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Chapter 10. Alkyl Halides

Chapter 10. Alkyl Halides. What Is an Alkyl Halide. An organic compound containing at least one carbon-halogen bond (C-X) X (F, Cl, Br, I) replaces H Can contain many C-X bonds Properties and some uses Fire-resistant solvents Refrigerants Pharmaceuticals and precursors.

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Chapter 10. Alkyl Halides

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  1. Chapter 10. Alkyl Halides

  2. What Is an Alkyl Halide • An organic compound containing at least one carbon-halogen bond (C-X) • X (F, Cl, Br, I) replaces H • Can contain many C-X bonds • Properties and some uses • Fire-resistant solvents • Refrigerants • Pharmaceuticals and precursors

  3. 10.1 Naming Alkyl Halides • Name is based on longest carbon chain • (Contains double or triple bond if present) • Number from end nearest any substituent (alkyl or halogen)

  4. Naming with Multiple Halides • If more than one of the same kind of halogen is present, use prefix di, tri, tetra

  5. Naming if Two Halides or Alkyl Are Equally Distant from Ends of Chain • If there are several different halogens, number them and list them in alphabetical order.

  6. Many Alkyl Halides That Are Widely Used Have Common Names • Chloroform • Methylene chloride • Carbon tetrachloride • Methyl iodide • Trichloroethylene

  7. 10.2 Structure of Alkyl Halides • C-X bonds are longer as you go down periodic table • C-X bonds are weaker as you go down periodic table • C-X bonds are polarized with slight positive on carbon and slight negative on halogen

  8. 10.3 Preparing Alkyl Halides • Alkyl halide is from addition of HCl, HBr, HI to alkenes to give Markovnikov product (see Alkenes chapter) • Alkyl dihalide from anti addition of bromine or chlorine

  9. Reaction of Alkanes with Halogens • Alkane + Cl2 or Br2, heat or light replaces C-H with C-X but Gives mixtures • Hard to control • Via free radical mechanism • See mechanism in Figure 1-1 • It is usually not a good idea to plan a synthesis that uses this method

  10. 10.4 Radical Halogenation of Alkanes • If there is more than one type of hydrogen in an alkane, reactions favor replacing the hydrogen at the most highly substituted carbons (not absolute)

  11. Relative Reactivity • Based on quantitative analysis of reaction products, relative reactivity is estimated • Order parallels stability of radicals • Reaction distinction is more selective with bromine than chlorine (See Figure 10-2)

  12. 10.5 Allylic Bromination of Alkenes • N-bromosuccinimide (NBS) selectively brominates allylic positions • Requires light for activation • A source of dilute bromine atoms

  13. Allylic Stabilization • Allyl radical is delocalized • More stable than typical alkyl radical by 40 kJ/mol (9 kcal/mol • Allylic radical is more stable than tertiary alkyl radical

  14. Stability of the Allyl Radical: Resonance Revisited • Three electrons are delocalized over three carbons • Spin density surface shows single electron is dispersed

  15. Use of Allylic Bromination • Allylic bromination with NBS creates an allylic bromide • Reaction of an allylic bromide with base produces a conjugated diene, useful in synthesis of complex molecules

  16. 10.6 Preparing Alkyl Halides from Alcohols • Reaction of tertiary C-OH with HX is fast and effective • Add HCl or HBr gas into ether solution of tertiary alcohol • Primary and secondary alcohols react very slowly and often rearrange, so alternative methods are used

  17. Preparation of Alkyl Halides from Primary and Secondary Alcohols • Specific reagents avoid acid and rearrangements of carbon skeleton • Thionyl chloride converts alcohols into alkyl chlorides (SOCl2 : ROH  RCl) • Phosphorus tribromide converts alcohols into alkyl bromides (PBr3: ROH  RBr)

  18. Preparation of Alkyl Halides from Primary and Secondary Alcohols

  19. 10.7 Reactions of Alkyl Halides: Grignard Reagents • Reaction of RX with Mg in ether or THF • Product is RMgX – an organometallic compound (alkyl-metal bond) • R is alkyl 1°, 2°, 3°, aryl, alkenyl • X = Cl, Br, I

  20. Reactions of Grignard Reagents • Many useful reactions • RMgX behaves as R- (adds to C=O) • RMgX + H3O+ R-H

  21. 10.9 Organometallic Coupling Reactions • Alkyllithium (RLi) forms from RBr and Li metal • RLi reacts with copper iodide to give lithium dialkylcopper (Gilman reagents) • Lithium dialkylcopper reagents react with alkyl halides to give alkanes

  22. Utility of Organometallic Coupling in Synthesis • Coupling of two organometallic molecules produces larger molecules of defined structure • Aryl and vinyl organometallics also effective • Coupling of lithium dialkylcopper molecules proceeds through trialkylcopper intermediate

  23. 10.10 Oxidation and Reduction in Organic Chemistry • In organic chemistry, we say that oxidation occurs when a carbon or hydrogen that is connected to a carbon atom in a structure is replaced by oxygen, nitrogen, or halogen • Not defined as loss of electrons by an atom as in inorganic chemistry • Oxidation is a reaction that results in loss of electron density at carbon (as more electronegative atoms replace hydrogen or carbon) Oxidation: break C-H (or C-C) and form C-O, C-N, C-X

  24. Reduction Reactions • Organic reduction is the opposite of oxidation • Results in gain of electron density at carbon (replacement of electronegative atoms by hydrogen or carbon) Reduction: form C-H (or C-C) and break C-O, C-N, C-X

  25. Oxidation Levels • Functional groups are associated with specific levels

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