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PHOTOCHEMICAL BROMINATION OF HYDROCARBONS

* PURPOSE

This experiment investigates which of a number of consumer products containing organic compounds are photochemically active in a bromination reaction.

* DESCRIPTION

This experiment is suitable for chemistry courses where photochemical reactions and/or hydrocarbons are part of the course content. Advanced Placement and second-year students are particularly ready for the chemical concepts underlying the experiments. Bromine reacts with a number of organic compounds found in consumer products in either a substitution reaction or an addition reaction. Several products are tested to determine which reactions are accelerated by exposure to light.

* TIME REQUIRED

One lab period.

* MATERIALS

Chemicals
KBr solution (1 g in 10 mL water)*
28% HCl solution
liquid bleach (solution of NaClO)
shellac thinner or stove fuel (contain methanol)
lacquer thinner (contains mixtures of toluene)
MinWax antique refinisher (contains methyl ethyl ketone)
paint thinner (contains ethyl amyl ketone)
ethyl acetate
methylene chloride
petroleum distillates
Equipment
test tubes with stoppers to fit
test tube rack
eye droppers
light source--75 watt (or larger) light bulb or photo flood light
*See Modifications/Substitutions

* HAZARDS

Care should be taken when handling the hydrochloric acid and hydrocarbons. HCl solutions will burn the skin; the fumes are very irritating. Most organic chemicals used in this experiment are very flammable, irritating to the skin, and toxic by inhalation or absorption through the skin. Work in a well-ventilated room or hood. Wear rubber gloves when handling the organic substances. Goggles must be worn throughout the experiment; Use caution to avoid shock and burns from the light source.

* MODIFICATIONS/SUBSTITUTIONS

  1. Potassium bromide, KBr, is available from photo supply stores.
  2. Hydrochloric acid, HCl, is available as muriatic acid from in hardware stores and from pool maintenance stores.

* PROCEDURE

Preparation of the source of free bromine
  1. Dissolve 1 gram of potassium bromide, KBr, in 10 mL of water.
  2. Add 5 drops HCl (28% muriatic acid) and then add liquid bleach dropwise until no further color change occurs This brown liquid will be the source of free bromine.

Identification of photochemical hydrocarbons

  1. Measure out two 3 mL samples of each product to be tested. Identify the sample by labeling the side of each test tube.
  2. Add 10 drops of the prepared free bromine solution to each test tube. Use a stopper and shake well.
  3. Place one sample in a dark drawer or cabinet and the other under a bright light bulb. Note the time each sample is placed under the light.
  4. When a sample gives the appearance of having reacted, check its color against the sample in the dark. If the sample under the light has become colorless while that in the dark has not, the reaction is photochemical. To check, take the sample from the dark drawer and place it beneath the light source. It should also gradually become colorless.
  5. Check as many different hydrocarbon samples as time permits.

Questions to be answered by students

  1. How are photochemical reactions related to the smog problem found in many large cities?
  2. On the sides of some cans of paint thinner or lacquer are written, "non-photochemically reactive." What does this mean?
  3. If a chemical reacts instantly with bromine, with and without light, would you classify it as photochemically active?
  4. What is the difference between an organic addition versus a substitution reaction? Which type of reaction do you think was occurring in this demonstration?
  5. Would you expect the same results if a "black-light" instead of a regular incandescent light bulb were used?

* DISPOSAL

Suggested disposal procedures are often listed on the cans of consumer products. Small quantities of organic solvents can be allowed to evaporated in a fume hood. If the amount of organic solvent is large, it can be absorbed on vermiculite, placed in an iron pan or glass dish, and allowed to evaporate in an efficient hood. Chemicals other than the solvents may be flushed down the drain followed by large amounts of water.

* DISCUSSION

Free chlorine is formed by the reaction between the hypochlorite ion from the bleach and the hydrochloric acid. The released chlorine oxidizes the bromide ion in solution to free bromine.
ClO-(aq) + H+(aq) + Cl-(aq) ---> OH-(aq) + Cl2(g)

Cl2(g) + 2 Br-(aq) ---> Br2(aq) + 2Cl-(aq)

Under the influence of ultraviolet light bromine undergoes a substitution reaction with alkane hydrocarbons converting them into bromo-alkanes and an equivalent amount of hydrogen bromide. Depending upon which hydrogen atom is replaced, a number of different isomeric products can be formed from a single alkane. A mixture of alkyl halide products is usually obtained.

Br2 (Colorless Products)
CH3CH2CH3 ---> CH3CH2CH2Br and CH3CH(Br)CH3
propane light

The halogenation mechanism involves the formation of a free radical following the absorption of ultraviolet light, Br2 --- uv ---> 2 (Br· ). The free radical is in turn involved in the substitution of a hydrogen atom on the original hydrocarbon. On toluene, C6H5CH3, found in many paint thinners and solvents, the substitution occurs on the methyl side chain and produces C6H5CH2Br.

If decolorization of the reddish bromine occurs very soon after the addition of the hydrocarbon, without the need of the light source, the reaction is likely occurring by addition instead of substitution process and the hydrocarbon shows some degree of unsaturation. Because addition reactions generally occur more rapidly than substitution, this behavior can be used to differentiate between saturated and unsaturated compounds. Both saturated and unsaturated compounds are found in the list of consumer chemicals.

Photochemistry and the oxides of nitrogen produced by the automobile, especially NO2, play a major role in the formation of "photochemical smog." The substances making up this smog are toxic and very irritating to the eyes, skin, and respiratory tract. They cause extensive crop damage and deterioration of materials. This type of air pollution is common in most larger cities.

Typically, photochemical smog develops on bright sunny mornings when the concentration of NO2 is relatively high.

NO2(g) ---uv light---> NO· (g) + O· (g)
The oxygen atoms react with oxygen molecules to form ozone,
O2(g) + O· ---> O3(g)
The product, ozone, is a major component of the photochemical smog. Ozone molecules, oxygen atoms, and nitric oxide molecules attack organic compounds in the air. Unsaturated hydrocarbons, containing multiple bonds, such as ethylene, CH2=CH2, and propylene, CH3CH=CH2, are particularly active. One compound often formed is peroxyacetyl nitrate, PAN, CH3C(=O)-O-O-N-O, which causes eye irritation.

The concern about the possible depletion of the ozone layer in the upper atmosphere due to excessive use of aerosol sprays or nitrogen supplying fertilizers is based upon the supposition that the strong absorption of ultraviolet radiation from the sun by ozone helps to protect us from certain types of skin cancer caused by uv-light-induced chemical changes in living tissue.

* REFERENCES

Alyea, H.N. and Dutton, F.B., Tested Demonstrations,Journal of Chemical Education, Easton, PA, 1960, p. 118.

Flynn Scientific, Inc., Chemical Catalog/Reference Manual, 1985.

Linstromberg, W. W. and Baumgarten, H. E., Organic Chemistry: A Brief Course, D. C. Heath and Company, Lexington, MA, 1978, p. 185.

Masterton, W.L., Slowinski, E.J., and Stanitski, C.L., Chemical Principles, Saunders College Pub., Philadelphia, 1981, p. 365.

Morrison, R.T. and Boyd, R.N., Organic Chemistry, Allyn & Bacon, Inc., Rockleigh, NJ, 1975, PP. 47, 383.

Mortimer, C.E., Chemistry, Wadsworth Publishing, Belmont, CA, 1986, p. 617.

Nebergall, W.H., Holtzclaw, H.F., and Robinson, W.R., College Chemistry, D.C. Heath & Co., Lexington, MA, 1980, p. 515.

Smooth, R.C., Price, S., and Smith, G., Chemistry: A Modern Course, Charles E. Merrill Publishing, Columbus, OH, 1983, p. 372.

Submitted by Bob Kemnitz

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