{"id":3290,"date":"2018-07-04T13:40:21","date_gmt":"2018-07-04T13:40:21","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/?post_type=chapter&p=3290"},"modified":"2020-06-24T01:50:56","modified_gmt":"2020-06-24T01:50:56","slug":"10-4-simple-addition-to-alkenes","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/chapter\/10-4-simple-addition-to-alkenes\/","title":{"raw":"10.4. Simple addition to alkenes","rendered":"10.4. Simple addition to alkenes"},"content":{"raw":"
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\r\n\r\nAlkenes undergo addition reactions. Often, they add a proton to one end of the double bond and another group to the other end. These reactions happen in slightly different ways, however.\r\n\r\n\"\"<\/a>\r\n\r\nAlkenes are reactive because they have a high-lying pair of \u03c0-bonding electrons. These electrons are loosely held, being high in energy compared to \u03c3-bonds. The fact that they are not located between the carbon nuclei, but are found above and below the plane of the double bond, also makes these electrons more accessible.\r\n\r\n\"\"<\/a>\r\n

Electrophilic addition of hydrogen halides (H-X) to alkenes<\/h1>\r\nHydrogen halides such as H-Br and H-Cl are suitable electrophiles for a simple addition to an alkene.\u00a0 Since the halogen is much more electronegative than the hydrogen, the H-X bond is quite polarized, with the H carrying a partial positive charge (\u03b4+) and serving as the electrophilic atom.\u00a0 In this example, HCl adds to but-1-ene to form racemic 2-chlorobutane:\r\n\r\n\"\"\r\n\r\nThe mechanism begins with an electrophilic addition elementary step, where the alkene forms a new sigma bond to the electrophilic H, and breaks the H-Cl bond displacing Cl\u00af.\u00a0 The second elementary step is a coordination step, where the chloride ion attacks the carbocation to form a second sigma bond.\u00a0 Note that overall we have lost one pi bond, but gained a sigma bond.\u00a0 Notice that Markovnikov's Rule is followed here, since the secondary carbocation formed is more stable than the alternative, which would have been primary.\r\n\r\n\"Butene\r\n

Video on addition of HX to alkenes<\/h4>\r\n\"\"\r\n\r\n[embed]https:\/\/www.youtube.com\/watch?v=vH--iR5jwSk[\/embed]\r\n
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Problem EA1.1.<\/h4>\r\n

The reaction of 2-methylpropene (or isobutylene) with HBr, as depicted above, is really a 2-step process.\u00a0 Draw this mechanism again and in each of the two steps label both the nucleophile and the electrophile (so, that's four labels).\u00a0 Name the two elementary steps.<\/p>\r\n\r\n<\/div>\r\n

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Problem EA1.2.<\/h4>\r\n

Draw a reaction progress diagram for the reaction of 2-methylpropene with hydrogen bromide.<\/p>\r\n\r\n<\/div>\r\n

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Problem EA1.3.<\/h4>\r\n

Predict the rate law for the reaction of 2-methylpropene with hydrogen bromide.<\/p>\r\n\r\n<\/div>\r\n

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Hydration of alkenes<\/h1>\r\nWater can be added to an alkene in the presence of an acid catalyst, to produce an alcohol.\r\n\r\n\"\"\r\n\r\nWater is not a strong enough electrophile to add an H+<\/sup> directly to an alkene, but H3<\/sub>O+<\/sup> is.\u00a0 The H3<\/sub>O+<\/sup> is. formed when sulfuric acid dissociates in water:\r\n\r\n\"\"\r\n\r\nThe alkene then reacts with the H3<\/sub>O+<\/sup> in the electrophilic addition elementary step.\u00a0 The resultant carbocation is reactive, and water then easily reacts as a nucleophile to perform the coordination step.\u00a0 A final acid-base step, regenerating the H3<\/sub>O+<\/sup> catalyst, produces the alcohol product.\r\n\r\n\"\"\r\n\r\nKhan Academy video on hydration of alkenes:\r\n\r\n\"\"\r\n\r\n<\/div>\r\n<\/div>\r\n
\r\n\r\n[embed]https:\/\/www.youtube.com\/watch?v=dJhxphep_gY[\/embed]\r\n\r\n<\/div>\r\n<\/section>","rendered":"
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Alkenes undergo addition reactions. Often, they add a proton to one end of the double bond and another group to the other end. These reactions happen in slightly different ways, however.<\/p>\n

\"\"<\/a><\/p>\n

Alkenes are reactive because they have a high-lying pair of \u03c0-bonding electrons. These electrons are loosely held, being high in energy compared to \u03c3-bonds. The fact that they are not located between the carbon nuclei, but are found above and below the plane of the double bond, also makes these electrons more accessible.<\/p>\n

\"\"<\/a><\/p>\n

Electrophilic addition of hydrogen halides (H-X) to alkenes<\/h1>\n

Hydrogen halides such as H-Br and H-Cl are suitable electrophiles for a simple addition to an alkene.\u00a0 Since the halogen is much more electronegative than the hydrogen, the H-X bond is quite polarized, with the H carrying a partial positive charge (\u03b4+) and serving as the electrophilic atom.\u00a0 In this example, HCl adds to but-1-ene to form racemic 2-chlorobutane:<\/p>\n

\"\"<\/p>\n

The mechanism begins with an electrophilic addition elementary step, where the alkene forms a new sigma bond to the electrophilic H, and breaks the H-Cl bond displacing Cl\u00af.\u00a0 The second elementary step is a coordination step, where the chloride ion attacks the carbocation to form a second sigma bond.\u00a0 Note that overall we have lost one pi bond, but gained a sigma bond.\u00a0 Notice that Markovnikov’s Rule is followed here, since the secondary carbocation formed is more stable than the alternative, which would have been primary.<\/p>\n

\"Butene<\/p>\n

Video on addition of HX to alkenes<\/h4>\n

\"\"<\/p>\n