Organic II lab 2 - MCMP 205 Laboratory Exercise 2 Exercise 2 Electrophilic Aromatic Substitution Nitration of Phenol Required Reading Chapter 16 in

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MCMP 205 Laboratory Exercise 2 1 Exercise 2. Electrophilic Aromatic Substitution: Nitration of Phenol Required Reading: Chapter 16 in the lecture text. Pay special attention to sections 16.4 and 16.5. Objective: This experiment has been designed to understand the mechanism of electrophilic aromatic substitution. In this two week lab exercise, you will carry out nitration of phenol analyze the products using thin-layer chromatography during the first week. During the second week of this lab you will be carrying out column chromatography to separate the various products formed in the reaction. This experiment will also demonstrate how thin-layer chromatography is used to monitor the progress of a column separation. Understanding the experiment Nitration of Phenol Aromatic compounds react with nitric acid, usually in the presence of sulfuric acid as a catalyst, to form nitroaromatic compounds. This reaction, called nitration , introduces a nitro group, —NO 2 , into the aromatic ring by electrophilic substitution. The electrophile in this reaction is the nitronium ion , NO 2 + , formed by the acid-catalyzed removal of the elements of water from HNO 3 . Phenol is a highly reactive aromatic compound that readily undergoes nitration at the ortho and para positions of the aromatic ring. The hydroxy group (—OH) donates electrons to the ring and activates it toward electrophilic substitution at ortho and para positions. Due to the highly activating nature of the hydroxyl group, phenol often undergoes multiple nitration to give 2,4-dinitrophenol (2,4,6-trinitrophenol at high temperature) along with the mononitro products. The highly activating —OH group of phenol necessitates the use of a mild nitrating reagent for the synthesis of mononitrated products. A convenient reagent for this purpose is nitrated silica gel , made by wetting a large excess of silica gel (SiO 2 ) a drying agent, with a dilute solution of HNO 3 . The silica gel reacts with the HNO 3 to form a salt between the nitric acid and the silicic acid in which the elements of water are eliminated. The resulting nitronium silicate, HSiO 3 NO 2 , present in the excess silica gel is a solid, mild nitrating agent that is easily dispensed. Furthermore, removal of any excess nitrating reagent is accomplished by a simple filtration.
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MCMP 205 Laboratory Exercise 2 2 Monitoring the Reaction by Thin-layer Chromatography (TLC) One of the major applications of thin-layer chromatography is to monitor the progress of a reaction. It is usually possible to withdraw a small amount of reaction mixture and spot it on the TLC plate along with standard solutions of reactants (and products, if available). The TLC plate is then developed using a suitable solvent system. Appearance of new spots and disappearance of the spots corresponding to the reactants indicates that the reaction has gone to completion. Nitration of phenol can also be monitored by TLC. Just as you observed the separation of
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This note was uploaded on 04/07/2008 for the course MCMP 204 taught by Professor Berkstrom during the Spring '08 term at Purdue University-West Lafayette.

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Organic II lab 2 - MCMP 205 Laboratory Exercise 2 Exercise 2 Electrophilic Aromatic Substitution Nitration of Phenol Required Reading Chapter 16 in

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