LAB 7 Materials

LAB 7 Materials - CHAPTER 7: AROMATIC SUBSTITUTION...

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1 CHAPTER 7: AROMATIC SUBSTITUTION REACTIONS, 7.1. BACKGROUND, Substituted aromatic compounds are of great practical importance because of their wide range of applications in chemical industry. For example, substituted benzene compounds are commonly used as solvents (e.g., toluene and xylenes) and important intermediates in many syntheses, including those of pharmaceutical agents such as acetaminophen (Tylenol ® ) and pseudoephedrine hydrochloride (Sudafed ® ). An aromatic compound can undergo a substitution reaction either by the action of an electrophilic or a nucleophilic agent. Hence, these methodologies are called electrophilic aromatic substitution (EAS) and nucleophilic aromatic substitution (NAS), respectively. It is important to note that the former is one of the primary methods of incorporating a functional group on an aromatic ring, while the latter provides access to only few useful benzene derivatives. 7.1.1. Electrophilic Aromatic Substitution (EAS) Reaction EAS involves a facile substitution of a hydrogen atom appended to an aromatic ring with an electrophile. The most important reactions include nitration, halogenations, sulfonation, and acylation and alkylating Friedel-Crafts reactions. The mechanism of EAS involves two steps: formation of a new σ bond from a C=C bond to generate a carbocation, removal of a proton by breaking the C-H σ bond to reform the C=C bond and restore aromaticity (Figure 7.1). The first step is slow because of the loss of aromaticity, even though the resulting cation is still resonance stabilized. This carbocation is often called a sigma-complex. HNO 3 H 2 SO 4 NO 2 Mechanism: NO 2 NO 2 H Slow SIGMA-COMPLEX Fast H NO 2 H NO 2 Resonance stabilization of the NO 2 H sigma-complex STEP-1 STEP-2 Figure 7.1. Mechanism of EAS reaction.
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2 By EAS, it is usually possible to achieve products with multiple substitution, but the position of the new group will be dictated by the activating/deactivating properties of the group already on the ring. The mechanisms shown below indicate that the presence of an electron donating group (which activates the ring) favors a mixture of ortho/para products, while an electron withdrawing group (which deactivates the ring) yields only the meta product. This is due to the greater stabilization and instability of the respective sigma- complexes (Figure 7.2). Groups with unshared pairs of electrons, such as amino and methoxy, are strongly activating and ortho/para directing. Such activating groups donate these unshared pair of electrons to the π -system, giving an additional resonance structure. This substantially enhances the stability of the cationic intermediate. Halogens are also ortho/para director, since they possess an unshared pair of electrons, just as nitrogen does. Other substituents, such as alkyl and aryl substituents, may also donate electron density to
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This note was uploaded on 03/03/2010 for the course CHEM 240 taught by Professor Mandal during the Spring '10 term at Illinois Tech.

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LAB 7 Materials - CHAPTER 7: AROMATIC SUBSTITUTION...

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