{"id":10497,"date":"2017-02-23T14:40:50","date_gmt":"2017-02-23T19:40:50","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=10497"},"modified":"2026-04-22T12:02:53","modified_gmt":"2026-04-22T17:02:53","slug":"rules-for-aromaticity","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2017\/02\/23\/rules-for-aromaticity\/","title":{"rendered":"Rules For Aromaticity"},"content":{"rendered":"<p><strong>Rules For Aromaticity: The 4 Key Factors <\/strong><\/p>\n<p>In the last post we introduced the concept of\u00a0<a href=\"https:\/\/www.masterorganicchemistry.com\/2017\/01\/20\/introducing-aromaticity\/\"><strong>aromaticity<\/strong><\/a>, a property of some unusually stable organic molecules such as benzene. Although some aromatic molecules are indeed fragrant (hello,\u00a0<a href=\"https:\/\/en.wikipedia.org\/wiki\/Vanillin\">vanillin<\/a>!\u00a0) the term &#8220;aromaticity&#8221; actually has nothing to do with smell. \u00a0We saw that aromatic molecules:<\/p>\n<ul>\n<li>have an extremely high <strong>resonance energy<\/strong> (36 kcal\/mol for benzene)<\/li>\n<li>undergo <strong>substitution<\/strong> rather than <strong>addition<\/strong> reactions<\/li>\n<li>have delocalized pi-electrons<\/li>\n<\/ul>\n<p>We also gave a few example of other molecules besides benzene which are aromatic, and some which look similar to benzene (e.g. cyclooctatetraene) which are not.<\/p>\n<p>So what are the rules? How can we predict whether a molecule is aromatic or not?<\/p>\n<p><img fetchpriority=\"high\" decoding=\"async\" class=\"alignnone wp-image-32130\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2022\/10\/0-summary-of-the-necessary-properties-for-aromatic-molecules-cyclic-conjugated-huckel-rule-flat.gif\" alt=\"summary of the necessary properties for aromatic molecules - cyclic - conjugated - huckel rule - flat\" width=\"641\" height=\"528\" \/><\/a><\/p>\n<p><strong>Table of Contents<\/strong><\/p>\n<ol>\n<li><a href=\"#one\">Four Key Rules for Aromaticity<\/a><\/li>\n<li><a href=\"#two\">Condition #1 for Aromaticity: The Molecule Must Be Cyclic<\/a><\/li>\n<li><a href=\"#three\">Condition #2: Every atom in the ring must be conjugated<\/a><\/li>\n<li><a href=\"#four\">Condition #3: The Molecule Must Have [4n+2] Pi Electrons<\/a><\/li>\n<li><a href=\"#five\">Which Electrons Count As &#8220;Pi Electrons&#8221;?<\/a><\/li>\n<li><a href=\"#six\">Pyridine and the Benzene Anion<\/a><\/li>\n<li><a href=\"#seven\">Some Examples With 5-Membered Rings<\/a><\/li>\n<li><a href=\"#eight\">Condition #4: The Molecule Must Be Flat<\/a><\/li>\n<li><a href=\"#nine\">Summary: Rules For Aromaticity<\/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. Four Key Rules For Aromaticity<\/h2>\n<p>There turn out to be\u00a04 conditions a molecule <strong>must<\/strong> meet in order for it to be aromatic.<\/p>\n<p>It&#8217;s all or nothing. If any of these conditions are violated, no aromaticity is possible.<\/p>\n<ul>\n<li><strong>First<\/strong>, it must be <strong>cyclic<\/strong>.<\/li>\n<li><strong>Second<\/strong>, every atom in the ring must be <strong>conjugated<\/strong>.<\/li>\n<li><strong>Third<\/strong>, the molecule must have<strong> [4n+2] pi electrons<\/strong> (we&#8217;ll explain in depth what that means, below)<\/li>\n<li><strong>Fourth<\/strong>, the molecule must be <strong>flat<\/strong> (<span style=\"color: #993366;\"><em>usually true if conditions 1-3 are met, but there are rare exceptions<\/em><\/span>)<\/li>\n<\/ul>\n<p><img decoding=\"async\" class=\"alignnone wp-image-15753\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/1-examples-of-aromatic-compounds-naphthalene-furan-indole-pyridine-but-not-cyclooctatetraene.gif\" alt=\"examples of aromatic compounds naphthalene furan indole pyridine but not cyclooctatetraene\" width=\"600\" height=\"278\" \/><\/p>\n<p>Let&#8217;s go into more detail.<\/p>\n<h2><strong><a id=\"two\"><\/a>2. Condition #1 for Aromaticity: The Molecule Must Be Cyclic<\/strong><\/h2>\n<p>Determining if a molecule is cyclic is pretty straightforward. Is there a ring? If yes, move to condition #2. If there&#8217;s no ring, forget it.<\/p>\n<p>Case in point: (<em>Z<\/em>)-1,3,5 hexatriene has the same number of pi bonds (and pi electrons) as benzene, but isn&#8217;t aromatic. No ring, no aromaticity.<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-15754\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/2-four-key-rules-for-aromaticity-molecule-must-be-cyclic-no-exceptions-hexatriene-not-aromatic.gif\" alt=\"four key rules for aromaticity molecule must be cyclic no exceptions hexatriene not aromatic\" width=\"600\" height=\"414\" \/><\/p>\n<h2><strong><a id=\"three\"><\/a>3. Condition #2 For Aromaticity: Every Atom In The Ring Must Be Conjugated<\/strong><\/h2>\n<p>Obviously, being cyclic isn&#8217;t a sufficient condition for aromaticity. Just\u00a0look at\u00a0cyclohexene, above right. Not aromatic.<\/p>\n<p>In order for aromaticty to exist, there must also be a continuous ring of p-orbitals around the ring that build up into a larger cyclic &#8220;pi system&#8221;.<\/p>\n<p>One way of saying this is\u00a0that every atom around the ring must be capable of\u00a0<strong>conjugation<\/strong> with each other\u00a0.<\/p>\n<p>There are a few alternative ways to say the same thing.<\/p>\n<p>We can also say:<\/p>\n<ul>\n<li>&#8220;Every atom in the ring must have an available p orbital&#8221;, or<\/li>\n<li>&#8220;Every atom in the ring must be able to participate in resonance&#8221;.<\/li>\n<\/ul>\n<p>Remember that the &#8220;available p orbital&#8221; condition applies not just to atoms\u00a0that are part of a pi bond, but also atoms bearing a lone pair, a radical, or an empty p orbital (e.g. carbocations). <span style=\"color: #993366;\"><em>[See this post on <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2017\/01\/24\/conjugation-and-resonance\/\">conjugation and resonance<\/a> if you are unsure].<\/em><\/span><\/p>\n<p>The key thing \u00a0that &#8220;kills&#8221; conjugation is an <strong>sp<sup>3<\/sup> hybridized atom with four bonds to atoms<\/strong>. Such an atom cannot participate in resonance.<\/p>\n<p>This is\u00a0why the lone pair on pyrrole (below) isn&#8217;t as basic as\u00a0you&#8217;d expect a nitrogen to be. Protonation of the nitrogen disrupts the conjugation around the ring, destroying aromaticity in the process. [<a href=\"#noteone\">Note 1<\/a>]<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15755\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/3-four-rules-for-aromaticity-rule-two-every-atom-in-ring-must-be-conjugated-no-sp3-carbons-with-4-atoms-attached.gif\" alt=\"four rules for aromaticity rule two every atom in ring must be conjugated no sp3 carbons with 4 atoms attached\" width=\"600\" height=\"603\" \/><\/p>\n<p><span style=\"color: #993366;\"><em>Advanced note. There&#8217;s actually several molecules which are conjugated around the perimeter of the ring but the interior carbons are sp<sup>3<\/sup> hybridized. Still counts as aromatic<\/em><\/span>.\u00a0[<a href=\"#notetwo\">Note 2<\/a>]<\/p>\n<h2><strong><a id=\"four\"><\/a>4. Condition #3 For Aromaticity: The Molecule Must Have [4n+2] Pi Electrons <\/strong><\/h2>\n<p>The third condition is that the cyclic, conjugated molecule must have the correct number of pi electrons. Benzene and cyclooctatetraene are both cyclic and conjugated, but benzene is aromatic and cyclooctatetraene is not. The difference is that benzene has 6 pi electrons, and cyclooctatetraene has 8. [We&#8217;ll explain why this makes a difference in subsequent posts].<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15756\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/4-condition-3-for-aromaticity-molecule-must-have-odd-number-of-pairs-of-pi-electrons-benzene-aromatic-cycloocatetraene-not.gif\" alt=\"condition 3 for aromaticity molecule must have odd number of pairs of pi electrons benzene aromatic cycloocatetraene not\" width=\"600\" height=\"225\" \/><\/p>\n<p>Our quick shorthand we often use is to say that\u00a0benzene has [4n+2] pi electrons and cyclooctatetraene does not.<\/p>\n<p>However, this term [4n+2] <a href=\"https:\/\/www.masterorganicchemistry.com\/2012\/06\/29\/what-does-4n2-mean\/\">causes a lot of confusion in organic chemistry<\/a>. I&#8217;ve known\u00a0students who will stare at a molecule and try to figure out what &#8220;n&#8221; is.<\/p>\n<p><strong>&#8220;n&#8221; is not a property of the molecule!<\/strong><\/p>\n<p>Commenter <a href=\"https:\/\/www.masterorganicchemistry.com\/2012\/06\/29\/what-does-4n2-mean\/\">Claire<\/a> had a great way of putting it:<\/p>\n<blockquote><p>&#8220;4n+2 is not a formula that you apply to see if your molecule is aromatic. It is a formula that tells you what numbers are in the magic series. If your pi electron value matches any number in this series then you have the capacity for aromaticity.&#8221;<\/p><\/blockquote>\n<p>Exactly! \u00a0The &#8220;magic series&#8221; is: 2, 6, 10, 14, 18, 22&#8230;.. (and counting up from 4 after that). [<a href=\"#notethree\">Note 3<\/a>]<\/p>\n<p>[4n+2] is mathematical shorthand for writing out the series [2, 6, 10, 14, 18, 22&#8230;] .<\/p>\n<p>We can generate this series by plugging in whole numbers (&#8220;n&#8221; = 0, 1, 2, 3, 4&#8230; ) to the [4n+2] formula. Those values of &#8220;n&#8221; have nothing to do with molecules. We are just using them to generate the series.<\/p>\n<p>So for n = 0 , we have [4 (0) + 2] =\u00a0<strong>2<\/strong><br \/>\nfor n = 1 , we have [4 (1) + 2 ] =\u00a0<strong>6<br \/>\n<\/strong>for n = 2, we have [4 (2) + 2 ] =\u00a0<strong>10<\/strong><br \/>\nfor n = 3, we have [4 (3) +2 ] =\u00a0<strong>14<\/strong><\/p>\n<p>And so on. See how it generates the series [2, 6, 10, 14&#8230;] ? That&#8217;s the point of [4n+2].<\/p>\n<p>The numbers in this &#8220;magic series&#8221; are sometimes referred to as &#8220;H\u00fcckel Numbers&#8221; after Erich\u00a0H\u00fcckel, who proposed this\u00a0rule\u00a0back in 1931.<\/p>\n<p>The condition that aromatic molecules must have [4n+2] pi electrons is sometimes called &#8220;H\u00fcckel&#8217;s rule&#8221;.<\/p>\n<p>In the figure below, molecules which fulfill H\u00fcckel&#8217;s rule are in green; those which do not fulfill H\u00fcckel&#8217;s rule are in red.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15757\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/5-for-aromaticity-molecule-must-have-4-n-2-pi-electrons-benzene-naphthalene-azulene-and-so-on.gif\" alt=\"for aromaticity molecule must have 4 n + 2 pi electrons benzene naphthalene azulene and so on\" width=\"600\" height=\"664\" \/><\/p>\n<p>Note that we can count electrons in pi bonds as well as electrons from lone pairs (so long as the carbon isn&#8217;t already participating in a pi bond &#8211; see below). So the cyclopentadiene anion has six pi electrons &#8211; 4 from the two double bonds, and two from the lone pair on carbon.<\/p>\n<h2><a id=\"five\"><\/a>5. Which Electrons Count As &#8220;Pi Electrons&#8221;, And Which Do Not?<\/h2>\n<p>That seems straightforward enough.\u00a0However, complications can arise when we have atoms in the ring which both participate in pi bonding and also have a lone pair. For example,<\/p>\n<ul>\n<li>how do we count electrons in the benzene anion (below left) or pyridine? Do we count the lone pair electrons as Pi electrons, giving a total of 8? Or do we ignore them?<\/li>\n<li>What about furan (middle) which has two lone pairs on oxygen?<\/li>\n<li>What about pyrrole, with its lone pair on nitrogen, or imidazole, with two nitrogens?<\/li>\n<\/ul>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15758\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/6-count-the-number-of-pi-electrons-in-these-molecules-furan-pyrrole-pyridine-.gif\" alt=\"count the number of pi electrons in these molecules furan pyrrole pyridine\" width=\"600\" height=\"235\" \/><\/p>\n<p>In order to answer these questions, it&#8217;s important to remind ourselves of how p orbitals contribute to aromaticity in benzene.<\/p>\n<p>In benzene, each p orbital is arranged at right angles (90\u00b0) to the plane of the ring. Each p orbital contains a single electron. We can verify the total number of pi electrons in benzene \u00a0by counting the pi bonds: 3 pi bonds times two electrons = 6 pi electrons total.\u00a0\u00a0<a href=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2017\/02\/5-p-orbitals-benzene-1.png\"><br \/>\n<\/a><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15759\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/7-p-orbitals-contribute-to-aromaticity-in-benzene-c-h-bonds-are-at-90-degrees-to-pi-system.gif\" alt=\"p orbitals contribute to aromaticity in benzene c-h bonds are at 90 degrees to pi system\" width=\"630\" height=\"220\" \/><\/p>\n<p>Note that the C-H bonds are at 90\u00b0 to the pi system. If there was a lone pair where the C-H bond is, then it wouldn&#8217;t be able to interact with the pi system at all. Which brings us to&#8230;.<\/p>\n<h2><a id=\"six\"><\/a>6. Some Electrons Don&#8217;t Count: Pyridine and the Benzene Anion<\/h2>\n<p>The benzene anion has a lone pair on one of the carbons. This lone pair <strong>can&#8217;t be in a p orbital<\/strong>, since the p-orbital is participating in the pi system. Instead, it&#8217;s at 90 degrees to the pi system, in the plane of the ring.<\/p>\n<p>In other words, <strong>the lone pair on carbon doesn&#8217;t count as a pair of pi electrons since it can&#8217;t overlap with the pi system.<\/strong><\/p>\n<p>The same is also true for <strong>pyridine<\/strong>, where the lone pair is also at right angles to the pi system.<\/p>\n<p>So while in each case you might be tempted to say that they have 8 pi electrons, <strong>the correct answer is\u00a06.\u00a0<\/strong>This is a H\u00fcckel number, and both of these molecules are in fact aromatic.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-38741\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/8-lone-pair-in-pyridine-does-not-count-towards-4n-2-aromaticity-since-it-is-at-90-degrees.gif\" alt=\"lone pair in pyridine does not count towards 4n + 2 aromaticity since it is at 90 degrees\" width=\"600\" height=\"484\" \/><\/a><\/p>\n<p>The bottom line: each ring atom can contribute a\u00a0<strong>maximum\u00a0<\/strong>of one p orbital toward the pi system.<\/p>\n<h2><a id=\"seven\"><\/a>7. Some Examples Of Aromatic 5-Membered Rings<\/h2>\n<p>Some molecules with five-membered rings can also present ambiguities.<\/p>\n<p>The cyclopentadiene anion (below) \u00a0has a\u00a0lone pair on one of the\u00a0carbons. Can this lone pair contribute to the pi system?<\/p>\n<p>Since that carbon is not involved in any pi-bonding, the answer is\u00a0<strong>yes<\/strong>.<\/p>\n<p>The total number of pi electrons for the cyclopentadiene anion equals 2 (from the lone pair) plus the 4 electrons in the two pi bonds, giving us a total of 6. This is a H\u00fcckel number and the cyclopentadiene anion is in fact aromatic.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15761\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/9-cyclopentadiene-anion-has-6-pi-electrons-aromatic.gif\" alt=\"cyclopentadiene anion has 6 pi electrons aromatic\" width=\"600\" height=\"173\" \/><\/p>\n<p>A similar situation arises for <strong><a href=\"https:\/\/en.wikipedia.org\/wiki\/Pyrrole\" target=\"_blank\" rel=\"noopener noreferrer\">pyrrole<\/a><\/strong>. The nitrogen bears a lone pair but \u00a0is not involved in a pi bond (unlike pyridine, above). Therefore it can contribute to the pi system and this gives us a total of 6 pi electrons once we account for the 4 electrons from the two pi bonds.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15762\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/10-pyrrole-has-6-pi-electrons-aromatic-2-electrons-contribute-to-pi-system.gif\" alt=\"pyrrole has 6 pi electrons aromatic 2 electrons contribute to pi system\" width=\"600\" height=\"158\" \/><\/p>\n<p>A curious case is<strong> <a href=\"https:\/\/en.wikipedia.org\/wiki\/Furan\" target=\"_blank\" rel=\"noopener noreferrer\">furan<\/a><\/strong>, where the oxygen bears\u00a0<strong>two<\/strong> lone pairs. Does this mean that furan has 8 pi electrons?\u00a0<strong>No!\u00a0<\/strong><\/p>\n<p>Why not? Because as we noted above, each atom can contribute a <strong>maximum<\/strong> of one p-orbital towards the pi system. In furan, one lone pair is in a p orbital, contributing\u00a0to the pi system; the other is in the plane of the ring. This gives us a total of 6 pi electrons. Furan is aromatic. (So is <a href=\"https:\/\/en.wikipedia.org\/wiki\/Thiophene\" target=\"_blank\" rel=\"noopener noreferrer\">thiophene<\/a>, the sulfur analog of furan).<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15763\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/11-furan-has-6-pi-electrons-only-one-lone-pair-contributes-to-pi-system.gif\" alt=\"furan has 6 pi electrons only one lone pair contributes to pi system\" width=\"600\" height=\"178\" \/><\/p>\n<p>Finally there is<strong> imidazole<\/strong>, which has two nitrogens. One nitrogen (the N-H) <strong>is not<\/strong> involved in a pi bond, and thus can contribute a full lone pair; the other <strong>is<\/strong> involved in a pi bond, and the lone pair is in the plane of the ring. This also gives us a total of 6 pi electrons once we account for the two pi bonds.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15764\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/12-imidazole-has-6-pi-electrons-aromatic-only-one-nitrogen-contributes-lone-pair-to-pi-system.gif\" alt=\"imidazole has 6 pi electrons aromatic only one nitrogen contributes lone pair to pi system\" width=\"600\" height=\"198\" \/><\/p>\n<h2><strong><a id=\"eight\"><\/a>8. Condition #4 For Aromaticity: The Molecule Must Be Flat<\/strong><\/h2>\n<p>The fourth condition for aromaticity is that the molecule must be flat (planar).<\/p>\n<p>Aromaticity is such a stabilizing property (worth 20-36 kcal\/mol) that generally a molecule that is<\/p>\n<ul>\n<li>cyclic<\/li>\n<li>conjugated<\/li>\n<li>has [4n+2] pi electrons<\/li>\n<\/ul>\n<p>will also be flat. \u00a0Give a molecule a large enough potential energy well, and it will fall into it eventually.<\/p>\n<p>It&#8217;s a bit like the punch line\u00a0to the old (crude) joke about why dogs adopt a certain energetically favourable\u00a0conformation: \u00a0&#8220;<em>Because they can&#8221;. \u00a0<\/em><\/p>\n<p>As with certain vertebrates, the only thing that preventing a molecule that fulfills the first three conditions from being flat is if the flat conformation is incredibly strained. \u00a0 One example\u00a0in this category is the molecule known as [10]-annulene, an isomer of which is drawn below left. In the <em>trans<\/em>, <em>cis<\/em>,\u00a0<em>trans, cis, cis<\/em> isomer, the molecule is cyclic, conjugated, and has 10 pi electrons, but the two marked hydrogens bump into one another when attempting to adopt a flat conformation.<\/p>\n<p>The molecule is prevented from adopting\u00a0planarity\u00a0due to this\u00a0punitive <a href=\"https:\/\/en.wikipedia.org\/wiki\/Van_der_Waals_strain\">Van Der Waals strain<\/a>\u00a0, and is therefore\u00a0<strong>not aromatic.\u00a0<\/strong><\/p>\n<p>Interestingly, if the hydrogens are removed and replaced with a bridging CH<sub>2<\/sub> group, the strain is relieved\u00a0and the pi bonds can adopt a planar conformation. The molecule below right shows the expected properties of an aromatic molecule.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15765\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/13-molecule-must-be-flat-isomer-of-10-annulene-is-aromatic-ring-currents.gif\" alt=\"molecule must be flat isomer of 10 annulene is aromatic ring currents\" width=\"600\" height=\"416\" \/><\/p>\n<h2><a id=\"nine\"><\/a>9. Summary: Rules For Aromaticity<\/h2>\n<p>This post went through the four conditions a molecule must meet to be aromatic.<\/p>\n<p>Generally, determining if a molecule is cyclic and conjugated isn&#8217;t what trips people up.<\/p>\n<p>It&#8217;s the damn pi electron-counting!<\/p>\n<p>So in the next post, we&#8217;ll play the game show &#8220;Is This Aromatic?&#8221; with a series of different contestants and try to demonstrate a relatively quick n&#8217; easy plan for determining if an unknown molecule is aromatic or not.<\/p>\n<p>After that, we&#8217;ll look at the molecular orbitals and attempt to understand what exactly is so special about the H\u00fcckel series.<\/p>\n<p><strong>Thanks to\u00a0Matt Knowe for assistance with this post.<\/strong><\/p>\n<p><a href=\"https:\/\/www.masterorganicchemistry.com\/2017\/03\/03\/is-this-molecule-aromatic-some-practice-problems\/\"><strong>Check out these worked examples\u00a0<\/strong><\/a><\/p>\n<hr \/>\n<h2><a id=\"notes\"><\/a>Notes<\/h2>\n<div class=\"related-articles\"><p><strong>Related Articles<\/strong><\/p><ul><li><a href=\"https:\/\/www.masterorganicchemistry.com\/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\/03\/03\/is-this-molecule-aromatic-some-practice-problems\/\" class=\"\"><span>\u201cIs This Molecule Aromatic?\u201d Some Practice Problems<\/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\/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\/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\/20\/introduction-aromaticity\/\" class=\"\"><span>Introduction To Aromaticity<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2017\/07\/11\/electrophilic-aromatic-substitution-introduction\/\" class=\"\"><span>Electrophilic Aromatic Substitution: Introduction<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/organic-chemistry-practice-problems\/aromaticity-practice-quizzes\/\" class=\"\"><span>Aromaticity Practice Quizzes (MOC Membership)<\/span><\/a><\/li><\/ul><\/div>\n<p><strong><a id=\"noteone\"><\/a>Note 1. <\/strong>Fun fact. Pyrrole actually reacts with acid on carbon, not nitrogen!<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15766\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F1-most-basic-site-on-pyrrole-is-c-3-not-nitrogen-since-nitrogen-contributes-to-aromaticity.gif\" alt=\"most basic site on pyrrole is c-3 not nitrogen since nitrogen contributes to aromaticity\" width=\"600\" height=\"205\" \/><\/p>\n<p><strong><a id=\"notetwo\"><\/a>Note 2. <\/strong>Weird case: the one with aromaticity around the exterior but not on the inside.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15767\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F2-interesting-case-of-aromatiicty-14-pi-system-double-bonds-outside-interior-atoms-sp3-hybridized.gif\" alt=\"interesting case of aromatiicty 14 pi system double bonds outside interior atoms sp3 hybridized\" width=\"600\" height=\"306\" \/><\/p>\n<p><strong><a id=\"notethree\"><\/a>Note 3. <\/strong>It should be noted that Huckel&#8217;s rule starts to break down for higher numbers of pi electrons (&gt;20) in polycyclic systems. I&#8217;m not going to go into the details here. <a href=\"https:\/\/en.wikipedia.org\/wiki\/H\u00fcckel's_rule#Refinement\">The wikipedia article covers it well<\/a>.<\/p>\n<hr \/>\n<h2><a id=\"quizzes\"><\/a>Quiz Yourself!<\/h2>\n<div class=\"wq-quiz-wrapper\" data-id=\"37962\"><style type=\"text\/css\" id=\"wq-flip-custom-css\">.wq-quiz-wrapper[data-id=\"37962\"] {\n--wq-question-width: 100%;\n--wq-question-color: #009cff;\n--wq-question-height: auto;\n--wq-font-color: #444;\n}\n\n\t\t\t.wq-quiz-wrapper[data-id=\"37962\"] {\n\t\t\t\t--wq-question-width: 600px;\n\t\t\t}\n\n\t\t\t@media screen and (max-width: 600px) {\n\t\t\t\t.wq-quiz-wrapper[data-id=\"37962\"] .wq_singleQuestionWrapper { width:100% !important; height:auto !important; }\n\t\t\t}\n\t\t<\/style><!-- wp quiz -->\n<div id=\"wp-quiz-37962\" class=\"wq_quizCtr single flip_quiz wq-quiz wq-quiz-37962 wq-quiz-flip wq-layout-single wq-skin-traditional wq-should-show-correct-answer\" data-quiz-id=\"37962\">\n<div class=\"wq-questions wq_questionsCtr\">\n\t<div class=\"wq-question wq_singleQuestionWrapper wq-question-8lnvv\" data-id=\"8lnvv\">\n\n\t\n\t<div class=\"item_top\">\n\t\t<div class=\"title_container\">\n\t\t\t<div class=\"wq_questionTextCtr\">\n\t\t\t\t<h4 class=\"wq-question-title\"><\/h4>\n\t\t\t<\/div>\n\t\t<\/div>\n\t<\/div>\n\n\t<div class=\"card \">\n\t\t<div class=\"front\" >\n\t\n\t\t\t\t\t<img decoding=\"async\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-images\/3012-Front.gif\" \/>\n\t\t\n\t\t\n\t\n\t\n\t\t\t<span class=\"top-desc\">Click to Flip<\/span>\n\t<\/div>\n\t\t<div class=\"back\" >\n\t\n\t\t\t\t\t<img decoding=\"async\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-images\/3012-Reverse.gif\" \/>\n\t\t\n\t\t\n\t\n\t<\/div>\n\t<\/div>\n\n\t\n<\/div>\n<\/div>\n<\/div>\n<!-- \/\/ wp quiz-->\n<\/div><!-- End .wq-quiz-wrapper -->\n<p><br \/>\n<img loading=\"lazy\" decoding=\"async\" class=\"wp-image-36214 aligncenter\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/1022-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\/1023-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\/1024-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\/1001-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\/1005-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\/3026-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><a id=\"references\"><\/a>(Advanced) References and Further Reading<\/h2>\n<ol>\n<li><strong>Quantentheoretische Beitr\u00e4ge zum Benzolproblem<br \/>\nDie Elektronenkonfiguration des Benzols und verwandter Verbindungen<br \/>\n<\/strong>Erich H\u00fcckel<strong><br \/>\n<\/strong><em>Zeitschrift f\u00fcr Physik <\/em><strong>1931, <\/strong><em>70<\/em><strong>, <\/strong>204\u2013286<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/link.springer.com\/article\/10.1007%2FBF01339530\">10.1007\/BF01339530<\/a><br \/>\nErich H\u00fcckel achieved recognition by elaborating, together with Peter Debye, the theory of strong electrolytes in 1923 and later by applying a simplified version of quantum theory to p-electrons in conjugated molecules, which became known as H\u00fcckel molecular orbital (HMO) theory. Although he never explicitly formulated a \u201c4n + 2 rule\u201d, this was obvious from his work. H\u00fcckel showed that monocyclic systems with continuous conjugation having 6, 10, 14, etc. p-electrons benefited from extra stabilization and were aromatic. But it is more accurate to refer to the \u201cH\u00fcckel 4n + 2 p-electron rule,\u201d rather than to \u201cH\u00fcckel\u2019s rule.\u201d<\/li>\n<li><strong>Aromaticity Today: Energetic and Structural Criteria<br \/>\n<\/strong>Mikhail Glukhovtsev<strong><br \/>\n<\/strong><em>Journal of Chemical Education<\/em><strong> 1997<\/strong>, <em>74<\/em> (1), 132<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/pubs.acs.org\/doi\/abs\/10.1021\/ed074p132\">1021\/ed074p132<\/a><br \/>\nThis paper discusses two of the criteria for establishing aromaticity \u2013 planarity and a positive stabilization energy. The latter can be verified by computational methods, as the article demonstrates.<\/li>\n<li><strong>Global aromaticity at the nanoscale<br \/>\n<\/strong>Rickhaus, M., Jirasek, M., Tejerina, L. et al.<br \/>\n<em>Nature Chem<\/em>. <em>12<\/em>, 236\u2013241 (<strong>2020<\/strong>)<br \/>\n<strong>DOI: <\/strong><a href=\"https:\/\/www.nature.com\/articles\/s41557-019-0398-3\">10.1038\/s41557-019-0398-3<\/a><br \/>\nThis paper was published in early 2020 and officially sets the record for the largest aromatic system, with 162 p electrons! Evidence for aromaticity comes from both NMR measurements and experimental (NICS) calculations.<\/li>\n<li><strong>Nucleus-Independent Chemical Shifts:\u2009 A Simple and Efficient Aromaticity Probe<br \/>\n<\/strong>Paul von Ragu\u00e9 Schleyer, Christoph Maerker, Alk Dransfeld, Haijun Jiao, and Nicolaas J. R. van Eikema Hommes<br \/>\n<em>Journal of the American Chemical Society<\/em> <strong>1996,<\/strong> <em>118<\/em> (26), 6317-6318<br \/>\n<strong>DOI<\/strong>: <a href=\"https:\/\/pubs.acs.org\/doi\/10.1021\/ja960582d\">10.1021\/ja960582d<\/a><br \/>\nThis paper is an advanced topic but worth including here, as it is one of Prof. Schleyer\u2019s most highly cited papers. Aromaticity is a difficult concept to accurately define, but one way to empirically measure it is to use computational methods. Here, Prof. Schleyer describes the \u201cNICS effect\u201d as a method of measuring aromaticity, based on <em>magnetic susceptibility exaltation.<\/em> Aromatic compounds have a \u2018ring current\u2019 due to the conjugation of the <em>p<\/em> orbitals and the presence of delocalized p electrons, and are therefore diamagnetic. This can be measured experimentally or probed computationally.<\/li>\n<li><strong>The Cyclodecapentaene System<br \/>\n<\/strong> Dr. E. Vogel, Dipl.\u2010Chem. H. D. Roth<strong><br \/>\n<\/strong><em>Angew. Chem. Int. Ed.<\/em><strong> 1964<\/strong>, <em>3<\/em> (3), 228-229<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/onlinelibrary.wiley.com\/doi\/10.1002\/anie.196402282\">10.1002\/anie.196402282<\/a><\/li>\n<li><strong>Perspektiven der Cycloheptatrien-Norcaradien-Valenztautomerie<br \/>\n<\/strong>Emanuel Vogel<br \/>\n<em>Pure Appl. Chem.<\/em> <strong>1969<\/strong>, <em>20<\/em> (3), 237-262<br \/>\n<strong>DOI<\/strong>: <a href=\"http:\/\/publications.iupac.org\/pac\/20\/3\/0237\/index.html\">10.1351\/pac196920030237<\/a><br \/>\nThe above two papers are on the synthesis and characterization of cyclodeca-1,3,5,7,9-pentaene. The NMR spectrum shows a diamagnetic ring current of the type expected in an aromatic system.<\/li>\n<li><strong>Synthesis of 1,6-didehydro[10]annulene. Observation of its exceptionally facile rearrangement to form the biradical 1,5-dehydronaphthalene<\/strong><br \/>\nAndrew G. Myers and Nathaniel S. Finney<br \/>\n<em>Journal of the American Chemical Society<\/em> <strong>1992,<\/strong> <em>114<\/em> (27), 10986-10987<br \/>\n<strong>DOI<\/strong>: <a href=\"https:\/\/pubs.acs.org\/doi\/abs\/10.1021\/ja00053a059\">10.1021\/ja00053a059<\/a><br \/>\nMyers (now at Harvard) started his career as a professor at Caltech, and his early papers are gems in physical organic chemistry. This is no exception, and the brevity of this communication belies the extreme difficulty of this experimental work, both in synthesis and characterization. The compound intramolecularly cyclizes via a diradical intermediate at low temperature to naphthalene, and this type of reaction is now known as a \u201cMyers cyclization\u201d after Prof. Myers.<\/li>\n<li><strong>HOMO-AROMATIC STRUCTURES<\/strong><br \/>\nS. Winstein<br \/>\n<em>Journal of the American Chemical Society<\/em> <strong>1959,<\/strong> <em>81<\/em> (24), 6524-6525<br \/>\n<strong>DOI<\/strong>: <a href=\"https:\/\/pubs.acs.org\/doi\/10.1021\/ja01533a052\">10.1021\/ja01533a052<\/a><br \/>\nThe term \u2018homoaromaticity\u2019 was first introduced to chemistry in this paper by organic chemist Prof. Saul Winstein (UCLA). Today, the prefix \u2018homo-\u2019 is commonly used in chemistry to denote extension by 1 methylene (-CH<sub>2<\/sub>-) group.<\/li>\n<li><strong>UNSATURATED MACROCYCLIC COMPOUNDS. XV.1 CYCLOTETRADECAHEPTAENE<br \/>\n<\/strong>Franz Sondheimer and Yehiel Gaoni<br \/>\n<em>Journal of the American Chemical Society<\/em> <strong>1960,<\/strong> <em>82<\/em> (21), 5765-5766<br \/>\n<strong>DOI<\/strong>: <a href=\"https:\/\/pubs.acs.org\/doi\/abs\/10.1021\/ja01506a061\">10.1021\/ja01506a061<\/a><br \/>\n[14]annulene has been synthesized but found to be unstable even though it has 14 p electrons as per H\u00fcckel 4n + 2 p-electron rule. This is because it cannot achieve planarity, thus proving that planarity is an essential criterion for achieving aromatic stabilization!<\/li>\n<li><strong>On the structure and thermochemistry of [18]annulene<br \/>\n<\/strong>Jerome M. Schulman and Raymond L. Disch<br \/>\n<em>Journal of Molecular Structure: THEOCHEM<\/em> <strong>1991, <\/strong><em>234<\/em>, 213-225<br \/>\n<strong>DOI<\/strong>: <a href=\"https:\/\/www.sciencedirect.com\/science\/article\/abs\/pii\/016612809189014R\">10.1016\/0166-1280(91)89014-R<\/a><br \/>\n[18]annulene is large enough to minimize steric interactions between the internal hydrogens in a geometry that is free of angle strain. The properties of [18]annulene are consistent with its being aromatic.<\/li>\n<li><strong>Ab Initio Density Funtional vs Hartree Fock Predictions for the Structure of [18]Annulene: Evidence for Bond Localization and Diminished Ring Currents in Bicycloannelated [18]Annulenes<\/strong><br \/>\nKim K. Baldridge Prof. Jay S. Siegel<br \/>\n<em>Angew. Chem. Int. Ed. <\/em><strong>1997<\/strong>, <em>36<\/em> (7), 745-748<br \/>\n<strong>DOI<\/strong>: <a href=\"https:\/\/onlinelibrary.wiley.com\/doi\/abs\/10.1002\/anie.199707451\">10.1002\/anie.199707451<\/a><br \/>\nAn stabilization energy of 18 kcal\/mol has been calculated for [18]annulene.<\/li>\n<\/ol>\n","protected":false},"excerpt":{"rendered":"<p>Rules For Aromaticity: The 4 Key Factors In the last post we introduced the concept of\u00a0aromaticity, a property of some unusually stable organic molecules such <\/p>\n","protected":false},"author":1,"featured_media":32130,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[844],"tags":[846,320,312,313,1184,1182,1183,845],"post_folder":[],"class_list":["post-10497","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-aromaticity-2","tag-4n2","tag-aromaticity","tag-aromatics","tag-benzene","tag-conjugated","tag-cyclooctatetraene","tag-flat","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>Rules for Aromaticity: The 4 Key Factors &#8211; 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