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- Section 7 Alkyl Halides 85 Section 7 Alkyl Halides/Haloalkanes 7.1 Structure and Properties of Alkyl Halides 7.1.1 Nomenclature The nomenclature

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Section 7 - Alkyl Halides 85 Section 7 – Alkyl Halides/Haloalkanes 7.1 Structure and Properties of Alkyl Halides 7.1.1 Nomenclature The nomenclature of these compounds, as it is for all other groups, is simply an extension of the IUPAC rules: Find the longest continuous chain or ring and name it as the parent. If there are double or triple bonds present in the system, the parent chain should contain them regardless of length. Number the chains starting from the end closest to the first substituent. Where a double or triple bond is present, the end closest to the double or triple bond takes precedence regardless of substituents. Assign numbers to the substituents. Trivial names. The most commonplace trivial names apply to the halogenated methane series: XC H 3 X H 2 C X X H C X X C X X X X halomethane methyl fluoride methyl chloride methyl bromide methyl iodide Generalized IUPAC trival names X = F X = Cl X = Br X = I dihalomethane methylene fluoride methylene chloride methylene bromide methylene iodide trihalomethane fluoroform chloroform bromoform iodoform tetrahalomethane carbon tetrafluoride carbon tetrachloride carbon tetrabromide
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Section 7 - Alkyl Halides 86 7.1.2 Physical Properties Physical Properties: Haloalkanes are dipolar, and generally have higher melting and boiling temperatures than their corresponding alkanes. Dipole-dipole intermolecular interactions are stronger than van der Waals dispersion forces. Compound C-X Bond Length C-X Bond Strength Molecular Dipole Moment Boiling Point (Å) (kJ/mol) (Debye) (K) Methane Fluoromethane Chloromethane Bromomethane Iodomethane 7.2 Preparation of Alkyl Halides Electrophilic Addition to Alkenes and Alkynes have been discussed in detail in Sections 4 and 5. 7.2.1 Free Radical Halogenation of Alkanes This is a chain reaction, with initiation, propagation and termination steps. Cl Cl h ν Cl + Cl 1. Initiation 2. Propagation 3. Termination H 3 CH Cl HC l CH 3 Atom abstraction reactions Cl Cl CH 3 + H 3 CC l Cl + Cl Cl Cl Cl CH 3 CH 3 CH 3 Cl H 3 l H 3 CCH 3
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Section 7 - Alkyl Halides 87 For thermodynamic reasons, only chlorination and bromination are preparatively useful – fluorination is dangerously exothermic, and iodination is thermally unfavorable. This reaction, whilst preparatively useful, is often not synthetically useful, as polychlorination may result: chloromethane also has abstractable hydrogens. Thus, free radical chlorination will often give mixtures.
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This note was uploaded on 10/28/2010 for the course CHEM 235 taught by Professor Dr.poole during the Spring '10 term at Ball State.

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- Section 7 Alkyl Halides 85 Section 7 Alkyl Halides/Haloalkanes 7.1 Structure and Properties of Alkyl Halides 7.1.1 Nomenclature The nomenclature

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