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Mechanism for Electrophilic Aromatic Substitution

Most of the reactions discussed in this chapter involve the attack of an electrophile on an aromatic compound. Although the initial step of the mechanism resembles that of the electrophilic addition reactions of carbon-carbon double bonds discussed in Chapter 11, the final product here results from substitution of the electrophile for a hydrogen on the aromatic ring rather than addition. Therefore, these reactions are called electrophilic aromatic substitutions. [Pg.671]

three different mechanisms for nucleophilic substitutions on aromatic rings are presented. These are followed by several other reactions that are useful in synthesis because they interconvert groups attached to aromatic rings. Finally, the use of combinations of all of these reactions to synthesize a variety of substituted aromatic compounds is discussed. [Pg.671]

O The electrophile reacts with a pair of pi electrons of the aromatic ring. This step resembles the first step of the reaction in which electrophiles react with alkenes, described in Chapter 11. This is the slow step of the reaction. [Pg.672]

This carbocation is called an arenium ion. Although it has considerable resonance stabilization, it has lost the aromatic resonance energy of benzene because the cycle of orbitals does not extend entirely around the ring. Note that the resonance structures have the positive charge located on the carbons ortho and para to the carbon that is bonded to the electrophile. [Pg.672]

Mechanism of a general electrophilic aromatic substitution reaction. [Pg.672]


Now that we ve outlined the general mechanism for electrophilic aromatic substitution we need only identify the specific electrophile m the nitration of benzene to have a fairly clear idea of how the reaction occurs... [Pg.477]

Scheme 10.2. Generalized Mechanism for Electrophilic Aromatic Substitution... Scheme 10.2. Generalized Mechanism for Electrophilic Aromatic Substitution...
At this point, attention can be given to specific electrophilic substitution reactions. The kinds of data that have been especially useful for determining mechanistic details include linear ffee-energy relationships, kinetic studies, isotope effects, and selectivity patterns. In general, the basic questions that need to be asked about each mechanism are (1) What is the active electrophile (2) Which step in the general mechanism for electrophilic aromatic substitution is rate-determining (3) What are the orientation and selectivity patterns ... [Pg.571]

The mechanism for electrophilic aromatic substitution is addition-elimination. Using these working hypotheses, Mills and Nixon explained the regioselectivity of electrophilic substitution in 5-hydroxyindan versus 6-hydroxytetralin. [Pg.174]

A priori, the two most likely mechanisms for electrophilic aromatic substitution on benzene, in the absence of strong base,156 are (1) direct displacement, the transition state for which is shown in 65, and (2) a two-step reaction in which... [Pg.384]

The most widely accepted mechanism for electrophilic aromatic substitution involves a change from sp2 to sps hybridization of the carbon under attack, with formation of a species (the Wheland or a complex) which is a real intermediate, i.e., a minimum in the energy-reaction coordinate diagram. In most of cases the rate-determining step is the formation of the a intermediate in other cases, depending on the structure of the substrate, the nature of the electrophile, and the reaction conditions, the decomposition of such an intermediate is kinetically significant. In such cases a positive primary kinetic isotope effect and a base catalysis are expected (as Melander43 first pointed out). [Pg.243]

The simplest and most general mechanism for electrophilic aromatic substitution in solution is the so-called arenium ion mechanism, depicted in Scheme 2 [54,254]. [Pg.24]

This two-step mechanism for electrophilic aromatic substitution applies to all of the electrophiles in Figure 18.1. The net result of addition of an electrophile (E ) followed by elimination of a proton (H ) is substitution of E for H. [Pg.643]

In bromination (Mechanism 18.2), the Lewis acid FeBr3 reacts with Br2 to form a Lewis acid-base complex that weakens and polarizes the Br- Br bond, making it more electrophilic. This reaction is Step [1] of the mechanism for the bromination of benzene. The remaining two steps follow directly from the general mechanism for electrophilic aromatic substitution addition of the electrophile (Br in this case) forms a resonance-stabilized carbocation, and loss of a proton regenerates the aromatic ring. [Pg.644]

These steps illustrate how to generate the electrophile E for nitration and sulfonation, the process that begins any mechanism for electrophilic aromatic substitution. To complete either of these mechanisms, you must replace the electrophile by either or S03H in the general mechanism (Mechanism 18.1). Thus, the two-step sequence that replaces H by E is the same r ardless of E. This is shown in Sample Problem 18.1 u.sing the reaction of benzene with the nitronium ion. [Pg.646]

We must first generate the electrophile and then write the two-step mechanism for electrophilic aromatic substitution using it. [Pg.647]

Rearrangements can occur even when no free carbocation is formed initially. For example, the 1° alkyl chloride in Equation [2] forms a complex with AICI3, which does not decompose to an unstable 1° carbocation, as shown in Mechanism 18.9. Instead, a 1,2-hydride shift forms a 2° carbocation, which then serves as the electrophile in the two-step mechanism for electrophilic aromatic substitution. [Pg.651]

In this chapter we shall examine the methods that are used to measure these effects on reactivity and orientation, the results of these measurements, and a theory that accounts for these results. The theory is, of course, based on the most likely mechanism for electrophilic aromatic substitution we shall see what this mechanism is, and some of the evidence supporting it. First let us look at the facts. [Pg.340]

Section 15.9 General Mechanism for Electrophilic Aromatic Substitution Reactions... [Pg.607]

We will look at each of these five electrophilic aromatic substitution reactions individually. As you study them, notice that they differ only in how the electrophile (Y" ") needed to start the reaction is generated. Once the electrophile is formed, all five reactions follow the same two-step mechanism for electrophilic aromatic substitution. [Pg.607]

The mechanism for electrophilic aromatic substitution with an arenediazonium ion electrophile is the same as the mechanism for electrophilic aromatic substitution with any other electrophile. [Pg.650]

In the mechanism for electrophilic aromatic substitution with a diazonium ion as the electrophile, why does nucleophilic attack occur on the terminal nitrogen atom of the diazonium ion rather than on the nitrogen atom bonded to the benzene ring ... [Pg.650]


See other pages where Mechanism for Electrophilic Aromatic Substitution is mentioned: [Pg.211]    [Pg.396]    [Pg.673]    [Pg.758]    [Pg.550]    [Pg.554]    [Pg.22]    [Pg.550]    [Pg.554]    [Pg.550]    [Pg.554]    [Pg.144]    [Pg.22]    [Pg.22]    [Pg.550]    [Pg.554]    [Pg.607]    [Pg.650]    [Pg.898]    [Pg.903]    [Pg.475]   


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A General Mechanism for Electrophilic Aromatic Substitution

Aromaticity electrophilic aromatic substitution

Aromatics electrophilic substitution

Electrophile Electrophilic aromatic substitution

Electrophile mechanism

Electrophilic aromatic mechanism

Electrophilic aromatic substitution mechanism

Electrophilic mechanism

For electrophilic aromatic

For electrophilic aromatic substitution

Generalized mechanism for electrophilic aromatic substitution

Mechanism aromatic

Mechanisms electrophiles

Substitutes for

Substitution electrophilic aromatic

Substitution electrophilic aromatic substitutions

Substitution electrophilic, mechanism

The General Mechanism for Electrophilic Aromatic Substitution Reactions

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