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7 Relative Rates of Bromination of Hydrocarbons

7 Relative Rates of Bromination of Hydrocarbons - Relative...

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Relative Rates of Bromination of Hydrocarbons. Free-Radical Substitution Mechanism James Mendoza March 31, 2008 Methods and Background The purpose of this lab is to experimentally determine the bromination of hydrocarbons through free-radical reactions, and the effects of temperature on the rate of bromination of the previously mentioned hydrocarbons. Normally, alkanes are chemically unreactive toward most reagents, but in this lab, reactions occur by free-radical chain mechanistic pathways making it possible to allow the introduction of certain functional groups into an alkane. Free-radical halogenation of saturated compounds generally yields products of both monosubstitution and polysubstitution. In this lab, the breaking of bonds is required to begin the chemical experiment we are trying to achieve. In this case, homolytic cleavage is observed when the sigma bond of molecular bromine is broken to generate free radical bromine atoms. In the first step of bromination of alkanes, this is known as initiation because it starts the free-radical chain reaction. This is achieved by heating a mixture of the alkane and bromine to 200-400ºC. This is necessary to form alkyl halides, which may then be transformed into different functional groups. These radicals are formed in low concentration from neutral molecules resulting in a net increase in the concentration of free radicals in the system. Initiation Step The second step in the free-radical chain reaction mechanism is the propagation step, where the radicals produced in the first step react with molecules to form a new set of molecules and radicals. This is where the substitution product, alkyl halide, is formed. Unlike the first step, there is no net change in the concentration of radicals. In the lab, the propagation step abstracts a hydrogen atom from the hydrocarbon by a bromine atom to make a new free radical. This new free radical then attacks the bromine atom to form an alkyl bromide and regenerate a bromine radical to continue the free-radical chain reaction. Propagation Steps The third and final step of the free-radical chain mechanism is the termination step. Normally in a chemical reaction, the limiting reagent of the reaction controls the
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length of the reaction, however the free-radical chain reaction is interrupted by a series of side reactions instead. Here, various radicals combine to give molecules resulting in a net decrease in radical concentration and a decrease in the rate of the reaction.
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