[QO3] Synthesis of Para Red (Mechanism) PDF

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Sintesis de Para Red colorsnte azoico...

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CENTRAL UNIVERSITY OF ECUADOR FACULTY OF CHEMICAL ENGINEERING ORGANIC CHEMISTRY III

Reaction mechanism for the synthesis of Para Red Richard Elvis Núñez Ochoa

25-05-2019

Synthesis of the azo dye known as Para Red from p-nitroaniline, β-naphthol (2-naphthol), sodium nitrite and muriatic acid. Procedure retrieved from "Introduction to Organic Laboratory Techniques", 3rd Edition - Pavia, Lampman & Kriz. Developed in Chem Draw Professional (ChemOffice Pro 16.0 Suite). Guiding teacher: Dr. Ullrich Stahl, Ph.D. 1. Electrophilic Generation Formation of nitrosonium ion [NO+] from a more stable compound such as sodium nitrite. The nitrite reacts with hydrochloric acid to form the nitrous acid that makes it possible to obtain the ion that will be used for diazotation.

2. Diazotation / Diazotization Para-nitroaniline reacts with the nitrosonium ion in such a way that the union between both nitrogens is formed, being a precursor for the appearance of the functional group (azo), inside the final molecule of the dye. In the process, a water molecule is eliminated and experimentally this reaction must be given at low temperature, since it is an exothermic reaction that if it is not controlled can decompose the dye. P-nitroaniline is a compound with which diazotization can occur due to the nitro group it possesses. This functional group pulls electrons and allows the amine to react with the nitrosonium ion. If it did not have the nitro group (-NO2), it simply could not have this reaction. The molecule must have a functional group that pulls electrons and helps the amine.

3. Copulation (Azo coupling) Due to the hydroxyl group present in naphthol, it is a strong activator that will orient in ortho and para position for its coupling with the diazo compound. Due to the disposition of the molecule, copulation will occur in the ortho position. In this step the resonance of the molecule must be performed to observe the positions where the diazo compound will attack. By movement of the oxygen free electrons in the hydroxyl group, a double bond is formed leaving the oxygen with a formal charge of 1+ and the adjacent carbon in the ortho position will be available to react. Teaching assistant: Richard E. Núñez O.

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CENTRAL UNIVERSITY OF ECUADOR FACULTY OF CHEMICAL ENGINEERING ORGANIC CHEMISTRY III

Even if α-naphthol (1-naphthol) was used for the coupling, it would occur in the ortho position because it has more space than the para position. It is enough to analyze this reasoning when observing Eriochrome Black T, which is an azo dye used as a complexometric indicator to analyze water hardness. Figure 1. Structure of Eriochrome Black T

Source: Sigma-Aldrich (2019) Eriochrome® Black T

References: • Pavia, Lampman & Kriz (2004) "Introduction to Organic Laboratory Techniques", Third edition. United States of America: Cengage Learning. • New Mexico Tech (undated) “Diazo Coupling: A Synthesis of Methyl Orange”. CHEM 334L. Organic Chemistry Laboratory. Revision 2.0. Retrieved April 29, 2018. Available at: https://infohost.nmt.edu/~jaltig/Diazo.pdf • Sigma-Aldrich (2019) Eriochrome® Black T. ACS reagent (indicator grade). Merck KGaA, Darmstadt, Germany and/or its affiliates. Retrieved May 25, 2019. Available at: https://www.sigmaaldrich.com/catalog/product/sial/858390?lang=en®ion=EC

Teaching assistant: Richard E. Núñez O.

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