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Unit 4

Unit 4 . Alkyl Halides: Nucleophilic Substitution and Elimination Reactions. Nomenclature of Alkyl Halides. Apply the same rules you learned for the alkanes. CH 3 CH 2 Br. IUPAC: Name the halogen as a substituent. bromoethane

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Unit 4

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  1. Unit 4 Alkyl Halides: Nucleophilic Substitution and Elimination Reactions

  2. Nomenclature of Alkyl Halides • Apply the same rules you learned for the alkanes. CH3CH2Br • IUPAC: Name the halogen as a substituent. bromoethane • Common name: The alkyl group is named as the substituent. Ethyl bromide

  3. Nomenclature of Alkyl Halides 3-(iodomethyl)pentane 4-(2-fluoroethyl)heptane

  4. Nomenclature of Alkyl Halides • An alkyl halide has a halogen bonded to an sp3 C. • Methyl halides, CH3X: the sp3 C is bonded only to H atoms. • X denotes a halogen: F, Cl, Br, I. • Primary (1°) halides, RCH2X: the sp3 C is bonded to one other C atom. CH3Br CH3CH2Br

  5. Nomenclature of Alkyl Halides methyl halide 2° halide 3° halide 1° halide

  6. Nomenclature of Alkyl Halides • A vicinyl dihalide has two halogens bonded to neighboring sp3 C atoms. • A geminal dihalide has two halogens on the same sp3 C.

  7. Nomenclature of Alkyl Halides • A allylic halogen is one that is bonded to the sp3 C atom immediately next to a double bond. • FYI: The allylic position can be occupied by a substituent other than a halogen.

  8. IR Spectrum of an Alkene (for comparison)

  9. IR Spectrum of an Allylic Chloride The C-Cl str is 600-800 cm-1.

  10. Nomenclature of Other Halides • A vinyl halide has a halogen bonded to an sp2 C of an alkene. • An aryl halide has a halogen bonded to one of the sp2 C atoms of the aromatic ring.

  11. toluene chlorobenzene

  12. Nomenclature of Other Halides • A benzylic halide has a halogen bonded to an sp3 C substituent of benzene.

  13. Nomenclature: Nitro, Phenyl • A benzene ring as a substituent is called phenyl, -C6H5. • Another substituent to know is the nitro group, -NO2. TNT trans-9-phenyl-2-methylundec-3-ene

  14. Uses of Alkyl Halides • Solvents • 1,1,1-trichloroethane and methylene chloride • Starting materials for syntheses. • Anesthetics • Halothane • Freons • Pesticides • DDT, lindane (lice), chlordane (termites)

  15. C-X is a Polar Bond • Polar bonds have dipole moments which are proportional to both partial charge and bond length. • Partial charge increases with electronegativity: I<Br<Cl<F. • Bond length increases down the group. • These two trends are in opposite directions, and results in the C-Cl bond being the most polar, with C-F and C-Br close behind.

  16. C-X is a Polar Bond

  17. Intermolecular Forces in Alkyl Halides • The strongest intermolecular force in alkyl halides is the London force (instantaneous dipole-induced dipole). This force increases with molecular weight and with surface area. • Since the C-X bond is polar, there will also be a contribution from the dipole-dipole attraction.

  18. Physical Properties of Alkyl Halides - Boiling Points • Here the surface area has more effect than the C-X dipole. Since atomic size increases down the halogen group, boiling points will, too, if the alkyl part of the compound is the same. • For compounds with similar molecular weights, branched compounds will have lower boiling points than compounds with straight chains.

  19. Physical Properties of Alkyl Halides - Densities • Alkyl fluorides are comparable to alkanes and are less dense than water. • Alkyl chlorides with one Cl are less dense than water. • Alkyl bromides and iodides are denser than water.

  20. Preparation of Alkyl Halides - Allylic Bromination • We will learn other ways to prepare alkyl halides in later units. • Free-radical halogenations often give a mixture of products. • Allylic bromination, a free-radical process, can be selective.

  21. Allylic Bromination - Mechanism hυ • Initiation • Propagation, step 1 Br2 2Br• allylic free radical

  22. Allylic Bromination - Mechanism • The allylic free radical is stabilized by resonance and will form two products.

  23. Allylic Bromination - Mechanism Propagation, step 2. + Br• + Br•

  24. Allylic Bromination - Overall Reaction • NBS, n-bromosuccinimide, is often used as the source of bromine. That way, the amount of bromine is kept low…why? H2C=CH-CH2-CH3 H2C=CH-CHBr-CH3 + H2CBr-CH=CH2-CH3 NBS, hv

  25. Allylic Bromination - NBS + + Br2 is present in trace amounts in NBS (for initiation), and HBr is formed as soon as any products are formed, and so is available to produce more Br2.

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