Electrophilic Aromatic Substitution Formal Lab free essay sample

The nitrating agent was prepared by slowly adding cold concentrated sulfuric acid (1. 7 mL, 0. 0319 mol) to cold concentrated nitric acid (0. 6 mL, 0. 0141 mol) in a 50 mL Erlenmeyer flask. The mixture was carefully swirled in an ice bath to ensure that the two concentrated acids were thoroughly mixed together. In a second 50 mL Erlenmeyer flask, acetanilide (1. 0 g, 0. 0074 mol) was dissolved in concentrated sulfuric acid (1. 5 mL, 0. 0281 mol) by having the acid slowly added to the solid while the mixture was swirled. Heating with a hot plate was also required to dissolve the acetanilide. When all of the solid had dissolved, the flask was cooled in an ice bath. The cold nitrating agent was added drop-wise to the cold acetanilide mixture. The flask was swirled after each addition of the nitrating agent. The flask was kept immersed in the ice bath so that the temperature of the reaction would not rise. The drop-wise addition of the nitrating agent took approximately 5-10 minutes. Including the time of adding the nitrating agent, the reaction was kept in the ice bath for a total of 20 minutes with intermittent swirling. Ice water (10 mL, 0. 555 mol) was carefully added to the flask. The mixture was thoroughly swirled to dilute the acids and was allowed to stand for about 5 minutes with occasional swirling. After the 5 minutes, solid nitroacetanilde formed. The solid was collected by vacuum filtration and rinsed with cold water. The solid was allowed to dry over the Buchner funnel for several minutes. A small amount of this solid was saved in a small test tube. The filtrate was disposed of in the waste jar in the hood. The filter flask was rinsed out with a small amount of water. The remainder of the solid was recrystallized from hot ethanol. The solid was collected by vacuum filtration. A TLC was ran on the crude solid, the recrystallized solid, and the filtrate from the recrystallization. The solid was allowed to dry until the following lab when it would be weighed and have a melting point taken. Calculations Theoretical Yield: 1. 0 g acetanilide x 1 mol/135. 16 g = 0. 0074 mol acetanilide 0. 0074 mol acetanilide x 1 mol p-nitroacetanilide/1 mol acetanilide = 0. 0074 mol p-nitroacetanilide 0. 0074 mol p-nitroacetanilide x 180. 16 g/1 mol = 1. 33 g p-nitroacetanilide Percent Yield: (Actual/Theoretical) x 100 = (0. 17 g/1. 33 g)x100 = 12. 8% yield Rf calculations: First spot: (center of first spot/ solvent front) = 3. 7 cm/ 5. 2 cm = 0. 71 Second spot: (center of second spot/ solvent front) = 4. 6 cm/ 5. 2 cm = 0. 88 Data Results The actual yield of p-nitroacetanilide was 0. 17 grams corresponding to a 12. 8% yield. The melting point was found to be 210-212 °C, confirming that the product’s identity was indeed the p-nitroacetanilide. A TLC was performed on the crude solid, the recrystallized solid, and the filtrate from the recrystallization. Each sample gave two distinct spots on the filter paper with Rf values of 0. 71 and 0. 88. Discussion Aromatic compounds can undergo electrophilic substitution reactions. In these reactions, the aromatic ring acts as a nucleophile (an electron pair donor) and reacts with an electrophilic reagent (an electron pair acceptor) resulting in the replacement of a hydrogen on the aromatic ring with the electrophile. Due to the fact that the conjugated 6? -electron system of the aromatic ring is so stable, the carbocation intermediate loses a proton to sustain the aromatic ring rather than reacting with a nucleophile. Ring substituents strongly influence the rate and position of electrophilic ttack. Electron-donating groups on the benzene ring speed up the substitution process by stabilizing the carbocation intermediate. Electron-withdrawing groups, however, slow down the aromatic substitution because formation of the carbocation intermediate is more difficult. The electron-withdrawing group withdraws electron density from a species that is already positively charged making it very electron d eficient. Therefore, electron-donating groups are considered to be â€Å"activating† and electron-withdrawing groups are â€Å"deactivating†. Activating substituents direct incoming groups to either the â€Å"ortho† or â€Å"para† positions. Deactivating substituents, with the exception of the halogens, direct incoming groups to the â€Å"meta† position. The experiment described above was an example of a specific electrophilic aromatic substitution reaction involving the nitration of acetanilde. This was formally the nitration of aniline, but some groups are not always compatible with electrophilic aromatic substitution. Aniline has an amino group (-NH2) which is electron donating and an ortho/para director. However, under the conditions of many electrophilic aromatic substitution reactions like those involving strongly acidic conditions, the amine becomes protonated (-NH3+) becoming electron withdrawing and a meta director. This is undesirable and would have led to a mixture of products. This problem can be avoided by converting the amine group into an amide (-NHCOCH3 in the case of acetanilide) prior to the electrophilic aromatic substitution reaction. The amide group is called a protecting group and will not form a salt under acidic conditions, is still activating and an ortho, para director. The protecting group can actually be easily removed after the substitution reaction to regenerate the original amine group if desired. In our experiment, we started with the protected aniline rather than have the acetanilide prepared from aniline and did not remove the protecting group at the end. The nitration was carried out with a mixture of concentrated nitric acid and concentrated sulfuric acid. This mixture gave NO2+ as the electrophilic species and the nitrating agent to be used in the reaction. The reaction yielded both ortho and para-nitroacetanilide as products. Para-nitroacetanilide was separated from the ortho form by recrystallization due to solubility properties. The melting point range obtained during this experiment was very important in determining the product’s identity since the two possible products had very different melting points. The product’s melting point range was found to be 210-212 °C, which is very close to the literature melting point range of p-nitroacetanilide (215-217 °C) confirming the product is essentially of the para form. Since the experimental melting point range is slightly lower than the literature value, there are still some impurities in the product. The impurities on the product are most likely o-nitroacetanilide. The TLC ran on the crude solid, the recrystallized solid, and the filtrate from the recrystallization was beneficial in determining what forms of nitroacetanilide was present at each step of the experiment. The TLC plate used had silica gel as the adsorbent which is highly polar. Ethyl acetate, polar, was used as the solvent. Therefore, the less polar substances would travel further up the plate with the more polar substances staying behind. P-nitroacetanilide is less polar than o-nitroacetanilide and was observed on the plate as the spot with the lower Rf value (0. 71). All three samples showed two spots at each of the Rf values (0. 71 and 0. 88), but the recrystallized solid sample’s second spot (Rf value of 0. 88) was very faint on the plate and was barely noticeable compared to the other two samples. This further supported that there was probably still a little bit of o-nitroacetanilide in the final recrystallized product, as previously shown by the melting point range. It was also expected that the crude solid sample and the filtrate from recrystallization sample would both contain both the ortho and para form and show the two distinct spots on the TLC plate. In conclusion, this lab proved to be successful in nitrating an aromatic compound and isolating the p-nitroacetanilide from the ortho/para mixture of products. The percent yield (12. 8%) was pretty low, but a low percent yield was expected. The sources of error which may have led to such a low percent yield include the fact that we had to separate the para product from the ortho product and most likely lost a good amount of product in this step. Also, the nitrating agent (NO2+) was gaseous and had to be prepared in â€Å"situ† from the reaction of sulfuric acid and nitric acid. It is very probable that we did not have a 100% yield of the nitrating agent prepared to be able to react in the nitrating reaction. As low as the percent yield may have been, I feel confident that the final product was pretty pure. This was evidenced by both the obtained melting point range, which was fairly close to the literature value, and the TLC plate which showed only a little bit of the ortho product. References 1) Sigma-Aldrich website