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Chapter 4 - “Alcohols and Alkyl Halides”

Chapter 4 - “Alcohols and Alkyl Halides”. Functional groups and nomenclature Role of intermolecular forces on properties Synthesis of alkyl halides from alcohols Two reaction mechanisms: S N 1 and S N 2 cation stability and the Hammond Postulate Additional ways to prepare RX from ROH

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Chapter 4 - “Alcohols and Alkyl Halides”

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  1. Chapter 4 - “Alcohols and Alkyl Halides” Functional groups and nomenclature Role of intermolecular forces on properties Synthesis of alkyl halides from alcohols Two reaction mechanisms: SN1 and SN2cation stability and the Hammond Postulate Additional ways to prepare RX from ROH Halogenation of alkanes: reactivity-selectivity principle, radical stability and chain reactions All sections are covered. Minto - Lectures 7-8

  2. Potential Energy Diagram of SN1 Reaction of an Alcohol with HX Rate = k[alkyloxonium ion] Mechanisms can be supported or disproved, but cannot be proven correct. Minto - Lectures 7-8

  3. Carbocation Stabilities Alkyl groups decrease the concentration of (a.k.a. delocalize) positive charge in the carbocation by inductive effect and hyperconjugation. Minto - Lectures 7-8

  4. Delocalization of Electrons b a b b This effect, hyperconjugation, depends upon the number ofsbonds betweentheand atomsnext to a carbocation. Ethyl (above) has 3, isopropyl cation has 6, and tert-butyl has 9. (Note: my naming differs slightly from the text) Minto - Lectures 7-8

  5. Molecular Orbital Diagram in a Hyperconjugation System Minto - Lectures 7-8

  6. Gas Phase Stabilities of Cations The gas phase stabilities of the cations differ greatly (last column in table), largely due to ionization potentials for R•. Polar solvents will reduce the differences, but only partially. Minto - Lectures 7-8

  7. How is cation stability be related to rates of SN1 reactions? Hammond postulate: the transition state will be more similar to the species that it is closest to energetically Exergonic reaction: early transition state (I) Endergonic reaction: late transition state (III) Minto - Lectures 7-8

  8. «–Thermoneutral reaction I: early transition state; ‡ structure resembles reactant II: mid-transition state; ‡ structure is the average between reactant and product III: late transition state; ‡ structure resembles product Minto - Lectures 7-8

  9. Let’s apply the Hammond postulate to the nucleophilic substitution of (CH3)2CHCH2OH by HX. According to the Postulate, the transition state leading to the tert-C4H9 cation is lower in energy than the barrier to the isobutyl cation, as it reflects the greater stability of the tert-C4H9 cation. To make this comparison, it was important that both oxonium ions are of similar stability. As the barrier to forming tert-C4H9 cation is lower, it is formed faster. SN1 reactions occur with 3° and 2° ROH. Minto - Lectures 7-8

  10. Minto - Lectures 7-8

  11. SN2 Mechanism for Forming RX from ROH Minto - Lectures 7-8

  12. Other Methods of Forming RX Minto - Lectures 7-8

  13. Using Relative Rate and Statistical Factors to Predict Product Distributions We are ignoring stereoisomers here. Bromination of the same alkane would lead to predicted yields of <1%, <1%, 8%, 8%, and 82% for the analogous bromination products. This reaction is much more selective than chlorination. These calculations actually underestimate the selectivity problem for chlorination because multiple chlorination also competes with monochlorination. The chlorination reaction is seldom used except in cases where only one monochlorination product is possible. Minto - Lectures 7-8

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