{"id":10352,"date":"2017-01-20T05:47:16","date_gmt":"2017-01-20T10:47:16","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=10352"},"modified":"2026-01-06T10:21:46","modified_gmt":"2026-01-06T16:21:46","slug":"introduction-aromaticity","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2017\/01\/20\/introduction-aromaticity\/","title":{"rendered":"Introduction To Aromaticity"},"content":{"rendered":"<p><strong>What Is &#8220;Aromaticity&#8221;, Anyway?\u00a0<\/strong><\/p>\n<p>In this post we introduce &#8220;aromaticity&#8221;, a term for describing a collection of three [<a href=\"#noteone\">Note 1<\/a>] main properties that distinguish benzene from (hypothetical) cyclohexatriene.<\/p>\n<ul>\n<li>Extremely large resonance energy (36 kcal\/mol)<\/li>\n<li>Delocalized electrons<\/li>\n<li>Undergoes substitution rather than addition reactions<\/li>\n<\/ul>\n<p><span style=\"color: #993366;\"><em>[This collection of features is not unique to benzene. We&#8217;ll describe other aromatic molecules in the next post. See &#8220;<a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2017\/02\/23\/rules-for-aromaticity\/\">Rules For Aromaticity<\/a>&#8220;].<\/em><\/span><\/p>\n<p>In the sections below we go through each of these three features in detail.<\/p>\n<p><img fetchpriority=\"high\" decoding=\"async\" class=\"alignnone wp-image-32129\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2022\/10\/0-summary-of-aromaticity-and-the-properties-of-aromatic-molecules.gif\" alt=\"summary of aromaticity and the properties of aromatic molecules\" width=\"640\" height=\"614\" \/><\/a><\/p>\n<p><strong>Table of Contents<\/strong><\/p>\n<ol>\n<li><a href=\"#one\">The Resonance Energy Of 1,3 Cyclohexadiene Is 2 kcal\/mol<\/a><\/li>\n<li><a href=\"#two\">The Resonance Energy Of &#8220;Cyclohexatriene&#8221; Is 36 kcal\/mol (!WOW!)<\/a><\/li>\n<li><a href=\"#three\">The &#8220;Emergent Property&#8221; Of Aromaticity<\/a><\/li>\n<li><a href=\"#four\">The Structure of <span class=\"s2\">Cyclohexatriene<\/span><span class=\"s1\"> Benzene And The &#8220;Delocalized&#8221; Nature Of Its Pi Bonds<\/span><\/a><\/li>\n<li><a href=\"#five\">Benzene Undergoes Substitution Reactions, Not Addition Reactions<\/a><\/li>\n<li><a href=\"#six\">This Collection of Three Special Properties Is Called &#8220;Aromaticity&#8221;<\/a><\/li>\n<li><a href=\"#seven\">Aromaticity Is Not Unique To Benzene<\/a><\/li>\n<li><a href=\"#eight\">So What Makes A Molecule Aromatic?<\/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><strong><a id=\"one\"><\/a>1. The Resonance Energy Of 1,3 Cyclohexadiene Is 2 kcal\/mol<\/strong><\/h2>\n<p>What&#8217;s &#8220;resonance energy&#8221; again? Let&#8217;s start with cyclohexene and build our way up.<\/p>\n<p>In case you can&#8217;t picture it, cyclohexene is a six membered ring containing a single pi ( \u03c0)bond.<\/p>\n<p>If you treat cyclohex<strong>ene<\/strong> with hydrogen gas (H<sub>2<\/sub>) \u00a0in the presence of a noble metal catalyst such as Pd supported on carbon (Pd\/C), you form cyclohex<strong>ane<\/strong>. The reaction liberates 28.6 kcal\/mol (120 kJ\/mol) of heat.<\/p>\n<p>If you&#8217;re in second semester organic chemistry, there&#8217;s no doubt you&#8217;ve\u00a0seen this reaction before.<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-15740\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/1-hydrogenation-of-cyclohexene-to-cyclohexane-releases-28-kcal-mol-exergonic.gif\" alt=\"hydrogenation of cyclohexene to cyclohexane releases 28 kcal mol exergonic\" width=\"600\" height=\"199\" \/><\/p>\n<p>Now for a slightly more interesting question. What happens to the enthalpy of hydrogenation when we add a second double bond adjacent to the first one: 1,3-cyclohexadiene. <span style=\"color: #993366;\"><em>(There&#8217;s another isomer &#8211;\u00a0<a style=\"color: #993366;\" href=\"https:\/\/en.wikipedia.org\/wiki\/1,4-Cyclohexadiene\">1,4 cyclohexadiene<\/a> &#8211; which we won&#8217;t concern ourselves with for this example, because the double bonds are\u00a0not conjugated).<\/em><\/span><\/p>\n<p>The enthalpy of hydrogenation in this case is 55.2 kcal\/mol (231 kJ\/mol) ; almost, but not quite double of that for cyclohexene (2 \u00d7 28.6 = 57.2 kcal\/mol).<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-32131\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2022\/10\/2-hydrogenation-of-1-3-hexadiene-to-cyclohexane-is-exothermic-by-55-kcal-mol-giving-resonance-energy-of-2-cal-mol-example-of-resonance-energy.gif\" alt=\"hydrogenation of 1 3 hexadiene to cyclohexane is exothermic by 55 kcal mol giving resonance energy of 2 cal mol example of resonance energy\" width=\"641\" height=\"245\" \/><\/a><\/p>\n<p>This is still an interesting result, because it tells us that having those two double bonds together results in a little bit of extra stability (2 kcal\/mol) versus two isolated pi bonds.<\/p>\n<p><strong>We call this extra stabilization (2 kcal\/ mol)\u00a0 &#8220;resonance energy&#8221; . It&#8217;s a consequence of conjugation &#8211; extending the p orbital overlap.<\/strong><\/p>\n<h2><a id=\"two\"><\/a>2. The Resonance Energy Of &#8220;Cyclohexatriene&#8221; Is 36 kcal\/mol (!WOW!)<\/h2>\n<p>Finally, let&#8217;s move on to cyclohexatriene. What happens when there are three pi bonds in the ring?<\/p>\n<p>We might naively expect the hydrogenation to liberate about (3 \u00d7 28.6 kcal\/mol = 85.8 kcal\/mol) \u00a0of heat.<\/p>\n<p>As it turns out, when we try to hydrogenate &#8220;cyclohexatriene&#8221; under normal conditions (Pd\/C, room temperature 1 atm of H<sub>2<\/sub>), nothing happens. It&#8217;s inert.<\/p>\n<p>Gradually we increase the heat and pressure. Still nothing happens. Cyclohexatriene refuses to hydrogenate.<\/p>\n<p>Something strange is clearly going on here.\u00a0Clearly, drastic measures are required.<\/p>\n<p>In desperation,\u00a0we\u00a0march to the stockroom and break out the heavy artillery.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15742\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/3-can-of-aldrich-whup-ass-98-per-cent-contents-one-foot.jpg\" alt=\"can of aldrich whup ass 98 per cent contents one foot\" width=\"600\" height=\"1067\" srcset=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/3-can-of-aldrich-whup-ass-98-per-cent-contents-one-foot.jpg 720w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/3-can-of-aldrich-whup-ass-98-per-cent-contents-one-foot-169x300.jpg 169w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/3-can-of-aldrich-whup-ass-98-per-cent-contents-one-foot-576x1024.jpg 576w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/3-can-of-aldrich-whup-ass-98-per-cent-contents-one-foot-320x569.jpg 320w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/3-can-of-aldrich-whup-ass-98-per-cent-contents-one-foot-640x1138.jpg 640w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/3-can-of-aldrich-whup-ass-98-per-cent-contents-one-foot-360x640.jpg 360w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/3-can-of-aldrich-whup-ass-98-per-cent-contents-one-foot-428x760.jpg 428w\" sizes=\"(max-width: 600px) 100vw, 600px\" \/><\/p>\n<p>At 180-220\u00b0C and 25-30 atmospheres of hydrogen gas, &#8220;cyclohexatriene&#8221;<strong> finally<\/strong> succumbs to hydrogenation to give us cyclohexane.<\/p>\n<p>The results of the enthalpy measurement are shocking. Recall that we expected the reaction to liberate 85 kcal\/mol of heat.<\/p>\n<p>In fact, the reaction liberates 49.8 kcal\/mol (208 kJ\/mol) of heat.\u00a0<strong>Our guess was off by 36 kcal\/mol<\/strong>.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15743\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/4-hydrogenation-of-theoretical-cyclohexatriene-to-cyclohexane-is-exothermic-by-50-kcal-mol-36-kcal-mol-less-than-expected-from-cyclohexene-resonance-energy.gif\" alt=\"hydrogenation of theoretical cyclohexatriene to cyclohexane is exothermic by 50 kcal mol 36 kcal mol less than expected from cyclohexene resonance energy\" width=\"630\" height=\"269\" \/><\/p>\n<p>Hot Damn!<\/p>\n<h2><a id=\"three\"><\/a>3. The &#8220;Emergent Property&#8221; Of Aromaticity<\/h2>\n<p>The heat liberated by hydrogenating cyclohexene and cyclohexadiene grew\u00a0in roughly linear fashion, but adding that third double bond unlocked an important\u00a0emergent property.<\/p>\n<p><span style=\"color: #993366;\"><em>[We&#8217;re going to discuss this special property a lot in the next few posts in this series &#8211; it&#8217;s called &#8220;aromaticity&#8221; &#8211; but let&#8217;s not get too ahead of ourselves.]<\/em><\/span><\/p>\n<p>First, it might help to take these energies and visualize them on a graph. Think of it as being a bit like standing on successive rungs of a ladder, with each &#8220;rung&#8221; representing a new pi bond. The &#8220;height above ground&#8221; corresponds to the energy released upon hydrogenation of the three successive molecules to the same final product (cyclohexane).<\/p>\n<p>Going from the &#8220;second rung&#8221; (cyclohexadiene) to the ground state released almost twice as much energy than did going from the &#8220;first rung&#8221; to the ground state. It was about 3% less (2 kcal\/mol) due to resonance energy.<\/p>\n<p>We expected\u00a0the &#8220;third rung&#8221; of our imaginary ladder to be another 26 kcal\/mol or so higher up. In fact, the third rung is about 6 kcal\/mol\u00a0<em>lower<\/em> than the second rung.<\/p>\n<p>Have you ever climbed a set of stairs with your eyes closed\u00a0and, reaching the top, \u00a0stumbled after\u00a0your foot\u00a0expected to land on\u00a0a step that&#8217;s not there? <strong>It&#8217;s a lot like that.\u00a0<\/strong><\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15744\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/5-visualization-of-resonance-energy-in-hydrogenation-of-cyclohexene-vs-cyclohexadiene-vs-cyclohexatriene.gif\" alt=\"visualization of resonance energy in hydrogenation of cyclohexene vs cyclohexadiene vs cyclohexatriene\" width=\"600\" height=\"466\" \/><\/p>\n<h2><a id=\"four\"><\/a>4. The Structure of <del>Cyclohexatriene<\/del> Benzene And The &#8220;Delocalized&#8221; Nature Of Its Pi Bonds<\/h2>\n<p>As you probably know by now, our &#8220;cyclohexatriene&#8221; actually goes by a different name:\u00a0<strong>benzene<\/strong>.<\/p>\n<p>Benzene has been known since at least the Middle Ages. Michael Faraday isolated it and originally determined its composition as C<sub>12<\/sub>H<sub>6<\/sub> (later revised to C<sub>6<\/sub>H<sub>6<\/sub> when the atomic\u00a0mass of carbon was\u00a0corrected) through burning a sample and <a href=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2017\/01\/Faraday_Benzene.png\">measuring the samples of CO<sub>2<\/sub> and H<sub>2<\/sub>O given off<\/a>. Here is Faradays sample of benzene, which he named &#8220;bicarburet of hydrogen&#8221;.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15745\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/6-Michael-Faradays-sample-of-benzene-bicarburet-of-hydrogen.jpg\" alt=\"Michael Faradays sample of benzene bicarburet of hydrogen\" width=\"630\" height=\"212\" srcset=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/6-Michael-Faradays-sample-of-benzene-bicarburet-of-hydrogen.jpg 1280w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/6-Michael-Faradays-sample-of-benzene-bicarburet-of-hydrogen-300x101.jpg 300w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/6-Michael-Faradays-sample-of-benzene-bicarburet-of-hydrogen-768x258.jpg 768w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/6-Michael-Faradays-sample-of-benzene-bicarburet-of-hydrogen-1024x344.jpg 1024w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/6-Michael-Faradays-sample-of-benzene-bicarburet-of-hydrogen-320x108.jpg 320w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/6-Michael-Faradays-sample-of-benzene-bicarburet-of-hydrogen-640x215.jpg 640w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/6-Michael-Faradays-sample-of-benzene-bicarburet-of-hydrogen-360x121.jpg 360w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/6-Michael-Faradays-sample-of-benzene-bicarburet-of-hydrogen-720x242.jpg 720w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/6-Michael-Faradays-sample-of-benzene-bicarburet-of-hydrogen-1080x363.jpg 1080w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/6-Michael-Faradays-sample-of-benzene-bicarburet-of-hydrogen-800x269.jpg 800w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/6-Michael-Faradays-sample-of-benzene-bicarburet-of-hydrogen-760x255.jpg 760w\" sizes=\"(max-width: 630px) 100vw, 630px\" \/><\/p>\n<p>By the 1860s it was known that the molecular formula of benzene was C<sub>6<\/sub>H<sub>6<\/sub>, all of the hydrogens were equivalent, and that valence bond theory required that there be 4 bonds to carbon.<\/p>\n<p>Many structures were proposed for this molecule. It fell to<a href=\"https:\/\/en.wikipedia.org\/wiki\/August_Kekul\u00e9\"> August Kekul\u00e9 <\/a>in 1865 to propose the correct, cyclic structure of benzene, which may or may not have been inspired by\u00a0a dream about a serpent eating its own tail.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15746\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/7-kekule-structure-of-benzene-snake-eating-own-tail.png\" alt=\"kekule structure of benzene snake eating own tail\" width=\"450\" height=\"224\" srcset=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/7-kekule-structure-of-benzene-snake-eating-own-tail.png 440w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/7-kekule-structure-of-benzene-snake-eating-own-tail-300x149.png 300w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/7-kekule-structure-of-benzene-snake-eating-own-tail-320x159.png 320w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/7-kekule-structure-of-benzene-snake-eating-own-tail-360x179.png 360w\" sizes=\"(max-width: 450px) 100vw, 450px\" \/><\/p>\n<p>Something still bothered Kekule about this structure, however: disubstituted benzenes always seemed to be missing an isomer.<\/p>\n<p>Take dichlorobenzene for example. From this structure, you&#8217;d expect\u00a0<strong>4<\/strong> isomers: 1,2, 1,3, 1,4, and 1,6 (the difference between 1,2 and 1,6 is subtle, but they are different nonetheless).<\/p>\n<p>The problem is that only 3 isomers of dichlorobenzene had ever been isolated and characterized.<\/p>\n<p>Kekule&#8217;s solution to this problem was to propose that 1,2 dichlorobenzene and 1,6 dichlorobenzene interconvert too rapidly to allow for either to be separated.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15747\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/8-according-to-kekule-proposal-4-isomers-of-disubstituted-benzenes-should-be-possible-but-only-3-had-ever-been-observed.gif\" alt=\"according to kekule proposal 4 isomers of disubstituted benzenes should be possible but only 3 had ever been observed\" width=\"600\" height=\"330\" \/><\/p>\n<p>It wasn&#8217;t until X-ray crystallography was developed that this picture was shown to be incorrect. In 1928<a href=\"https:\/\/en.wikipedia.org\/wiki\/Kathleen_Lonsdale\"> Kathleen Lonsdale<\/a> showed that all six C-C bond lengths in hexamethylbenzene were equivalent. She later determined the structure of\u00a0benzene itself, which\u00a0established the length of each C-C bond as 1.40\u00a0\u00c5 (140 picometers).<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15748\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/9-evidence-for-aromaticity-1928-x-ray-crystallography-of-benzene-determines-that-all-c-c-bond-lengths-are-of-equal-length-not-an-equilibrium.gif\" alt=\"evidence for aromaticity 1928 x ray crystallography of benzene determines that all c c bond lengths are of equal length not an equilibrium\" width=\"600\" height=\"240\" \/><\/p>\n<p>Equal bond lengths is\u00a0inconsistent with two interconverting molecules.<\/p>\n<p>Rather, it suggests that\u00a0&#8220;1,2&#8221; and &#8220;1,6&#8221; dichlorobenzene are actually\u00a0<strong>resonance forms of the same<\/strong>\u00a0<strong>molecule<\/strong><strong>\u00a0<\/strong>\u00a0and the &#8220;true&#8221; structure of benzene is a\u00a0<strong>hybrid<\/strong> of these two structures.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15749\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/10-1-2-and-1-6-isomers-of-dichlorobenzene-are-actually-resonance-forms-of-same-molecule-not-different-molecules.gif\" alt=\"1 2 and 1 6 isomers of dichlorobenzene are actually resonance forms of same molecule not different molecules\" width=\"600\" height=\"198\" \/><\/p>\n<p>One way\u00a0of describing this is that the pi electrons in benzene are\u00a0<em>delocalized<\/em> throughout the perimeter of the molecule, forming a kind of circle, or torus.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15750\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/11-way-to-show-delocalization-of-electrons-in-benzene-is-to-draw-circle-inside-six-membered-ring-resonance.gif\" alt=\"way to show delocalization of electrons in benzene is to draw circle inside six membered ring resonance\" width=\"630\" height=\"74\" \/><\/p>\n<p>This is\u00a0sometimes conveyed by drawing benzene as\u00a0a hexagon with a circle in the middle.<\/p>\n<h2><strong><a id=\"five\"><\/a>5. Benzene Undergoes Substitution Reactions, Not Addition Reactions<\/strong><\/h2>\n<p>Let&#8217;s go back to reactions.<\/p>\n<p>We saw that benzene is a lot less reactive in hydrogenation reactions than what we&#8217;d expect from an alkene. In order for hydrogenation to occur, you have to subject it to tremendous heat and pressure.\u00a0<a href=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2017\/01\/1-Whup-Ass-e1484851918939.jpg\"><br \/>\n<\/a><\/p>\n<p>You might reasonably ask: <strong>does benzene differ in its reactivity from alkenes in other ways as well?<\/strong><\/p>\n<p>You bet it does.<\/p>\n<p>Here&#8217;s one of the most prominent differences.<\/p>\n<p>As we&#8217;ve seen, alkenes typically react with electrophiles to give &#8220;addition&#8221;\u00a0products (break C-C pi, form two new adjacent single bonds to carbon).\u00a0\u00a0Case in point: chlorination (see below).<\/p>\n<p>Benzene is much less reactive toward chlorine than a typical alkene &#8211; it generally requires a catalyst in order for chlorination to occur\u00a0(FeCl<sub>3<\/sub> is a popular choice here). When it does react, instead of giving addition products, we <strong>instead see a\u00a0substitution\u00a0product<\/strong> (break C-H, form C-Cl).<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15751\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/12-reactivity-of-benzene-compared-to-alkenes-does-not-give-addition-reactions-but-substitution-reactions.gif\" alt=\"reactivity of benzene compared to alkenes does not give addition reactions but substitution reactions\" width=\"600\" height=\"362\" \/><\/p>\n<p>This turns out to be a very common class of reaction with benzene and its derivatives, a reaction we call &#8220;Electrophilic Aromatic Substitution&#8221;.<\/p>\n<p>We&#8217;re not going to get into\u00a0<em>why<\/em> this happens just yet. For now, lets just satisfy ourselves with the\u00a0<em>what<\/em>.<\/p>\n<p>More on that in a few posts.<\/p>\n<h2><strong><a id=\"six\"><\/a>6. This Collection of Three Special Properties Is Called &#8220;Aromaticity&#8221;<\/strong><\/h2>\n<p>Let&#8217;s review the three* [<a href=\"#notetwo\">Note 2 <\/a>] important properties\u00a0we&#8217;ve noticed about benzene so far.<\/p>\n<ul>\n<li>Extremely large resonance energy (36 kcal\/mol)<\/li>\n<li>Delocalized electrons<\/li>\n<li>Undergoes substitution rather than addition reactions<\/li>\n<\/ul>\n<p>This collection of special properties has been known since the late 1800&#8217;s, and was given a special name that we are now stuck with:\u00a0<strong>aromaticity<\/strong>. It actually has nothing to do with their aroma, although it was noticed that a number of sweet-smelling molecules all had these properties in common.<\/p>\n<p>You might reasonably ask:<strong> is aromaticity unique to benzene and its derivatives?<\/strong><\/p>\n<h2><strong><a id=\"seven\"><\/a>7. Aromaticity Is Not Unique To Benzene<\/strong><\/h2>\n<p>The answer is no &#8211; it\u00a0turns out that aromaticity is such a fundamental and important property that it deserves its own chapter(s) in introductory textbooks, which is why we will be writing a whole series on this phenomenon.<\/p>\n<p>Aromaticity is a property shared by a large number of molecules, including<\/p>\n<ul>\n<li>molecules with multiple rings (e.g. naphthalene, anthracene, indole)<\/li>\n<li>molecules with ring sizes other than 6<\/li>\n<li>molecules with rings containing atoms other than carbon (furan, indole, pyridine)<em> [aromatic molecules containing an atom other than carbon are often called &#8220;heterocycles&#8221;].<\/em><\/li>\n<li>certain molecules bearing a positive charge (the &#8220;tropylium ion&#8221;)<\/li>\n<li>certain molecules bearing a negative charge (the &#8220;cyclopentadienyl anion&#8221;)<\/li>\n<\/ul>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15752\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/13-aromaticity-is-not-unique-to-benzene-there-are-many-other-aromatic-molecules-like-naphthalene-furan-indole-pyridine-and-more.gif\" alt=\"aromaticity is not unique to benzene there are many other aromatic molecules like naphthalene furan indole pyridine and more\" width=\"600\" height=\"326\" \/><\/p>\n<p>There&#8217;s even molecules entirely lacking in carbon which have some properties of aromaticity (<a href=\"https:\/\/en.wikipedia.org\/wiki\/Borazine\">borazine<\/a>). The bases of DNA (adenine, cytosine, guanine, thymine) have aromatic character.<\/p>\n<p>However, molecules are not aromatic simply by virtue of being cyclic and having double bonds at the perimeter. Cyclooctatetraene (above right) is\u00a0<strong>not<\/strong> aromatic, and behaves like a normal conjugated alkene.<\/p>\n<p>It&#8217;s enough to make you scratch your head.<\/p>\n<h2><a id=\"eight\"><\/a>8. So What Makes A Molecule Aromatic?<\/h2>\n<p>So why is benzene aromatic, but cyclooctatetraene is not? How can we tell if a molecule is aromatic? What are the rules?<\/p>\n<p><strong>Great, great question.<\/strong>\u00a0But we&#8217;ve run out of time to answer that today. More on that in the next post. See next: <a href=\"https:\/\/www.masterorganicchemistry.com\/2017\/02\/23\/rules-for-aromaticity\/\">Rules For Aromaticity<\/a><\/p>\n<p><strong>Thanks to Matt Knowe\u00a0for assistance with writing this post.\u00a0<\/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\/02\/23\/rules-for-aromaticity\/\" class=\"\"><span>Rules For Aromaticity<\/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\/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\/2018\/01\/29\/ortho-para-and-meta-directors-in-electrophilic-aromatic-substitution\/\" class=\"\"><span>Ortho-, Para- and Meta- Directors in Electrophilic Aromatic Substitution<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/02\/08\/markovnikovs-rule-1\/\" class=\"\"><span>Markovnikov Addition Of HCl To Alkenes<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/04\/02\/epoxidation-hydroxylation-cyclopropanation-alkene-mechanism\/\" class=\"\"><span>Alkene Addition Pattern #3: The \u201cConcerted\u201d Pathway<\/span><\/a><\/li><\/ul><\/div>\n<p><strong><a id=\"noteone\"><\/a>Note 1. <\/strong>\u00a036 kcal\/mol is a huge deal in energy terms. A C-C pi bond is worth about 60 kcal\/mol, so the energy stabilization of aromaticity is worth about half that number.<\/p>\n<p>Even 3 kcal\/mol difference between two compounds in equilibrium will result in about a 99:1 distribution towards the more stable component (remember cyclohexane chair <a href=\"https:\/\/www.masterorganicchemistry.com\/2014\/07\/01\/substituted-cyclohexanes-a-values\/\">A-values<\/a>?)<\/p>\n<p><strong><a id=\"notetwo\"><\/a>Note 2. <\/strong>There&#8217;s a fourth diagnostic property of aromaticity &#8211;<a href=\"https:\/\/en.wikipedia.org\/wiki\/Aromatic_ring_current\">\u00a0ring currents <\/a>&#8211; which is observed in an applied magnetic field, such as when obtaining nuclear magnetic resonance (NMR) spectra. \u00a0There&#8217;s no need to get into this right now, so it will get no further comment<\/p>\n<hr \/>\n<h2><strong><a id=\"quizzes\"><\/a>Quiz Yourself!\u00a0<\/strong><\/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\/3619-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\/3620-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\/3621-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\/3627-Front-Image-Only.png\" alt=\"\" width=\"640\" height=\"616\" \/><\/p>\n<p><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a\u00a0 MOC member<\/strong><\/a> to see the clickable quiz with answers on the back. <\/p>\n<hr \/>\n<h2><a id=\"references\"><\/a>(Advanced) References and Further Reading<\/h2>\n<ol>\n<li><strong>Sur la constitution des substances aromatiques<br \/>\n<\/strong>A. Kekul\u00e9<br \/>\n<em>Bull. Soc. Chim. Fr.<\/em> <strong>1865<\/strong>, <em>3<\/em>, 98\u2013110<br \/>\n<strong><a href=\"https:\/\/gallica.bnf.fr\/ark:\/12148\/bpt6k281952v\/f102.image\">LINK<\/a><\/strong><strong><br \/>\n<\/strong>Kekul\u00e9\u2019s famous paper on the structure of benzene and aromatic compounds, in French.<\/li>\n<li><strong>Zur Constitution des Benzols<br \/>\n<\/strong> Ladenburg<strong><br \/>\n<\/strong><em>Just. Lieb. Ann. Chem.<\/em><strong> 1874<\/strong>, <em>172<\/em> (3), 331-356<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/chemistry-europe.onlinelibrary.wiley.com\/doi\/abs\/10.1002\/jlac.18741720314\">10.1002\/jlac.18741720314<\/a><br \/>\nLadenburg proved all 6 positions of benzene were equivalent (this paper is in German).<\/li>\n<li><strong>Ueber die Constitution des Benzols. Ueber die Hexahydroisophtals\u00e4ure<br \/>\n<\/strong>Adolf Baeyer, Victor Villiger<strong><br \/>\n<\/strong><em> Lieb. Ann. Chem. <\/em><strong>1893<\/strong>, <em>276<\/em> (3), 255-265<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/chemistry-europe.onlinelibrary.wiley.com\/doi\/abs\/10.1002\/jlac.18932760302\">10.1002\/jlac.18932760302<\/a><br \/>\nThis paper is by the same Adolf von Baeyer and Victor Villiger of Baeyer-Villiger reaction fame. Here, they describe the reduction of various benzene derivatives to cyclohexadiene, cyclohexene or cyclohexane compounds and determined the constitutions by standard methods. They also disproved a prism structure for benzene (this paper is in German).<\/li>\n<li><strong>Zur Kenntniss der unges\u00e4ttigten Verbindungen. Theorie der unges\u00e4ttigten und aromatischen Verbindungen<br \/>\n<\/strong>Johannes Thiele<strong><br \/>\n<\/strong><em> Lieb. Ann. Chem.<\/em><strong> 1899<\/strong>, <em>306<\/em> (1-2), 87-142<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/chemistry-europe.onlinelibrary.wiley.com\/doi\/abs\/10.1002\/jlac.18993060107\">10.1002\/jlac.18993060107<\/a><br \/>\nThiele was the first person to discuss &#8220;conjugation&#8221; between double bonds, and potentially might have been the first to draw benzene with a circle in the middle (this is in German).<\/li>\n<li><strong>The structure of the benzene ring in C<sub>6<\/sub>(CH<sub>3<\/sub>)<sub>6<\/sub><br \/>\n<\/strong>Kathleen Lonsdale<strong><br \/>\n<\/strong><em>Proc. Royal Soc. A<\/em><strong> 1929, <\/strong><em>123<\/em> (792), 494-515<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/royalsocietypublishing.org\/doi\/10.1098\/rspa.1929.0081\">10.1098\/rspa.1929.0081<\/a><strong><br \/>\n<\/strong>Paper that describes the X-ray structure of hexamethylbenzene, showing that all the aromatic C-C bonds are the same length, 1.42 \u00c5.<\/li>\n<li><strong>The Nature of the Chemical Bond. VI. The Calculation from Thermochemical Data of the Energy of Resonance of Molecules Among Several Electronic Structures<br \/>\n<\/strong>Linus Pauling and J. Sherman<br \/>\n<em>J. Chem. Phys.<\/em><strong> 1933<\/strong>, <em>1<\/em>, 606<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/aip.scitation.org\/doi\/10.1063\/1.1749335\">10.1063\/1.1749335<\/a><br \/>\nThis paper is the origin of the term \u2018resonance energy\u2019.<\/li>\n<li><strong>Heats of Organic Reactions. IV. Hydrogenation of Some Dienes and of Benzene<br \/>\n<\/strong> B. Kistiakowsky, John R. Ruhoff, Hilton A. Smith, and W. E. Vaughan<br \/>\n<em>Journal of the American Chemical Society<\/em><strong> 1936<\/strong>, <em>58<\/em> (1), 146-153<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/pubs.acs.org\/doi\/10.1021\/ja01292a043\">10.1021\/ja01292a043<\/a><strong><br \/>\n<\/strong>The commonly cited value of 49.8 kcal\/mol for the complete hydrogenation of benzene is from this paper.<\/li>\n<li><strong>The use of 90\u00b0-1,3-butadiene as a reference structure for the evaluation of stabilization energies for benzene and other conjugated cyclic hydrocarbons<br \/>\n<\/strong>Philip George, Mendel Trachtman, Charles W. Bock, Alistair M. Brett<strong><br \/>\n<\/strong><em>Tetrahedron<\/em><strong> 1976, <\/strong><em>32<\/em> (12), 1357-1362<strong><br \/>\nDOI: <\/strong><a href=\"https:\/\/www.sciencedirect.com\/science\/article\/abs\/pii\/0040402076850107\">1016\/0040-4020(76)85010-7<\/a><strong><br \/>\n<\/strong>The <em>isodesmic<\/em> reaction approach has been applied to the calculation of the resonance stabilization of benzene. This approach can be taken using either experimental thermochemical data or energies obtained by MO calculations. If the resonance energy of butadiene is assigned as zero, then the resonance energy of benzene would be 21.2 kcal\/mol. If butadiene is considered to have a delocalization energy, the calculation must be modified to reflect that fact. Using 7.2 kcal\/mol as the butadiene delocalization energy gives a value of 42.8 kcal\/mol as the benzene resonance energy.<\/li>\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>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>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<\/ol>\n","protected":false},"excerpt":{"rendered":"<p>What Is &#8220;Aromaticity&#8221;, Anyway?\u00a0 In this post we introduce &#8220;aromaticity&#8221;, a term for describing a collection of three [Note 1] main properties that distinguish benzene <\/p>\n","protected":false},"author":1,"featured_media":32129,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[844],"tags":[320,313,319,1160,1162,1161],"post_folder":[],"class_list":["post-10352","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-aromaticity-2","tag-aromaticity","tag-benzene","tag-electrophilic-aromatic-substitution","tag-resonance-energy","tag-ring-current","tag-whup-ass"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.7 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Introduction To Aromaticity &#8211; Master Organic Chemistry<\/title>\n<meta name=\"description\" content=\"What is aromaticity? 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