{"id":799,"date":"2018-11-28T15:30:30","date_gmt":"2018-11-28T15:30:30","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry2\/?post_type=chapter&p=799"},"modified":"2019-01-07T15:45:45","modified_gmt":"2019-01-07T15:45:45","slug":"18-1-what-is-a-free-radical","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry2\/chapter\/18-1-what-is-a-free-radical\/","title":{"raw":"18.1. What is a free radical?","rendered":"18.1. What is a free radical?"},"content":{"raw":"
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In\u00a0<\/span>chemistry<\/a>, a\u00a0<\/span>radical<\/strong>\u00a0(more precisely, a\u00a0<\/span>free radical<\/strong>) is an\u00a0<\/span>atom<\/a>,\u00a0<\/span>molecule<\/a>, or\u00a0<\/span>ion<\/a>\u00a0that has\u00a0<\/span>unpaired valence electrons<\/a>\u00a0or an\u00a0<\/span>open electron shell<\/a>, and therefore may be seen as having one or more \"dangling\"\u00a0<\/span>covalent bonds<\/a>.<\/span><\/div>\r\n<\/div>\r\n<\/header>
With some exceptions, these \"dangling\" bonds make free radicals highly\u00a0chemically reactive<\/a>\u00a0towards other substances, or even towards themselves: their molecules will often spontaneously\u00a0dimerize<\/a>\u00a0or\u00a0polymerize<\/a>\u00a0if they come in contact with each other. Most radicals are reasonably stable only at very low concentrations in inert media or in a vacuum.Free radicals may be created in a number of ways, including synthesis with very dilute or rarefied reagents, reactions at very low temperatures, or breakup of larger molecules. The last can be effected by any process that puts enough energy into the parent molecule, such as\u00a0ionizing radiation<\/a>, heat, electrical discharges,\u00a0electrolysis<\/a>, and chemical reactions. Indeed, radicals are intermediate stages in many chemical reactions.Until late in the 20th century the word \"radical\" was used in chemistry to indicate any connected group of atoms, such as a\u00a0methyl group<\/a>\u00a0or a\u00a0carboxyl<\/a>, whether it was part of a larger molecule or a molecule on its own. The qualifier \"free\" was then needed to specify the unbound case. Following recent nomenclature revisions, a part of a larger molecule is now called a\u00a0functional group<\/a>\u00a0or\u00a0substituent<\/a>, and \"radical\" now implies \"free\". However, the old nomenclature may still occur in the literature.\r\n
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History<\/span><\/h2>\r\nThe first organic free radical identified was\u00a0triphenylmethyl radical<\/a>. This species was discovered by\u00a0Moses Gomberg<\/a>\u00a0in 1900 at the\u00a0University of Michigan<\/a>\u00a0USA. Historically, the term\u00a0radical<\/em>\u00a0in\u00a0radical theory<\/a>\u00a0was also used for bound parts of the molecule, especially when they remain unchanged in reactions. These are now called\u00a0functional groups<\/a>. For example,\u00a0methyl alcohol<\/a>\u00a0was described as consisting of a methyl \"radical\" and a hydroxyl \"radical\". Neither are radicals in the modern chemical sense, as they are permanently bound to each other, and have no unpaired, reactive electrons; however, they can be observed as radicals in\u00a0mass spectrometry<\/a>\u00a0when broken apart by irradiation with energetic electrons.\r\n\r\n<\/div>\r\n
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Depiction in chemical reactions<\/span><\/h2>\r\nIn chemical equations, free radicals are frequently denoted by a dot placed immediately to the right of the atomic symbol or molecular formula as follows:\r\n
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\"\\mathrm{Cl}_2<\/dd>\r\n \t
Chlorine gas can be broken down by ultraviolet light to form atomic chlorine radicals<\/em>.<\/dd>\r\n<\/dl>\r\nRadical\u00a0reaction mechanisms<\/a>\u00a0use single-headed arrows to depict the movement of single electrons:\r\n
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\"Radical.svg\"<\/a><\/div>\r\n<\/div>\r\nThe\u00a0homolytic<\/a>\u00a0cleavage of the breaking bond is drawn with a 'fish-hook' arrow to distinguish from the usual movement of two electrons depicted by a standard curly arrow. It should be noted that the second electron of the breaking bond also moves to pair up with the attacking radical electron; this is not explicitly indicated in this case.\r\n\r\nFree radicals also take part in\u00a0radical addition<\/a>\u00a0and\u00a0radical substitution<\/a>\u00a0as\u00a0reactive intermediates<\/a>. Chain reactions involving free radicals can usually be divided into three distinct processes. These are\u00a0initiation<\/em>,\u00a0propagation<\/em>, and\u00a0termination<\/em>., discussed in detail in section 18.4<\/a>.\r\n\r\n<\/div>\r\n
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  1. <\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n<\/section><\/article>","rendered":"
    \n
    \n
    <\/div>\n
    In\u00a0<\/span>chemistry<\/a>, a\u00a0<\/span>radical<\/strong>\u00a0(more precisely, a\u00a0<\/span>free radical<\/strong>) is an\u00a0<\/span>atom<\/a>,\u00a0<\/span>molecule<\/a>, or\u00a0<\/span>ion<\/a>\u00a0that has\u00a0<\/span>unpaired valence electrons<\/a>\u00a0or an\u00a0<\/span>open electron shell<\/a>, and therefore may be seen as having one or more “dangling”\u00a0<\/span>covalent bonds<\/a>.<\/span><\/div>\n<\/div>\n<\/header>\n
    \n
    With some exceptions, these “dangling” bonds make free radicals highly\u00a0chemically reactive<\/a>\u00a0towards other substances, or even towards themselves: their molecules will often spontaneously\u00a0dimerize<\/a>\u00a0or\u00a0polymerize<\/a>\u00a0if they come in contact with each other. Most radicals are reasonably stable only at very low concentrations in inert media or in a vacuum.Free radicals may be created in a number of ways, including synthesis with very dilute or rarefied reagents, reactions at very low temperatures, or breakup of larger molecules. The last can be effected by any process that puts enough energy into the parent molecule, such as\u00a0ionizing radiation<\/a>, heat, electrical discharges,\u00a0electrolysis<\/a>, and chemical reactions. Indeed, radicals are intermediate stages in many chemical reactions.Until late in the 20th century the word “radical” was used in chemistry to indicate any connected group of atoms, such as a\u00a0methyl group<\/a>\u00a0or a\u00a0carboxyl<\/a>, whether it was part of a larger molecule or a molecule on its own. The qualifier “free” was then needed to specify the unbound case. Following recent nomenclature revisions, a part of a larger molecule is now called a\u00a0functional group<\/a>\u00a0or\u00a0substituent<\/a>, and “radical” now implies “free”. However, the old nomenclature may still occur in the literature.<\/p>\n
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    History<\/span><\/h2>\n

    The first organic free radical identified was\u00a0triphenylmethyl radical<\/a>. This species was discovered by\u00a0Moses Gomberg<\/a>\u00a0in 1900 at the\u00a0University of Michigan<\/a>\u00a0USA. Historically, the term\u00a0radical<\/em>\u00a0in\u00a0radical theory<\/a>\u00a0was also used for bound parts of the molecule, especially when they remain unchanged in reactions. These are now called\u00a0functional groups<\/a>. For example,\u00a0methyl alcohol<\/a>\u00a0was described as consisting of a methyl “radical” and a hydroxyl “radical”. Neither are radicals in the modern chemical sense, as they are permanently bound to each other, and have no unpaired, reactive electrons; however, they can be observed as radicals in\u00a0mass spectrometry<\/a>\u00a0when broken apart by irradiation with energetic electrons.<\/p>\n<\/div>\n

    \n

    Depiction in chemical reactions<\/span><\/h2>\n

    In chemical equations, free radicals are frequently denoted by a dot placed immediately to the right of the atomic symbol or molecular formula as follows:<\/p>\n

    \n
    \"\\mathrm{Cl}_2<\/dd>\n
    Chlorine gas can be broken down by ultraviolet light to form atomic chlorine radicals<\/em>.<\/dd>\n<\/dl>\n

    Radical\u00a0reaction mechanisms<\/a>\u00a0use single-headed arrows to depict the movement of single electrons:<\/p>\n

    \n
    \"Radical.svg\"<\/a><\/div>\n<\/div>\n

    The\u00a0homolytic<\/a>\u00a0cleavage of the breaking bond is drawn with a ‘fish-hook’ arrow to distinguish from the usual movement of two electrons depicted by a standard curly arrow. It should be noted that the second electron of the breaking bond also moves to pair up with the attacking radical electron; this is not explicitly indicated in this case.<\/p>\n

    Free radicals also take part in\u00a0radical addition<\/a>\u00a0and\u00a0radical substitution<\/a>\u00a0as\u00a0reactive intermediates<\/a>. Chain reactions involving free radicals can usually be divided into three distinct processes. These are\u00a0initiation<\/em>,\u00a0propagation<\/em>, and\u00a0termination<\/em>., discussed in detail in section 18.4<\/a>.<\/p>\n<\/div>\n

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    1. <\/li>\n<\/ol>\n<\/div>\n<\/div>\n<\/section>\n<\/article>\n\n\t\t\t
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      Candela Citations<\/h3>\n\t\t\t\t\t
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