Grignard Reaction
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Grignard Reaction – Lab ReportTable 1 Results for 2-Methyl-3-Heptanol:Theoretical Yield (g)Amount Observed (g)Percent Yield (%)Boiling Point (°C)AppearanceSheenViscosity2.631.2647.9140-145Lit. (167)The mixture was colorless and had a cloudy opaquenessSolution lacked any shine Seemingly thick solution[pic 1][pic 2]Scheme 19.5[pic 3]Scheme 19.9[pic 4] Fig. 19.1 Scheme 19. 13
Discussion: In the preparation leading to the synthesis of 2-methyl-3-heptanol a Grignard reaction was preformed between 1-bromobutane and magnesium turnings as scheme 19.5 indicates. With the addition of anhydrous diethyl ether the product desired was 1-butylmagnesium bromide. However, one problem that arose shortly into the experiment called for the magnesium turnings to be freshly crushed. The purpose of this part was to expose fresh reactive magnesium that was located underneath the oxidized surface of the unreactive magnesium. Without the tools to properly grind magnesium, scissors were used to finely chop the magnesium into individual pieces. This small detail is one plausible reason why the organometallic was slow to react. As the experiment continued an ethereal solution composed of the alkyl halide, 1-bromobutane, and anhydrous ether was added with imprecision. As figure 19.1 indicates a syringe is preferred in the apparatus as exact amounts of ethereal solution can be added to the magnesium. Instead a Pasteur pipette transferred small amounts of the solution onto the magnesium. While great for transferring the liquid, the Pasteur pipette delivered varied amounts. This slight problem most likely resulted in a side reaction known as the Wurtz-type mechanism (scheme 19.9). As higher concentrations of the 1-bromobutane were added at varied times this increased the concentration of the halide bromine. As the solution under went reflux, part of the product then became a symmetrical hydrocarbon known as n-octane. Evidence of octane was observed in the formation of 2-methyl-3-heptanol as distillation also occurred in part at 124°C. This value is close to the literature boiling point of n-octane at 125°C.