The objective of this lab was to prepare n-butyl bromide or nbromobutane, which is derived from an alcohol and an acid. In this case, n-butyl alcohol and sulfuric acid were the reagents. There were two methods of distillation that was involved in this experiment. The first was by reflux distillation, which is used to speed up a chemical reaction without having the reactants/ products evaporate or explode. Data Table 1 indicates the amount of each reagents that was prepared for the reflux apparatus. However, the reagents, sodium bromide, water, and butanol, were combined and cooled in an ice bath previously before transferred to the apparatus.
Sulfuric acid was then slowly added to the cooled mixture, causing the solution to turn a dark yellow. As the temperature decreases, the solution changed to a pale yellow. This is an indication of a reaction between the sulfuric acid and the sodium bromide. Sulfuric acid acts as the excess reagent and shifts the equilibrium to the right to produce a higher concentration of the product, hydrobromic acid. The hydrobromic acid is needed to react with n-butyl alcohol via an SN2 mechanism to produce n-butyl bromide.
The cooled, pale-yellow solution was transferred to the reflux apparatus and was brought to a boil, while the water circulated in the condenser for about 70 minutes. During the reflux period, there have not been any signs of errors; the reflux distillation process occurred smoothly. Once reflux distillation was completed, the mixture was cooled to room temperature, then extracted thrice by a separatory funnel. Before any extractions proceeded though, the mixture already had two distinct layers – a top dark brown layer, and a bottom light yellow layer. This mixture was extracted with 9M sulfuric acid, and the aqueous ayer, containing butanol, sulfuric acid, and water, was removed and discarded.
The organic layer, consisting of the n-butyl bromide, went through a second extraction with water. This resulted in an organic layer of n-butyl bromide and water and an aqueous layer of water and a little amount of alcohol, which was discarded. The organic layer was then dried by extracting it with sodium bicarbonate; this is an extra step for drying the alkyl halide. The resulting organic layer contained the crude nbutyl bromide, and the drying procedure was completed by adding anhydrous calcium chloride until the solution turned clear.
The drying agent was used to absorb any excess water and thus, resulted in a clear solution of n-butyl bromide. The second technique of distillation that was involved in this experiment was to distill the dried mixture by a simple distillation apparatus. No errors occurred during the course of this procedure. The clear liquid form of n-butyl bromide was collected. Based on Data Table 2, about 10. 133 g of the alkyl halide was produced. From the pre-lab calculations, the mixture had about 14. 973 or 15g of n-butanol. As a result, the percent/ theoretical yield of n-butyl bromide was determined to be 68%.
Though there have not been any issues throughout the experiment, this was not a low theoretical yield. There could have been little mistakes during the addition of the acid or during the extractions, but nothing major that could have decreased the mass of the alkyl halide. Apparatuses should have always been checked and assembled thoroughly before the start of each experiment, though. Figure 1 shows an image of the mass spectroscopy that was taken of n-bromobutane.
There are two spectrums shown: the top spectrum indicates the largest peak in n-bromobutane, which has a retention time of 0. 5. The spectrum, below, is the mass spectroscopy for the large peak (above). Mass spectroscopy is a technique to determine the molecular weight of a compound and its composition. Based on this figure, the farthest peak marks the molecular weight for n-bromobutane, which is 139. 03g. The highest peak on the spectrum at 57. 12 was then subtracted from the total mass, giving 78. 88g – the atomic mass of bromine; this indicated the presence of a bromine atom in the n-bromobutane molecule.
The highest peak of 57. 12 on the spectrum also indicated the amount of carbons in the molecule. 7. 12 grams is the atomic mass of a butyl group; therefore, there are four carbons, nine hydrogens, and one bromine atom in the final product. The literature molecular weight for n-bromobutane is 137. 02 g/mol; it was close to the determined weight from the spectrum. A mass spectrum can also take the isotopes into effect, but by observing the peaks, this is a simpler method to figure the amount of atoms in the molecule. As a result, the alkyl halide, n-butyl bromide/ n-bromobutane, was successfully produced.