Chem 323
B. Terem
PHOTOBROMINATION OF 1,2-DIPHENYLETHANE (BIBENZYL)
Br

+
h
Br2
Br
This is the first step of a multi-step synthesis in which the target molecule is diphenylacetylene.
However, the preparation of 1,2-dibromo-1,2-diphenylethane (also called stilbene dibromide)
photochemically is a remarkable reaction in itself. There are a number of interesting key features
about this reaction, which is worth reviewing:



The presence of sunlight is very effective in initiating a free radical chain reaction1.
Benzylic positions of bibenzyl are extremely reactive (BDE’s of C-H bonds are low)2.
The reaction is stereoselective: only the (optically inactive) meso-dibromide is formed.
The reaction proceeds via a free radical mechanism, the initiation step of which is the formation
of bromine radicals under sunlight. It should be noted that the dissociation energy (BDE) of the
Br-Br bond is 189 kJ/mol, which is well within the radiant energy of sunlight transmitted through
Pyrex glass (up to ca. 370 kJ/mol).
Initiation Step
Br
2 Br
Br
The propagation step occurs rapidly in view of the stability of the benzylic free radical formed.
The attack of the benzylic free radical to bromine is exothermic, since a C-Br bond (the bond
being formed) is stronger than a Br-Br bond (the bond being broken). The same reasoning can be
generalized also to other halogens.
Propagation Steps
H H
Br
+
H
H H
H H
H
Br
Br
+
+
HBr
Br H
H H
H H
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+
Br
Chem 323
B. Terem
The following steps are similar to previous ones. The only distinguishing feature is the
regioselectivity of the attack by the second bromine, which occurs at the adjacent carbon to the
first site of attack.
Br H
Br
+
Br H
H H
Br H
Br
Br
+
+
H
HBr
Br H
H
H Br
+
Br
The stereoselectivity of the second attack can be explained in terms of the anti-conformation
between the first bromo-substituent and the orbital of the adjacent carbon which accommodates
the single electron.
H
H
Br
Note that the addition reaction of trans-1,2-diphenylethylene (trans-stilbene), shown below, also
gives meso-1,2-dibromo-1,2-diphenylethane, although the mechanism in this case is different3,4.
Br H
Br2
CH2Cl2
H Br
meso
(1R,2S)
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Chem 323
B. Terem
PROCEDURE:
Precautions:
Carry out all transfers and other operations in this experiment, except the photobromination, in
the hood. Wear safety glasses at all times (including when you are exposing your reaction flask to
the sunlight in the courtyard). Wear gloves during transferring chemicals from one container to
the other.
Hazards:
Bromine is a toxic, corrosive compound. Take great care to avoid breathing any vapors from your
reaction flask. Avoid even the smallest amount of bromine solution to come into contact with
your eyes or skin. Immediately wash any spill on skin with sodium bicarbonate solution.
Hydrogen bromide is an acidic gas! The product, 1,2-dibromo-1,2-diphenylethane, is relatively
less hazardous; however, you may have minor products in your reaction mixture which may be
corrosive and/or lachrymatory. Take necessary precautions when filtering and transferring the
product.
Weigh out 1.0 g (0.0055 mol) of bibenzyl in a 125-mL Erlenmeyer flask. Add 10 mL of
dichloromethane and swirl the flask to dissolve the bibenzyl.
The following step will be carried out by your instructor:
Take your flask to the fume hood.
Your instructor will add to each flask 6.0 mL of a 10% (v/v) bromine-dichloromethane solution
(contains 0.31 g Br2 per mL)
Before you go back to your bench, swirl the flask to mix the layers. Cork the flask and wrap it
completely in aluminum foil to protect it from light.
Carry the flask into the direct sunlight outside the lab in Henry Hall Courtyard. Uncork and
unwrap the flask, set your flask on the outside window sill of the lab (HH 49).
Notice that a vigorous reaction begins at this point, signaled by the fading of the bromine color,
precipitation of solid product, and vigorous evolution of hydrogen bromide. Keep your
distance from the flask until the evolution of hydrogen bromide ceases,
After about 10 minutes, when the bromine color has completely disappeared, re-cork the flask,
take it back to the lab, and chill it for a few minutes in an ice bath. Collect the solid product by
vacuum filtration (save some of the filtrate for tlc analysis), wash it with approximately 5 mL of
ice-cold dichloromethane, and air dry it for a few minutes on the Büchner funnel. Label and store
the crude white (or off-white) solid. The solid product will have dried to constant weight by next
week. Weigh your product (next week) and calculate the percent yield.
References (to Carey, Organic Chemistry):
1
Sect. 4.17, p. 407; 2Sect. 11.12, p. 439; 3Sect. 7.13 pp 307-309;
See Sect. 7.11 “Achiral Molecules with two Chirality Centers” for a discussion on the
stereochemistry of meso-compounds.
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