Question

Bromination of cis alkene compound give meso or dl mixture

Answer 1

X

2

Addition to Alkenes: Bromination

of

trans

-S tilbene

Reference:

Organic Chemistry, 4

th

ed., M. Jones:

Section 10.2b, Addition of X

2

to Alkenes, pp. 414-421, esp. Figs. 10.7 – 10.14.

Section 4.8, Diastereomers...

meso

compounds..., p

p. 164-169.

Thi

s procedure has been adapted from the online procedure of Professor Veljka Dragojlovic, at

the Oceanographic Center, Nova Southeastern University.

Bac

kground

In t

his experiment, you will perform the addition of Br

2

to

trans

-stilbene

(Figure 1)

.

C

C

Br

Br

C

C

Br

Br

trans

-Stilbene

MW 180.25

mp 122-124

o

C

1,2-Dibromo-1,2-diphenylethane

mp 238

o

C for the

meso

compound

mp 113

o

C for a racemic mixture

H

H

H

H

Fi

gure 1.

The general reaction for the addition of Br

2

to

trans

-stilbene.

Mo

lecular b

romine (Br

2

) is a brown, highly corrosive

, fuming liquid. R

ather than use it

directly,

Br

2

will be generated

in situ

in

this reaction from the reaction of hydrobromic

acid and hydrogen peroxide (Figure 2).

H

Br

+

H

2

O

2

Br

2

+ 2 H

2

O

Bromine

d = 3.12

MW = 159.808

47% hydrobromic

acid (by weight)

d = 1.49

30% hydrogen

peroxide (by

weight) d = 1.11

Fi

gure 2.

The generation of Br

2

in situ

.

In a

nother lab use

, Br

2

is used as an indicator for the presence of excess

alkenyl and

alkynyl

π

bonds by the observation of a color change from brown to clear

in. Bromine

reacts with these

bonds,

but not aromatic

rings

, making it

possible to distinguish between

unsaturated molecules containing aromatic

rings and those

containing carbon-

car

bon

π

bonds