lecture_04_chapter_16

lecture_04_chapter_16 - Lecture IV, Chapter 16, Chem 308...

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Lecture IV, Chapter 16, Chem 308 1 ORGANIC CHEMISTRY 308 LECTURE IV CHAPTER 16 Let us continue to explore electrophilic aromatic substitution, a major area of synthetic and mechanistic organic chemistry. Review the general mechanism of this reaction. We wish to focus now on the course of these reactions with substituted benzenes. A. Two fundamental questions about the effects of substituents already on the ring in electrophilic aromatic substitution of the compound: 1. Relative reactivity of the benzene derivative compared to benzene itself? 2. Regioselectivity as the result of the substituent? We already have enough background to predict (to some extent) and certainly to understand the substituent effects. B. Let’s look at relative reactivity first. 1. It should be very simple. Electron donation enhances reactivity in an electrophilic reaction and electron withdrawal results in lower reactivity. When a group enhances reactivity we say it is activating. When it decreases reactivity, we say it is deactivating. 2. Not so simple, however, because there are two ways in which a group can donate or withdraw electrons: induction and resonance. Induction is electron distribution through σ bonds and resonance is electron distribution through π bonds. 3. The reason it is not so simple is because many groups display inductive and resonance effects in the opposite direction. As a general rule in such cases, resonance usually wins, but there are important exceptions, as we shall see.
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Lecture IV, Chapter 16, Chem 308 2 4. Inductive effects are relatively straightforward. a. Electronegative atoms (X, O, N, and S) are electron withdrawing especially when they are substituted directly on the phenyl ring. Thus virtually all functional groups are inductively withdrawing. Also positively charged species are electron withdrawing by induction. The positive charge attracts the electrons, which are negatively charged. b. Alkyl groups are electron donating. Alkyl groups are electron donating, as we know, in part because of hyperconjugation. But haloalkyl groups are electron withdrawing. Because the halogen is not substituted directly on the phenyl ring they tend to be less electron withdrawing. But a group such as CF 3 is an extreme example and quite a powerful electron withdrawing group by induction. Here is a summary of inductive effects: 5. Resonance effects are more complex. Remember that resonance is transmission of electron density through a π system. In order for a substituent to participate in resonance with the phenyl ring it must have either nonbonding electrons or π electrons. Let’s look at some examples
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Lecture IV, Chapter 16, Chem 308 3 a. Most common and strongest is the donation of a nonbonding electron pair of a hetero atom substituted directly on the phenyl ring: Note that R can be alkyl or H in these cases. Thus aniline φNH 2 and phenol φOH both have considerable electron donation by resonance into the phenyl ring. b. Electron withdrawal also occurs by resonance when
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lecture_04_chapter_16 - Lecture IV, Chapter 16, Chem 308...

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