{"id":11439,"date":"2018-03-23T08:03:11","date_gmt":"2018-03-23T12:03:11","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=11439"},"modified":"2026-04-17T21:17:53","modified_gmt":"2026-04-18T02:17:53","slug":"molecular-orbitals-in-the-diels-alder-reaction","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2018\/03\/23\/molecular-orbitals-in-the-diels-alder-reaction\/","title":{"rendered":"HOMO and LUMO In the Diels Alder Reaction"},"content":{"rendered":"<p><strong>The HOMO and LUMO In The Diels Alder Reaction<\/strong><\/p>\n<ul>\n<li>The Diels-Alder reaction is a concerted reaction between a diene (with 4 pi-electrons) and a dienophile (2 pi-electrons) that forms a new 6-membered ring<\/li>\n<li>The new C-C sigma bonds are formed from the\u00a0<strong>overlap\u00a0<\/strong>of the pi molecular orbitals at C1 and C4 of the diene and C1 and C2 of the dienophile.<\/li>\n<li>In order for the new bonds to form, there has to be\u00a0<strong>constructive\u00a0<\/strong>overlap between these orbitals &#8211; that is, they have to have the\u00a0<strong>same phase<\/strong>.<\/li>\n<li>By analyzing the molecular orbitals of the diene and dienophile in the Diels-Alder reaction, it becomes clear that constructive overlap is possible between the highest-occupied molecular orbital (HOMO) of the <strong>diene\u00a0<\/strong>with the lowest-unoccupied molecular orbital (LUMO) of the <strong>dienophile<\/strong>.<\/li>\n<li>This type of overlap is not possible in a reaction between two alkenes.<\/li>\n<\/ul>\n<p><img fetchpriority=\"high\" decoding=\"async\" class=\"alignnone wp-image-34098\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/02\/0-Summary-of-the-molecular-orbital-interactions-in-the-diels-alder-reaction.gif\" alt=\"Summary of the molecular orbital interactions in the diels alder reaction\" width=\"640\" height=\"582\" \/><\/a><\/p>\n<p><strong>Table of Contents<\/strong><\/p>\n<ol>\n<li><a href=\"#one\">A Quick Recap Of The Diels-Alder Reaction<\/a><\/li>\n<li><a href=\"#two\"><span class=\"s1\"><span class=\"s1\">Bond Formation Requires Overlap Between The HOMO Of One Molecule (The Nucleophile) With The LUMO Of Another Molecule (The Electrophile)<\/span><\/span><\/a><\/li>\n<li><a href=\"#three\">Concerted Reactions: When Two Bonds Form At The Same Time, Multiple Orbitals Must Overlap<\/a><\/li>\n<li><a href=\"#four\">Molecular Orbitals In The [2+2] Cycloaddition Between Ethene And Ethene Show Why The Reaction Is Unfavorable Under &#8220;Thermal&#8221; Conditions<\/a><\/li>\n<li><a href=\"#five\">Molecular Orbitals In The Diels-Alder Reaction: Interaction of the Diene HOMO with the Dienophile LUMO Is Favorable At Both Bond-Forming Sites<\/a><\/li>\n<li><a href=\"#six\">Under &#8220;Photochemical&#8221; Conditions, The [2+2] Actually Works Pretty Well<\/a><\/li>\n<li><a href=\"#seven\">Summary &#8211; The Key Role Of Orbital Symmetry In Concerted Reactions Of Pi-Systems<\/a><\/li>\n<li><a href=\"#notes\">Notes<\/a><\/li>\n<li><a href=\"#quizzes\">Quiz Yourself!\u00a0<\/a><\/li>\n<\/ol>\n<hr \/>\n<h2><a id=\"one\"><\/a>1. A Quick Recap Of The Diels-Alder Reaction<\/h2>\n<p>Today we&#8217;re going to go into the mechanism of the Diels-Alder reaction from a molecular orbital perspective. Using our previous posts on how to build up molecular orbitals, we&#8217;ll show how the Diels-Alder results from the constructive orbital overlap between the highest-occupied molecular orbital (HOMO) of the diene with the lowest-unoccupied molecular orbital (LUMO) of the dienophile.<\/p>\n<p>But first, let&#8217;s recap where we are with the Diels-Alder so far:<\/p>\n<ul>\n<li>The Diels-Alder reaction combines a diene with a dienophile to form a new six-membered ring\u00a0<em>[see: <a href=\"https:\/\/www.masterorganicchemistry.com\/2017\/08\/30\/the-diels-alder-reaction\/\">Introduction to the Diels-Alder reaction<\/a>]\u00a0<\/em><\/li>\n<li>three bonds form (two sigma bonds and a pi bond) and three bonds break (three pi bonds)<\/li>\n<li>the stereochemistry of the product can be reliably predicted from analyzing the stereochemistry of the diene and dienophile [<em>see: <a href=\"https:\/\/www.masterorganicchemistry.com\/2017\/11\/13\/stereochemistry-of-the-diels-alder-reaction\/\">Stereochemistry of the Diels-Alder Reaction<\/a>]<\/em><\/li>\n<li>in certain cases, mixtures of diastereomers (<em>exo<\/em>&#8211; and\u00a0<em>endo-<\/em>\u00a0) are obtained.\u00a0 Generally the\u00a0<em>endo<\/em>&#8211; is favored over the exo.\u00a0<em>[see this post on <a href=\"https:\/\/www.masterorganicchemistry.com\/2018\/02\/09\/exo-vs-endo-products-in-the-diels-alder-how-to-tell-them-apart\/\">exo and endo<\/a>]\u00a0<\/em><\/li>\n<\/ul>\n<p>What we haven&#8217;t really covered is <em>why the Diels-Alder actually\u00a0works<\/em>. After all, we&#8217;ve seen plenty of examples of things that don&#8217;t work; two alkenes, for example, don&#8217;t combine to form four membered rings upon heating in the way that a diene and a dienophile combine to form a six-membered ring. Nor do two dienes combine easily upon heating to give eight-membered rings.<\/p>\n<p><strong>Why is the Diels-Alder so easy, and many seemingly related reactions so hard?<\/strong><\/p>\n<p>The answer to this question lies in the arrangement of<strong> pi molecular orbitals<\/strong> in the two components of this reaction.<\/p>\n<p>In the 1960&#8217;s a theory of\u00a0<a href=\"https:\/\/en.wikipedia.org\/wiki\/Woodward%E2%80%93Hoffmann_rules\"><em>orbital symmetry\u00a0<\/em><\/a>was developed to understand these reactions, which provided a very useful set of predictive rules &#8211; the <a href=\"https:\/\/en.wikipedia.org\/wiki\/Woodward%E2%80%93Hoffmann_rules\">Woodward-Hoffmann rules<\/a>.<\/p>\n<p>We will scratch the surface of the orbital symmetry rules here and use them to show why the reaction of dienes with alkenes (the Diels-Alder) occurs readily upon heating,\u00a0 but the reaction of alkenes with alkenes (a.k.a. [2+2] cycloadditions) does not.<\/p>\n<h2><strong><a id=\"two\"><\/a>2. Bond Formation Requires Overlap Between The HOMO Of One Molecule (The Nucleophile) With The LUMO Of Another Molecule (The Electrophile)<\/strong><\/h2>\n<p>Most reactions we&#8217;ve seen involve a nucleophile (an electron-pair donor) reacting with an electrophile (an electron-pair acceptor) to form ONE new bond.<\/p>\n<p>In order for that bond to form, the <strong>filled<\/strong> orbital on the nucleophile containing the electron pair has to come into contact (overlap) with the <strong>empty<\/strong> orbital on the electrophile which can accept the electron pair.<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-15689\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/1-perfect-orbital-overlap-analogy-slipper-disney-diels-alder.jpg\" alt=\"perfect orbital overlap analogy slipper disney diels alder\" width=\"325\" height=\"177\" srcset=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/1-perfect-orbital-overlap-analogy-slipper-disney-diels-alder.jpg 304w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/1-perfect-orbital-overlap-analogy-slipper-disney-diels-alder-300x164.jpg 300w\" sizes=\"(max-width: 325px) 100vw, 325px\" \/><\/p>\n<p style=\"text-align: center;\"><em>[Perfect orbital overlap between nucleophile and electrophile.]<\/em><\/p>\n<ul>\n<li>The pair of electrons on the nucleophile almost always comes from the highest-energy occupied molecular orbital (HOMO) of the nucleophile. Why? Because these are the electrons that are the least tightly held.<\/li>\n<li>\u00a0The orbital on the electrophile that accepts the pair of electrons is almost always the lowest-energy unoccupied molecular orbital (LUMO), because this will result in the lowest-energy transition state (<em>and the fastest reaction).\u00a0<\/em><\/li>\n<\/ul>\n<p>In most reactions (such as the S<sub>N<\/sub>2) only one bond is forming at a given center at any one time:<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-15690\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/2-reivew-of-orbital-overlap-in-the-sn2-reaction-homo-lumo-nucleophile-electrophile.gif\" alt=\"review of orbital overlap in the sn2 reaction homo lumo nucleophile electrophile\" width=\"630\" height=\"361\" \/><\/p>\n<p>One little note.\u00a0\u00a0In the S<sub>N<\/sub>2 we make the assumption that the HOMO and LUMO have the same phase. This is perfectly valid &#8211;\u00a0<em>so long as we&#8217;re only dealing with one bond being formed at a time.\u00a0<\/em><\/p>\n<h2><strong><a id=\"three\"><\/a>3. Concerted Reactions:\u00a0<\/strong><strong>When Two Bonds Form At The Same Time, Multiple Orbitals Must Overlap<\/strong><\/h2>\n<p>Things get more complex when we have a reaction where two or more bonds are formed at the exact same time. This is known as a\u00a0<em>concerted\u00a0<\/em>reaction (as opposed to &#8220;stepwise&#8221;).<\/p>\n<p>Take the combination of two alkenes to give a cyclobutane ring.\u00a0<span style=\"color: #993366;\"><em> (This is often called a [2+2] <strong>cycloaddition<\/strong>.)<\/em><\/span><\/p>\n<p>Since we have two bonds forming at the same time, we have\u00a0<em>two<\/em> orbital interactions to consider.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15691\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/3-concerted-22-cycloaddition-bond-formation-occurs-at-two-different-sites-at-the-same-time.gif\" alt=\"concerted 2+2 cycloaddition bond formation occurs at two different sites at the same time\" width=\"630\" height=\"265\" \/><\/p>\n<p>What&#8217;s the nucleophile and the electrophile here?<\/p>\n<p>Ethene <em>and<\/em> ethene.\u00a0 : &#8211; )<\/p>\n<p>More specifically, the nucleophile is the HOMO of one ethene molecule, and the electrophile is the LUMO of another ethene.<\/p>\n<p>The HOMO of one ethene molecule <strong>must<\/strong> combine with the LUMO of another ethene molecule. <span style=\"color: #993366;\"><em>[We can&#8217;t combine two occupied orbitals &#8211; Nature has a strict 2-electron occupancy limit per orbital. And since we can&#8217;t form a bond without electrons, combining two LUMOs would be silly]<\/em><\/span><\/p>\n<h2><a id=\"four\"><\/a>4. Molecular Orbitals In The [2+2] Cycloaddition Between Ethene And Ethene Show Why The Reaction Is Unfavorable Under &#8220;Thermal&#8221; Conditions<\/h2>\n<p>Let&#8217;s look at the \u03c0 molecular orbitals of ethene. The HOMO has zero nodes, and the LUMO has a single node.<em> [<a href=\"https:\/\/www.masterorganicchemistry.com\/2017\/02\/14\/molecular-orbital-pi-bond\/\">We learned how to build up molecular orbitals of ethene in this post<\/a>].\u00a0<\/em><\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15692\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/4-molecular-orbitals-homo-and-lumo-of-22-cycloaddition-orbital-overlap.gif\" alt=\"molecular orbitals homo and lumo of 2+2 cycloaddition orbital overlap\" width=\"600\" height=\"313\" \/><\/p>\n<p>In order for the reaction to occur in a concerted fashion, we must have <em>constructive overlap<\/em> between each of the lobes where the bonds are being formed. [If the phases are opposite, there is destructive interference between the orbitals and therefore zero electron density between the atoms]<\/p>\n<p>Now let&#8217;s bring the two molecules of ethene together:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15693\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/5-molecular-orbital-overlap-in-the-22-cycloaddition-shows-why-reaction-failes-antibonding-interaction-homo-lumo.gif\" alt=\"molecular orbital overlap in the 2+2 cycloaddition shows why reaction failes antibonding interaction homo lumo\" width=\"600\" height=\"247\" \/><\/p>\n<p>Note that only <strong>one<\/strong> of\u00a0the interactions between the lobes has lobes of like phase interacting (bonding). The other interaction has lobes of opposite phase interacting, which will not result in a bond. [<a href=\"#noteone\">Note 1<\/a>]<\/p>\n<p>This helps us understand why [2+2] cycloadditions don&#8217;t generally occur under &#8220;thermal&#8221; conditions (i.e.\u00a0 heating). The orbitals don&#8217;t both overlap! [<a href=\"#notetwo\">Note 2<\/a>]<\/p>\n<p>[2+2] cycloadditions\u00a0<em>do\u00a0<\/em>occur under photochemical conditions, however. More on that in a moment.<\/p>\n<h2><strong><a id=\"five\"><\/a>5. Molecular Orbitals In The Diels-Alder Reaction: Interaction of the Diene HOMO with the Dienophile LUMO Is Favorable At Both Bond-Forming Sites<\/strong><\/h2>\n<p>Now let&#8217;s perform the same kind of analysis on the Diels-Alder reaction.<\/p>\n<p>Since we&#8217;ve already seen the molecular orbitals of ethene, let&#8217;s look at butadiene. [<em>Relevant post: <a href=\"https:\/\/www.masterorganicchemistry.com\/2017\/02\/28\/pi-molecular-orbitals-of-butadiene\/\">The Pi Molecular Orbitals of Butadiene<\/a><\/em>].<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15694\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/6-molecular-orbitals-of-butadiene-showing-homo-and-lumo-orbitals.gif\" alt=\"molecular orbitals of butadiene showing homo and lumo orbitals\" width=\"600\" height=\"431\" \/><\/p>\n<p>Now let&#8217;s see what happens when we try to line up the HOMO of butadiene with the LUMO of ethene.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15695\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/7-molecular-orbital-interactions-in-diels-alder-reaction-showing-homo-of-butadiene-and-lumo-of-dienophile-orbital-symmetry.gif\" alt=\"molecular orbital interactions in diels alder reaction showing homo of butadiene and lumo of dienophile orbital symmetry\" width=\"600\" height=\"468\" \/><\/p>\n<p><span style=\"color: #993366;\"><em>[Why not the other way around, with the LUMO of butadiene and the HOMO of ethene? See <a style=\"color: #993366;\" href=\"#notethree\">Note 3<\/a> ]\u00a0<\/em><\/span><\/p>\n<p>Here we have the\u00a0 diene (in the green plane) approaching the dienophile (orange plane) from the top, as a helicopter might approach a landing pad.\u00a0 New bonds form between C<sub>1\u00a0<\/sub>\u2013C<sub>6<\/sub> and C<sub>4<\/sub>\u2013C<sub>5<\/sub> . Note that the phases of the lobes for each pair of interactions match and thus have constructive orbital overlap.<\/p>\n<p>[Also note that although the diene is depicted as being on &#8220;top&#8221; here,\u00a0 it works equally well if it&#8217;s on the bottom\u00a0 <a href=\"\" class=\"custom-tooltip\" data-image=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/Supp-1-showing-diels-alder-orbital-interactions-from-other-face.gif\" data-link=\"\" data-title=\"\" data-text=\"\"> hover here for a pop-up image\u00a0 <\/a> or click this <a href=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/Supp-1-showing-diels-alder-orbital-interactions-from-other-face.gif\">link<\/a>. <span style=\"color: #993366;\"><em>The symmetry works out in both cases &#8211; just like it does for two Lego blocks, even though the phases of the &#8220;lobes&#8221; on each face are opposite ]<\/em><\/span><\/p>\n<p>This helps us understand why the Diels-Alder reaction works &#8211; <strong>the orbital interactions are favorable.<\/strong><\/p>\n<p>We&#8217;ll stop with the Diels-Alder, but\u00a0 [<a href=\"#notefour\">Note 4<\/a>] continues the discussion [nerds only].<\/p>\n<h2><strong><a id=\"six\"><\/a>6. Under &#8220;Photochemical&#8221; Conditions, The [2+2] Actually Works Pretty Well\u00a0<\/strong><\/h2>\n<p>Above, I said the [2+2] cycloaddition doesn&#8217;t work under &#8220;normal&#8221; conditions, by which I meant &#8220;heating&#8221;.<span style=\"color: #993366;\"><em> [Organic chemists\u00a0 usually use the term &#8220;thermal&#8221; conditions]<\/em><\/span><\/p>\n<p>However, it&#8217;s been observed that if one exposes the reaction to ultraviolet (UV) light, the reaction can proceed quite well. <span style=\"color: #993366;\"><em>[These are called, &#8220;photochemical conditions&#8221;].<\/em><\/span><\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15696\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/8-under-photochemical-conditions-the-22-cycloaddition-works-well-uv-light.gif\" alt=\"under photochemical conditions the 2+2 cycloaddition works well uv light\" width=\"600\" height=\"169\" \/><\/p>\n<p>Why?<\/p>\n<p>Ultraviolet light promotes an electron from the HOMO to the LUMO, resulting in a &#8220;new&#8221; HOMO. <span style=\"color: #993366;\"><em>[sometimes called HOMO-prime, or <a style=\"color: #993366;\" href=\"http:\/\/www.chem.ucla.edu\/~harding\/IGOC\/S\/somo.html\">SOMO<\/a> (for &#8220;singly occupied molecular orbital&#8221;)]<\/em>. [<a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2016\/09\/16\/introduction-to-uv-vis-spectroscopy\/\"><em>Here is a previous post on UV spectroscopy<\/em><\/a>].<\/span><\/p>\n<p>Now there are two bonding interactions between the lobes. And the reaction actually works!<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15697\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/9-why-does-22-cycloaddition-work-when-light-is-applied-promotes-electron-from-homo-to-lumo-two-bonding-interactions.gif\" alt=\"why does 2+2 cycloaddition work when light is applied promotes electron from homo to lumo two bonding interactions\" width=\"600\" height=\"419\" \/><\/p>\n<p>Not that one would want to mess with perfection, but it&#8217;s at least worth a brief note that promotion of the Diels-Alder reaction is done through heating,\u00a0<em>not<\/em> via photochemical means. An attempt to run a Diels-Alder under &#8220;photochemical conditions&#8221; would be met with the same failure as a [2+2] cycloaddition under thermal conditions, and for the same reasons &#8211; because the orbital symmetry is wrong.<\/p>\n<h2><strong><a id=\"seven\"><\/a>7. Summary &#8211; The Key Role Of Orbital Symmetry In Concerted Reactions Of Pi-Systems<\/strong><\/h2>\n<p>Under &#8220;thermal conditions&#8221; (heating, no UV light) the [2+2] is &#8220;forbidden&#8221; and the Diels-Alder is &#8220;allowed&#8221;. [<a href=\"#notefour\">Note 4<\/a>]<\/p>\n<p>The situation reverses in the presence of ultraviolet light, where an electron can be promoted to give a new HOMO with different orbital symmetry.<\/p>\n<p>Under photochemical conditions, the [2+2] cycloaddition between two alkenes is &#8220;allowed&#8221; and the Diels-Alder is &#8220;forbidden&#8221;.<\/p>\n<p>We can boil this all down to a simple table:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15702\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/10-summary-table-of-22-and-diels-alder-thermal-photochemical-forbidden-allowed-1.gif\" alt=\"summary table of 2+2 and diels alder thermal photochemical forbidden allowed\" width=\"600\" height=\"352\" \/><\/p>\n<p>As we continue to explore this topic, we&#8217;ll revisit this table and make updates, because there&#8217;s a whole family of reactions where orbital symmetry plays a crucial role.<\/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\/14\/molecular-orbital-pi-bond\/\" class=\"\"><span>Bonding And Antibonding Pi Orbitals<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2017\/02\/16\/molecular-orbitals-of-the-allyl-cation-allyl-radical-and-allyl-anion\/\" class=\"\"><span>Molecular Orbitals of The Allyl Cation, Allyl Radical, and Allyl Anion<\/span><\/a><\/li><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\/2018\/02\/09\/endo-exo-diels-alder-telling-them-apart\/\" class=\"\"><span>Exo vs Endo Products In The Diels Alder: How To Tell Them Apart<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2018\/09\/03\/diels-alder-kinetic-thermodynamic-exo-endo\/\" class=\"\"><span>Diels-Alder Reaction: Kinetic and Thermodynamic Control<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2018\/11\/05\/regiochemistry-in-the-diels-alder-reaction\/\" class=\"\"><span>Regiochemistry In The Diels-Alder Reaction<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2019\/12\/06\/the-intramolecular-diels-alder-reaction\/\" class=\"\"><span>The Intramolecular Diels Alder Reaction<\/span><\/a><\/li><\/ul><\/div>\n<p><a id=\"noteone\"><\/a><strong>Note 1<\/strong>. We&#8217;re making the assumption here that one molecule of ethene approaches the other molecule of ethene in the same way we&#8217;d bring together two pieces of Lego. The bottom face of one component joins with the top face of another.<\/p>\n<p>Each pair of lobes involved in bonding is on the same face of the molecule.\u00a0This arrangement is called\u00a0<em>suprafacial .\u00a0<\/em>It&#8217;s analogous to &#8220;<em>syn<\/em>&#8220;.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15699\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F1-definition-of-suprafacial-and-antarafacial-22-forbidden-under-thermal-conditions-suprafacial.gif\" alt=\"definition of suprafacial and antarafacial 2+2 forbidden under thermal conditions suprafacial\" width=\"600\" height=\"340\" \/><\/p>\n<p>There&#8217;s another possibility. What if, instead of the &#8220;shaded&#8221; lobe of the ethene HOMO combining with the &#8220;white&#8221; lobe of the LUMO, it instead got together with the &#8220;shaded&#8221; lobe on the\u00a0<em>other face\u00a0<\/em>of the LUMO. Since both lobes have the same phase, this would be a bonding interaction!<\/p>\n<p>There&#8217;s a name for the situation where lobes on opposite faces of a reactant participate in a reaction: it&#8217;s called, &#8220;antarafacial&#8221; (similar to &#8220;<em>anti<\/em>&#8220;).<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15700\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F2-definition-of-antarafacial-showing-22-antarafacial-reaction-very-strained-transition-state.gif\" alt=\"definition of antarafacial showing 2+2 antarafacial reaction very strained transition state\" width=\"600\" height=\"301\" \/><\/p>\n<p>You might ask why this doesn&#8217;t happen in the [2+2] cycloaddition between alkenes. If you build a model however, you&#8217;ll quickly see that the answer is that it <em>ain&#8217;t so frickin&#8217; easy!\u00a0<\/em>The transition state for a [2+2] between two alkenes with a single antarafacial component is highly strained.<\/p>\n<p><span style=\"color: #993366;\"><em>[There\u00a0are examples of [2+2] cycloadditions that work under thermal conditions, <a style=\"color: #993366;\" href=\"https:\/\/en.wikipedia.org\/wiki\/Ketene_cycloaddition\">such as those involving ketenes<\/a>, that do have an antarafacial component. That&#8217;s not a topic for today. ]<\/em><\/span><\/p>\n<p><a id=\"notetwo\"><\/a><strong>Note 2<\/strong>. The success of this analysis implies is that during these types of reactions, the symmetry of the molecular orbitals is\u00a0<em>conserved &#8211;\u00a0<\/em>in other words, we can treat the relative phases of the lobes on the orbitals as constant on the timescale of the transition state. This is why these rules are titled, &#8220;The Conservation of Orbital Symmetry&#8221;.<\/p>\n<p><a id=\"notethree\"><\/a><strong>Note 3.<\/strong>\u00a0 Interactions between the HOMO of the dienophile and the LUMO of the diene are just as favorable from an &#8220;orbital symmetry&#8221; perspective. The reaction rate, however, will be fastest in situations where the energies of the HOMO\/LUMO pair are close together. Most Diels-Alder reactions you&#8217;ll see will be of electron-rich dienes (high-energy HOMO) with electron-poor dienophiles (low-energy LUMO).<\/p>\n<p>There are also favorable Diels-Alder reactions between electron-poor dienes (low-energy LUMO) with electron-rich dienophiles (high-energy HOMO). These are known as <a href=\"https:\/\/en.wikipedia.org\/wiki\/Inverse_electron-demand_Diels%E2%80%93Alder_reaction\">inverse electron-demand Diels-Alder reactions<\/a>.<\/p>\n<p><a id=\"notefour\"><\/a><strong>Note 4<\/strong> &#8211;\u00a0The pattern continues to alternate as additional pi bonds are added; the [4+4] is &#8220;thermally forbidden&#8221; and the [6+4] is &#8220;thermally allowed&#8221;. The [6+6] is &#8220;thermally forbidden&#8221; again, and so on. The cycloaddition with the largest number of pi electrons I am aware of is a [14+2] cycloaddition. This is thermally allowed only because one of the reaction components (heptafulvalene) reacts in an antarafacial fashion.<\/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\/0332-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\/0333-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>&nbsp;<\/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\/3360-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\/3361-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\/3362-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\/3363-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\/3364-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>The HOMO and LUMO In The Diels Alder Reaction The Diels-Alder reaction is a concerted reaction between a diene (with 4 pi-electrons) and a dienophile <\/p>\n","protected":false},"author":1,"featured_media":34098,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1163],"tags":[1325,1326,631,940,941,612,1323,1327,1324],"post_folder":[],"class_list":["post-11439","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-dienes-and-mo-theory","tag-antarafacial","tag-concerted-reactions","tag-diels-alder","tag-homo","tag-lumo","tag-molecular-orbitals","tag-orbital-symmetry","tag-photochemical","tag-suprafacial"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.7 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>HOMO &amp; 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