Heterocyclic compound - Aromaticity, Structure, Reactivity

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class="md-content"> <div class="md-article-container template-desktop infinite-pagination"> <div class="infinite-scroll-container article last"> <article class="article-content container-lg qa-content px-0 pt-0 pb-40 py-lg-20 content md-expanded" data-topic-id="264227"> <div class="grid gx-0"> <div class="col-auto"> <div class="topic-left-rail md-article-drawer position-relative d-flex border-right-sm border-left-sm open"> <div class="drawer d-flex flex-column open"> <div class="left-rail-section-content"> <div class="topic-left-rail-header text-truncate bg-gray-50 position-relative text-right d-flex align-items-center"> <div class="tlr-title px-20 py-15 text-left"> <em class="material-icons text-gray-400 d-lg-none" data-icon="toc"></em> <a class="font-serif font-weight-bold text-black link-blue" href="https://www.britannica.com/science/heterocyclic-compound">heterocyclic compound</a> </div> <button aria-label="Close" class="js-sections-close-button btn-link btn-sm btn d-lg-none position-absolute top-0 p-10 right-0" > <em class="material-icons font-26" data-icon="close"></em> </button> </div> <div class="section-content pl-10 pr-20 pl-sm-50 pr-sm-60 pl-lg-5 pr-lg-10 pt-10 pt-lg-0 bg-gray-50 clear-catfish-ad"> <div class="toc mb-20"> <div class="font-serif font-14 font-weight-bold mx-15 mb-15 mt-20"> Table of Contents </div> <ul class="list-unstyled my-0" data-level="h1"><li data-target="#ref1"><div class="pl-25"><a class="link-gray-900 w-100" href="/science/heterocyclic-compound">Introduction</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li><li data-target="#ref277860"><div class="d-flex align-items-center"><div class="ml-25"></div><a class="w-100 link-gray-900" href="/science/heterocyclic-compound#ref277860">General aspects of heterocyclic compounds</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li><li data-target="#ref277861"><div class="d-flex align-items-center"><div class="ml-25"></div><a class="w-100 link-gray-900" href="/science/heterocyclic-compound/Comparison-with-carbocyclic-compounds">Comparison with carbocyclic compounds</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li><li data-target="#ref277862"><div class="d-flex align-items-center"><div class="ml-25"></div><a class="w-100 link-gray-900" href="/science/heterocyclic-compound/Comparison-with-carbocyclic-compounds#ref277862">Nomenclature of heterocyclic compounds</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li><li data-target="#ref277863"><div class="d-flex align-items-center"><div class="ml-25"></div><a class="w-100 link-gray-900" href="/science/heterocyclic-compound/The-nature-of-heteroaromaticity">The nature of heteroaromaticity</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li><li data-target="#ref277864"><div class="d-flex align-items-center"><button class="h1-link-drawer-button btn btn-xs btn-circle d-flex rounded" type="button" aria-label="Toggle Heading"><em class="material-icons font-18" data-icon="keyboard_arrow_right"></em></button><a class="w-100 link-gray-900" href="/science/heterocyclic-compound/The-nature-of-heteroaromaticity#ref277864">Physical properties of heterocyclic compounds</a></div><div class="ml-40 toc-drawer sub-toc-drawer"><ul class="list-unstyled" data-level="h2"><li data-target="#ref277865"><a class="w-100 link-gray-900" href="/science/heterocyclic-compound/Melting-and-boiling-points">Melting and boiling points</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref277866"><a class="w-100 link-gray-900" href="/science/heterocyclic-compound/Melting-and-boiling-points#ref277866">Ultraviolet, infrared, nuclear magnetic resonance, and mass spectra</a></li></ul></div></li><li data-target="#ref277867"><div class="d-flex align-items-center"><button class="h1-link-drawer-button btn btn-xs btn-circle d-flex rounded" type="button" aria-label="Toggle Heading"><em class="material-icons font-18" data-icon="keyboard_arrow_right"></em></button><a class="w-100 link-gray-900" href="/science/heterocyclic-compound/Melting-and-boiling-points#ref277867">Synthesis and modification of heterocyclic rings</a></div><div class="ml-40 toc-drawer sub-toc-drawer"><ul class="list-unstyled" data-level="h2"><li data-target="#ref277868"><a class="w-100 link-gray-900" href="/science/heterocyclic-compound/Nucleophilic-ring-closure">Nucleophilic ring closure</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref277869"><a class="w-100 link-gray-900" href="/science/heterocyclic-compound/Nucleophilic-ring-closure#ref277869">Electrophilic ring closure</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref277870"><a class="w-100 link-gray-900" href="/science/heterocyclic-compound/Nucleophilic-ring-closure#ref277870">Ring closure by way of cyclic transition states</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref277871"><a class="w-100 link-gray-900" href="/science/heterocyclic-compound/Nucleophilic-ring-closure#ref277871">Conversion of one heterocyclic ring into another</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref277872"><a class="w-100 link-gray-900" href="/science/heterocyclic-compound/Nucleophilic-ring-closure#ref277872">Modification of an existing ring</a></li></ul></div></li><li data-target="#ref277873"><div class="d-flex align-items-center"><button class="h1-link-drawer-button btn btn-xs btn-circle d-flex rounded" type="button" aria-label="Toggle Heading"><em class="material-icons font-18" data-icon="keyboard_arrow_right"></em></button><a class="w-100 link-gray-900" href="/science/heterocyclic-compound/Major-classes-of-heterocyclic-compounds">Major classes of heterocyclic compounds</a></div><div class="ml-40 toc-drawer sub-toc-drawer"><ul class="list-unstyled" data-level="h2"><li data-target="#ref277874"><a class="w-100 link-gray-900" href="/science/heterocyclic-compound/Major-classes-of-heterocyclic-compounds#ref277874">Three-membered rings</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref277875"><a class="w-100 link-gray-900" href="/science/heterocyclic-compound/Major-classes-of-heterocyclic-compounds#ref277875">Four-membered rings</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref277876"><a class="w-100 link-gray-900" href="/science/heterocyclic-compound/Five-membered-rings-with-one-heteroatom">Five-membered rings with one heteroatom</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref277877"><a class="w-100 link-gray-900" href="/science/heterocyclic-compound/Six-membered-rings-with-one-heteroatom">Six-membered rings with one heteroatom</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref277878"><a class="w-100 link-gray-900" href="/science/heterocyclic-compound/Five-and-six-membered-rings-with-two-or-more-heteroatoms">Five- and six-membered rings with two or more heteroatoms</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref277879"><a class="w-100 link-gray-900" href="/science/heterocyclic-compound/Five-and-six-membered-rings-with-two-or-more-heteroatoms#ref277879">Rings with seven or more members</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref277880"><a class="w-100 link-gray-900" href="/science/heterocyclic-compound/Rings-with-uncommon-heteroatoms">Rings with uncommon heteroatoms</a><ul class="list-unstyled" data-level="h3"><li data-target="#ref277881"><a class="w-100 link-gray-900" href="/science/heterocyclic-compound/Rings-with-uncommon-heteroatoms#ref277881">Halogens, selenium, and tellurium</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref277882"><a class="w-100 link-gray-900" href="/science/heterocyclic-compound/Rings-with-uncommon-heteroatoms#ref277882">Phosphorus, arsenic, antimony, and bismuth</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref277883"><a class="w-100 link-gray-900" href="/science/heterocyclic-compound/Rings-with-uncommon-heteroatoms#ref277883">Silicon, germanium, tin, and lead</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref277884"><a class="w-100 link-gray-900" href="/science/heterocyclic-compound/Rings-with-uncommon-heteroatoms#ref277884">Boron</a></li></ul></li></ul></div></li></ul> <a class="toc-extra-link link-gray-900" href="https://www.britannica.com/science/heterocyclic-compound/additional-info">References & Edit History</a> <a class="toc-extra-link link-gray-900" href="/facts/heterocyclic-compound">Quick Facts & Related Topics</a> </div> <div class="tlr-media-slider pb-10 mb-30"> <a class="section-header link-gray-900 font-serif font-14 font-weight-bold mb-10 mx-10" 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Editor of <em>Advances in Heterocyclic Chemistry.</em></div> </a> <div data-popper-arrow></div> </div> <span class="btn btn-link editor-link p-0 qa-byline-link gtm-byline font-12 byline-contributor text-decoration-underline"> Alan Roy Katritzky</span>, <div class="editor-popover popover p-0"> <a class="d-block p-20 gtm-byline font-12 byline-contributor" href="/contributor/Olga-V-Denisko/5488" > <div class="editor-title font-16 font-weight-bold">Olga V. Denisko</div> <div class="editor-description font-12 font-serif mt-5 clamp-description text-black">Associate scientific information analyst, Chemical Abstracts Service, Columbus, Ohio, U.S.</div> </a> <div data-popper-arrow></div> </div> <span class="btn btn-link editor-link p-0 qa-byline-link gtm-byline font-12 byline-contributor text-decoration-underline"> Olga V. Denisko</span><span class="text-gray-700 mx-5">•</span><a class="see-all border-gray-700 gtm-byline" rel="nofollow" href="https://www.britannica.com/science/heterocyclic-compound/additional-info#contributors">All</a> </div> <div class="font-serif font-12 text-gray-700"> <span class="qa-fact-checked-by">Fact-checked by</span> <div class="editor-popover popover p-0"> <a class="d-block p-20 font-12" href="/editor/The-Editors-of-Encyclopaedia-Britannica/4419" > <div class="editor-title font-16 font-weight-bold">The Editors of Encyclopaedia Britannica</div> <div class="editor-description font-12 font-serif mt-5 text-black">Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree. They write new content and verify and edit content received from contributors.</div> </a> <div data-popper-arrow></div> </div> <span class="btn btn-link editor-link p-0 qa-byline-link font-12 "> The Editors of Encyclopaedia Britannica</span></div> <div class="last-updated font-12 font-serif"> <span class="text-gray-700"> Last Updated: <time datetime="2024-10-18T00:00:00CDT" >Oct 18, 2024</time> •</span> <a class="byline-edit-history" href="https://www.britannica.com/science/heterocyclic-compound/additional-info#history" rel="nofollow">Article History</a> </div></div> </div> <button class="d-flex d-lg-none btn btn-outline-blue border rounded-sm shadow-sm mobile-toc-button gtm-mobile-toc-inline-button d-none d-sm-block js-sections-inline-button module-spacing btn d-lg-none"> <em class="material-icons mr-5 ml-n10 my-n5 md-icon" data-icon="toc"></em> Table of Contents </button> <div class="d-flex d-sm-none flex-row"> <button class="d-flex d-lg-none btn btn-outline-blue border rounded-sm shadow-sm mobile-toc-button gtm-mobile-toc-inline-button js-sections-inline-button module-spacing"> <em class="material-icons mr-5 ml-n10 my-n5 md-icon" data-icon="toc"></em> Table of Contents </button> <button class="ai-ask-button btn border-2 ai-ask-button btn border-2 module-spacing btn-sm js-inline-ai-ask-button btn-outline-red-400 border-red-400 p-10 ml-5"> Ask the Chatbot a Question </button> </div> <div class="js-qf-module qf-module px-40 px-sm-20 py-15 mx-auto module-spacing font-14 bg-gray-50 rounded"> <div class="facts-list mt-10"> <div class=""> <div class="js-fact mb-10 line-clamp clamp-3"> <dl> <dt>Also called: </dt> <dd> heterocycle</dd> </dl> <button class="js-more-btn d-none btn btn-unstyled font-12 bg-gray-50" aria-label="Toggle more/less fact data"> <em class="js-content link-blue">(Show more)</em> </button> </div> </div> <div class=""> <div class="js-fact mb-10 line-clamp clamp-3"> <dl> <dt>Related Topics: </dt> <dd><a href="/science/melamine" topicid="373631">melamine</a></dd> <dd><a href="/science/saccharin" topicid="515193">saccharin</a></dd> <dd><a href="/science/cyclamate" topicid="147871">cyclamate</a></dd> <dd><a href="/science/furan" topicid="222526">furan</a></dd> <dd><a href="/science/pyridine" topicid="484880">pyridine</a></dd> </dl> <button class="js-more-btn d-none btn btn-unstyled font-12 bg-gray-50" aria-label="Toggle more/less fact data"> <em class="js-content link-blue">(Show more)</em> </button> </div> <div class="text-center"> <a class="btn btn-sm btn-link p-0" href="/facts/heterocyclic-compound"> See all related content </a> </div> </div> </div> </div><span class="marker before-article"></span><span class="marker h5"></span><section data-level="1" id="ref277863">  <span class="marker PREMOD1 mod-inline"></span><p class="topic-paragraph"><span id="ref1004795"></span><a href="https://www.britannica.com/science/aromatic-compound" class="md-crosslink " data-show-preview="true">Aromaticity</a> denotes the significant stabilization of a ring <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="compound" href="https://www.merriam-webster.com/dictionary/compound" data-type="MW">compound</a> by a system of alternating single and double bonds—called a cyclic <a href="https://www.britannica.com/science/conjugated-system" class="md-crosslink " data-show-preview="true">conjugated system</a>—in which six π electrons generally participate. A <a href="https://www.britannica.com/science/nitrogen" class="md-crosslink autoxref " data-show-preview="true">nitrogen</a> atom in a ring can carry a positive or a negative charge, or it can be in the neutral form. An <a href="https://www.britannica.com/science/oxygen" class="md-crosslink autoxref " data-show-preview="true">oxygen</a> or <a href="https://www.britannica.com/science/sulfur" class="md-crosslink autoxref " data-show-preview="true">sulfur</a> atom in a ring can either be in the neutral form or carry a positive charge. A fundamental distinction is usually made between (1) those heteroatoms that participate in a cyclic conjugated system by means of a lone, or unshared, pair of electrons that are in an <a href="https://www.britannica.com/science/orbital" class="md-crosslink " data-show-preview="true">orbital</a> perpendicular to the plane of the ring and (2) those heteroatoms that do so because they are connected to another atom by means of a double bond.</p><span class="marker MOD1 mod-inline"></span> <span class="marker PREMOD2 mod-inline"></span><p class="topic-paragraph">An example of an atom of the first type is the nitrogen atom in <span id="ref1004797"></span><a href="https://www.britannica.com/science/pyrrole" class="md-crosslink " data-show-preview="true">pyrrole</a>, which is linked by single covalent bonds to two <a href="https://www.britannica.com/science/carbon-chemical-element" class="md-crosslink autoxref " data-show-preview="true">carbon</a> atoms and one hydrogen atom. Nitrogen has an outermost shell of five electrons, three of which can enter into three covalent bonds with other atoms. After the bonds are formed, as in the case of pyrrole, there remains an unshared electron pair that can engage in cyclic conjugation. The aromatic sextet in pyrrole is made up of two electrons from each of the two carbon-carbon double bonds and the two electrons that compose the unshared electron pair of the nitrogen atom. As a consequence, there tends to be a net flow of electron density from the nitrogen atom to the carbon atoms as the nitrogen’s electrons are drawn into the aromatic sextet. Alternatively, the pyrrole molecule may be described as a <a href="https://www.britannica.com/science/theory-of-resonance" class="md-crosslink " data-show-preview="true">resonance</a> hybrid—that is, a molecule whose true structure can only be approximated by two or more different forms, called <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="resonance" href="https://www.merriam-webster.com/dictionary/resonance" data-type="MW">resonance</a> forms.</p><span class="marker MOD2 mod-inline"></span> <span class="marker PREMOD3 mod-inline"></span><p class="topic-paragraph">An example of a heteroatom of the second type is the nitrogen atom in <a href="https://www.britannica.com/science/pyridine" class="md-crosslink " data-show-preview="true">pyridine</a>, which is linked by covalent bonds to only two carbon atoms. Pyridine also has a π-electron sextet, but the nitrogen atom contributes only one electron to it, one additional electron being contributed by each of the five carbon atoms in the ring. In particular, the unshared electron pair of the nitrogen atom is not involved. Moreover, because nitrogen’s attraction for electrons (its <a href="https://www.britannica.com/science/electronegativity" class="md-crosslink " data-show-preview="true">electronegativity</a>) is greater than that of carbon, electrons tend to move toward the nitrogen atom rather than away from it, as in pyrrole.</p><span class="marker MOD3 mod-inline"></span> <span class="marker PREMOD4 mod-inline"></span><p class="topic-paragraph">Quite generally, heteroatoms may be referred to as pyrrolelike or pyridine-like, depending on whether they fall into the first or second class described above. The pyrrolelike heteroatoms ―NR― (R being hydrogen or a <a href="https://www.britannica.com/science/hydrocarbon" class="md-crosslink autoxref " data-show-preview="true">hydrocarbon</a> group), ―N<sup>−</sup>―, ―O―, and ―S― tend to donate electrons into the π-electron system, whereas the pyridine-like heteroatoms ―N=, ―N<sup>+</sup>R=, ―O<sup>+</sup>=, and ―S<sup>+</sup>= tend to attract the π electrons of a double bond.</p><span class="marker MOD4 mod-inline"></span> <span class="marker PREMOD5 mod-inline"></span><p class="topic-paragraph">In six-membered heteroaromatic rings, the heteroatoms (usually nitrogen) are pyridine-like—for example, the <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="compounds" href="https://www.merriam-webster.com/dictionary/compounds" data-type="MW">compounds</a> <a href="https://www.britannica.com/science/pyrimidine" class="md-crosslink " data-show-preview="true">pyrimidine</a>, which contains two nitrogen atoms, and 1,2,4-triazine, which contains three nitrogen atoms.</p><div class="module-spacing"> </div><span class="marker MOD5 mod-inline"></span> <span class="marker PREMOD6 mod-inline"></span><p class="topic-paragraph">Six-membered heteroaromatic compounds cannot normally contain pyrrolelike heteroatoms. Five-membered heteroaromatic rings, however, always contain one pyrrolelike nitrogen, oxygen, or sulfur atom, and they may also contain up to four pyridine-like heteroatoms, as in the compounds <a href="https://www.britannica.com/science/thiophene" class="md-crosslink " data-show-preview="true">thiophene</a> (with one sulfur atom), 1,2,4-oxadiazole (with one oxygen atom and two nitrogen atoms), and pentazole (with five nitrogen atoms).</p><span class="marker MOD6 mod-inline"></span> <span class="marker PREMOD7 mod-inline"></span><p class="topic-paragraph">The quantitative measurement of aromaticity—and even its precise definition—has challenged chemists since German chemist <a href="https://www.britannica.com/biography/August-Kekule-von-Stradonitz" class="md-crosslink " data-show-preview="true">August Kekule</a> formulated the ring structure for benzene in the mid-19th century. Various methods based on energetic, structural, and magnetic <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="criteria" href="https://www.merriam-webster.com/dictionary/criteria" data-type="MW">criteria</a> have been widely used to measure the aromaticity of carbocyclic compounds. All of them, however, are difficult to apply quantitatively to heteroaromatic systems because of complications arising from the presence of heteroatoms.</p><span class="marker MOD7 mod-inline"></span> <span class="marker PREMOD8 mod-inline"></span><p class="topic-paragraph"><span id="ref1004799"></span><a href="https://www.britannica.com/science/chemical-reactivity" class="md-crosslink ">Chemical reactivity</a> can provide a certain qualitative insight into aromaticity. The reactivity of an <a href="https://www.britannica.com/science/aromatic-compound" class="md-crosslink autoxref " data-show-preview="true">aromatic compound</a> is affected by the extra stability of the conjugated system that it contains; the extra stability in turn determines the tendency of the compound to react by substitution of hydrogen—i.e., replacement of a singly bonded hydrogen atom with another singly bonded atom or group—rather than by addition of one or more atoms to the molecule via the breaking of a double bond (<em>see</em> <a href="https://www.britannica.com/science/substitution-reaction" class="md-crosslink " data-show-preview="true">substitution reaction</a>; <a href="https://www.britannica.com/science/addition-reaction" class="md-crosslink " data-show-preview="true">addition reaction</a>). In terms of reactivity, therefore, the degree of aromaticity is measured by the relative tendency toward substitution rather than addition. By this <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="criterion" href="https://www.merriam-webster.com/dictionary/criterion" data-type="MW">criterion</a>, pyridine is more aromatic than <a href="https://www.britannica.com/science/furan" class="md-crosslink " data-show-preview="true">furan</a>, but it is difficult to say just how much more aromatic.</p><span class="marker MOD8 mod-inline"></span> </section> <span class="marker h6"></span><section data-level="1" id="ref277864"> <h2 class="h1">Physical properties of heterocyclic compounds</h2> <span class="marker PREMOD9 mod-inline"></span><p class="topic-paragraph">Physical properties are important as criteria for judging the purity of heterocycles just as for other <a href="https://www.britannica.com/science/organic-compound" class="md-crosslink " data-show-preview="true">organic compounds</a>. Organic compounds generally show great regularity in their physical properties, and heterocycles are no exception.</p><span class="marker MOD9 mod-inline"></span> <span class="marker PREMOD10 mod-inline"></span><p class="topic-paragraph">The <a href="https://www.britannica.com/science/melting-point" class="md-crosslink " data-show-preview="true">melting point</a> was once a widely used criterion for purity, but it has been increasingly superseded by optical spectra, based on light absorption; mass spectra, based on relative masses of molecular fragments; and <a href="https://www.britannica.com/science/magnetic-resonance" class="md-crosslink autoxref " data-show-preview="true">magnetic resonance</a> spectra, based on nuclear properties (<em>see</em> <a href="https://www.britannica.com/science/spectroscopy" class="md-crosslink " data-show-preview="true">spectroscopy</a>). Nevertheless, knowledge of melting and boiling points is still helpful for judging the purity of a compound.</p><span class="marker MOD10 mod-inline"></span> </section><span class="marker end-of-content"></span><span class="marker after-article"></span></div> <div id="chatbot-root"></div> </div> </div> </div> <div class="ai-dialog-placeholder"></div> </div> </div> <aside class="col-md-da-320"></aside> </div> </div> </div> </div> </article></div> </div></div> </div> </main> <div id="md-footer"></div> <noscript><iframe src="//www.googletagmanager.com/ns.html?id=GTM-5W6NC8" height="0" width="0" style="display:none;visibility:hidden"></iframe></noscript> <script type="text/javascript" id="_informizely_script_tag"> var IzWidget = IzWidget || {}; (function (d) { var scriptElement = d.createElement('script'); scriptElement.type = 'text/javascript'; scriptElement.async = true; scriptElement.src = "https://insitez.blob.core.windows.net/site/f780f33e-a610-4ac2-af81-3eb184037547.js"; var node = d.getElementById('_informizely_script_tag'); node.parentNode.insertBefore(scriptElement, node); } )(document); </script>  <script> window.ap3c = window.ap3c || {}; var ap3c = window.ap3c; ap3c.cmd = ap3c.cmd || []; ap3c.cmd.push(function() { ap3c.init('ZO4siT4cLwnykPnzZWJtd3Byb2Q', 'https://engage.email.britannica.com/'); ap3c.track({v: 0}); }); ap3c.activity = function(act) { ap3c.act = (ap3c.act || []); ap3c.act.push(act); }; var s, t; s = document.createElement('script'); s.type = 'text/javascript'; s.src = "https://engage.email.britannica.com/app.js"; t = document.getElementsByTagName('script')[0]; t.parentNode.insertBefore(s, t); </script> <script class="marketing-page-info" type="application/json"> {"pageType":"Topic","templateName":"DESKTOP","pageNumber":3,"pagesTotal":10,"pageId":264227,"pageLength":833,"initialLoad":true,"lastPageOfScroll":false} </script> <script class="marketing-content-info" type="application/json"> [] </script> <script src="https://cdn.britannica.com/mendel-resources/3-130/js/libs/jquery-3.5.0.min.js?v=3.130.14"></script> <script type="text/javascript" data-type="Init Mendel Code Splitting"> (function() { $.ajax({ dataType: 'script', cache: true, url: 'https://cdn.britannica.com/mendel-resources/3-130/dist/topic-page.js?v=3.130.14' }); })(); </script> <script class="analytics-metadata" type="application/json"> {"leg":"C","adLeg":"C","userType":"ANONYMOUS","pageType":"Topic","pageSubtype":null,"articleTemplateType":"PAGINATED","gisted":false,"pageNumber":3,"hasSummarizeButton":false,"hasAskButton":false} </script> <script type="text/javascript"> EBStat={accountId:-1,hostnameOverride:'webstats.eb.com',domain:'www.britannica.com', json:''}; </script> <script type="text/javascript"> ( function() { $.ajax( { dataType: 'script', cache: true, url: '//www.britannica.com/webstats/mendelstats.js?v=1' } ) .done( function() { try {writeStat(null,EBStat);} catch(err){} } ); })(); </script> <div id="bc-fixed-dialogue"></div> </body> </html>

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Heterocyclic compound - Aromaticity, Structure, Reactivity | Britannica