Peter Defnet and Cody Krepps
Department of Chemstry
September 18, 2012
Abstract: Nitration of 2-benzylpyridine is supposed to yield 2-(2,4-Dinitrobenzyl) pyridine, when electrophilic aromatic substitution is the mechanism. Experiencing many pitfalls, however, has lead to the actual product obtained to contain the expected product, as well as many impurities. This report examines the supposed mechanism for the electrophilic substitution, a mechanism for the product’s photochromic property, as well as an explanation as to why the mechanism happens in this particular sequence.
Nitration of 2-benzylpyridine through electrophilic aromatic substitution (EAS) is expected to yield 2-(2,4-Dinitrobenzyl) pyridine, a photochromic product. This reaction was first discovered by Tshitschibabin in 19253, and is frequently used today for a variety of purposes, including use with photo recording media4. Being photochromic means that when exposed to UV light, or when exposed to darkness, the color of the compound changes in response to the presence or absence light. The mechanism for the creation of 2-(2,4-Dinitrobenzyl) pyridine from 2-benzylpyridine, is a basic form of EAS, and is shown in Figure 1, below.
Results and Discussion:
Concentrated sulfuric acid was obtained and placed in a round bottom flask, which was placed in an ice bath. A stir bar was added to the solution and the ice bath was placed on a combo plate to induce the stirring mechanism. While stirring, 2-benzylpyridine and 70% nitric acid were added dropwise to the cold sulfuric acid. The solution was observed to be a yellow color. The flask was then placed on an already hot heating mantle, which immediately changed the color of the solution from yellow to a dark brown color. The mantle then had changed electrical sockets and was plugged into a variac with a range of 35-40, in attempt to cool the solution to appropriate temperature. The initial heating, however, could have denatured the chemicals in the solution, providing a basis for error.
The mixture was heated and stirred for 20 minutes, from which it was poured onto ice in a 500ml beaker, and rinsed with a little water to ensure all of the reactants were in the ice mixture. The stir bar was supposed to have been left behind, however, it got mixed in with the ice mixture. This prompted its removal by dipping a large magnet typically used to remove stir bars from solution into our solution and removing it. This act could have contaminated the solution if the large magnet was not sufficiently clean, providing another basis of error.
Small measured volumes of 10% Sodium hydroxide were added to the ice mixture until a yellow opaque color was observed. The pH was observed with use of Litmus paper. The color of the litmus paper when the solution was added turned a dark blue color. Perhaps the solution became too basic, despite adding the sodium hydroxide slowly until the solution turned the appropriate color, because the procedure called for a color that was ‘somewhat blue.’ Also it noted that if our solution became too basic, then our product would not remain solid, which is what would eventually happen. Despite the indicator recommending that the reaction be restarted; it was continued.
The icy solution was stirred constantly for 25 minutes and rapidly turned from a yellow color to a dark brown color with small brown specks forming in solution. Let it be noted that these specks never formed one large solid, but rather stayed separated. This suggests that the pH of the solution was, in fact, too high. After 25 minutes, the solution was then vacuum filtered and the solid was isolated. The solid was added to ether and triturated for about 1 minute, and then the ether was then removed from the solid.
An IR spectrum and a melting point were obtained from the isolated product. Both the melting...
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