Bromination of Alkenes: Tomatoes
Bromination of Tomato Pigments with Br2
Purpose:
To brominate lycopene in tomatoes using liquid Bromine.
Safety Concerns:
Toxicity of bromine.
Discussion:
The two main pigment components of tomato paste are beta-carotene (yellow-orange) and
lycopene (red) pigments.
Beta-Carotene
H3C
CH3
CH3
H3C
CH3
CH3
CH3
H3C
CH3
H 3C
CH3
CH3
Lycopene
H 3C
CH3
CH3
CH3
CH3
CH3
CH3
H3C
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
The colors of these pigments are due to the numerous double bonds in their structure. When bromine is
added to double bonds it saturates them and the color changes accordingly. In the tomato juice “rainbow”
experiment we stir bromine water into the tomato juice. The slow stirring allows the bromine water to
penetrate deeper and deeper into the cylinder in which the tomato juice was placed. As the bromine
penetrates, more and more double bonds will be saturated. Therefore, you may be able to observe a
continuous change, a “rainbow” of colors starting with the reddish tomato color at the bottom of the
cylinder where no reaction occurred since the bromine did not reach the bottom. Lighter colors will be
observed on the top of the cylinder where most of the double bonds have been saturated.
Procedures:
1. Weigh about 15 g tomato paste in a beaker.
2. Add about 30 mL of water and stir. Transfer the tomato juice into a 50 mL cylinder.
3. With a pipet add 10 mL of saturated bromine water drop-wise. With a glass rod moving in a
downward spiral swirl very gently to expose the tomato lycopenes to the bromine.
4. Observe the colors and their positions in the cylinder.
Post Lab Questions:
1. Explain why/how the reaction of lycopene with bromine makes a variety of colors.
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CH241 Lab 7: Bromination of Alkenes: Tomatoes (F14)
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CH241 Lab 7: Bromination of Alkenes: Tomatoes (F14)
CH241 Lab 7: Bromination of Tomatoes
Pre Lab Exercises
Name_______
1.
Give toxicity/hazard data for Bromine.
2.
What safety procedures are necessary beyond wearing goggles and gloves?
3.
Label the isoprene units in the following structures of beta-carotene and lycopene:
Beta-Carotene
H3C
CH3
CH3
H3C
CH3
CH3
CH3
H3C
CH3
H 3C
CH3
CH3
Lycopene
H 3C
CH3
CH3
CH3
H 3C
CH3
CH3
CH3
4.
Why is lycopene red?
5.
Show the structure of a possible product that would result from reaction of lycopene with 2
equivalents of bromine.
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CH241 Lab 7: Bromination of Alkenes: Tomatoes (F14)