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Nitrating - REAC modular publisher H.A Neidig laboratory...

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Nitrating Acetanilide or Methyl Benzoate: Electrophilic Aromatic Substitution prepared by Carl T. Wigal , Lebanon Valley College PURPOSE OF THE EXPERIMENT Demonstrate the regiochemistry of electrophilic aromatic substitution reactions for monosubstituted aromatic compounds. EXPERIMENTAL OPTIONS Nitrating Acetanilide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Nitrating Methyl Benzoate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 BACKGROUND REQUIRED You should be familiar with vacuum filtration, melting point measure- ment, and recrystallization techniques. BACKGROUND INFORMATION Most substitution reactions at aliphatic carbon atoms are nucleophilic. However, aromatic substitution reactions are generally electrophilic, due to the high electron density of the benzene ring. The species reacting with the aromatic ring is usually a positive ion or the positive end of a di- pole. This electron-deficient species, or electrophile , may be produced in various ways, but the reaction between the electrophile and the aro- matic ring is essentially the same in all cases. The most common electro- philic aromatic substitution mechanism is the arenium ion mechanism , shown in Figure 1. REAC 716 m o d u l a r · l a b o r a t o r y · p r o g r a m · i n · c h e m i s t r y publisher: H.A. Neidig organic editor: Joe Jeffers Copyright 1998 by Chemical Education Resources, Inc., P.O. Box 357, 220 South Railroad, Palmyra, Pennsylvania 17078 No part of this laboratory program may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photo- copying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Printed in the United States of America 02 01 00 99 98 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 Figure 1 The arenium ion mechanism for electrophilic aromatic substitution Step 1 benzene arenium ion Step 2 –H + + E + substituted benzene + + H E + E H E H E H E
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In the first step of the reaction, benzene donates an electron pair to the electrophilic species, designated E + . A carbocation intermediate is formed, called an arenium ion or sigma complex . This arenium ion can be written in three resonance forms. Although the arenium ion is stabi- lized by these resonance forms, it is destabilized by the loss of aromatic stability (~ 36 kcal/mol). This aromatic stability is regained in the second step of the reaction, consisting of elimination of a proton from the arenium ion, forming a substituted benzene. To accurately design an experiment involving an electrophilic substi- tution reaction performed on a monosubstituted benzene, several factors must be considered. One factor is the relative rate of the reaction. The sub- stituent group already present on the ring may cause the substitution re- action to be slower or faster than the initial reaction with benzene. Substituent groups that increase the reaction rate relative to the reaction rate with benzene are called activators . Activators donate electrons, in- creasing the electron density of the aromatic ring and thus stabilizing the arenium ion. Activators can donate electrons to the aromatic ring in either of two ways. Most activators donate electrons by resonance. Resonance do-
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