{"id":10600,"date":"2017-03-27T16:56:38","date_gmt":"2017-03-27T20:56:38","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=10600"},"modified":"2026-04-22T12:29:14","modified_gmt":"2026-04-22T17:29:14","slug":"antiaromaticity","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2017\/03\/27\/antiaromaticity\/","title":{"rendered":"Antiaromatic Compounds and Antiaromaticity"},"content":{"rendered":"<p><strong>Antiaromatic Compounds Are <em>Unusually<\/em> Unstable<\/strong><\/p>\n<p>What are the key factors that determine whether a molecule is antiaromatic?<\/p>\n<p>It is similar to the requirements for aromaticity, except for one key factor (in <span style=\"color: #ff0000;\"><strong>red<\/strong><\/span>).<\/p>\n<ul>\n<li>The molecule must be\u00a0<strong>cyclic<\/strong><\/li>\n<li>The molecule must be\u00a0<strong>conjugated<\/strong> all the way around the ring<\/li>\n<li>The molecule must be <strong>flat<\/strong><\/li>\n<li>The molecule must have (<span style=\"color: #ff0000;\"><strong>4, 8, 12, 16&#8230;<\/strong><\/span>) pi electrons (summarized algebraically as &#8220;4n where n is a natural number&#8221;). Contrast this with aromatic molecules, which must have (2, 6, 10, 14, 18&#8230;) pi electrons.<\/li>\n<\/ul>\n<p>Anti-aromatic molecules are unusually <strong>unstable<\/strong>.<\/p>\n<p><img fetchpriority=\"high\" decoding=\"async\" class=\"alignnone wp-image-15787\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/0-summary-of-anti-aromaticity-unusually-unstable-cyclic-conjugated-4n-pi-electrons-flat-example-cyclobutadiene.gif\" alt=\"summary of anti aromaticity unusually unstable cyclic conjugated 4n pi electrons flat example cyclobutadiene\" width=\"600\" height=\"436\" \/><\/p>\n<p>In our previous posts in our series on aromaticity [<a href=\"https:\/\/www.masterorganicchemistry.com\/2017\/01\/20\/introducing-aromaticity\/\">intro to aromaticity<\/a>], [<a href=\"https:\/\/www.masterorganicchemistry.com\/2017\/02\/23\/rules-for-aromaticity\/\">rules for aromaticity<\/a>], we\u00a0saw that <strong>aromatic molecules are unusually stable<\/strong>. They have particularly large\u00a0resonance energies, tend to undergo substitution rather than addition reactions, and have delocalized pi electrons (all the C-C bond lengths in benzene are equal, for example).<\/p>\n<p>In order to be aromatic, a molecule must\u00a0possess the following four structural characteristics:<\/p>\n<ul>\n<li>cyclic<\/li>\n<li>conjugated all around the ring<\/li>\n<li>have [4n+2] pi electrons \u00a0[equivalently: the number of pi electrons must equal <em>twice an odd number<\/em>]<\/li>\n<li>flat (planar)<\/li>\n<\/ul>\n<p>If these four conditions are met, the molecule is aromatic.<\/p>\n<p><strong>If it flunks even one question on the test, it&#8217;s non-aromatic.<\/strong><\/p>\n<p>That seems to be pretty clear.\u00a0Or is it?<\/p>\n<p>You might guess from the way I&#8217;m framing this question\u00a0that the answer is &#8220;not quite&#8221;. : &#8211; )<\/p>\n<p><strong>Table of Contents<\/strong><\/p>\n<ol>\n<li><a href=\"#one\">Some Molecules Are So Spectacularly And Unusually Unstable And Hard-To-Isolate That They Deserve Their Own Category:Antiaromatic<\/a><\/li>\n<li><a href=\"#two\">Cyclopentadienyl Cation: The &#8220;Resonance-Stabilized&#8221; Carbocation That Isn&#8217;t Stable<\/a><\/li>\n<li><a href=\"#three\">Antiaromatic Three-Membered Rings:\u00a0 Oxirene, 1H-Azirene, Thiirene<\/a><\/li>\n<li><a href=\"#four\">Cyclobutadiene Is Antiaromatic<\/a><\/li>\n<li><a href=\"#five\">An 8 Pi-Electron Example of an Antiaromatic Compound: Pentalene<\/a><\/li>\n<li><a href=\"#six\">What Makes A Molecule Anti-Aromatic?<\/a><\/li>\n<li><a href=\"#seven\">Cyclooctatetraene &#8220;Escapes&#8221; Anti-Aromaticity Through Twisting Out Of Flatness<\/a><\/li>\n<li><a href=\"#eight\">Conclusion: Antiaromatic Compounds<\/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. Spectacularly Unstable Molecules That Form A Category Unto Themselves: &#8220;Anti-Aromatic&#8221;<\/h2>\n<p>Here&#8217;s the thing. There&#8217;s\u00a0a small number of molecules that flunk the aromaticity test that aren&#8217;t <em>just<\/em> non-aromatic:<strong> they have the property of being so spectacularly and\u00a0<em>unusually unstable<\/em> and difficult to isolate\u00a0that they deserve another name.<\/strong><\/p>\n<p>We call these molecules &#8220;<strong>anti-aromatic<\/strong>&#8220;.<\/p>\n<p>What&#8217;s interesting is that in many cases the molecules themselves don&#8217;t <em>look<\/em> to be particularly unstable from first principles, in contrast with, say, <a href=\"https:\/\/en.wikipedia.org\/wiki\/Cubane\">cubane<\/a>. \u00a0Funny enough, you can buy cubane (more or less)<a href=\"https:\/\/www.sigmaaldrich.com\/catalog\/product\/aldrich\/901000?lang=en&amp;region=US\"> from Aldrich<\/a>, but most of the anti-aromatic molecules we&#8217;ll discuss below are only stable at extremely low temperatures &#8211; if they can be isolated at all!<\/p>\n<h2><strong><a id=\"two\"><\/a>2. Cyclopentadienyl Cation: <\/strong><strong>The &#8220;Resonance-Stabilized&#8221; Carbocation That Isn&#8217;t Stable\u00a0<\/strong><\/h2>\n<p>Let&#8217;s start our journey down the rabbit hole with a simple example\u00a0that should seem familiar from first-semester organic chemistry.<\/p>\n<p>Remember the S<sub>N<\/sub>1 reaction?<\/p>\n<p><a href=\"\" class=\"custom-tooltip\" data-image=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/1-using-cats-to-compare-sn1-and-sn2-reactions-master-organic-chemistry.png\" data-link=\"\" data-title=\"\" data-text=\"\">The cat that leaves the comfy chair first?<\/a> <span style=\"color: #993366;\">(<em>see post: <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2012\/07\/13\/the-sn1-mechanism\/\">The S<sub>N<\/sub>1 Reaction<\/a>)<\/em><\/span><\/p>\n<p>Quick review: Start with an alkyl halide. Leaving group leaves, forming a carbocation. Nucleophile attacks. It&#8217;s called the S<sub>N<\/sub>1 because the rate-determining step (carbocation formation) is unimolecular. Bottom line: the more stable the carbocation, the faster the reaction proceeds. Hence, the reaction rate for alkyl halides is tertiary &gt; secondary &gt; primary, and the carbocation gets special bonus points if it&#8217;s stabilized by resonance.<\/p>\n<p>Look at these two S<sub>N<\/sub>1 reactions. Which one do you think would happen faster &#8211; the S<sub>N<\/sub>1 reaction which goes through a secondary carbocation (A), or the S<sub>N<\/sub>1 reaction which goes through a secondary carbocation and can form multiple resonance forms (B)?<\/p>\n<p>From everything you&#8217;ve learned so far, you&#8217;d expect the answer to be B).<a href=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2017\/03\/0-trick-question.png\"><br \/>\n<\/a> <img decoding=\"async\" class=\"alignnone wp-image-15788\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/1-trick-question-which-sn1-reaction-is-faster-cyclopentadienyl-cation-is-antiaromatic.gif\" alt=\"trick question which sn1 reaction is faster cyclopentadienyl cation is antiaromatic\" width=\"600\" height=\"396\" \/><a href=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2017\/03\/0-trick-question-1-e1490296675241.png\"><br \/>\n<\/a>It&#8217;s not.<\/p>\n<p>The answer is that reaction A) happens faster.<em> Reaction B does not happen at all.\u00a0<\/em>[<a href=\"#noteone\">Note 1<\/a>]<\/p>\n<p>This should strike you as deeply weird.<\/p>\n<p><strong>I<\/strong><strong>t is!\u00a0<\/strong><\/p>\n<p>How is it that this supposedly resonance-stabilized carbocation is\u00a0<em>less stable<\/em> than a secondary carbocation?<\/p>\n<p>After all, how\u00a0many times is it drilled into your head that resonance stabilizes molecules, especially carbocations?<\/p>\n<p>But here, it seems to be\u00a0<em>destabilizing.\u00a0<\/em>\u00a0It&#8217;s&#8230; it&#8217;s&#8230;<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-15789\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/2-resonance-stabilized-carbocation-cyclobutadienyl-cation-is-actually-unstable.jpg\" alt=\"resonance stabilized carbocation cyclobutadienyl cation is actually unstable\" width=\"325\" height=\"244\" \/><\/p>\n<p>But unstable it is! The cyclopentadienyl cation is\u00a0<strong>incredibly unstable and difficult to make<\/strong> &#8211; and it&#8217;s not for lack of trying.\u00a0There&#8217;s something very strange about the structure of the cyclopentadienyl cation that gives it unusual instability.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15790\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/3-cyclopentadienyl-cation-is-unusually-unstable-cyclic-conjugated-4-pi-electrons-and-flat.gif\" alt=\"cyclopentadienyl cation is unusually unstable cyclic conjugated 4 pi electrons and flat\" width=\"600\" height=\"132\" \/><\/p>\n<p>Cyclic, conjugated, flat&#8230;. and has 4 pi electrons. Interesting!<\/p>\n<h2><a id=\"three\"><\/a>3. Antiaromatic Three-Membered Rings:\u00a0 Oxirene, 1H-Azirene, Thiirene<\/h2>\n<p>Another class of &#8220;elusive&#8221; molecules is a family of three-membered rings. \u00a0You might recall from Org 1 that it&#8217;s easy to make epoxides from alkenes using an oxidant like <a href=\"https:\/\/www.masterorganicchemistry.com\/2011\/06\/17\/reagent-friday-m-cpba-meta-chloroperoxybenzoic-acid\/\"><em>m<\/em>-CPBA<\/a>.<\/p>\n<p>Did you ever wonder why we never covered the same reaction for alkynes?<\/p>\n<p>Well, it&#8217;s not for lack of trying. \u00a0Chemists have tried all kinds of methods for epoxidizing alkynes, and you know what? The reaction just doesn&#8217;t frickin&#8217; work.<\/p>\n<p>For instance, epoxidation of acetylene would give the molecule below (oxirene).<\/p>\n<p>Oxirene itself has never been observed, although there are tantalizing traces of its fleeting existence. \u00a0 And by the way, neither has the nitrogen analogue, \u00a01H-azirene, or the thiirene. \u00a0 [<a href=\"#notetwo\">Note 2<\/a>]<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15791\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/4-oxidrene-1h-azirene-and-thiirene-have-never-been-made-antiaromatic.gif\" alt=\"oxidrene 1h-azirene and thiirene have never been made antiaromatic\" width=\"600\" height=\"198\" \/><\/p>\n<p>Why not? What&#8217;s special about each of these cases?<\/p>\n<p>You might notice that, like the cyclopentadienyl cation, these molecules are <strong>cyclic, conjugated, flat, and have 4 pi electrons <\/strong>(two in the pi bond, and two from a lone pair).\u00a0\u00a0[<a href=\"#notethree\">Note 3<\/a>]<\/p>\n<p>OK. What else might be cyclic, conjugated, flat, and have 4 pi electrons?<\/p>\n<h2><a id=\"four\"><\/a>4. Cyclobutadiene Is Antiaromatic<\/h2>\n<p><a href=\"https:\/\/en.wikipedia.org\/wiki\/Cyclobutadiene\">Cyclobutadiene<\/a> looks like a simple enough molecule, but\u00a0it\u00a0wasn&#8217;t actually\u00a0synthesized\u00a0until 1965. And even then, it was found that it isn&#8217;t stable at temperatures above 35 Kelvin. [<a href=\"#notefour\">Note 4<\/a>]<\/p>\n<p>The question is, why?<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15792\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/5-cyclobutadiene-is-unusually-unstable-antiaromatic.gif\" alt=\"cyclobutadiene is unusually unstable antiaromatic\" width=\"600\" height=\"167\" \/><\/p>\n<p>Sure, it&#8217;s a four-membered ring, and yes, it has a lot of\u00a0ring strain, but <a href=\"https:\/\/en.wikipedia.org\/wiki\/Prismane\">more strained molecules have been made<\/a>\u00a0that are actually stable at room temperature.<\/p>\n<p>You might also note that like the above examples, cyclobutadiene is another example of a molecule that is cyclic, conjugated, has 4 pi-electrons, and is flat.<\/p>\n<p>What&#8217;s even more interesting is what was learned about\u00a0the\u00a0<em>geometry<\/em> of cyclobutadiene. Rather than being a molecule with identical bond lengths (like benzene), cyclobutadiene was found to have a rectangular shape, indicating that the electrons were\u00a0<em>not<\/em> delocalized. [<a href=\"#notethree\">Note 3<\/a>]<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15793\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/6-cyclobutadiene-is-rectangular-electrons-are-not-delocalized-unlike-aromatic-molecules.gif\" alt=\"cyclobutadiene is rectangular electrons are not delocalized unlike aromatic molecules\" width=\"600\" height=\"203\" \/><\/p>\n<p>What this tells us is that even when a molecule fulfills all the conditions (cyclic, conjugated, flat, 4 pi electrons), the symmetric geometry is particularly unstable. <a href=\"#notefive\">[Note 5]<\/a><strong>.<\/strong> Very interesting!<\/p>\n<h2><strong><a id=\"five\"><\/a>5. An 8 Pi-Electron Example of an Antiaromatic Compound: Pentalene<\/strong><\/h2>\n<p>So far all the examples we&#8217;ve seen so far had 4 pi electrons. You might be wondering: are there any examples of anti-aromatic molecules that have more than 4 pi electrons? Why, yes.<\/p>\n<p>The molecule below is called &#8220;Pentalene&#8221;. It has been synthesized, but is only stable below \u2013100 \u00b0C. Above this temperature it combines with another molecule of itself \u00a0(<a href=\"http:\/\/www-chem.unifr.ch\/tb\/bally\/pdf\/pentalene.pdf\">reference<\/a>). Another example of spectacular instability.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15794\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/7-pentalene-is-antiaromatic-unstable-above-100-degrees-c.gif\" alt=\"pentalene is antiaromatic unstable above -100 degrees c\" width=\"600\" height=\"149\" \/><\/p>\n<p>Pentalene has 8 pi electrons. This might set off some alarm bells of recognition. Remember how the number of pi-electrons in aromatic molecules followed the sequence (2, 6, 10, 14&#8230;.) ?<\/p>\n<p>This example suggests that the number of pi electrons in anti-aromatic molecules follows the sequence (4, 8, 12&#8230;)<\/p>\n<h2><strong><a id=\"six\"><\/a>6. What Makes A Molecule Anti-Aromatic?<\/strong><\/h2>\n<p>So what do all of these molecules in this rogues&#8217; gallery have in common?<\/p>\n<p>Each of them is <strong>cyclic, conjugated, and flat<\/strong> &#8211; and when you count the number of pi electrons, it&#8217;s <strong>multiples of 4<\/strong>.\u00a0So while aromatic molecules have (4n+2) pi electrons, the &#8220;rule&#8221; for anti aromatic molecules is (4n).\u00a0<strong>\u00a0<\/strong><em>(a<\/em><em>nother way to look at it: the number of pi electrons will be twice an even number).\u00a0<\/em><\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15795\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/8-each-of-these-unusually-unstable-molecules-is-cyclic-conjugated-and-flat-the-number-of-pi-electrons-is-4-or-8-and-they-are-antiaromatid.gif\" alt=\"each of these unusually unstable molecules is cyclic conjugated and flat the number of pi electrons is 4 or 8 and they are antiaromatid\" width=\"600\" height=\"226\" \/><\/p>\n<p>This unusual instability is called &#8220;anti-aromaticity&#8221;.<\/p>\n<p>This means that we can now draw up three categories for molecules according to the following criteria:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15796\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/9-criteria-for-aromaticity-antiaromaticity-and-non-aromaticity.gif\" alt=\"criteria for aromaticity antiaromaticity and non-aromaticity\" width=\"600\" height=\"332\" \/><\/p>\n<ul>\n<li>Aromatic molecules are cyclic, conjugated, have (4n+2) pi electrons, and are flat.<\/li>\n<li>Anti-aromatic molecules are cyclic, conjugated, have (4n) pi electrons, and are flat.<\/li>\n<li>Non-aromatic molecules are every other molecule that fails one of these conditions.<\/li>\n<\/ul>\n<p>Wait a second &#8211; you might wonder why cyclooctatetraene is classified as &#8220;non-aromatic&#8221;. With 8 pi electrons (twice an even number) shouldn&#8217;t it be &#8220;anti-aromatic&#8221;?<\/p>\n<h2><strong><a id=\"seven\"><\/a>7. Cyclooctatetraene &#8220;Escapes&#8221; Anti-Aromaticity Through Twisting Out Of Flatness<\/strong><\/h2>\n<p>Think of fulfilling the conditions for \u00a0&#8220;anti-aromaticity&#8221; \u00a0as a bit like qualifying for an extremely punishing income tax. Given the opportunity to find a loophole to get out of the tax, would you do it? Probably.<\/p>\n<p>Cyclooctatetraene is anti-aromatic <em>only\u00a0if it is flat<\/em>. However, the relatively &#8220;floppy&#8221; structure of cyclooctatetraene allows for some flexibility. The bonds can rotate away from flatness such that the molecule adopts a &#8220;tub-like&#8221; shape, thereby avoiding the &#8220;antiaromaticity tax&#8221; of <del>18<\/del> <a href=\"#refone\">12 kcal\/mol<\/a> that would be paid if all the p-orbitals on the molecule were conjugated with each other.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15797\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/10-is-cyclooctatetraene-anti-aromatic-no-becuase-it-isnt-flat-it-twists.gif\" alt=\"is cyclooctatetraene anti aromatic no - becuase it isnt flat it twists\" width=\"600\" height=\"270\" \/><\/p>\n<p>It turns out that cyclooctatetraene has been synthesized, is stable<em> (<span style=\"color: #993366;\">you can buy it <a style=\"color: #993366;\" href=\"http:\/\/www.sigmaaldrich.com\/catalog\/product\/aldrich\/138924?lang=en&amp;region=US\">here<\/a>, for example<\/span>)<\/em>\u00a0and behaves like a &#8220;normal&#8221; alkene, undergoing addition reactions, hydrogenation, and so on.<\/p>\n<p>Pentalene (above) which also has 8 pi-electrons, has a very rigid bicyclic structure that prevents bond-rotation away from flatness. Hence, it&#8217;s stuck in its anti-aromatic conformation.<\/p>\n<p>Anti-aromaticity for molecules with more than 8 pi electrons is known, but very unusual. <em>(<span style=\"color: #993366;\">Here&#8217;s a <span style=\"text-decoration: underline;\"><a style=\"color: #993366; text-decoration: underline;\" href=\"http:\/\/pubs.acs.org\/doi\/abs\/10.1021\/ja0291991\">fun example<\/a><\/span> of another &#8220;escape&#8221; from aromaticity for a molecule with 18 pi electrons.<\/span>)<\/em><\/p>\n<h2><strong><a id=\"eight\"><\/a>8. Conclusion: Antiaromatic Compounds<\/strong><\/h2>\n<p>So far our treatment of aromaticity and anti-aromaticity has been purely\u00a0descriptive and empirical. \u00a0We&#8217;ve shown lots of examples, and given lots of rules, but missing from the discussion has been any deep explanation of &#8220;<strong>why<\/strong>&#8220;.<\/p>\n<p>What&#8217;s so special about the benzene ring that makes it so stable?\u00a0<strong><em>Why<\/em><\/strong> is it stable?<\/p>\n<p>What&#8217;s so special about the cyclobutadiene system that makes it so\u00a0<em>unstable<\/em>. <em><strong>Why<\/strong><\/em> is it unstable?<\/p>\n<p>In order to answer these deeper questions, we&#8217;re going to have to step back and examine the molecular orbitals of these two molecules, and then come to a deeper understanding of aromaticity and anti-aromaticity.<\/p>\n<p>That&#8217;s what we&#8217;ll do in our next post.<\/p>\n<p><strong>Thanks to Matthew Knowe for help in preparing this post.<\/strong><\/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\/01\/20\/introduction-aromaticity\/\" class=\"\"><span>Introduction To Aromaticity<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2017\/03\/03\/aromatic-antiaromatic-nonaromatic-some-practice-problems\/\" class=\"\"><span>Aromatic, Non-Aromatic, or Antiaromatic? Some Practice Problems<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2012\/06\/29\/huckels-rule-what-does-4n2-mean\/\" class=\"\"><span>Huckel\u2019s Rule: What Does 4n+2 Mean?<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2017\/05\/10\/the-pi-molecular-orbitals-of-cyclobutadiene\/\" class=\"\"><span>The Pi Molecular Orbitals of Cyclobutadiene<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2017\/05\/05\/the-pi-molecular-orbitals-of-benzene\/\" class=\"\"><span>The Pi Molecular Orbitals of Benzene<\/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><\/ul><\/div>\n<p><strong><a id=\"noteone\"><\/a>Note 1. <\/strong>\u00a0For simplicity, the image shown in the main article fudges on the actual reaction conditions. The actual experimental conditions are shown below. The solvent is propionic acid, and the Lewis acid silver perchlorate is added to assist with pulling off the iodine. [The idea is that Ag+ coordinates to the iodine, and then the C-I bond breaks. Silver iodide is extremely insoluble, and precipitates out of solution, driving the reaction towards completion].<\/p>\n<p>Even under these conditions the cyclopentadienyl carbocation is not formed, which is a testament to its extreme instability.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15798\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F1-footnote-difficulty-of-formation-of-cyclopentadienyl-cation-never-formed.gif\" alt=\"footnote difficulty of formation of cyclopentadienyl cation never formed\" width=\"600\" height=\"227\" \/><\/p>\n<p><strong><a id=\"notetwo\"><\/a>Note 2.\u00a0<\/strong> Epoxidation on ethene (aka ethylene) is done to the tune of 15 million tons, annually. Epoxidation of acetylene to form oxirene is unknown.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15799\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F2-epoxidation-of-acetylene-very-difficult-because-it-would-lead-to-oxirene-antiaromatic.gif\" alt=\"epoxidation of acetylene very difficult because it would lead to oxirene antiaromatic\" width=\"600\" height=\"330\" \/><a href=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2017\/03\/F2-acetylene-oxide-e1490385668428.png\"><br \/>\n<\/a>Fleeting evidence of the existence of oxirenes has been found. \u00a0There is good evidence from isotope labelling studies that oxirenes are fleeting intermediates in certain <a href=\"https:\/\/www.masterorganicchemistry.com\/reaction-guide\/wolff-rearrangement\/\">Wolff rearrangements<\/a>. There are also two examples of oxirenes that have been trapped at extremely low temperature, but decomposed above 35 Kelvin. <em>[For the curious, <a href=\"http:\/\/pubs.acs.org\/doi\/abs\/10.1021\/cr00057a002\">here&#8217;s a review<\/a>]<\/em><\/p>\n<p>The fleeting existence of a 1H-azirine has been<a href=\"http:\/\/pubs.rsc.org\/en\/content\/articlelanding\/1969\/c2\/c29690000147\"> postulated <\/a>in the addition of a nitrene to an acetylene, which quickly rearranges to a (stable) 2H-azirene (below).<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15800\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F3-1H-azirine-never-been-formed-antiaromatic.gif\" alt=\"1H azirine never been formed antiaromatic\" width=\"600\" height=\"169\" \/><\/p>\n<p>Note that in 2H-azirene the lone pair on the nitrogen is at right angles to the pi system, so this system is non-aromatic as opposed to anti-aromatic.<\/p>\n<p>For more discussion, see March&#8217;s Advanced Organic Chemistry. I&#8217;ve been working off the 5th edition, pages 62-63.<\/p>\n<p><strong><a id=\"notethree\"><\/a>Note 3. <\/strong>\u00a0A very clever experiment determined that 1,2-dideutero cyclobutadiene has two isomers, not one (see below). This indicates that the double bonds in cyclobutadiene are not delocalized as they are in benzene, but are more like double bonds in a conventional diene.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15801\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F4-double-bonds-in-cyclobutadiene-are-not-delocalized-two-deuterated-isomers-observed.gif\" alt=\"double bonds in cyclobutadiene are not delocalized two deuterated isomers observed\" width=\"630\" height=\"251\" \/><\/p>\n<p>Cyclobutadiene reacts with itself at 35 K to form a &#8220;dimer&#8221;, in an example of a Diels-Alder reaction.<\/p>\n<p><strong><a id=\"notefour\"><\/a>Note 4<\/strong>. Yes, it&#8217;s true that the lone pair in azirene can be put into an sp<sup>3<\/sup> hybrid orbital (below). This likely reduces the anti aromatic &#8220;penalty&#8221; somewhat, but the molecule is still incredibly unstable. Anti-aromaticity is the simplest way to account for this.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15802\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F5-lone-pair-in-1h-azirene-drawn-in-sp3-hybrid-orbital.gif\" alt=\"lone pair in 1h azirene drawn in sp3 hybrid orbital\" width=\"600\" height=\"156\" \/><\/p>\n<p><strong><a id=\"notefive\"><\/a>Note 5<\/strong>. The rectangular geometry is due to an effect known as the <a href=\"https:\/\/en.wikipedia.org\/wiki\/Jahn\u2013Teller_effect\">Jahn-Teller effect<\/a>. Remember the Pauli exclusion principle, where electrons can&#8217;t have the same quantum number? Well, as we&#8217;ll see when we look at the molecular orbital diagram of cyclobutadiene, &#8220;square&#8221; cyclobutadiene has two &#8220;degenerate&#8221; electrons, i.e. they have the same energies and quantum numbers. This leads to a phenomenon known as &#8220;Pauli repulsion&#8221;, where elongation of the bonds\u00a0occur until the energy levels of the two electrons are differentiated. Definitely an advanced topic.<\/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\/1033-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\/1035-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\/1013-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\/1016-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\/0331-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\/3204-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<hr \/>\n<h2><strong><a id=\"references\"><\/a>(Advanced) References and Further Reading<\/strong><\/h2>\n<ol>\n<li class=\"highwire-cite-title media__headline__title\"><strong><a id=\"refone\"><\/a>Transition-State Spectroscopy of Cyclooctatetraene<br \/>\n<\/strong>Paul G. Wenthold, David A. Hrovat, Weston T. Borden, W. C. Lineberger.<br \/>\nScience<strong>,\u00a0<\/strong><strong>1996<\/strong>, <em>272<\/em>, 1456-1459.<br \/>\n<strong>DOI<\/strong>: <a href=\"https:\/\/science.sciencemag.org\/content\/272\/5267\/1456\/tab-pdf\">10.1126\/science.272.5267.1456<\/a><br \/>\nThis study is the basis for the statement that the non-aromatic ground state of cyclooctatetraene is 12 kcal\/mol below the &#8220;antiaromatic&#8221; conformation.<\/li>\n<li><strong>The Taming of Cyclobutadiene<br \/>\n<\/strong>Donald J. Cram, Martin E. Tanner, Robert Thomas<strong><br \/>\n<\/strong><em> Chem. Int. Ed.<\/em><strong> 1991, <\/strong><em>30<\/em> (8), 1024-1027<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/onlinelibrary.wiley.com\/doi\/10.1002\/anie.199110241\">10.1002\/anie.199110241<\/a><br \/>\nThis paper by Nobel Laureate Prof. Donald J. Cram (UCLA) describes the first spectroscopic characterization of cyclobutadiene. This is accomplished by enclosing the precursor in a hemicarcerand (cage complex), and then irradiating it with UV light, thus creating cyclobutadiene, which is unable to do anything since it is trapped.<\/li>\n<li><strong>Cyclobutadiene<br \/>\n<\/strong>Thomas Bally, Satoru Masamune<strong><br \/>\n<\/strong><em>Tetrahedron<\/em> <strong>1980<\/strong>, <em>36<\/em> (3), 343-370<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/www.sciencedirect.com\/science\/article\/abs\/pii\/0040402080870037\">1016\/0040-4020(80)87003-7<\/a><br \/>\nThis paper from 1980 reviews work done on cyclobutadiene up to that time. This is divided into 2 parts \u2013 experimental synthetic efforts and theoretical calculations.<\/li>\n<li><strong>Tetrahedrane and Cyclobutadiene<br \/>\n<\/strong>G\u00fcnther Maier<strong><br \/>\n<\/strong><em> Chem. Int. Ed.<\/em><strong> 1988, <\/strong><em>27<\/em> (3), 309-332<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/onlinelibrary.wiley.com\/doi\/10.1002\/anie.198803093\">10.1002\/anie.198803093<\/a><strong><br \/>\n<\/strong>This paper reviews synthetic efforts towards tetrahedrane and cyclobutadiene \u2013 both molecules have a formula of C<sub>4<\/sub>H<sub>4<\/sub>. Only substituted (tetra <em>t<\/em>-butyl and tetra trimethylsilyl) tetrahedranes have been successfully synthesized and isolated to date.<\/li>\n<li><strong>Cyclobutadiene- and Benzocyclobutadiene-Iron Tricarbonyl Complexes<br \/>\n<\/strong> F. Emerson, L. Watts, and R. Pettit<strong><br \/>\n<\/strong><em>Journal of the American Chemical Society<\/em><strong> 1965, <\/strong><em>87<\/em> (1), 131-133<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/pubs.acs.org\/doi\/abs\/10.1021\/ja01079a032\">10.1021\/ja01079a032<\/a><\/li>\n<li><strong>Cyclobutadieneiron Tricarbonyl. A New Aromatic System<br \/>\n<\/strong> D. Fitzpatrick, L. Watts, G. F. Emerson, and R. Pettit<strong><br \/>\n<\/strong><em>Journal of the American Chemical Society<\/em><strong> 1965, <\/strong><em>87<\/em> (14), 3254-3255<strong><br \/>\nDOI: <\/strong><a href=\"http:\/\/10.1021\/ja01092a050\">10.1021\/ja01092a050<\/a><strong><br \/>\n<\/strong>A bonus topic: cyclobutadiene can be stabilized with metals \u2013 the organometallic complex cyclobutadiene iron tricarbonyl is a stable, crystalline solid. As the authors mention, it is unlikely that free cyclobutadiene is generated in the synthesis.<\/li>\n<li><a href=\"https:\/\/www.asu.edu\/courses\/chm233\/notes\/aromatic\/aromaticRL1\/antiaromatic.html\">Olanzapine<\/a>.<br \/>\nThe antipsychotic olanzapine adopts a \u201cbutterfly\u201d shape to avoid antiaromatic effects, similar to cyclooctatetraene. This is apparently critical to its function.<\/li>\n<li><strong>Antiaromaticity<br \/>\n<\/strong>Ronald Breslow<strong><br \/>\n<\/strong><em>Accounts of Chemical Research<\/em><strong> 1973, <\/strong><em>6<\/em> (12), 393-398<strong><br \/>\nDOI: <\/strong><a href=\"http:\/\/10.1021\/ar50072a001\">1021\/ar50072a001<\/a><strong><br \/>\n<\/strong>Interestingly, Prof. Breslow (Columbia) states in this account that he was the person to first suggest the term \u201cantiaromatic\u201d for especially destabilized cyclic compounds with 4n p electrons. He also cites other work by Prof. M. J. S. Dewar and others where the antiaromatic destabilization of cyclobutadiene is calculated to be between 18-33 kcal\/mol relative to linear butadiene.<\/li>\n<li><strong>Experimental Determination of the Antiaromaticity of Cyclobutadiene<br \/>\n<\/strong>Ashok A. Deniz, Kevin S. Peters, Gary J. Snyder<strong><br \/>\n<\/strong><em>Science<\/em> <strong>1999<\/strong>, <em>286<\/em> (5442), 1119-1122<br \/>\n<strong>DOI: <\/strong><a href=\"https:\/\/science.sciencemag.org\/content\/286\/5442\/1119.abstract\">1126\/science.286.5442.1119<\/a><strong><br \/>\n<\/strong>This is a very rigorous paper that uses novel spectroscopic techniques to determine antiaromatic destabilization of cyclobutadiene. Relative to a hypothetical strain-less, conjugated diene reference, cyclobutadiene is destabilized by a total of 87 kcal\/mol, 32 kcal\/mol of which can be attributed to ring strain and 55 kcal\/mol to antiaromaticity (compared with 21 kcal\/mol for the aromatic stabilization of benzene).<\/li>\n<li><strong>Unsubstituted cyclopentadienyl cation, a ground-state triplet<br \/>\n<\/strong> Saunders, R. Berger, A. Jaffe, J. M. McBride, J. O&#8217;Neill, R. Breslow, J. M. Hoffmann, C. Perchonock, E. Wasserman, R. S. Hutton, V. J. Kuck<strong><br \/>\n<\/strong><em>Journal of the American Chemical Society<\/em><strong> 1973, <\/strong><em>95<\/em> (9), 3017-3018<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/pubs.acs.org\/doi\/abs\/10.1021\/ja00790a049\">10.1021\/ja00790a049<\/a><br \/>\nThe cyclopentadienyl cation can only be prepared with difficulty, as this paper describes. Once it is formed, it has a triplet electronic ground state (with 2 unpaired electrons), and can be characterized by EPR (electron paramagnetic resonance) spectroscopy.<\/li>\n<li><strong>Oxirenes<br \/>\n<\/strong>Errol G. Lewars<strong><br \/>\n<\/strong><em>Chemical Reviews<\/em> <strong>1983,<\/strong> <em>83<\/em> (5), 519-534<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/pubs.acs.org\/doi\/abs\/10.1021\/cr00057a002\">10.1021\/cr00057a002<\/a><\/li>\n<\/ol>\n<div><\/div>\n<p class=\"highwire-cite-metadata byline\">\n","protected":false},"excerpt":{"rendered":"<p>Antiaromatic Compounds Are Unusually Unstable What are the key factors that determine whether a molecule is antiaromatic? It is similar to the requirements for aromaticity, <\/p>\n","protected":false},"author":1,"featured_media":15787,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[844],"tags":[846,929,320,1194,1193,845],"post_folder":[],"class_list":["post-10600","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-aromaticity-2","tag-4n2","tag-antiaromatic","tag-aromaticity","tag-azirene","tag-cyclobutadiene","tag-huckels-rule"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.7 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Antiaromaticity and Antiaromatic Compounds &#8211; Master Organic Chemistry<\/title>\n<meta name=\"description\" content=\"What makes a compound antiaromatic? What are some examples of antiaromatic compounds? 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