{"id":3234,"date":"2018-06-22T20:35:18","date_gmt":"2018-06-22T20:35:18","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/?post_type=chapter&#038;p=3234"},"modified":"2020-06-23T04:52:25","modified_gmt":"2020-06-23T04:52:25","slug":"5-6-reactive-intermediates","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/chapter\/5-6-reactive-intermediates\/","title":{"raw":"5.6. Reactive intermediates","rendered":"5.6. Reactive intermediates"},"content":{"raw":"<div id=\"bodyContent\" class=\"mw-body-content\">\r\n<div id=\"mw-content-text\" class=\"mw-content-ltr\" dir=\"ltr\" xml:lang=\"en\">\r\n<div class=\"mw-parser-output\">\r\n\r\nIn chemistry, a reactive intermediate or an intermediate is a short-lived, high-energy, highly reactive molecule. When generated in a chemical reaction, it will quickly convert into a more stable molecule. Only in exceptional cases can these compounds be isolated and stored, e.g. low temperatures, matrix isolation. When their existence is indicated, reactive intermediates can help explain how a chemical reaction takes place.\r\n\r\nMost chemical reactions take more than one elementary step to complete, and a reactive intermediate is a high-energy, yet stable, product that exists only in one of the intermediate steps. The series of steps together make a reaction mechanism. A reactive intermediate differs from a reactant or product or a simple reaction intermediate only in that it cannot usually be isolated but is sometimes observable only through fast spectroscopic methods. It is stable in the sense that an elementary reaction forms the reactive intermediate and the elementary reaction in the next step is needed to destroy it.\r\n\r\nWhen a reactive intermediate is not observable, its existence must be inferred through experimentation. This usually involves changing reaction conditions such as temperature or concentration and applying the techniques of chemical kinetics, chemical thermodynamics, or spectroscopy. We will often refer to certain reactive intermediates based on carbon, viz., \u00a0carbocations, radicals, carbanions and carbenes.\r\n\r\n<\/div>\r\n<h3 class=\"mw-parser-output\"><span id=\"Common_features\" class=\"mw-headline\">Common features<\/span><\/h3>\r\n<div class=\"mw-parser-output\">\r\n\r\nReactive intermediates have several features in common:\r\n<ul>\r\n \t<li>\u00a0low concentration with respect to reaction substrate and final reaction product<\/li>\r\n \t<li>\u00a0often generated on chemical decomposition of a chemical compound<\/li>\r\n \t<li>\u00a0it is often possible to prove the existence of this species by spectroscopic means<\/li>\r\n \t<li>\u00a0cage effects have to be taken into account<\/li>\r\n \t<li>\u00a0often stabilization by conjugation or resonance<\/li>\r\n \t<li>\u00a0often difficult to distinguish from a transition state<\/li>\r\n \t<li>\u00a0prove existence by means of chemical trapping<\/li>\r\n<\/ul>\r\n<h3><span id=\"References\" class=\"mw-headline\">References<\/span><\/h3>\r\n<div class=\"reflist\">\r\n<div class=\"mw-references-wrap\">\r\n<ol class=\"references\">\r\n \t<li id=\"cite_note-1\"><span class=\"mw-cite-backlink\"><b><a href=\"#cite_ref-1\">^<\/a><\/b><\/span> <span class=\"reference-text\">Carey, Francis A.; Sundberg, Richard J.; (1984). Advanced Organic Chemistry Part A Structure and Mechanisms (2nd ed.). New York N.Y.: Plenum Press. <a title=\"International Standard Book Number\" href=\"\/wiki\/International_Standard_Book_Number\">ISBN<\/a><a title=\"Special:BookSources\/0-306-41198-9\" href=\"BookSources\/0-306-41198-9\">0-306-41198-9<\/a>.<\/span><\/li>\r\n \t<li id=\"cite_note-2\"><span class=\"mw-cite-backlink\"><b><a href=\"#cite_ref-2\">^<\/a><\/b><\/span> <span class=\"reference-text\">March Jerry; (1885). Advanced Organic Chemistry reactions, mechanisms and structure (3rd ed.). New York: John Wiley &amp; Sons, inc. <a title=\"International Standard Book Number\" href=\"\/wiki\/International_Standard_Book_Number\">ISBN<\/a><a title=\"Special:BookSources\/0-471-85472-7\" href=\"BookSources\/0-471-85472-7\">0-471-85472-7<\/a><\/span><\/li>\r\n \t<li id=\"cite_note-3\"><span class=\"mw-cite-backlink\"><b><a href=\"#cite_ref-3\">^<\/a><\/b><\/span> <span class=\"reference-text\"><cite class=\"citation book\">Gilchrist, T. L. (1966). <i>Carbenes nitrenes and arynes<\/i>. Springer US. <a title=\"International Standard Book Number\" href=\"\/wiki\/International_Standard_Book_Number\">ISBN<\/a><a title=\"Special:BookSources\/9780306500268\" href=\"BookSources\/9780306500268\">9780306500268<\/a>.<\/cite><\/span><\/li>\r\n \t<li id=\"cite_note-4\"><span class=\"mw-cite-backlink\"><b><a href=\"#cite_ref-4\">^<\/a><\/b><\/span> <span class=\"reference-text\"><cite class=\"citation book\">Moss, Robert A.; Platz, Matthew S.; Jones, Jr., Maitland (2004). <i>Reactive intermediate chemistry<\/i>. Hoboken, N.J.: Wiley-Interscience. <a title=\"International Standard Book Number\" href=\"\/wiki\/International_Standard_Book_Number\">ISBN<\/a><a title=\"Special:BookSources\/9780471721499\" href=\"BookSources\/9780471721499\">9780471721499<\/a>.<\/cite><\/span><\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<h1>Carbocations (R+)<\/h1>\r\n<section class=\"mt-content-container\">A\u00a0<strong>carbocation<\/strong> is an\u00a0ion\u00a0with a positively-charged\u00a0carbon\u00a0<a class=\"external\" title=\"Atom\" href=\"http:\/\/en.wikipedia.org\/wiki\/Atom\" target=\"_blank\" rel=\"external nofollow noopener\">atom<\/a>. Among the simplest examples are\u00a0methenium\u00a0<span class=\"chemf\">CH<sub>3<\/sub><\/span><sup>+<\/sup>,\u00a0methanium\u00a0<span class=\"chemf\">CH<sub>5<\/sub><\/span><sup>+<\/sup>, and\u00a0ethanium\u00a0<span class=\"chemf\">C<sub>2<\/sub><\/span>H<sub>7<\/sub><sup>+<\/sup>.<sup>\u00a0 <\/sup>Until the early 1970s, all carbocations were called\u00a0carbonium ions.[1]\u00a0In present-day chemistry, a carbocation is any positively charged carbon atom, classified in two main categories according to the\u00a0valence\u00a0of the charged carbon:\r\n<div class=\"mt-section\">\r\n<ul>\r\n \t<li>+3 in carbenium ions\u00a0(protonated\u00a0carbenes),<\/li>\r\n \t<li>+5 or +6 in the\u00a0carbonium ions\u00a0(protonated\u00a0alkanes, named by analogy to\u00a0ammonium).\u00a0 These are much less common.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<div class=\"mt-section\">\r\n<h3 class=\"editable\"><span class=\"mw-headline\">Structure and properties<\/span><\/h3>\r\n<span class=\"mw-headline\">The charged carbon atom in a carbocation is a \"sextet\", i.e. it has only six\u00a0electrons\u00a0in its outer\u00a0valence shell\u00a0instead of the eight valence electrons that ensures maximum stability (octet rule). Therefore, carbocations are often reactive, seeking to fill the octet of valence electrons as well as regain a neutral\u00a0charge. One could reasonably assume a carbocation to have $$sp^3$$ hybridization with an empty $$sp_3$$ orbital giving positive charge. However, the reactivity of a carbocation more closely resembles $$sp^2$$ hybridization\u00a0with atrigonal planar\u00a0molecular geometry. An example is the methyl cation, $$CH_3^+$$.<\/span>\r\n\r\n<img class=\"wp-image-5096 aligncenter\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/3369\/2018\/06\/23043929\/Carbocations.png\" alt=\"Structures of tertiary, secondary, primary and methyl carbocations\" width=\"656\" height=\"121\" \/>\r\n\r\nOrder of stability of examples of tertiary (3<sup>o<\/sup>), secondary (2<sup>o<\/sup>), and primary (1<sup>o<\/sup>) alkyl<a class=\"external\" title=\"Carbenium ion\" href=\"http:\/\/en.wikipedia.org\/wiki\/Carbenium_ion\" target=\"_blank\" rel=\"external nofollow noopener\">carbenium<\/a><a class=\"external\" title=\"Carbenium ion\" href=\"http:\/\/en.wikipedia.org\/wiki\/Carbenium_ion\" target=\"_blank\" rel=\"external nofollow noopener\"> ions<\/a>, as well as the methyl cation (far right).\u00a0 The methyl group is so unstable it is only observed in the gas phase.\r\n\r\nCarbocations are often the target of nucleophilic attack by\u00a0<a class=\"external\" title=\"Nucleophile\" href=\"http:\/\/en.wikipedia.org\/wiki\/Nucleophile\" target=\"_blank\" rel=\"external nofollow noopener\">nucleophiles<\/a> such as water\u00a0or halide\u00a0ions.\r\n\r\nCarbocations typically undergo\u00a0<a class=\"external\" title=\"Rearrangement reaction\" href=\"http:\/\/en.wikipedia.org\/wiki\/Rearrangement_reaction\" target=\"_blank\" rel=\"external nofollow noopener\">rearrangement reactions<\/a>\u00a0from less stable structures to equally stable or more stable ones with\u00a0<a class=\"mw-redirect external\" title=\"Rate constant\" href=\"http:\/\/en.wikipedia.org\/wiki\/Rate_constant\" target=\"_blank\" rel=\"external nofollow noopener\">rate constants<\/a>\u00a0in excess of 10<sup>9<\/sup>\/sec. This fact complicates synthetic pathways to many compounds. For example, when 3-pentanol is heated with aqueous HCl, the initially formed 3-pentyl carbocation rearranges to a statistical mixture of the 3-pentyl and 2-pentyl. These cations react with chloride ion to produce about 1\/3 3-chloropentane and 2\/3 2-chloropentane.\r\n\r\nA carbocation may be stabilized by\u00a0<a class=\"external\" title=\"Resonance (chemistry)\" href=\"http:\/\/en.wikipedia.org\/wiki\/Resonance_(chemistry)\" target=\"_blank\" rel=\"external nofollow noopener\">resonance<\/a>\u00a0by a carbon-carbon double bond next to the ionized carbon. Such cations as\u00a0<em><a class=\"external\" title=\"Allyl\" href=\"http:\/\/en.wikipedia.org\/wiki\/Allyl\" target=\"_blank\" rel=\"external nofollow noopener\">allyl<\/a><\/em>\u00a0cation CH<sub>2<\/sub>=CH\u2013CH<sub>2<\/sub><sup>+<\/sup>\u00a0and\u00a0<em><a class=\"external\" title=\"Benzyl\" href=\"http:\/\/en.wikipedia.org\/wiki\/Benzyl\" target=\"_blank\" rel=\"external nofollow noopener\">benzyl<\/a><\/em>\u00a0cation C<sub>6<\/sub>H<sub>5<\/sub>\u2013CH<sub>2<\/sub><sup>+<\/sup>\u00a0are more stable than most other carbocations. Molecules that can form allyl or benzyl carbocations are especially reactive. These carbocations where the C+ is adjacent to another carbon atom that has a double or triple bond have extra stability because of the overlap of the empty p orbital of the carbocation with the p orbitals of the \u03c0 bond. This overlap of the orbitals allows the charge to be shared between multiple atoms \u2013 delocalization of the charge - and, therefore, stabilizes the carbocation.\r\n\r\n<\/div>\r\n<div class=\"mt-section\">\r\n<h3 class=\"editable\"><span class=\"mw-headline\">References<\/span><\/h3>\r\n<div class=\"reflist\">\r\n<ol class=\"references\">\r\n \t<li id=\"cite_note-2\"><span class=\"reference-text\"><a class=\"mw-redirect external\" title=\"Gold Book\" href=\"http:\/\/en.wikipedia.org\/wiki\/Gold_Book\" target=\"_blank\" rel=\"external nofollow noopener\">Gold Book<\/a>\u00a0definition for <a href=\"http:\/\/goldbook.iupac.org\/terms\/view\/C00817\">carbocation<\/a><\/span><\/li>\r\n<\/ol>\r\n<h1>Radicals<\/h1>\r\n<img class=\"alignnone size-full wp-image-4416\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/3369\/2018\/06\/31065843\/Radical.png\" alt=\"\" width=\"108\" height=\"107\" \/>\r\n\r\nA radicals is a seven electron intermediate that adopts a flat, sp<sup>2<\/sup> structure despite the fact that it has four electron groups; the lone electron resides in a half-filled p-orbital.\u00a0 This sp<sup>2<\/sup> structure allows radicals to delocalize the single electron through resonance.\u00a0 We will study radical reactions in detail in the second semester.\r\n\r\nBeing short of the octet, radicals are electrophilic, and therefore they are stabilized by alkyl groups.\u00a0 Thus the order for stability is the same as for carbocations, namely <strong>tertiary &gt; secondary &gt; primary &gt; methyl <\/strong>.\r\n\r\n<\/div>\r\n<\/div>\r\n<h1>Carbanions<\/h1>\r\n<img class=\"alignnone size-medium wp-image-4418\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/3369\/2018\/06\/31070905\/CarbanionStructures-300x131.png\" alt=\"\" width=\"300\" height=\"131\" \/>\r\n\r\nA carbanion is an eight electron intermediate with an sp<sup>3<\/sup> structure as shown in A. Despite its full octet, it is very reactive due to the fact that carbon is not very electronegative.\u00a0 Although it is sp<sup>3<\/sup>, it can participate in resonance because it can easily re-hybridize to an sp<sup>2<\/sup> structure (see B), which allows overlap.\r\n\r\nCarbanions are electron-rich and nucleophilic, so in fact they are destabilized by alkyl groups.\u00a0 This means that the order for stability is the opposite of that for carbocations, namely\u00a0<strong> methyl &gt; primary &gt; secondary &gt; tertiary<\/strong>.\r\n<h1>Carbenes<\/h1>\r\n<img class=\"alignnone size-full wp-image-4419\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/3369\/2018\/06\/31071530\/Carbene.png\" alt=\"\" width=\"102\" height=\"110\" \/>\r\n\r\nCarbenes are the least obvious of the four common intermediates; in most cases they have a six-electron sp<sup>2<\/sup> structure that has a lone pair but no overall charge.\u00a0 Although they are short of a full octet, they also have a reactive lone pair, so (depending on structure) carbenes can be either electrophilic or nucleophilic, or sometimes both - they just like to react with almost anything!\u00a0 We will learn about carbene reactions in <a href=\"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/chapter\/10-7-additions-involving-cyclic-intermediates\/\">section 10.7. <\/a>\r\n\r\n<\/section><\/div>\r\n<\/div>","rendered":"<div id=\"bodyContent\" class=\"mw-body-content\">\n<div id=\"mw-content-text\" class=\"mw-content-ltr\" dir=\"ltr\" xml:lang=\"en\">\n<div class=\"mw-parser-output\">\n<p>In chemistry, a reactive intermediate or an intermediate is a short-lived, high-energy, highly reactive molecule. When generated in a chemical reaction, it will quickly convert into a more stable molecule. Only in exceptional cases can these compounds be isolated and stored, e.g. low temperatures, matrix isolation. When their existence is indicated, reactive intermediates can help explain how a chemical reaction takes place.<\/p>\n<p>Most chemical reactions take more than one elementary step to complete, and a reactive intermediate is a high-energy, yet stable, product that exists only in one of the intermediate steps. The series of steps together make a reaction mechanism. A reactive intermediate differs from a reactant or product or a simple reaction intermediate only in that it cannot usually be isolated but is sometimes observable only through fast spectroscopic methods. It is stable in the sense that an elementary reaction forms the reactive intermediate and the elementary reaction in the next step is needed to destroy it.<\/p>\n<p>When a reactive intermediate is not observable, its existence must be inferred through experimentation. This usually involves changing reaction conditions such as temperature or concentration and applying the techniques of chemical kinetics, chemical thermodynamics, or spectroscopy. We will often refer to certain reactive intermediates based on carbon, viz., \u00a0carbocations, radicals, carbanions and carbenes.<\/p>\n<\/div>\n<h3 class=\"mw-parser-output\"><span id=\"Common_features\" class=\"mw-headline\">Common features<\/span><\/h3>\n<div class=\"mw-parser-output\">\n<p>Reactive intermediates have several features in common:<\/p>\n<ul>\n<li>\u00a0low concentration with respect to reaction substrate and final reaction product<\/li>\n<li>\u00a0often generated on chemical decomposition of a chemical compound<\/li>\n<li>\u00a0it is often possible to prove the existence of this species by spectroscopic means<\/li>\n<li>\u00a0cage effects have to be taken into account<\/li>\n<li>\u00a0often stabilization by conjugation or resonance<\/li>\n<li>\u00a0often difficult to distinguish from a transition state<\/li>\n<li>\u00a0prove existence by means of chemical trapping<\/li>\n<\/ul>\n<h3><span id=\"References\" class=\"mw-headline\">References<\/span><\/h3>\n<div class=\"reflist\">\n<div class=\"mw-references-wrap\">\n<ol class=\"references\">\n<li id=\"cite_note-1\"><span class=\"mw-cite-backlink\"><b><a href=\"#cite_ref-1\">^<\/a><\/b><\/span> <span class=\"reference-text\">Carey, Francis A.; Sundberg, Richard J.; (1984). Advanced Organic Chemistry Part A Structure and Mechanisms (2nd ed.). New York N.Y.: Plenum Press. <a title=\"International Standard Book Number\" href=\"\/wiki\/International_Standard_Book_Number\">ISBN<\/a><a title=\"Special:BookSources\/0-306-41198-9\" href=\"BookSources\/0-306-41198-9\">0-306-41198-9<\/a>.<\/span><\/li>\n<li id=\"cite_note-2\"><span class=\"mw-cite-backlink\"><b><a href=\"#cite_ref-2\">^<\/a><\/b><\/span> <span class=\"reference-text\">March Jerry; (1885). Advanced Organic Chemistry reactions, mechanisms and structure (3rd ed.). New York: John Wiley &amp; Sons, inc. <a title=\"International Standard Book Number\" href=\"\/wiki\/International_Standard_Book_Number\">ISBN<\/a><a title=\"Special:BookSources\/0-471-85472-7\" href=\"BookSources\/0-471-85472-7\">0-471-85472-7<\/a><\/span><\/li>\n<li id=\"cite_note-3\"><span class=\"mw-cite-backlink\"><b><a href=\"#cite_ref-3\">^<\/a><\/b><\/span> <span class=\"reference-text\"><cite class=\"citation book\">Gilchrist, T. L. (1966). <i>Carbenes nitrenes and arynes<\/i>. Springer US. <a title=\"International Standard Book Number\" href=\"\/wiki\/International_Standard_Book_Number\">ISBN<\/a><a title=\"Special:BookSources\/9780306500268\" href=\"BookSources\/9780306500268\">9780306500268<\/a>.<\/cite><\/span><\/li>\n<li id=\"cite_note-4\"><span class=\"mw-cite-backlink\"><b><a href=\"#cite_ref-4\">^<\/a><\/b><\/span> <span class=\"reference-text\"><cite class=\"citation book\">Moss, Robert A.; Platz, Matthew S.; Jones, Jr., Maitland (2004). <i>Reactive intermediate chemistry<\/i>. Hoboken, N.J.: Wiley-Interscience. <a title=\"International Standard Book Number\" href=\"\/wiki\/International_Standard_Book_Number\">ISBN<\/a><a title=\"Special:BookSources\/9780471721499\" href=\"BookSources\/9780471721499\">9780471721499<\/a>.<\/cite><\/span><\/li>\n<\/ol>\n<\/div>\n<\/div>\n<\/div>\n<h1>Carbocations (R+)<\/h1>\n<section class=\"mt-content-container\">A\u00a0<strong>carbocation<\/strong> is an\u00a0ion\u00a0with a positively-charged\u00a0carbon\u00a0<a class=\"external\" title=\"Atom\" href=\"http:\/\/en.wikipedia.org\/wiki\/Atom\" target=\"_blank\" rel=\"external nofollow noopener\">atom<\/a>. Among the simplest examples are\u00a0methenium\u00a0<span class=\"chemf\">CH<sub>3<\/sub><\/span><sup>+<\/sup>,\u00a0methanium\u00a0<span class=\"chemf\">CH<sub>5<\/sub><\/span><sup>+<\/sup>, and\u00a0ethanium\u00a0<span class=\"chemf\">C<sub>2<\/sub><\/span>H<sub>7<\/sub><sup>+<\/sup>.<sup>\u00a0 <\/sup>Until the early 1970s, all carbocations were called\u00a0carbonium ions.[1]\u00a0In present-day chemistry, a carbocation is any positively charged carbon atom, classified in two main categories according to the\u00a0valence\u00a0of the charged carbon:<\/p>\n<div class=\"mt-section\">\n<ul>\n<li>+3 in carbenium ions\u00a0(protonated\u00a0carbenes),<\/li>\n<li>+5 or +6 in the\u00a0carbonium ions\u00a0(protonated\u00a0alkanes, named by analogy to\u00a0ammonium).\u00a0 These are much less common.<\/li>\n<\/ul>\n<\/div>\n<div class=\"mt-section\">\n<h3 class=\"editable\"><span class=\"mw-headline\">Structure and properties<\/span><\/h3>\n<p><span class=\"mw-headline\">The charged carbon atom in a carbocation is a &#8220;sextet&#8221;, i.e. it has only six\u00a0electrons\u00a0in its outer\u00a0valence shell\u00a0instead of the eight valence electrons that ensures maximum stability (octet rule). Therefore, carbocations are often reactive, seeking to fill the octet of valence electrons as well as regain a neutral\u00a0charge. One could reasonably assume a carbocation to have $$sp^3$$ hybridization with an empty $$sp_3$$ orbital giving positive charge. However, the reactivity of a carbocation more closely resembles $$sp^2$$ hybridization\u00a0with atrigonal planar\u00a0molecular geometry. An example is the methyl cation, $$CH_3^+$$.<\/span><\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-5096 aligncenter\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/3369\/2018\/06\/23043929\/Carbocations.png\" alt=\"Structures of tertiary, secondary, primary and methyl carbocations\" width=\"656\" height=\"121\" \/><\/p>\n<p>Order of stability of examples of tertiary (3<sup>o<\/sup>), secondary (2<sup>o<\/sup>), and primary (1<sup>o<\/sup>) alkyl<a class=\"external\" title=\"Carbenium ion\" href=\"http:\/\/en.wikipedia.org\/wiki\/Carbenium_ion\" target=\"_blank\" rel=\"external nofollow noopener\">carbenium<\/a><a class=\"external\" title=\"Carbenium ion\" href=\"http:\/\/en.wikipedia.org\/wiki\/Carbenium_ion\" target=\"_blank\" rel=\"external nofollow noopener\"> ions<\/a>, as well as the methyl cation (far right).\u00a0 The methyl group is so unstable it is only observed in the gas phase.<\/p>\n<p>Carbocations are often the target of nucleophilic attack by\u00a0<a class=\"external\" title=\"Nucleophile\" href=\"http:\/\/en.wikipedia.org\/wiki\/Nucleophile\" target=\"_blank\" rel=\"external nofollow noopener\">nucleophiles<\/a> such as water\u00a0or halide\u00a0ions.<\/p>\n<p>Carbocations typically undergo\u00a0<a class=\"external\" title=\"Rearrangement reaction\" href=\"http:\/\/en.wikipedia.org\/wiki\/Rearrangement_reaction\" target=\"_blank\" rel=\"external nofollow noopener\">rearrangement reactions<\/a>\u00a0from less stable structures to equally stable or more stable ones with\u00a0<a class=\"mw-redirect external\" title=\"Rate constant\" href=\"http:\/\/en.wikipedia.org\/wiki\/Rate_constant\" target=\"_blank\" rel=\"external nofollow noopener\">rate constants<\/a>\u00a0in excess of 10<sup>9<\/sup>\/sec. This fact complicates synthetic pathways to many compounds. For example, when 3-pentanol is heated with aqueous HCl, the initially formed 3-pentyl carbocation rearranges to a statistical mixture of the 3-pentyl and 2-pentyl. These cations react with chloride ion to produce about 1\/3 3-chloropentane and 2\/3 2-chloropentane.<\/p>\n<p>A carbocation may be stabilized by\u00a0<a class=\"external\" title=\"Resonance (chemistry)\" href=\"http:\/\/en.wikipedia.org\/wiki\/Resonance_(chemistry)\" target=\"_blank\" rel=\"external nofollow noopener\">resonance<\/a>\u00a0by a carbon-carbon double bond next to the ionized carbon. Such cations as\u00a0<em><a class=\"external\" title=\"Allyl\" href=\"http:\/\/en.wikipedia.org\/wiki\/Allyl\" target=\"_blank\" rel=\"external nofollow noopener\">allyl<\/a><\/em>\u00a0cation CH<sub>2<\/sub>=CH\u2013CH<sub>2<\/sub><sup>+<\/sup>\u00a0and\u00a0<em><a class=\"external\" title=\"Benzyl\" href=\"http:\/\/en.wikipedia.org\/wiki\/Benzyl\" target=\"_blank\" rel=\"external nofollow noopener\">benzyl<\/a><\/em>\u00a0cation C<sub>6<\/sub>H<sub>5<\/sub>\u2013CH<sub>2<\/sub><sup>+<\/sup>\u00a0are more stable than most other carbocations. Molecules that can form allyl or benzyl carbocations are especially reactive. These carbocations where the C+ is adjacent to another carbon atom that has a double or triple bond have extra stability because of the overlap of the empty p orbital of the carbocation with the p orbitals of the \u03c0 bond. This overlap of the orbitals allows the charge to be shared between multiple atoms \u2013 delocalization of the charge &#8211; and, therefore, stabilizes the carbocation.<\/p>\n<\/div>\n<div class=\"mt-section\">\n<h3 class=\"editable\"><span class=\"mw-headline\">References<\/span><\/h3>\n<div class=\"reflist\">\n<ol class=\"references\">\n<li id=\"cite_note-2\"><span class=\"reference-text\"><a class=\"mw-redirect external\" title=\"Gold Book\" href=\"http:\/\/en.wikipedia.org\/wiki\/Gold_Book\" target=\"_blank\" rel=\"external nofollow noopener\">Gold Book<\/a>\u00a0definition for <a href=\"http:\/\/goldbook.iupac.org\/terms\/view\/C00817\">carbocation<\/a><\/span><\/li>\n<\/ol>\n<h1>Radicals<\/h1>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-4416\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/3369\/2018\/06\/31065843\/Radical.png\" alt=\"\" width=\"108\" height=\"107\" \/><\/p>\n<p>A radicals is a seven electron intermediate that adopts a flat, sp<sup>2<\/sup> structure despite the fact that it has four electron groups; the lone electron resides in a half-filled p-orbital.\u00a0 This sp<sup>2<\/sup> structure allows radicals to delocalize the single electron through resonance.\u00a0 We will study radical reactions in detail in the second semester.<\/p>\n<p>Being short of the octet, radicals are electrophilic, and therefore they are stabilized by alkyl groups.\u00a0 Thus the order for stability is the same as for carbocations, namely <strong>tertiary &gt; secondary &gt; primary &gt; methyl <\/strong>.<\/p>\n<\/div>\n<\/div>\n<h1>Carbanions<\/h1>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-medium wp-image-4418\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/3369\/2018\/06\/31070905\/CarbanionStructures-300x131.png\" alt=\"\" width=\"300\" height=\"131\" \/><\/p>\n<p>A carbanion is an eight electron intermediate with an sp<sup>3<\/sup> structure as shown in A. Despite its full octet, it is very reactive due to the fact that carbon is not very electronegative.\u00a0 Although it is sp<sup>3<\/sup>, it can participate in resonance because it can easily re-hybridize to an sp<sup>2<\/sup> structure (see B), which allows overlap.<\/p>\n<p>Carbanions are electron-rich and nucleophilic, so in fact they are destabilized by alkyl groups.\u00a0 This means that the order for stability is the opposite of that for carbocations, namely\u00a0<strong> methyl &gt; primary &gt; secondary &gt; tertiary<\/strong>.<\/p>\n<h1>Carbenes<\/h1>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-4419\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/3369\/2018\/06\/31071530\/Carbene.png\" alt=\"\" width=\"102\" height=\"110\" \/><\/p>\n<p>Carbenes are the least obvious of the four common intermediates; in most cases they have a six-electron sp<sup>2<\/sup> structure that has a lone pair but no overall charge.\u00a0 Although they are short of a full octet, they also have a reactive lone pair, so (depending on structure) carbenes can be either electrophilic or nucleophilic, or sometimes both &#8211; they just like to react with almost anything!\u00a0 We will learn about carbene reactions in <a href=\"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/chapter\/10-7-additions-involving-cyclic-intermediates\/\">section 10.7. <\/a><\/p>\n<\/section>\n<\/div>\n<\/div>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-3234\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>Reactive_intermediate. <strong>Authored by<\/strong>: Wikipedia contributors. <strong>Provided by<\/strong>: Wikimedia Foundation. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"https:\/\/en.wikipedia.org\/wiki\/Reactive_intermediate\">https:\/\/en.wikipedia.org\/wiki\/Reactive_intermediate<\/a>. <strong>Project<\/strong>: Wikipedia. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by-sa\/4.0\/\">CC BY-SA: Attribution-ShareAlike<\/a><\/em><\/li><li>Carbocations. <strong>Authored by<\/strong>: Libretexts contributors. <strong>Provided by<\/strong>: UC Davis. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"https:\/\/chem.libretexts.org\/Textbook_Maps\/Organic_Chemistry\/Supplemental_Modules_(Organic_Chemistry)\/Fundamentals\/Reactive_Intermediates\/Carbocations\">https:\/\/chem.libretexts.org\/Textbook_Maps\/Organic_Chemistry\/Supplemental_Modules_(Organic_Chemistry)\/Fundamentals\/Reactive_Intermediates\/Carbocations<\/a>. <strong>Project<\/strong>: Libretexts. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by-nc\/4.0\/\">CC BY-NC: Attribution-NonCommercial<\/a><\/em><\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section>","protected":false},"author":311,"menu_order":6,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"Reactive_intermediate\",\"author\":\"Wikipedia contributors\",\"organization\":\"Wikimedia Foundation\",\"url\":\"https:\/\/en.wikipedia.org\/wiki\/Reactive_intermediate\",\"project\":\"Wikipedia\",\"license\":\"cc-by-sa\",\"license_terms\":\"\"},{\"type\":\"cc\",\"description\":\"Carbocations\",\"author\":\"Libretexts contributors\",\"organization\":\"UC Davis\",\"url\":\"https:\/\/chem.libretexts.org\/Textbook_Maps\/Organic_Chemistry\/Supplemental_Modules_(Organic_Chemistry)\/Fundamentals\/Reactive_Intermediates\/Carbocations\",\"project\":\"Libretexts\",\"license\":\"cc-by-nc\",\"license_terms\":\"\"}]","CANDELA_OUTCOMES_GUID":"","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-3234","chapter","type-chapter","status-publish","hentry"],"part":22,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/pressbooks\/v2\/chapters\/3234","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/wp\/v2\/users\/311"}],"version-history":[{"count":15,"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/pressbooks\/v2\/chapters\/3234\/revisions"}],"predecessor-version":[{"id":5098,"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/pressbooks\/v2\/chapters\/3234\/revisions\/5098"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/pressbooks\/v2\/parts\/22"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/pressbooks\/v2\/chapters\/3234\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/wp\/v2\/media?parent=3234"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/pressbooks\/v2\/chapter-type?post=3234"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/wp\/v2\/contributor?post=3234"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/wp-json\/wp\/v2\/license?post=3234"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}