TTh_13 - Organic Lecture Series CH 310/318 M LECTURE 13...

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Organic Lecture Series 1 CH 310/318 M LECTURE 13 Textbook Assignment: Chapter 7- Begin Today’s Topics: Alkenes Alkenes - Reactions Reactions Notice & Announcements: Organic Lecture Series 2 • Treatment of an alkene with Br 2 or Cl 2 in water forms a halohydrin. • Halohydrin Halohydrin : A compound containing -OH and -X on adjacent carbons. CH 3 CH=CH 2 Cl 2 H 2 OC H 3 CH-CH 2 HO HCl 1-Chloro-2-propanol (a chlorohydrin) Propene + + + Additions of Additions of HOCl HOCl and and HOBr HOBr
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Organic Lecture Series 3 – Reaction is both regioselective (OH adds to the more substituted carbon) and anti stereoselective. – Both selectivities are illustrated by the addition of HOBr to 1-methylcyclopentene. – To account for the regioselectivity and the anti stereoselectivity, chemists propose a three-step mechanism (next two screen). 2-Bromo-1-methylcyclopentanol ( a racemic mixture ) Br 2 /H 2 O OH 1-Methylcyclopentene + HBr + H H Additions of HOCl HOCl and and HOBr HOBr Organic Lecture Series 4 Step 1: Formation of a bridged halonium ion intermediate CC R H H H R H H H R H H H -Br - bridged bromonium ion minor contributing structure : : : : : : : : : Additions of Additions of HOCl HOCl and and HOBr HOBr
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Organic Lecture Series 5 Step 2: Attack of H 2 O on the more substituted carbon opens the three-membered ring. CC Br O H H H R O H HH H + R H H H : : : : : : Additions of Additions of HOCl HOCl and and HOBr HOBr Organic Lecture Series 6 – Reaction is both regioselective (OH adds to the more substituted carbon) and anti stereoselective. – Both selectivities are illustrated by the addition of HOBr to 1-methylcyclopentene.
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This note was uploaded on 10/28/2010 for the course BIO 325 taught by Professor Saxena during the Spring '08 term at University of Texas.

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TTh_13 - Organic Lecture Series CH 310/318 M LECTURE 13...

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