lecture 18 (ch 8) - Organic Lecture Series CH 310/318 M...

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Organic Lecture Series 1 CH 310/318 M LECTURE 18 Textbook Assignment: Chapter 8- Cont’d Today’s Topics: Haloalkanes Haloalkanes -the free radical reaction the free radical reaction Notice & Announcements: Exam II: Pick up exams AFTER class Organic Lecture Series 2 Haloalkanes Haloalkanes Chapter 8 Chapter 8
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Organic Lecture Series 3 • Bond Dissociation Enthalpies (BDE) CH 2 =CH- H 2 =CH 3 - H 3 3 2 - H 3 2 (CH 3 ) 2 CH- H 3 ) 2 3 ) 3 C- H 3 ) 3 C C 6 H 5 2 - H C 6 H 5 2 2 =CHCH 2 - H 2 =CHCH 2 Benzyl Allyl Vinyl Methyl Ethyl Isopropyl tert- Butyl 372 (89) 376 (90) 405 (97) 414 (99) 421 (101) 439 (105) 464 (111) Hydrocarbon Radical Name of Radical Δ H 0 kJ (kcal)/mol Vinylic Benzylic Allylic Type of Energetics Energetics Increasing E Increasing Stability Organic Lecture Series 4 Radical: Radical: any chemical species that contains one or more unpaired electrons. –Radicals are formed by homolytic homolytic bond bond cleavage cleavage. –The order of stability of alkyl radicals is 3° > 2° > 1° > methyl. Mechanisms Mechanisms
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Organic Lecture Series 5 – The order of stability of alkyl radicals is 3° > 2° > 1° > methyl. CH 3 2 OO C H 2 3 Cl 3 3 3 3 2 O 3 OCH 2 3 Δ H 0 = +159 kJ/mol (+38 kcal/mol) Ethoxy radicals Diethyl peroxide 80° + Δ H 0 = +376 kJ/mol (+90 kcal/mol) Methyl radicals Ethane heat + Chlorine atoms Chlorine light + Δ H 0 = +247 kJ/mol (+59 kcal/mol) Mechanisms Mechanisms Organic Lecture Series 6 Chain initiation: Chain initiation: A step in a chain reaction characterized by formation of reactive intermediates (radicals, anions, or cations) from nonradical or noncharged molecules. light + •• or heat Step 1: Radical Chain Mechanism Radical Chain Mechanism
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Organic Lecture Series 7 Chain propagation: Chain propagation: A step in a chain reaction characterized by the reaction of a reactive intermediate and a molecule to form a new radical or reactive intermediate and a new molecule. Chain length: Chain length: The number of times the cycle of chain propagation steps repeats in a chain reaction. CH 3 2 3 2 HC H 3 2 3 2 Cl l + + Step 2: Step 3: + + Radical Chain Mechanism Radical Chain Mechanism Organic Lecture Series 8 Chain termination: Chain termination: A step in a chain reaction that involves destruction of reactive intermediates.
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This note was uploaded on 03/20/2011 for the course CHEMISTRY 310m taught by Professor Iverson during the Fall '08 term at University of Texas at Austin.

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lecture 18 (ch 8) - Organic Lecture Series CH 310/318 M...

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