Nitration of Bromobenzene Lab

The purpose of this lab was to observe where substituents are added on a benzene ring, by way of electrophilic aromatic substitution. The electrophilic source in the nitration of bromobenzene comes from a nitronium ion, NO2+, which acts as the electrophilic species. This nitronium ion is generated when sulfuric acid, H2SO4, is added to nitric acid, HNO3. Nitric acid is the weaker of the two acids in this reaction, thus it acts as the base, and it takes a hydrogen atom from H2SO4 to produce NO2+ and H2O. This is then followed by the rate determining step: nucleophilic attack of the aromatic ring on the nitronium ion to produce a sigma complex. The bromine on the benzene ring is a ortho-para director in this instance, because of the three resonance structures in the sigma complex, the ortho and para precursor products are more stables than the meta precursor. Thus, they have lower activation energies due to the participation of the bromines nonbonding electrons in delocalizing the positive charges of these two precursor products. In a laboratory setting, there should be a 100:1 preference of ortho-para over meta, however, the ortho:para ratio is a bit trickier. Because there are two ortho carbons and only one para carbon, there should hypothetically be a 2:1 ratio (ortho:para). However, because of steric bulk and the electron-withdrawing inductive effect, the ratio is actually observed to be 1:2, ortho: para. So, the o- bromonitrobenzene product is a by-product of the reaction to isolate the pbromonitrobenzene product. Two important aspects of this lab allowed for the observed ratio of 2.76:1, from the 0.50g ortho product, and 1.38 g para product. First, by performing the nitration at a temperature below 60 ºC, there was a suppression of dinitration to produce 1-bromo-2,4-dinitrobenzene and 1-bromo2,6-dinitrobenzene. Second, by relying on the difference in polarity between the ortho and para product, ortho-1-Bromo-2nitrobenzene being more polar than para-1-Bromo-4nitrobenzene, the products can be isolated by fractional crystallization. The first addition of hot 95% ethanol dissolved the ortho product because it was more soluble, and the para isomer selectively crystallized from solution and was isolated by filtration in the first crop. Then concentration of the filtrate allowed isolation of the second crop, which was supposed to be the ortho product. Thin layer chromatography relies on similar differences in polarity to identify the products at the end of an experiment. The para-1-Bromo-4-nitrobenzene was less polar than the meta and ortho products, so it should hypothetically have a greater Rf value because it will move with the non-polar mobile phase. However, the ortho-1-Bromo-2-nitrobenzene and meta-1-Bromo-3nitrobenzene are more polar, ortho being more polar than meta. So, the ortho product should have the lowest Rf value, with meta being second, as they are more soluble in the polar mobile phase. In the results, the para product from the first crop had an Rf value of 0.457 , which was similar to the genuine para-1-Bromo-4nitrobenzene value of 0.478. The ortho product from the second crop had an Rf value of 0.325, which was identical to the genuine ortho-1-Bromo-2-nitrobenzene sample. However, there was streaking on the TLC paper. This may have occurred because of the presence of multiple compounds or too large of a sample. To prove the existence of multiple compounds, it was shown that the filtrate TLC contained multiple spots on it with three respective Rf values: 0.0714, 0.381, and 0.5. These other products could have included other byproducts such as 1-bromo-2,4-dinitrobenzene and 1-bromo-2,6-dinitrobenzene, from dinitration, or H2SO4 and HNO3, from the starting reaction. The possibility of dinitration products could be a result of allowing the original nitration reaction to go over 60 ºC, and the presence of H2SO4 and HNO3 could be a result of not mixing well enough in the initial reaction of the addition of bromobenzene, or allowing the reaction to exceed 55 ºC. Melting point determination was also done in addition to TLC anaylsis, and the melting points of the para-1-Bromo-4nitrobenzene and ortho-1-Bromo-2-nitrobenzene sample were found to be 115-120 ºC, and 41-42 ºC respectively. The ortho product has a similar melting point to its theoretical 41-43 ºC melting point. However, the para product had a significant melting point depression when compared to its theoretical melting point of 125-127 ºC. Impurities in the sample, which may have included the dinitration products, the original reactants, or maybe even unreacted bromobenzene, were found and explained earlier in TLC to have existed. These impurities will disrupt the crystal lattice structure, so the intramolecular forces will not be as strong, thus depressing the melting point. It was also observed that the para product had a much greater melting point that the ortho product, which can also be explained through intramolecular forces. The para product has symmetry to it, and this symmetry allows for stronger intramolecular forces, which will raise the melting point. The overall yield of the experiment was 53%, and this is based on the limiting reagent bromobenzene. The lower yield of product could have been because the various issues discussed above and a few more. High temperatures could have promoted dinitration, too much ethanol could have limited the yield of product, and not proper mixing could have reduced the amount of product as well. In conclusion, this experiment successfully showed favoring of the para isomer over the ortho isomer, 2:1, which is because of steric bulk and the electron-withdrawing inductive effect. Also because of the symmetry and polarity, the melting point was much higher and the Rf value was higher as well. Furthermore, it also showed the ortho-para directing ability of bromine because there was not a significant amount of meta product formation, which is theoretically supposed to be 1:100 (meta:orth;para).