Electrophilic addition of hydrogen halides (H-X) to alkenes
المؤلف:
University of Missouri System
المصدر:
Organic Chemistry
الجزء والصفحة:
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17-9-2020
1779
Electrophilic addition of hydrogen halides (H-X) to alkenes
Hydrogen halides such as H-Br and H-Cl are suitable electrophiles for a simple addition to an alkene. 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 (δ+) and serving as the electrophilic atom. In this example, HCl adds to but-1-ene to form racemic 2-chlorobutane:
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-. The second elementary step is a coordination step, where the chloride ion attacks the carbocation to form a second sigma bond. Note that overall we have lost one pi bond, but gained a sigma bond. Notice that Markovnikov’s Rule is followed here, since the secondary carbocation formed is more stable than the alternative, which would have been primary.
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