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Conrotatory and disrotatory
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<h2 id="example-of-a-photochemical-reaction">Example of a photochemical reaction</h2> <p>Analysis of a <a href="/facts/Electrocyclic_reaction/DjXPWxjw">photochemical electrocyclic reaction</a> involves the <a href="/facts/HOMO/XyofpH0p">HOMO</a>, the <a href="/facts/LUMO/XyofpH0p">LUMO</a>, and correlations diagrams. </p><p>An electron is promoted into the LUMO changing the frontier molecular orbital involved in the reaction. </p> <h2 id="example-of-a-thermal-reaction">Example of a thermal reaction</h2> <p>Suppose that trans-cis-trans-2,4,6-octatriene is converted to dimethylcyclohexadiene under thermal conditions. Since the substrate octatriene is a "4n + 2" molecule, the <a href="/facts/Woodward%25E2%2580%2593Hoffmann_rules/4tW2tBIF">Woodward–Hoffmann rules</a> predict that the reaction happens in a disrotatory mechanism. </p><p>Since thermal electrocyclic reactions occur in the HOMO, it is first necessary to draw the appropriate <a href="/facts/Molecular_orbital_diagram/pJRHMzwJ">molecular orbitals</a>. Next, the new carbon-carbon bond is formed by taking two of the p-orbitals and rotating them 90 degrees (see diagram). Since the new bond requires constructive overlap, the orbitals must be rotated in a certain way. Performing a disrotation will cause the two black lobes to overlap, forming a new bond. Therefore, the reaction with octatriene happens through a disrotatory mechanism. </p><p>In contrast, if a conrotation had been performed then one white lobe would overlap with one black lobe. This would have caused destructive interference and no new carbon-carbon bond would have been formed. </p><p>In addition, the cis/trans geometry of the product can also be determined. When the p-orbitals were rotated inwards it also caused the two methyl groups to rotate upwards. Since both methyls are pointing "up", then the product is cis-dimethylcyclohexadiene. </p> <ul><li>Carey, Francis A.; Sundberg, Richard J.; (1984). Advanced Organic Chemistry Part A Structure and Mechanisms (2nd ed.). New York N.Y.: Plenum Press. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 0-306-41198-9.</li> <li>March Jerry; (1985). Advanced Organic Chemistry reactions, mechanisms and structure (3rd ed.). New York: John Wiley & Sons, inc. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 0-471-85472-7</li></ul>