DiphenylacetyleneDiphenylacetylene from from StilbeneStilbene

OverviewOverview

DibromideDibromide AlkyneAlkyneAlkeneAlkene

Addition EliminationX 2

Ph Ph

H

Br

H

Br

PhPhC CPh

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

Carbocationintermediate

E

BrominationBromination of Alkenesof 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 --- Brδ + δ −

Minor Shifts inElectron Density

BrominationBromination of of transtrans--StilbeneStilbene

1R, 2S 1S, 2R

MesoMeso Compounds: IdenticalCompounds: 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 Elimination of DibromideDibromide

H

Br

H

Br

PhPh

OH H

Ph

Ph

Br

Ph Ph

OH

DiphenylacetyleneDiphenylacetyleneE2 E2

E2: Bimolecular EliminationE2: Bimolecular Elimination1-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, 25-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.

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

Reflux ApparatusReflux Apparatus

Caution: Caution: Reflux gently because the overheating may allow bromine to escape. Observe a reflux ring!!!

Sand bathon top of magnetic stirrer

Set up the reflux apparatus same as the figure shown in the book using a long condenser with the sand bath as a heat source. (Use a thermometer to measure temperature in the sand bath. Circulate the water through the condenser.

Use 400 mg of meso-dibromide, 25-mL round bottom flask, 2 mL of

triethylene glycol, 2 pellets of KOH.

Heat to 185-195 ºC with sand bath (thermometer!!).

When mixture turns dark (after 1-2 min), swirl, heat for 15 more min instead

of 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!!!