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Simple aromatic ring
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<h2 id="heterocyclic-aromatic-rings">Heterocyclic aromatic rings</h2> <table><tbody><tr><td>Table of simple aromatic rings<table><tbody><tr><td colspan="3"> </td></tr><tr><th>Five-membered rings and </th><th colspan="2">Fused five-membered rings</th><td> </td></tr><tr><td><a href="/facts/Pyrrole/7c7AbyN8">Pyrrole</a></td><td><a href="/facts/Indole/HT72P6e6">Indole</a></td><td><a href="/facts/Isoindole/eTxUlq54">Isoindole</a></td></tr><tr><td><a href="/facts/Furan/XCEMqxER">Furan</a></td><td><a href="/facts/Benzofuran/02NubrIR">Benzofuran</a></td><td><a href="/facts/Isobenzofuran/B5humJt1">Isobenzofuran</a></td></tr><tr><td><a href="/facts/Thiophene/pQhcmsCK">Thiophene</a></td><td><a href="/facts/Benzothiophene/96Ybno7S">Benzothiophene</a></td><td><a href="/facts/Benzo(c)thiophene/r9rWGSEq">Benzo[<i>c</i>]thiophene</a></td></tr><tr><td><a href="/facts/Imidazole/5MybM6Hh">Imidazole</a></td><td><a href="/facts/Benzimidazole/21Ml91Tr">Benzimidazole</a></td><td><a href="/facts/Purine/9ogLp0Wc">Purine</a></td></tr><tr><td><a href="/facts/Pyrazole/mNToRYlQ">Pyrazole</a></td><td><a href="/facts/Indazole/cxuExFaI">Indazole</a></td><td> </td></tr><tr><td><a href="/facts/Oxazole/nWp31hxW">Oxazole</a></td><td><a href="/facts/Benzoxazole/JnuprXN3">Benzoxazole</a></td><td> </td></tr><tr><td><a href="/facts/Isoxazole/uxqUPMdP">Isoxazole</a></td><td><a href="/facts/Benzisoxazole/4Qbzxl7t">Benzisoxazole</a></td><td> </td></tr><tr><td><a href="/facts/Thiazole/xtUANXCR">Thiazole</a></td><td><a href="/facts/Benzothiazole/Tzq9jB6B">Benzothiazole</a></td><td> </td></tr><tr><td colspan="3"> </td></tr><tr><th>Six-membered rings and </th><th colspan="2">Fused six-membered rings</th><td> </td></tr><tr><td><a href="/facts/Benzene/Fguue7nr">Benzene</a></td><td><a href="/facts/Naphthalene/RU2sV8mN">Naphthalene</a></td><td><a href="/facts/Anthracene/mV8Sblcq">Anthracene</a></td></tr><tr><td><a href="/facts/Pyridine/X817hCch">Pyridine</a></td><td><a href="/facts/Quinoline/kJyUItFu">Quinoline</a></td><td><a href="/facts/Isoquinoline/ygj32c57">Isoquinoline</a></td></tr><tr><td><a href="/facts/Pyrazine/fxCa3uuI">Pyrazine</a></td><td><a href="/facts/Quinoxaline/DPKE9hnT">Quinoxaline</a></td><td><a href="/facts/Acridine/1yp7JF2T">Acridine</a></td></tr><tr><td><a href="/facts/Pyrimidine/uFRSB8Cq">Pyrimidine</a></td><td><a href="/facts/Quinazoline/1Ho3cd9r">Quinazoline</a></td><td><a href="/facts/Phenazine/n33XHo5l">Phenazine</a></td></tr><tr><td><a href="/facts/Pyridazine/jLflt9M1">Pyridazine</a></td><td><a href="/facts/Cinnoline/h2Quq3pg">Cinnoline</a></td><td><a href="/facts/Phthalazine/tbdmao3y">Phthalazine</a></td></tr><tr><td><a href="/facts/1,2,3-Triazine/IxQSQjk9">1,2,3-Triazine</a></td><td><a href="/facts/1,2,4-Triazine/IxQSQjk9">1,2,4-Triazine</a></td><td><a href="/facts/1,3,5-Triazine/sXzSRezw">1,3,5-Triazine</a>(s-triazine)</td></tr></tbody></table></td></tr></tbody></table> <p>The nitrogen (N)-containing aromatic rings can be separated into <a href="/facts/Basic_aromatic_ring/Q2YWfFdk">basic aromatic rings</a> that are easily <a href="/facts/Protonation/q5Gf9z0M">protonated</a>, and form aromatic <a href="/facts/Cation/wrbwhF3z">cations</a> and <a href="/facts/Salts/lsSKwGpn">salts</a> (e.g., <a href="/facts/Pyridinium/txxbe0vf">pyridinium</a>), and non-basic aromatic rings. </p> <ul><li>In the basic aromatic rings, the <a href="/facts/Lone_pair/Q6kEAC4g">lone pair</a> of <a href="/facts/Electron/L0KBo5cW">electrons</a> is not part of the aromatic system and extends in the plane of the ring. This lone pair is responsible for the basicity of these <a href="/facts/Nitrogenous_base/zj1Vl9Qh">nitrogenous bases</a>, similar to the nitrogen atom in <a href="/facts/Amine/JvfYJfP6">amines</a>. In these compounds, the nitrogen atom is <i>not</i> connected to a hydrogen atom. Examples of basic aromatic rings are <a href="/facts/Pyridine/X817hCch">pyridine</a> or <a href="/facts/Quinoline/kJyUItFu">quinoline</a>. Several rings contain basic as well as non-basic nitrogen atoms, e.g., <a href="/facts/Imidazole/5MybM6Hh">imidazole</a> and <a href="/facts/Purine/9ogLp0Wc">purine</a>.</li> <li>In the non-basic rings, the lone pair of electrons of the nitrogen atom is delocalized and contributes to the aromatic pi-electron system. In these compounds, the nitrogen atom <i>is</i> connected to a hydrogen atom. Examples of non-basic nitrogen-containing aromatic rings are <a href="/facts/Pyrrole/7c7AbyN8">pyrrole</a> and <a href="/facts/Indole/HT72P6e6">indole</a>.</li></ul> <p>In the oxygen- and sulfur-containing aromatic rings, one of the electron pairs of the heteroatoms contributes to the aromatic system (similar to the non-basic nitrogen-containing rings), whereas the second lone pair extends in the plane of the ring (similar to the primary nitrogen-containing rings). </p> <h2 id="criteria-for-aromaticity">Criteria for aromaticity</h2> <p class="note">Main article: <a href="/facts/Aromaticity/wK0DzRY6">Aromaticity</a></p> <ul><li>Molecule must be cyclic.</li> <li>Every atom in the ring must have an occupied <a href="/facts/P_orbital/tKhgl6mL">p orbital</a>, which overlaps with p orbitals on either side (completely <a href="/facts/Conjugated_system/Ddx9qzox">conjugated</a>).</li> <li>Molecule must be planar.</li> <li>It must contain an odd number of pairs of pi electrons; must satisfy <a href="/facts/H%C3%BCckel%27s_rule/5BDPHGnk">Hückel's rule</a>: (4n+2) pi electrons, where n is an integer starting at zero.</li></ul> <p>In contrast, molecules with 4n pi electrons are <a href="/facts/Antiaromatic/rYPfTvFk">antiaromatic</a>. </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Polycyclic_aromatic_hydrocarbons/K52xSfNC">Polycyclic aromatic hydrocarbons</a> (PAH)</li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Clayden, J.; Greeves, N.; Warren, S.; Wothers, P. (2001). Organic Chemistry. Oxford, Oxfordshire: Oxford University Press. ISBN 0-19-850346-6. <a href="0-19-850346-6" target="_blank">0-19-850346-6</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Eicher, T.; Hauptmann, S. (2003). The Chemistry of Heterocycles: Structure, Reactions, Syntheses, and Applications (2nd ed.). Wiley-VCH. ISBN 3-527-30720-6. <a href="3-527-30720-6" target="_blank">3-527-30720-6</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>"Aromatic Hydrocarbon - an overview | ScienceDirect Topics". www.sciencedirect.com. Retrieved 2021-05-06. <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/aromatic-hydrocarbon" target="_blank">https://www.sciencedirect.com/topics/earth-and-planetary-sciences/aromatic-hydrocarbon</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> </ol>