Ch06 - Alkyl Halides Alkyl halides are a class of compounds...

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Alkyl Halides Alkyl halides are a class of compounds where a halogen atom or atoms are bound to an sp 3 orbital of an alkyl group. CHCl 3 (Chloroform: organic solvent) CF 2 Cl 2 (Freon-12: refrigerant CFC) CF 3 CHClBr (Halothane: anesthetic) Halogen atoms are more electronegative than carbon atoms, and so the C-Hal bond is polarized. CC H H H μ δ + δ - l The C-X bond is polarized in such a way that there is partial positive charge on the carbon and partial negative charge on the halogen. Dipole moment, μ = 4.8 x δ x d δ = charge d = bond length. Electronegativities decrease in the order of: F > Cl > Br > I Carbon-halogen bond lengths increase in the order of: C-F < C-Cl < C-Br < C-I Bond Dipole Moments decrease in the order of: C-Cl > C-F > C-Br > C-I μ = 1.56D 1.51D 1.48D 1.29D Ch06 Alkyl Halides Page 1
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Typically the chemistry of alkyl halides is dominated by this effect, and usually results in the C-X bond being broken (either in a substitution or elimination process). This reactivity makes alkyl halides useful chemical reagents. Nomenclature According to IUPAC, alkyl halides are treated as alkanes with a halogen substituent. The halogen prefixes are Fluoro-, Chloro-, Bromo- and Iodo-. Examples: FC H 2 CH 3 H Cl HC H 3 fluoroethane trans-1-chloro-3-methylcyclopentane Often compounds of CH 2 X 2 type are called methylene halides. (CH 2 Cl 2 is methylene chloride). CHX 3 type compounds are called haloforms. (CHI 3 is iodoform). CX 4 type compounds are called carbon tetrahalides. (CF 4 is carbon tetrafluoride). Alkyl halides can be primary (1°), secondary (2°) or tertiary (3°). R H H X 1 o R R H X 2 o R R R X 3 o Ch06 Alkyl Halides Page 2
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Other types: A geminal (gem) dihalide has two halogens on the same carbon. A vicinal dihalide has halogens on adjacent carbon atoms. R Br Br R gem-dibromide R R Cl R Cl R vicinal dichloride Preparation of Alkyl Halides Numerous ways to make alkyl halides. (1a) Free Radical Halogenation Usually this method gives mixtures of mono-, di-, tri- etc halogenated compounds, which is considered an inefficient method for the synthesis of a desired compound. Consider propane: + Cl 2 h ν CH 3 -CH 2 -CH 2 Cl + CH 3 -CHCl-CH 3 CH -CH -CHCl +CH -CCl -CH and others 3 2 2 3 2 3 Sometimes if there can be control over the selectivity of halogenation this is a useful route. H Cl + Cl 2 h ν Chlorocyclohexane (50%) H H CH 3 H 3 C CH 3 H + Br 2 h ν CH 3 H 3 C CH 3 Br t-Butylbromide (90%) Ch06 Alkyl Halides Page 3
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(1b) Allylic Bromination (Allylic means adjacent to a C=C double bond) The bromination of cyclohexene produces a high yield of 3- bromocyclohexene. H H + Br 2 h ν Br H + H-Br (80%) An allylic hydrogen has been substituted for a bromine. The bromine atom abstracts an allylic hydrogen because the allylic radical is resonance stabilized. H H H H Br -HBr H H H H H H Br Br Br H H H Br The radical then reacts with a bromine molecule to continue the chain.
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This note was uploaded on 11/17/2011 for the course CHEM 355 taught by Professor Staff during the Fall '11 term at Rutgers.

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Ch06 - Alkyl Halides Alkyl halides are a class of compounds...

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