Formal Report
Alkyl Halides Alkyl halides are a class of compounds where a halogen atom or atoms are bound to an sp3 orbital of an alkyl group. CHCl3 (Chloroform: organic solvent) CF2Cl2 (Freon-12: refrigerant CFC) CF3CHClBr (Halothane: anesthetic) Halogen atoms are more electronegative than carbon atoms, and so the C-Hal bond is polarized.
H H μ
C + C-l δ δ H
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: μ= C-Cl > 1.56D C-F > 1.51D C-Br > 1.48D C-I 1.29D
Ch06 Alkyl Halides
Page 1
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:
H Cl CH3 H
F CH2CH3 fluoroethane
trans-1-chloro-3-methylcyclopentane
Often compounds of CH2X2 type are called methylene halides. (CH2Cl2 is methylene chloride). CHX3 type compounds are called haloforms. (CHI3 is iodoform). CX4 type compounds are called carbon tetrahalides. (CF4 is carbon tetrafluoride). Alkyl halides can be primary (1°), secondary (2°) or tertiary (3°).
H R H 1 o R X R H 2 o R X R R 3o X
Ch06 Alkyl Halides
Page 2
Other types: A geminal (gem) dihalide has two halogens on the same carbon. A vicinal dihalide has halogens on adjacent carbon atoms.
Br Br R R R gem-dibromide Cl Cl R R R vicinal dichloride
Preparation of Alkyl Halides Numerous ways to make alkyl halides. (1a) Free Radical Halogenation Usually this method
H H μ
C + C-l δ δ H
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: μ= C-Cl > 1.56D C-F > 1.51D C-Br > 1.48D C-I 1.29D
Ch06 Alkyl Halides
Page 1
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:
H Cl CH3 H
F CH2CH3 fluoroethane
trans-1-chloro-3-methylcyclopentane
Often compounds of CH2X2 type are called methylene halides. (CH2Cl2 is methylene chloride). CHX3 type compounds are called haloforms. (CHI3 is iodoform). CX4 type compounds are called carbon tetrahalides. (CF4 is carbon tetrafluoride). Alkyl halides can be primary (1°), secondary (2°) or tertiary (3°).
H R H 1 o R X R H 2 o R X R R 3o X
Ch06 Alkyl Halides
Page 2
Other types: A geminal (gem) dihalide has two halogens on the same carbon. A vicinal dihalide has halogens on adjacent carbon atoms.
Br Br R R R gem-dibromide Cl Cl R R R vicinal dichloride
Preparation of Alkyl Halides Numerous ways to make alkyl halides. (1a) Free Radical Halogenation Usually this method