Bromination And Elimination
Transcript of Bromination And Elimination
Diphenylacetylene from Diphenylacetylene from StilbeneStilbene
OverviewOverview
DibromideDibromide AlkyneAlkyneAlkeneAlkene
Addition EliminationX 2
Ph Ph
H
Br
H
Br
PhPhC C
Ph
H
H
Ph
Br2-OH
Electrophilic Addition
The most common applications of alkene halogenation are chlorination
and bromination, which are electrophilic addition reactions.
General mechanism of electrophilic addition:
C=C
E+ Nu- Nu-
ENu
Carbocation intermediate
E
Bromination of AlkenesBromination of Alkenes
Bromonium ion intermediate formation
Anti-addition of nucleophilic Br
Pyridinium tribromide (PyH+Br3−) is used as a source for bromine.
BrBr
Br
Br Br
Br
Br
Br
a
b
b
a
Bromonium Ion
Br --- Br
Minor Shifts inElectron Density
Bromination of Bromination of transtrans-Stilbene-Stilbene
1R, 2S 1S, 2R
Meso Compounds: IdenticalMeso Compounds: Identical
rotate
An internal plane of symmetry
C C
H
HBr Br
N
H
Br3-
Br
C C
Ph H
H Ph
Br-
a b
a b
C C
Br
Br
PhH
HPh
C C
Br
PhH
HPh
Br
C C
Br
HPh
Br
HPh
Ph
Ph
Elimination of DibromideElimination of Dibromide
H
Br
H
Br
PhPh
OH H
Ph
Ph
Br
Ph Ph
OH
DiphenylacetyleneDiphenylacetyleneE2 E2
E2: Bimolecular EliminationE2: Bimolecular Elimination
1-step process.
Rate = k[RX][Base]
Strong base takes away H, a pi bond forms, and X leaves at the same time.
H and X have to be anti-coplanar.
Use 400 mg of (E)-Stilbene, 50-mL round bottom flask, 8 mL of glacial acetic
acid, 800 mg of pyridinium tribromide (Weigh in the hood!).
Reflux for at least 15 min instead of 5-10 min. The flask must be placed
deeply in the sand bath (really close to the bottom of the heat source) to
make sure the heat distributes fast.
Add 8 mL of distilled water instead of 4 mL.
Cool in an ice bath. Collect crystals and use cold water to rinse flask.
Wash crystals twice with 1 mL each of cold methanol.
Suction dry as much as possible. Do NOT heat to dry.
Measure both mp and mass.
Part A ChangesPart A Changes
Use 400 mg of meso-dibromide, 25-mL round bottom flask, 2 mL of
triethylene glycol, 4 pellets of KOH.
Heat to 185-195 ºC with sand bath (thermometer!!).
When mixture turns dark (after 1-2 min), swirl, increase temperature to
Reflux for at least 15 more min instead of just heating for 5 min.
Add 12 mL of distilled water instead of 6 mL.
Rinse the flask with cold distilled water.
Recrystallize from 95% methanol, except do NOT heat to dry.
Let crystals dry in drawer for a week. Then take mp and mass.
Part BPart B
Heating sand is very HOT!
Glacial acetic acid, pyridinium tribromide and KOH are all caustic
and corrisive.
Use Gloves!
WARNING!!!WARNING!!!