School of Mathematics and Natural Sciences

Bromination of Heptane through irradiation with blue and with red light



n-Heptane - DANGER

H225, H304, H315, H336, H410
P210, P240, P273, P301+P330+P331, P302+P352, P403+P233

Bromine - DANGER

H330, H314, H400
P210, P273, P304+P340, P305+P351+P338, P308+P310, P403+P2338

Ammonia solution, conc. - DANGER

H290, H314, H335, H400
P260, P273, P280, P301+P330+P331, P303+P361+P353, P305+P351+P3388

Silver nitrate - DANGER

H272, H290, H314, H410
P210, P220, P260, P280, P305+P351+P338, P370+P378, P308+P310

In a fume hood 30 mL of n-heptane and 5 to 6 drops of elementary bromine are filled into a dry 100 mL erlenmeyer flask. The flask is swung to reach a good mixture of the two substances. The produced solution is distributed to two dry erlenmeyer flasks. Both flasks are covered with a watch glass each and are irradiated at the same time. The irradiation can be done with a blue or a red LED-flashlight. (Alternatively an overhead projector can be used by covering the light-surface with aluminium foil that has two holes in it. The holes are covered with a blue and a red glass respectively and the flasks are placed on the blue and the red glass.)

  • Only the sample that is irradiated with blue light becomes colorless over time. An indicator paper held to the opening of the flask with the now colorless solution will show an acidic reaction. A glass rod with a drop of concentrated ammonia solution held to the flasks opening, will cause white smoke to form.

30 mL of water are added to the erlenmeyer flask with the now colorless solution and the flask is swung a few times. The mixture is put into a phase separator and the phases are separated. The aqueous phase is tested with an indicator paper and a few drops of silver nitrate solution are added.

  • The test with the indicator paper shows that the aqueous solution reacts acidic and the addition of silver nitrate solution causes the formation of a white precipitate.

Under the fume hood the Beilstein test is applied to the organic phase. The same test is applied to pure heptane in comparison.

For the Beilstein test a strip of copper metal is held into the pale blue flame of a gas burner until it doesn't cause any flame coloration. A few drops of the substance that is to be examined is placed onto the cooled copper strip, and the strip is held into the flame once more. A green flame is evidence for a halognenated organic compound.

  • The beilstein test for the organic phase from the experiment is positive while the same test with pure heptane is negative.

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