Lab Report - Nitration of Bromobenzene

Lab Report: Nitration of Bromobenzene Abstract: In this experiment, the electrophilic aromatic substitution mechanism was used to obtain 4-nitrobromobenzene and 2-nitrobromobenzene. Recrystallization was used to purify 4-nitrobromobenzene. The melting points for both products were obtained and compared to the melting points found in literature. Thin layer chromatography was performed to calculate the Rf values. The Rf values for 4-nitrobromobenzene and 2-nitrobromobenzene were then compared to the Rf values of the pure substances to determine similarity and purity. Introduction: The electrophilic aromatic substitution reaction involves an atom that is attached to an aromatic system and is replaced by an electrophile. This type of reaction is what occurs in this specific experiment. It has been found that cationic intermediates represent satisfactory comparison models for the transition states of electrophilic aromatic substitutions [1]. Figure 1: Electrophilic aromatic substitution reaction example Recrystallization is used to purify solid compounds. There are various techniques that can be used to recrystallize, such as single-solvent recrystallization, multi-solvent recrystallization, and hot filtration-recrystallization. In this experiment, single-solvent recrystallization was used in order to purify 4-nitrobromobenzene. Regardless of the type of recrystallization, it tends to form random textures [2]. However, there are a few pitfalls of recrystallization that can damage the purity of the product. Pitfalls include adding too much solvent and not filtering the solution fast enough to where the solvent cools. Figure 2: 4-nitrobromobenzene after recrystallization Melting point is defined as the point at which a compound or substance changes from a solid to a liquid. Having a minimal difference between the melting point from literature and the melting point of a product means that the product is pure. Usually, the melting point range is 0.5°C to 2°C for a substance to be considered pure. For an impure substance, the melting point range is more than 5°C. Thin layer chromatography, often abbreviated as TLC, can be used to identify certain compounds as well as confirm the presence of specific compounds when compared to a control group. Using thin layer chromatography allows for retention factors (Rf) values to be calculated. The Rf values are ratios between the distance traveled by the compound and the distance traveled by the solvent. The values can indicate polarity and similarity between two compounds. In this experiment, 4-nitrobromobenzene and 2-nitrobromobenzene are expected to be obtained and compared to the pure substance using thin layer chromatography. Discussion/Results: After the 4-nitrobromobenzene and 2-nitrobromobenzene products were obtained, melting point, percent yield, and thin layer chromatography were completed and the results were analyzed. The yield of 4-nitrobromobenzene was found to be 0.21. The melting point for 4-nitrobromobenzene was found to be 127°C. A TLC plate for 4-nitrobromobenzene and 2-nitrobromobenzene, as well as the pure substances, was also obtained. Figure 3: TLC Plate As previously stated, using the TLC plate allows for Rf values to be calculated. The Rf values were calculated by finding the distance the spot traveled divided by the distance the solvent traveled. The calculated Rf values are as follows: Pure 4-nitrobromobenzene: 48.26 mm/53 mm = 0.91 Pure 2-nitrobromobenzene: 43.18 mm/53 mm = 0.81 4-nitrobromobenzene product: 46.99 mm/53 mm = 0.89 2-nitrobromobenzene product: 40.64 mm/53 mm = 0.77 By comparing the Rf values, it can be seen that the products are very similar to each other, meaning that the experiment was performed properly. The melting point for 4-nitrobromobenzene was found to be 127°C. The melting point from literature for 4-nitrobromobenzene is 125-127°C. Because the melting point of the product and the melting point in literature are very similar, the product is considered to be pure. The melting point for the 2-nitrobromobenzene product was unable to be determined because not enough crystals formed to test for melting point. The melting point in literature for 2-nitrobromobenzene is 40-41°C.