exp6w08

exp6w08 - Experiment 6 Butene Synthesis by E1 dehydration...

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Experiment 6: Butene Synthesis by E1 dehydration of 2- butanol (GC) : Mohrig, Chapter 16-17 In this experiment, the non destructive technique of gas chromatography will be used to monitor the progress of a reaction over time. In gas chromatography, the mobile phase is a gas rather than a liquid as in liquid chromatography. Separation is achieved by the rela- tive affinities of the various components of a mixture for the stationary phase as it is car- ried by the moving gaseous phase. It has several distinct advantages over other forms of chromatography (greater number of theoretical plates, good sensitivity, very rapid). Ana- lytical scale GC involves the syringe injection of a small volume of liquid or gas through a rubber gasket (injection septum) into a stream of inert gas such as helium, which flows at a pre-adjusted rate. The injection port is maintained at a temperature high enough to vaporize the liquid which is then swept by the helium flow into a heated metal column (6’ x 1/8”) maintained at an appropriate temperature. The column is packed with the solid support. The various components of the sample mixture pass through the column at different velocities depending upon their relative gas-liquid phase partition coefficients. In general, non-polar substrates separate on the basis of boiling points while polar substrates separate more (but not exclusively) on the basis of polarity. This stationary phase, which is commercially produced by Union Carbide, is a non-volatile polymer with structure R- (CH 2 -CH 2 -O) n -H, [n ~100]. The area under the peak recorded by the detector is pro- portional to its quantity in a mixture of similar components.* The retention time T R of a given compound varies for a given column and the flow rate and column. The formation of an alkene from an alcohol catalyzed by acid with loss of water is called a dehydration. In this particular instance, the mechanism is classified as an E1 unimolecular process. In the first step, the oxygen atom of 2-butanol is rapidly and reversibly proto- nated by sulfuric acid to give an oxonium cationic intermediate. In the second and rate- determining step, water dissociates to give a 2° carbocation. In a third rapid step, the cat-
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exp6w08 - Experiment 6 Butene Synthesis by E1 dehydration...

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