Chapter 6
Alkyl Halides: Nucleophilic Substitution and Elimination
亲核取代和消除
1
Classes of Alkyl Halides
?Methyl halides: only one C, CH
X
3
?Primary: C to which X is bonded has only one C-C bond.
?Secondary: C to which X is bonded has two C-C bonds.
?Tertiary: C to which X is bonded has three C-C bonds.
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3
Common Names of Halides
?CH
2X
2
called methylene halide.
?CHX
3
is a haloform.
?CX
4
is carbon tetrahalide.?Examples:
?CH
2Cl
2
is methylene chloride
?CHCl
3
is chloroform
?CCl
4
is carbon tetrachloride.
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9Uses of Alkyl Halides
?Solvents -degreasers and dry cleaning fluid
?Reagents for synthesis of other
compounds
?Anesthetics: Halothane is CF 3CHClBr
?CHCl 3used originally (toxic and carcinogenic)?Freons, chlorofluorocarbons or CFC’s ?Freon 12, CF 2Cl 2, now replaced with Freon 22, CF 2CHCl, not as harmful to ozone layer.
?
Pesticides -DDT banned （禁止）in U.S.
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10
氟里昂与臭氧层：Freons, CF 2Cl 2
C H Cl
Cl CCl3
DDT
DichloroDiphenylTrichloroethane
11
12
Chemical Warfare--Phosgene
CHCl 3+ O 2Cl ?C ?O ?OH
Cl Cl
-HOCl Cl ?C ?Cl O 光气
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内蒙古自治区发现的日军丢弃的芥子气、路易氏气混合毒剂
Mustard Gas
芥子气
14
Polarity and Reactivity
?Halogens are more electronegative than C.?Carbon-halogen bond is polar, so carbon has partial positive charge.
?Carbon can be attacked by a nucleophile.?
Halogen can leave with the electron pair.
15
Dipole Moments ?
μ= 4.8 x δx d , where δis the charge (proportional to ΔEN) and d is the distance (bond length) in Angstroms.
?Electronegativities: F > Cl > Br > I
?Bond lengths: C-F < C-Cl < C-Br < C-I
?Bond dipoles: C-Cl > C-F > C-Br > C-I
1.56 D 1.51 D 1.48 D 1.29 D
?Molecular dipoles depend on shape, too!
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Boiling Points
?Greater intermolecular forces, higher b.p.
?dipole-dipole attractions not significantly different for
different halides
?London forces greater for larger atoms
?Greater mass, higher b.p.
?Spherical shape decreases b.p.
(CH3)3CBr CH3(CH2)3Br
73°C 102°C
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Densities
?Alkyl fluorides and chlorides less dense than water.
?Alkyl dichlorides, bromides, and iodides more dense than water.
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Preparation of RX
?Free radical halogenation(Chapter 4)?produces mixtures, not good lab synthesis
?unless: all H’s are equivalent, or
?halogenation is highly selective.
?Free radical allylic halogenation
?produces alkyl halide with double bond on the neighboring carbon.
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Halogenation
of Alkanes
3ocarbons > 2ocarbons > 1ocarbons
19
?Allylic radical is resonance stabilized.
?Bromination occurs with good yield at the allylic position (sp3C next to C C).