Heterocyclic compound - Melting, Boiling, Points

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The melting points or boiling points of common heteroaromatic compounds and their substituted derivatives are compared with those of benzene and its derivatives in the second table. Replacement of a two-carbon unit (two carbon and two hydrogen atoms, molecular weight equal to 26) by a single sulfur atom (atomic weight 32) has little effect on the melting or boiling point. 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The melting points or boiling points of common heteroaromatic compounds and their substituted derivatives are compared with those of benzene and its derivatives in the second table. Replacement of a two-carbon unit (two carbon and two hydrogen atoms, molecular weight equal to 26) by a single sulfur atom (atomic weight 32) has little effect on the melting or boiling point. On the other hand, replacement of a two-carbon unit by an oxygen atom"/> <meta property="og:type" content="ARTICLE"/> <meta property="og:title" content="Heterocyclic compound - Melting, Boiling, Points | Britannica"/> <meta property="og:description" content="Heterocyclic compound - Melting, Boiling, Points: The boiling points of certain saturated heterocycles are listed in the first table and are compared with those of the corresponding cycloalkanes (rightmost column of the table). The melting points or boiling points of common heteroaromatic compounds and their substituted derivatives are compared with those of benzene and its derivatives in the second table. Replacement of a two-carbon unit (two carbon and two hydrogen atoms, molecular weight equal to 26) by a single sulfur atom (atomic weight 32) has little effect on the melting or boiling point. 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The melting points or boiling points of common heteroaromatic compounds and their substituted derivatives are compared with those of benzene and its derivatives in the second table. Replacement of a two-carbon unit (two carbon and two hydrogen atoms, molecular weight equal to 26) by a single sulfur atom (atomic weight 32) has little effect on the melting or boiling point. 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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. 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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><section data-level="1"><span class="marker before-article"></span><section data-level="2" id="ref277865">  <span class="marker PREMOD1 mod-inline"></span><p class="topic-paragraph"><span id="ref3773"></span><span id="ref3774"></span>The <span id="ref1004800"></span><a href="https://www.britannica.com/science/boiling-point" class="md-crosslink " data-show-preview="true">boiling points</a> of certain saturated heterocycles are listed in the first table and are compared with those of the corresponding cycloalkanes (rightmost column of the table). The <span id="ref1004801"></span><a href="https://www.britannica.com/science/melting-point" class="md-crosslink " data-show-preview="true">melting points</a> or boiling points of common heteroaromatic <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> and their substituted derivatives are compared with those of <a href="https://www.britannica.com/science/benzene" class="md-crosslink " data-show-preview="true">benzene</a> and its derivatives in the second table.</p><span class="marker MOD1 mod-inline"></span><span class="md-raw-html"><div class="md-drag md-table-wrapper"><table> <caption> Melting and boiling points* of heteroaromatic compounds </caption> <thead> <tr> <th id="r" rowspan="2" scope="col"> ring system (with position of substituent) </th> <th colspan="11" id="s" scope="col"> substituent </th> </tr> <tr> <th id="2" scope="col"> H </th> <th id="3" scope="col"> CH<sub>3</sub> </th> <th id="4" scope="col"> C<sub>2</sub>H<sub>5</sub> </th> <th id="5" scope="col"> CO<sub>2</sub>H </th> <th id="6" scope="col"> CO<sub>2</sub>C<sub>2</sub>H<sub>5</sub> </th> <th id="7" scope="col"> CONH<sub>2</sub> </th> <th id="8" scope="col"> NH<sub>2</sub> </th> <th id="9" scope="col"> OH </th> <th id="10" scope="col"> OCH<sub>3</sub> </th> <th id="11" scope="col"> Cl </th> <th id="12" scope="col"> Br </th> </tr> </thead> <tfoot> <tr> <td colspan="12" headers="s"> *In °C. Boldface indicates the melting points. A dash indicates that a compound is unstable or unknown or that data are not readily available. </td> </tr> <tr> <td colspan="12" headers="s"> **Calculated using the experimentally obtained boiling point at reduced pressure. </td> </tr> <tr> <td colspan="12" headers="s"> ***Compound melts with decomposition. </td> </tr> </tfoot> <tbody> <tr> <td headers="r" scope="row"> <a href="https://www.britannica.com/science/benzene" class="md-crosslink " data-show-preview="true">benzene</a> </td> <td headers="2"> 80 </td> <td headers="3"> 111 </td> <td headers="4"> 136 </td> <td headers="5"> <strong>122</strong> </td> <td headers="6"> 212 </td> <td headers="7"> <strong>129</strong> </td> <td headers="8"> 184 </td> <td headers="9"> <strong>41</strong> </td> <td headers="10"> 154 </td> <td headers="11"> 132 </td> <td headers="12"> 156 </td> </tr> <tr> <td headers="r" scope="row"> <a href="https://www.britannica.com/science/pyridine" class="md-crosslink " data-show-preview="true">pyridine</a> (2) </td> <td headers="2"> 115 </td> <td headers="3"> 129 </td> <td headers="4"> 148 </td> <td headers="5"> <strong>137</strong> </td> <td headers="6"> 243 </td> <td headers="7"> <strong>107</strong> </td> <td headers="8"> <strong>57</strong> </td> <td headers="9"> <strong>107</strong> </td> <td headers="10"> 140 </td> <td headers="11"> 170 </td> <td headers="12"> 193 </td> </tr> <tr> <td headers="r" scope="row"> pyridine (3) </td> <td headers="2"> 115 </td> <td headers="3"> 144 </td> <td headers="4"> 165 </td> <td headers="5"> <strong>237</strong> </td> <td headers="6"> 224 </td> <td headers="7"> <strong>130</strong> </td> <td headers="8"> <strong>65</strong> </td> <td headers="9"> <strong>127</strong> </td> <td headers="10"> 179 </td> <td headers="11"> 148 </td> <td headers="12"> 173 </td> </tr> <tr> <td headers="r" scope="row"> pyridine (4) </td> <td headers="2"> 115 </td> <td headers="3"> 145 </td> <td headers="4"> 168 </td> <td headers="5"> <strong>315</strong> </td> <td headers="6"> 219 </td> <td headers="7"> <strong>156</strong> </td> <td headers="8"> <strong>158</strong> </td> <td headers="9"> <strong>148</strong> </td> <td headers="10"> 190 </td> <td headers="11"> 147 </td> <td headers="12"> 174 </td> </tr> <tr> <td headers="r" scope="row"> <a href="https://www.britannica.com/science/pyrrole" class="md-crosslink " data-show-preview="true">pyrrole</a> (1) </td> <td headers="2"> 130 </td> <td headers="3"> 113 </td> <td headers="4"> 129 </td> <td headers="5"> <strong>95</strong> </td> <td headers="6"> 178 </td> <td headers="7"> <strong>166</strong> </td> <td headers="8"> 175 </td> <td headers="9"> 185** </td> <td headers="10"> — </td> <td headers="11"> — </td> <td headers="12"> — </td> </tr> <tr> <td headers="r" scope="row"> pyrrole (2) </td> <td headers="2"> 130 </td> <td headers="3"> 148 </td> <td headers="4"> 164 </td> <td headers="5"> <strong>208</strong> </td> <td headers="6"> <strong>39</strong> </td> <td headers="7"> <strong>174</strong> </td> <td headers="8"> 285** </td> <td headers="9"> — </td> <td headers="10"> — </td> <td headers="11"> — </td> <td headers="12"> — </td> </tr> <tr> <td headers="r" scope="row"> pyrrole (3) </td> <td headers="2"> 130 </td> <td headers="3"> 143 </td> <td headers="4"> 179 </td> <td headers="5"> <strong>148</strong> </td> <td headers="6"> <strong>40</strong> </td> <td headers="7"> <strong>152</strong> </td> <td headers="8"> — </td> <td headers="9"> — </td> <td headers="10"> — </td> <td headers="11"> — </td> <td headers="12"> — </td> </tr> <tr> <td headers="r" scope="row"> <a href="https://www.britannica.com/science/furan" class="md-crosslink " data-show-preview="true">furan</a> (2) </td> <td headers="2"> 31 </td> <td headers="3"> 65 </td> <td headers="4"> 92 </td> <td headers="5"> <strong>133</strong> </td> <td headers="6"> <strong>34</strong> </td> <td headers="7"> <strong>142</strong> </td> <td headers="8"> — </td> <td headers="9"> — </td> <td headers="10"> 110 </td> <td headers="11"> 78 </td> <td headers="12"> 103 </td> </tr> <tr> <td headers="r" scope="row"> furan (3) </td> <td headers="2"> 31 </td> <td headers="3"> 66 </td> <td headers="4"> 92 </td> <td headers="5"> <strong>122</strong> </td> <td headers="6"> 175 </td> <td headers="7"> <strong>168</strong> </td> <td headers="8"> — </td> <td headers="9"> — </td> <td headers="10"> 110 </td> <td headers="11"> 80 </td> <td headers="12"> 103 </td> </tr> <tr> <td headers="r" scope="row"> <a href="https://www.britannica.com/science/thiophene" class="md-crosslink " data-show-preview="true">thiophene</a> (2) </td> <td headers="2"> 84 </td> <td headers="3"> 113 </td> <td headers="4"> 134 </td> <td headers="5"> <strong>129</strong> </td> <td headers="6"> 218 </td> <td headers="7"> <strong>180</strong> </td> <td headers="8"> — </td> <td headers="9"> 218 </td> <td headers="10"> 151 </td> <td headers="11"> 128 </td> <td headers="12"> 150 </td> </tr> <tr> <td headers="r" scope="row"> thiophene (3) </td> <td headers="2"> 84 </td> <td headers="3"> 115 </td> <td headers="4"> 136 </td> <td headers="5"> <strong>138</strong> </td> <td headers="6"> 208 </td> <td headers="7"> <strong>178</strong> </td> <td headers="8"> <strong>146</strong> </td> <td headers="9"> 270** </td> <td headers="10"> 156** </td> <td headers="11"> 136 </td> <td headers="12"> 159 </td> </tr> <tr> <td headers="r" scope="row"> <a href="https://www.britannica.com/science/pyrazole" class="md-crosslink " data-show-preview="true">pyrazole</a> (1) </td> <td headers="2"> <strong>68</strong> </td> <td headers="3"> 127 </td> <td headers="4"> 136 </td> <td headers="5"> <strong>102</strong> </td> <td headers="6"> <strong>213</strong> </td> <td headers="7"> <strong>141</strong> </td> <td headers="8"> 185** </td> <td headers="9"> <strong>72</strong> </td> <td headers="10"> — </td> <td headers="11"> — </td> <td headers="12"> — </td> </tr> <tr> <td headers="r" scope="row"> pyrazole (3) </td> <td headers="2"> <strong>68</strong> </td> <td headers="3"> 204 </td> <td headers="4"> 209 </td> <td headers="5"> <strong>214</strong> </td> <td headers="6"> <strong>158</strong> </td> <td headers="7"> <strong>159</strong> </td> <td headers="8"> <strong>38</strong> </td> <td headers="9"> — </td> <td headers="10"> — </td> <td headers="11"> <strong>40</strong> </td> <td headers="12"> <strong>70</strong> </td> </tr> <tr> <td headers="r" scope="row"> pyrazole (4) </td> <td headers="2"> <strong>68</strong> </td> <td headers="3"> 206 </td> <td headers="4"> 247** </td> <td headers="5"> <strong>275</strong> </td> <td headers="6"> <strong>78</strong> </td> <td headers="7"> — </td> <td headers="8"> <strong>81</strong> </td> <td headers="9"> <strong>118</strong> </td> <td headers="10"> <strong>60</strong> </td> <td headers="11"> <strong>77</strong> </td> <td headers="12"> <strong>97</strong> </td> </tr> <tr> <td headers="r" scope="row"> isoxazole (3) </td> <td headers="2"> 95 </td> <td headers="3"> 118 </td> <td headers="4"> 138 </td> <td headers="5"> <strong>149</strong> </td> <td headers="6"> — </td> <td headers="7"> <strong>134</strong> </td> <td headers="8"> — </td> <td headers="9"> <strong>98</strong> </td> <td headers="10"> — </td> <td headers="11"> — </td> <td headers="12"> — </td> </tr> <tr> <td headers="r" scope="row"> isoxazole (5) </td> <td headers="2"> 95 </td> <td headers="3"> 122 </td> <td headers="4"> 138** </td> <td headers="5"> <strong>146</strong> </td> <td headers="6"> — </td> <td headers="7"> <strong>174</strong> </td> <td headers="8"> <strong>77</strong> </td> <td headers="9"> — </td> <td headers="10"> 200** </td> <td headers="11"> — </td> <td headers="12"> — </td> </tr> <tr> <td headers="r" scope="row"> <a href="https://www.britannica.com/science/imidazole-organic-compound-class" class="md-crosslink " data-show-preview="true">imidazole</a> (1) </td> <td headers="2"> <strong>90</strong> </td> <td headers="3"> 196 </td> <td headers="4"> 208 </td> <td headers="5"> — </td> <td headers="6"> 218 </td> <td headers="7"> — </td> <td headers="8"> 315** </td> <td headers="9"> <strong>93</strong> </td> <td headers="10"> 252** </td> <td headers="11"> — </td> <td headers="12"> — </td> </tr> <tr> <td headers="r" scope="row"> imidazole (2) </td> <td headers="2"> <strong>90</strong> </td> <td headers="3"> <strong>144</strong> </td> <td headers="4"> <strong>80</strong> </td> <td headers="5"> <strong>164</strong> </td> <td headers="6"> <strong>178</strong> </td> <td headers="7"> <strong>312</strong> </td> <td headers="8"> — </td> <td headers="9"> <strong>251</strong> </td> <td headers="10"> <strong>71</strong> </td> <td headers="11"> <strong>165</strong> </td> <td headers="12"> <strong>207</strong> </td> </tr> <tr> <td headers="r" scope="row"> imidazole (4) </td> <td headers="2"> <strong>90</strong> </td> <td headers="3"> <strong>56</strong> </td> <td headers="4"> <strong>76</strong> </td> <td headers="5"> <strong>281</strong> </td> <td headers="6"> <strong>157</strong> </td> <td headers="7"> <strong>215</strong> </td> <td headers="8"> — </td> <td headers="9"> — </td> <td headers="10"> — </td> <td headers="11"> <strong>117</strong> </td> <td headers="12"> <strong>130</strong> </td> </tr> <tr> <td headers="r" scope="row"> <a href="https://www.britannica.com/science/pyrimidine" class="md-crosslink " data-show-preview="true">pyrimidine</a> (2) </td> <td headers="2"> 124 </td> <td headers="3"> 138 </td> <td headers="4"> 152 </td> <td headers="5"> <strong>197</strong> </td> <td headers="6"> <strong>64</strong> </td> <td headers="7"> <strong>166</strong> </td> <td headers="8"> <strong>127</strong> </td> <td headers="9"> <strong>180</strong> </td> <td headers="10"> 175** </td> <td headers="11"> <strong>65</strong> </td> <td headers="12"> <strong>56</strong> </td> </tr> <tr> <td headers="r" scope="row"> pyrimidine (4) </td> <td headers="2"> 124 </td> <td headers="3"> 141 </td> <td headers="4"> 140 </td> <td headers="5"> <strong>240</strong> </td> <td headers="6"> <strong>39</strong> </td> <td headers="7"> <strong>194</strong> </td> <td headers="8"> <strong>151</strong> </td> <td headers="9"> <strong>164</strong> </td> <td headers="10"> 152 </td> <td headers="11"> — </td> <td headers="12"> — </td> </tr> <tr> <td headers="r" scope="row"> pyrimidine (5) </td> <td headers="2"> 124 </td> <td headers="3"> 153 </td> <td headers="4"> 175 </td> <td headers="5"> <strong>270</strong> </td> <td headers="6"> <strong>38</strong> </td> <td headers="7"> <strong>212</strong> </td> <td headers="8"> <strong>170</strong> </td> <td headers="9"> <strong>210</strong> </td> <td headers="10"> <strong>47</strong> </td> <td headers="11"> <strong>37</strong> </td> <td headers="12"> <strong>75</strong> </td> </tr> <tr> <td headers="r" scope="row"> <a href="https://www.britannica.com/science/pyrazine" class="md-crosslink " data-show-preview="true">pyrazine</a> (2) </td> <td headers="2"> <strong>55</strong> </td> <td headers="3"> 137 </td> <td headers="4"> 155 </td> <td headers="5"> <strong>225</strong>*** </td> <td headers="6"> <strong>50</strong> </td> <td headers="7"> <strong>189</strong> </td> <td headers="8"> <strong>118</strong> </td> <td headers="9"> <strong>188</strong> </td> <td headers="10"> 187 </td> <td headers="11"> 152 </td> <td headers="12"> 180 </td> </tr> </tbody> </table></div> </span><span class="md-raw-html"><div class="md-drag md-table-wrapper"><table> <caption> Boiling points (°C) of saturated heterocycles and corresponding carbocycles </caption> <thead> <tr> <th id="r" rowspan="2" scope="col"> ring size </th> <th id="n" rowspan="2" scope="col"> number (and position) of heteroatoms </th> <th colspan="3" id="t" scope="col"> type of heteroatom </th> <th id="s" rowspan="2" scope="col"> saturated cycloalkane </th> </tr> <tr> <th headers="t" id="N" scope="col"> N (as NH) </th> <th headers="t" id="O" scope="col"> O </th> <th headers="t" id="S" scope="col"> S </th> </tr> </thead> <tfoot> <tr> <td colspan="6" headers="t S"> *Calculated using the experimentally obtained point at reduced pressure. </td> </tr> </tfoot> <tbody> <tr> <td headers="r" scope="row"> 3 </td> <td headers="n"> one </td> <td headers="t N"> 56 </td> <td headers="t O"> 11 </td> <td headers="t S"> 55 </td> <td headers="s"> −33 </td> </tr> <tr> <td headers="r" scope="row"> 4 </td> <td headers="n"> one </td> <td headers="t N"> 63 </td> <td headers="t O"> 48 </td> <td headers="t S"> 94 </td> <td headers="s"> 13 </td> </tr> <tr> <td headers="r" scope="row"> 5 </td> <td headers="n"> one </td> <td headers="t N"> 87 </td> <td headers="t O"> 65 </td> <td headers="t S"> 121 </td> <td headers="s"> 49 </td> </tr> <tr> <td headers="r" scope="row"> 6 </td> <td headers="n"> one </td> <td headers="t N"> 106 </td> <td headers="t O"> 88 </td> <td headers="t S"> 141 </td> <td headers="s"> 80 </td> </tr> <tr> <td headers="r" scope="row"> 6 </td> <td headers="n"> two (1,2) </td> <td headers="t N"> 150 </td> <td headers="t O"> 116 </td> <td headers="t S"> 190* </td> <td headers="s"> 80 </td> </tr> <tr> <td headers="r" scope="row"> 6 </td> <td headers="n"> two (1,3) </td> <td headers="t N"> 150 </td> <td headers="t O"> 106 </td> <td headers="t S"> 207 </td> <td headers="s"> 80 </td> </tr> <tr> <td headers="r" scope="row"> 6 </td> <td headers="n"> two (1,4) </td> <td headers="t N"> 145 </td> <td headers="t O"> 101 </td> <td headers="t S"> 200 </td> <td headers="s"> 80 </td> </tr> <tr> <td headers="r" scope="row"> 7 </td> <td headers="n"> one </td> <td headers="t N"> 138 </td> <td headers="t O"> 120 </td> <td headers="t S"> 174 </td> <td headers="s"> 119 </td> </tr> </tbody> </table></div> </span> <span class="marker PREMOD2 mod-inline"></span><p class="topic-paragraph">Replacement of a two-<a href="https://www.britannica.com/science/carbon-chemical-element" class="md-crosslink " data-show-preview="true">carbon</a> unit (two carbon and two <a href="https://www.britannica.com/science/hydrogen" class="md-crosslink " data-show-preview="true">hydrogen</a> atoms, <a href="https://www.britannica.com/science/molecular-weight" class="md-crosslink autoxref " data-show-preview="true">molecular weight</a> equal to 26) by a single <a href="https://www.britannica.com/science/sulfur" class="md-crosslink " data-show-preview="true">sulfur</a> atom (<a href="https://www.britannica.com/science/atomic-weight" class="md-crosslink " data-show-preview="true">atomic weight</a> 32) has little effect on the melting or boiling point. On the other hand, replacement of a two-carbon unit by an <a href="https://www.britannica.com/science/oxygen" class="md-crosslink " data-show-preview="true">oxygen</a> atom (atomic weight 16) lowers the boiling point by about 40 °C (72 °F), which is to be expected because of the decreased molecular weight of the furan compounds (lighter compounds being more volatile). Introduction of nitrogen atoms into the benzene ring is accompanied by less-regular changes. Replacement of a two-carbon unit by an imino (NH) group, or of a single carbon by a nitrogen atom, increases the boiling point. Furthermore, making these two changes simultaneously increases the boiling point even more, probably as the result of intermolecular association by <a href="https://www.britannica.com/science/hydrogen-bonding" class="md-crosslink autoxref " data-show-preview="true">hydrogen bonding</a> (a weak form of attachment via certain types of hydrogen atoms; <em>see</em> <a href="https://www.britannica.com/science/chemical-bonding" class="md-crosslink " data-show-preview="true">chemical bonding</a>) between the pyridine-like nitrogen atom and the imino group.</p><span class="marker MOD2 mod-inline"></span> <span class="marker PREMOD3 mod-inline"></span><p class="topic-paragraph">The effects of substituent groups in heteroaromatic rings show considerable regularity. <a href="https://www.britannica.com/science/methyl-group" class="md-crosslink " data-show-preview="true">Methyl</a> (CH<sub>3</sub>) and ethyl (C<sub>2</sub>H<sub>5</sub>) groups attached to ring carbon atoms usually increase the boiling point by about 20–30 °C (36–54 °F) and 50–60 °C (90–108 °F), respectively, whereas a similar attachment to a ring nitrogen atom (e.g., pyrrole → 1-methylpyrrole) significantly decreases the boiling point because of decreased ease of intermolecular association by hydrogen bonding (the active hydrogen having been replaced by a <a href="https://www.britannica.com/science/hydrocarbon" class="md-crosslink autoxref " data-show-preview="true">hydrocarbon</a> group). Heterocyclic <a href="https://www.britannica.com/science/carboxylic-acid" class="md-crosslink " data-show-preview="true">carboxylic acids</a> and <a href="https://www.britannica.com/science/amide" class="md-crosslink " data-show-preview="true">amides</a> are all solids at room temperature. Carboxylic acids of heterocycles containing a ring nitrogen atom usually melt at higher temperatures than those containing ring oxygen or sulfur atoms, because of hydrogen bonding. Compounds containing both a ring nitrogen atom and a hydroxyl (OH) or amino (NH<sub>2</sub>) group are usually relatively high-melting solids. Compounds containing <a href="https://www.britannica.com/science/chlorine" class="md-crosslink " data-show-preview="true">chlorine</a> (Cl) usually have boiling points similar to those of the corresponding ethyl-substituted compounds.</p><span class="marker MOD3 mod-inline"></span> </section> <section data-level="2" id="ref277866"> <h2 class="h2">Ultraviolet, infrared, nuclear magnetic resonance, and mass spectra</h2> <span class="marker PREMOD4 mod-inline"></span><p class="topic-paragraph">Spectroscopic studies of heterocyclic compounds, like those of other organic compounds, have became of great importance as means of identification of unknown materials, as <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> for purity, and as probes for investigating the electronic structures of molecules, thereby explaining and helping to predict their reactions. The <span id="ref1004802"></span><a href="https://www.britannica.com/science/ultraviolet-spectroscopy" class="md-crosslink ">ultraviolet</a> <a href="https://www.britannica.com/science/spectrum" class="md-crosslink " data-show-preview="true">spectrum</a> of an <a href="https://www.britannica.com/science/organic-compound" class="md-crosslink autoxref " data-show-preview="true">organic compound</a> (the pattern of its light absorption in the ultraviolet region of the spectrum) is characteristic of the π-<a href="https://www.britannica.com/science/electron" class="md-crosslink " data-show-preview="true">electron</a> system of the molecule—i.e., of the arrangement of double bonds within the structure. The ultraviolet spectra of heteroaromatic compounds show general similarity to those of benzenoid compounds (compounds with one or more <a href="https://www.britannica.com/science/benzene" class="md-crosslink " data-show-preview="true">benzene</a> rings), and the effects of substituents can usually be rationalized in a similar way.</p><span class="marker MOD4 mod-inline"></span> <span class="marker PREMOD5 mod-inline"></span><p class="topic-paragraph">The <span id="ref1004803"></span><a href="https://www.britannica.com/science/infrared-radiation" class="md-crosslink " data-show-preview="true">infrared</a> spectrum of an organic <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>, with its complexity of bands, provides an excellent “fingerprint” of the compound—far more characteristic than a melting point. It also can be used to identify certain common groups, such as <a href="https://www.britannica.com/science/carbonyl-group" class="md-crosslink " data-show-preview="true">carbonyl</a> (C=O) and imino, as well as various heterocyclic ring systems.</p><div class="module-spacing"> </div><span class="marker MOD5 mod-inline"></span> <span class="marker PREMOD6 mod-inline"></span><p class="topic-paragraph"><span id="ref1004804"></span><a href="https://www.britannica.com/science/magnetic-resonance" class="md-crosslink " data-show-preview="true">Magnetic resonance</a> spectra are indispensable today for studies in heterocyclic <a href="https://www.britannica.com/science/chemistry" class="md-crosslink autoxref " data-show-preview="true">chemistry</a>. <a href="https://www.britannica.com/science/proton-subatomic-particle" class="md-crosslink " data-show-preview="true">Proton</a> <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> spectra, the most common type, yield information regarding the number of hydrogen atoms in the molecule, their chemical <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="environment" href="https://www.merriam-webster.com/dictionary/environment" data-type="MW">environment</a>, and their relative orientation in space. Mass spectra are used to determine not only the complete molecular formula of the compound but also the molecular structure from the way the molecule fragments.</p><span class="marker MOD6 mod-inline"></span> </section> </section> <span class="marker h7"></span><section data-level="1" id="ref277867"> <h2 class="h1"><span id="ref1004806"></span><a href="https://www.britannica.com/science/chemical-synthesis" class="md-crosslink " data-show-preview="true">Synthesis</a> and modification of heterocyclic rings</h2> <span class="marker PREMOD7 mod-inline"></span><p class="topic-paragraph">The important methods for synthesizing heterocyclic compounds can be classified under five headings. Three are ways of forming new heterocyclic rings from <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="precursors" href="https://www.merriam-webster.com/dictionary/precursors" data-type="MW">precursors</a> containing either no rings (acyclic precursors) or one fewer ring than the desired product; one is a way of obtaining a heterocyclic ring from another heterocyclic ring or from a carbocyclic ring; and one involves the modification of substituents on an existing heterocyclic ring.</p><span class="marker MOD7 mod-inline"></span> <span class="marker PREMOD8 mod-inline"></span><p class="topic-paragraph">In the formation of rings from acyclic precursors, the key step is frequently the formation of a carbon-heteroatom linkage (C―Z, in which Z represents an atom of <a href="https://www.britannica.com/science/nitrogen" class="md-crosslink " data-show-preview="true">nitrogen</a>, <a href="https://www.britannica.com/science/oxygen" class="md-crosslink " data-show-preview="true">oxygen</a>, <a href="https://www.britannica.com/science/sulfur" class="md-crosslink " data-show-preview="true">sulfur</a>, or a more unusual element). The actual <span id="ref1004814"></span>ring closure, or cyclization, however, may involve the formation of a carbon-carbon bond. In any case, ring formation reactions are divided into three general categories according to whether the cyclization reaction occurs primarily as a result of nucleophilic or electrophilic attack or by way of a cyclic <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="transition" href="https://www.britannica.com/dictionary/transition" data-type="EB">transition</a> state.</p><span class="marker MOD8 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":4,"pagesTotal":10,"pageId":264227,"pageLength":753,"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":"D","adLeg":"D","userType":"ANONYMOUS","pageType":"Topic","pageSubtype":null,"articleTemplateType":"PAGINATED","gisted":false,"pageNumber":4,"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 - Melting, Boiling, Points | Britannica