Fading of Ketchup using Bromine

Demonstration Experiment on Video

Fading of Ketchup using Bromine

Objective: Charge-Transfer-Complex, Electrophilic Addition to the C=C Double Bond

Peter Keusch

German version

Supermarket product: Ketchup

Chemicals:
aqueous bromine solution (2 g bromine / 100 mL H₂O)

Glaswaren:
gas collecting glass cylinder h = 20 cm d = 5 cm
buret 50 mL
beaker 100 mL
beaker 500 mL
cylinder graduated 250 mL
2 cylinder graduated 50 mL
glass rod

Hazards and safety precautions::

Bromine is highly toxic if inhaled, ingested or comes in contact with the skin.

Bromine water is harmful if ingested or inhaled. Prolonged skin contact can cause burns. Eye irritant - lengthy contact will lead to eye damage.

Safety goggles and protective gloves must be worn when handling bromine. The experiment should be performed in a laboratory fume cupboard!

Experimental procedure:

40 ml Ketchup are dissolved in 500 mL of deionized water.

A buret is filled with 50 mL of bromine water. The stopcock is opened to allow the bromine water to drain rapidly into a glass cylinder containing aqueous ketchup. Using a glass rod the top layer of the mixture is stirred gently.

Results:

The red color of ketchup turns green. After some minutes a red (unchanged ketchup), a blue, a green and a yellow layer can be observed from the bottom to the top. If the mixture is stirred, it will look dark green. The next day, the solution is yellow or nearly colourless and yellow residue has gathered at the bottom of the flask.

Video clip (Download RealPlayer .rm Datei)

Discussion:

Like b-carotene, which gives carrots their orange colour, also lycopene the dye of the tomato is a polyene. The lycopene molecule has 11 conjugated C=C double bonds. Each double bond reduces the energy required for electrons to transition to higher energy states, allowing the molecule to absorb visible light of progressively longer wavelengths. Lycopene absorbs most of the visible spectrum, so it appears orange red.

Lycopin

Lycopin

If lycopene is treated with an aqueous solution of bromine the double bonds between carbon atoms will be converted to single bonds, dinimishing the ability to absorb light. The disruption of the conjugated system, depending on the extent of the bromine addition causes the colour change from red to yellow and then to colourless.

The change in color of the red ketchup solution to green and blue is to be attributed to the formation of charge-transfer-complexes. Lycopene acts as an electron donor and bromine acts as an electron acceptor.

References:
Demonstration Experiment on Video Reaction of Cyclohexene with Bromine and Potassium Permanganate

Index of Lecture Experiments