chapter 6 - Start of Chapter 6 Lecture Notes Methyl halide...

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Unformatted text preview: Start of Chapter 6 Lecture Notes Methyl halide primary halide secondary halide Tertiary halide Allylic halogen Aryl halides Vicinal dihalide geminal dihalide H 3 C Cl H 2 C Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Vinyl halogen Different designations for halogens Structure of alkyl halides F Cl Br I Most electronegative Atom Smallest atomic radii Shortest sigma bond Length (C-X) Least electronegative Atom Largest atomic radii Longest sigma bond Length (C-X) Biggest dipole moment between a C-X bond Lower dipole moment between a C-X bond Making alkyl halides Starting material reaction process Alkanes radical formation & chain reaction mechanism Alkenes addition reaction Alkenes allylic halogenation Alcohols substitution reactions Alkyl halides substitution reactions In chapter 4 in your text Cl H H H H H H H + Cl-Cl HCl + Light Occurs through a chain reaction mechanism 3 steps in a chain reaction: 1. Initiation step produces reactive intermediate a radical Converting an alkane to an alkyl halide Note: this information is on pages 128 132 (section 4-3) & 146 150 (section 4-13) in the 7 th edition of the text or pages 126 130 (section 4-3) & 144 148 (section 4- 13) in the 6 th edition of the text Cl Cl Light (photon), h v 2 Cl 1. Propagation steps reactive intermediate reacts with a stable molecule and produces another stable molecule The overall reaction is the sum of the propagation steps What is the most stable type of C atom to put a free radical intermediate on? Tertiary > secondary > primary C H H H H Cl C H H H + HCl Cl Cl C H H H C H H Cl H + Cl 1. Termination steps- side reactions that destroy the reactive intermediates- slows or stops the reaction Cl + Cl . . Cl-Cl + Another possibility is a reaction between the free radical and a contaminant producing a stable product but no new reactive free radical Adding a halogen to an alkane details Making a free radical intermediate on a tertiary C is best Experimentally Cl results in mixtures of products - it will give primary as well as other types of products- it is not selective in its formation Experimentally Br results in mainly only one product being formed- it adds to the most substituted C atom- it adds selectively Cl 2 hv Cl Cl Cl Cl Br 2 hv Br Br Main product VERY minor product Wade 7 th edition chapter 4 section 13 Br NBS One way to add only ONE Br to a SPECIFIC place N O O Br light + Br 2 Here only one Br is added to the allylic position Here a free radical C intermediate is formed on an allylic C RESONACE IS POSSIBLE! Br Also, there is a second product made after resonance occurs on the reaction intermediate & Source of Br Adds Br to allylic position Through a free-radical mechanism H 2 C C H 2 C H 2 C Br Br Details on NBS addition Circled are all possible products Cis-alkene trans-alkene C* Br C* is a chiral C & this C could be in the R- or S- configuration NBS light 4 products Making alkyl halides Starting material reaction process Alkanes radical formation...
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This note was uploaded on 02/13/2011 for the course CHEM 2344 taught by Professor Slack,wu during the Spring '11 term at University of Houston.

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chapter 6 - Start of Chapter 6 Lecture Notes Methyl halide...

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