08 - Chemistry 5.12 Spring 2003, 3/7/03 Handout #8: Lecture...

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Chemistry 5.12 Spring 2003, 3/7/03 Handout #8: Lecture 12 Outline E. Chlorination of Propane (4-13) 1. Inequivalent Hydrogens (1°,2°,3°) (3-3) 2. Relative Reactivity (4-13A) 3. Selectivity F. Bromination of Propane (4-13C) 1. Selectivity (Hammond Postulate) (4-14) G. Radical Stability (4-13B, 4-16) H. General Selectivity of Radical Halogenations Suggested Problems: 4-39,41,44–46,54 E. Radical Chlorination of Propane: Inequivalent Hydrogens Cl Cl Δ or hv Cl Cl 40% (1°) 60% (2°) • Propane has two inequivalent types of hydrogens that can be abstracted in the first propagation step. • The first propagation step determines which product will be formed (1° or 2°). There are two possible products! Why? 1
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Primary (1°): There are six primary hydrogens. Me Me H H H H H H Cl H H H H H H Cl H H Cl H Cl Me Me H Secondary (2°): There are two secondary hydrogens. 1° radical 2° radical • Abstraction of a primary hydrogen leads to the primary product. • Abstraction of a secondary hydrogen leads to the secondary product. 1. Inequivalent Hydrogens (1°, 2°, 3°) Primary (1°) Carbon: carbon that has only one other carbon attached to it Secondary (2°) Carbon: carbon that has two other carbons attached to it Tertiary (3°) Carbon: carbon that has three other carbons attached to it C H H H R C R H H R C R H R R 2
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• We also refer to substituents as primary, secondary or tertiary. Me Br Me Me Me Me Br Me Me Br Me Br n -butyl bromide ( ) iso -butyl bromide ( ) sec -butyl bromide ( ) tert -butyl bromide ( ) Radical Chlorination of Propane (cont.) Cl Cl Δ or hv Cl Cl (40%) (60%) • There are six primary hydrogens and two secondary hydrogens, but the secondary chloride is favored. • How can we explain the selectivity of this reaction? • Radical formation is under kinetic control , so abstraction of the 2° hydrogen to form the 2° radical must be faster than abstraction of the 1° hydrogen to form the 1° radical.
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This note was uploaded on 11/27/2011 for the course CHEMICAL E 20.410j taught by Professor Rogerd.kamm during the Spring '03 term at MIT.

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08 - Chemistry 5.12 Spring 2003, 3/7/03 Handout #8: Lecture...

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