lecture 17 - Organic Lecture Series CH 310/318 M LECTURE 17...

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Organic Lecture Series 1 CH 310/318 M LECTURE 17 Textbook Assignment: Chap 8-finish Today’s Topics: Hammond Postulate; Hammond Postulate; allylic allylic bromination bromination Notice & Announcements: Grading in Progress Organic Lecture Series 2 Haloalkanes Haloalkanes Chapter 8 Chapter 8
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Organic Lecture Series 3 Regioselectivity is high for bromination , but chlorination is NOT as selective CH 3 2 3 Br 2 Cl 2 3 2 3 3 CHCH 3 3 3 3 2 2 3 2 2 H Br HCl Propane + heat or light ++ 2-Bromopropane (92%) 1-Bromopropane (8%) Propane + 2-Chloropropane (57%) 1-Chloropropane (43%) Regioselectivity Regioselectivity Organic Lecture Series 4 • The regioselectivity of chlorination and bromination can be accounted for in terms of the relative stabilities of alkyl radicals (3° > 2° > 1° > methyl). • But how do we account for the greater regioselectivity of bromination (1600:80:1) compared with chlorination (5:4:1)? Regioselectivity Regioselectivity
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Organic Lecture Series 5 • Hammond Hammond ’s Postulate: s Postulate: The structure of the transition state: –for an exothermic step is reached relatively early in the reaction, and resembles the reactants of that step more than the products. endothermic step is reached relatively late in the reaction and resembles the products of that step more than the reactants. Hammond Hammond ’s Postulate s Postulate Organic Lecture Series 6 • Hammond Hammond ’s Postulate: s Postulate: This postulate applies equally well to the transition state for one-step reactions and to each transition state in a multi-step reaction. Hammond Hammond ’s Postulate s Postulate
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Organic Lecture Series 7 A highly exothermic reaction: Hammond Hammond ’s Postulate s Postulate (chlorination) (chlorination) Organic Lecture Series 8 Transition state Transition state “looks looks ” more the alkane (sp more the alkane (sp 3 hybridized C) hybridized C)
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Organic Lecture Series 9 (chlorination) (chlorination) Organic Lecture Series 10 A highly endothermic reaction: Hammond Hammond ’s Postulate s Postulate (bromination bromination )
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lecture 17 - Organic Lecture Series CH 310/318 M LECTURE 17...

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