{"id":1119,"date":"2018-11-28T16:28:38","date_gmt":"2018-11-28T16:28:38","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry2\/?post_type=chapter&p=1119"},"modified":"2019-01-08T14:57:58","modified_gmt":"2019-01-08T14:57:58","slug":"19-5-reductions-of-aromatic-rings","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry2\/chapter\/19-5-reductions-of-aromatic-rings\/","title":{"raw":"19.5. Reductions of Aromatic Rings","rendered":"19.5. Reductions of Aromatic Rings"},"content":{"raw":"
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Reduction of aromatic compounds<\/h2>\r\n<\/header>
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Study Notes<\/p>\r\nCatalytic hydrogenation of aromatic rings requires forcing conditions (high heat and hydrogen pressure).\"\"\r\n\r\n \r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n

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Hydrogenation of benzene at high pressure<\/h3>\r\nAlthough it does so less readily than simple alkenes or dienes, benzene adds hydrogen at high pressure in the presence of Pt, Pd or Ni catalysts. The product is cyclohexane and the heat of reaction provides evidence of benzene's thermodynamic stability<\/span>. Substituted benzene rings may also be reduced in this fashion, and hydroxy-substituted compounds such as phenol give carbonyl products resulting from the fast tautomerization of intermediate enols. Nickel or palladium on carbon catalysts are often used for this purpose, as noted in the following equations.\r\n\r\n\"Hydrogenation\r\n\r\n<\/div>\r\n
\r\n\r\nUnder low pressure conditions at room temperature, H2<\/sub>\/Pd will reduce a ketone carbonyl group when it is directly attached to an aromatic ring.\u00a0 This reduction of the $\\ce{\\sf{C=O}}$ group next to an aromatic ring is an important synthetic tool, as we saw in Section 16.2<\/a>.\r\n\r\n<\/div>\r\n<\/section><\/div>\r\n
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The Birch Reduction<\/strong><\/h3>\r\nAnother way of adding hydrogen to the benzene ring is by treatment with the electron rich solution of alkali metals, usually lithium or sodium, in liquid ammonia.\u00a0 This general type of reaction is known as the Birch reduction<\/b><\/a>\u00a0after the Australian chemist, A. J. Birch. With benzene, reduction with metals leads to 1,4-cyclohexadiene:\r\n\r\n\"\"\r\n\r\nThe initial step of the Birch reduction is an electron transfer to the lowest unoccupied molecular orbital of the benzene $$\\pi$$ system (see Figure 21-5) to form a radical anion:\r\n\r\n\"\"\r\n\r\nSubsequent steps include a sequence of proton- and electron-transfer steps as follows:\r\n\r\n\"\"\r\n\r\nSubstituent effects observed for this reaction are entirely consistent with those described for electrophilic substitution and addition - only reversed. That is, the reactivity of an arene in metal reductions is increased by electron-withdrawing groups and decreased by electron-donating groups. Substituents that can stabilize the anion-radical intermediate facilitate the reduction.\r\n\r\n
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Contributors<\/h3>\r\n