"><span data-mce-type=\"bookmark\" style=\"display: inline-block; width: 0px; overflow: hidden; line-height: 0;\" class=\"mce_SELRES_start\">\ufeff<\/span><\/iframe><\/p>\n<p>Allyl radical (<a href=\"https:\/\/www.rowansci.com\/blog\/electronegativity-and-conjugation-in-bonding\">source<\/a>)<\/p>\n<p>Calculated C\u2013C bond lengths: 1.377 \u00c5<br \/>\nCalculated C\u2013C\u2013C bond angle:\u00a0 125.3\u00b0<\/p>\n<p><iframe title=\"allyl radical\" src=\"https:\/\/labs.rowansci.com\/public\/orbitals\/14696149-989a-4e86-ae8c-69926d029f5f\" width=\"640\" height=\"640\"><span data-mce-type=\"bookmark\" style=\"display: inline-block; width: 0px; overflow: hidden; line-height: 0;\" class=\"mce_SELRES_start\">\ufeff<\/span><\/iframe><\/p>\n<p>Allyl anion: (<a href=\"https:\/\/www.rowansci.com\/blog\/electronegativity-and-conjugation-in-bonding\">source<\/a>)<\/p>\n<p>Calculated C\u2013C bond length: 1.388 \u00c5<br \/>\nCalculated C\u2013C\u2013C bond angle: 132.8\u00b0.<\/p>\n<p><iframe title=\"allyl anion\" src=\"https:\/\/labs.rowansci.com\/public\/orbitals\/fc29f8d8-0b56-4995-b4c4-0eea53a91f9c\" width=\"640\" height=\"640\"><\/iframe><\/p>\n<hr \/>\n<h2><a id=\"quizzes\"><\/a>Quiz Yourself!<\/h2>\n\n<p class=\"p1\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-26714\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/3511-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\/3512-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\/3513-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\/3514-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\/3515-Front-Image-Only.png\" alt=\"\" width=\"640\" height=\"616\" \/><\/p>\n<p><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a\u00a0 MOC member<\/strong><\/a> to see the clickable quiz with answers on the back. <\/p>\n<hr \/>\n<h2><a id=\"references\"><\/a>(Advanced) References and Further Reading<\/h2>\n<p>This topic is covered in several textbooks, and the best sources of information on this topic is books.<\/p>\n<ol>\n<li>Streitwieser, A., Jr. <strong>Molecular Orbital Theory for Organic Chemists<\/strong>; Wiley: New York, 1961.<\/li>\n<li>Fleming, I. <strong>Frontier Orbitals and Organic Chemical Reactions<\/strong>; Wiley: New York, 1991<\/li>\n<li>Streitweiser, A. Jr. <strong>Supplemental Tables of Molecular Orbital Calculations, Volume 2<\/strong>; Pergamon Press: Oxford, 1965<\/li>\n<li><strong>Recognition of stereochemical paths by orbital interaction<\/strong><br \/>\nKenichi Fukui<br \/>\n<em>Accounts of Chemical Research<\/em> <strong>1971<\/strong> <em>4<\/em> (2), 57-64<br \/>\n<strong>DOI<\/strong>: <a href=\"https:\/\/pubs.acs.org\/doi\/10.1021\/ar50038a003\">10.1021\/ar50038a003<\/a><br \/>\nThis paper discusses the molecular orbitals of butadiene within the context of Frontier Molecular Orbital theory and the \u2018Woodward-Hoffmann\u2019 Rules. Prof. Fukui received the Nobel Prize in Chemistry in 1981 along with Prof. Roald Hoffmann for his contributions to the development of Frontier Molecular Orbital (FMO) theory.<\/li>\n<li><strong> 2. Examination of the energetic preference for coplanarity of double bonds. Comparison of butadiene, acrolein, and vinylamine<\/strong><br \/>\nKenneth B. Wiberg, Robert E. Rosenberg, and Paul R. Rablen<br \/>\n<em>Journal of the American Chemical Society<\/em> <strong>1991,<\/strong> <em>113<\/em> (8), 2890-2898<br \/>\n<strong>DOI<\/strong>: <a href=\"https:\/\/pubs.acs.org\/doi\/abs\/10.1021\/ja00008a016\">10.1021\/ja00008a016<\/a><br \/>\nThis paper examines the MO\u2019s of butadiene and the isoelectronic molecules vinylamine and acrolein. Butadiene has a barrier to rotation along the C2-C3 bond, which is predicted by MO theory.<\/li>\n<\/ol>\n","protected":false},"excerpt":{"rendered":"<p>How To Draw The Molecular Orbitals of The Allyl Cation, Allyl Radical And Allyl Anion Drawing the molecular orbitals of a pi system like allyl <\/p>\n","protected":false},"author":1,"featured_media":33851,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1163],"tags":[1178,1179,1177,195,363,940,941,1164,612,1180,267],"post_folder":[],"class_list":["post-10495","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-dienes-and-mo-theory","tag-allyl-anion","tag-allyl-cation","tag-allyl-radical","tag-conjugation","tag-dienes","tag-homo","tag-lumo","tag-mo-theory","tag-molecular-orbitals","tag-pi-electrons","tag-resonance"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin 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