Chem_233_lab5_Bromination_hydrocarbons2

Chem_233_lab5_Bromination_hydrocarbons2 - Lab #5: Relative...

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Lab #5: Relative Rates of Bromination of Hydrocarbons Aliasghar Husain TA: Reena Patel 21-02-2006 Tuesday 8:00-12:00
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Introduction: In this experiment, the relative rates of the free-radical chain reactions of bromine with different hydrocarbons, containing various types of hydrogen atoms was determined. Since alkanes are relatively non-reactive in the lab addition of various compounds is hard except for when free radical bromination or chlorination is used. The six hydrocarbons used in this lab were: toluene, ethylbenzene, isopropylbenzene, tert- Butylbenzene, cyclohexane, and methylcyclohexane. The reaction occurred by a free radical mechanic in which the initiation step involved the homolytic cleavage of bromine molecules. In this lab the cleavage was promoted by light and heat at room temperature and ice bath. Finally the overall rate is determined by the stability of the radical in the produced, then it was regenerated, and the entire chain process was carried on. The process of bromination involves three steps: initiation, propagation, and termination. The initiation step involves homolysis, the propagation step is produced with the substitution product, R-Br and regenerate Br to continue the chain reaction and then comes the final step which is termination. Chlorine and bromine are the only halides that are used to form alkyl halides because fluorine reacts explosive with alkanes while iodine does not react at all. Bromine is preferred in formation of alkyl halides because chlorine will often form a mixture of products, while bromine generally forms a single bromoalkane. This happens because the propagation steps are the rate-limiting steps and it takes more energy for bromine atoms to abstract hydrogen atoms from alkanes than it takes chlorine atom. Procedure: 1. Seven 13mm x l 00mm test tubes were obtained and were filled with 10 drops of each of the six different hydrocarbons. 2. 5 drops of dichloromethane was added to each test tube with the different hydrocarbons. 3. The test tubes were taken under the hood to put 5 drops of bromine in each test tube. 4. Then the test tubes are observed and timed to see which hydrocarbon would react with bromine faster. 5.
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Chem_233_lab5_Bromination_hydrocarbons2 - Lab #5: Relative...

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