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Prepform synthesis lab 1b experiment 4.3 part 1
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Synthesis lab 1b (MOL126)Preparation form experiment 4.3a (LJV009)
Lisa Verhoeven, s1014716
20-2-2022
Synthesis of meso-stilbene
dibromide by bromination of E-
stilbene
Experimental aim
The aim of this experiment is to synthesize and purify meso-stilbene dibromide ((1R,2S/1S,2R)1,2-
dibromo-1,2-diphenylethane) by bromination of E-stilbene (E-diphenylethene). Moreover, the aim is
the analyse the obtained product by IR and C-NMR spectroscopy, TLC and melting point
measurement and to determine its purity.
Background
Bromine is a nucleophilic reagent that is frequently used for the efficient bromination of double
bonds. This particular reaction is stereoselective, due to its mechanism. The bromination of double
bonds namely proceeds via a bromonium ion intermediate, due to which subsequent nucleophilic
attack by bromine can only proceed from the backside of this intermediate. The bromination of E-
stilbene (E-diphenylethene) will therefore yield the two meso-stilbene dibromide diastereomers
(1R,2S)-dibromo-1,2-diphenylethane and (1S,2R)-dibromo-1,2-diphenylethane.
Scheme 1. Bromination reaction of E-stilbene. As a bromonium ion intermediate is formed, only meso-stilbene dibromide,
(1R,2S)-1,2-dibromo-1,2-diphenylethane and (1S,2R)-1,2-dibromo-1,2-diphenylethane (not depicted), is formed.
Experimental
Bromination of E-stilbene
1. Build the experimental set up.
2. Add 0.75g E-stilbene to 10mL DCM in an Erlenmeyer.
3. Prepare 10mL of 1M bromine in DCM by dissolving 1.55g (=0.5mL) bromine into 9.5mL DCM.
4. Titrate bromine solution until the reaction mixture keeps its orange colour.
5. Stir the reaction for 30 minutes, a precipitate will form if possible perform a TLC in the
meanwhile.
6. Cool the reaction mixture on ice.
7. Filter off the precipitate.
8. Wash the product with 3x25mL cold DCM.
9. Dissolve the residue in a minimal volume of warm toluene and recrystallise it.
10. Filter off the formed crystals.
, 11. Analyse the product with IR and C-NMR spectroscopy, melting point measurement, and TLC.
Moreover, weigh the product.
Expected result(s)
It is expected that the melting point analysis will yield a melting point of around 241 ◦C. Moreover, it
is expected that TLC will show only one spot resulting from pure meso-stilbene dibromide.
Furthermore, it is expected that IR and C-NMR spectroscopy will yield the following spectra:
IR spectroscopy:
3106-3009: aromatic C-H stretch
2992-2928: alkane C-H stretch
1953, 1885: not defined
1600, 1588, 1498, 1453: aromatic C=C stretch
or 1453: alkane C-H bend, Rock
1305, 1232, 1200: haloalkane C-H wag
1164-1000: haloalkane C-H wag (?)
914, 812: alkene (aromatic) C-H bend
763, 693: aromatic (monosubstituted) out-of-plane C-H bending
626, 603, 553: C-Br stretch
507 – 426: not defined
C-NMR spectroscopy:
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