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While alkene undergo an addition reaction when treated with bromine, benzene is inert under the same conditions

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Addition Reactions and Benzene

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When Fe (iron) is introduced into the mixture, bromine will react with benzene by replacing one of the aromatic protons

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Electrophilic Aromatic Substitution

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Iron first reacts with Br2 to generate iron tribromide(FeBr3)(1st example in image). Iron tribromide, a Lewis acid, is the real catalyst in the reaction between benzene and bromine. Specifically, FeBr3 interacts with Br2 to from a complex which reacts as if it were Br^+(2nd example in image)

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Iron Tribromide

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Aluminum tribromide(AlBr3) is another common Lewis acid that can serve as a suitable alternative to FeBr3

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Common Reactant for Bromination of Benzene

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Notice that, overall, substitution is an exergonic process(downhill in energy), while addition is an endergonic process(uphill in energy). For this reason, only substitution is observed

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Energy Diagram for Substitution and Addition to Benzene

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Chlorination of benzene is accomplished with a suitable Lewis acid, such as aluminum trichloride(1st example in image). Chlorine reacts with AlCl3 to form a complex, which reacts as if it were Cl^+(2nd example in image)

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Chlorination of Benzene

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This mechanism has two general steps: (1) the aromatic ring functions as a nucleophile and attacks an electrophile to form a sigma complex followed by (2) deprotonation of the sigma complex to restore aromaticity

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A General Mechanism for Electrophilic Aromatic Substitution

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When benzene is treated with fuming sulfuric acid and benzene-sulfonic acid is obtained

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Sulfonation

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Fuming sulfuric acid is a mixture of H2SO4 and SO3 gas. Sulfur trioxide(SO3) is a very powerful electrophile, as can be seen in the elctrostratic potential map in this image

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Fuming Sulfuring Acid

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The reaction between benzene and SO3 is highly sensitive to the concentrations of the reagents and is, therefore, reversible

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Reversibility of Sulfonation

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A reaction that occurs which forming nitrobenzene when it is treated with a mixture of nitric acid and sulfuric acid

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Nitration

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Nitration proceeds via an electrophilic aromatic substitution in which a *nitronium ion* (NO2^+) is believed to be the electrophile. This strong electrophile is formed from the acid-base reaction that takes place between HNO3 and H2SO4

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Nitronium Ion (NO2^+)

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Once the nitro group(NO2) is one the ring, it can be reduced to give an amino group(NH2)(1st example in image). This provides us with a two-step method for installing an amino group on an aromatic ring: (1) nitration, followed by (2) reduction of the nitro group(2nd example in image)

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Installing and Amino Group(NH2) on an Aromatic Ring

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Makes possible the installation of an alkyl group on an aromatic ring(1st example in image). In this example, 2-chlorobutane is not sufficiently electrophilic to react with benzene without the presence of a Lewis acid, such as aluminum trichloride, which converts the alkyl halide into a carbocation(2nd example in image)

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Friedel-Crafts Alkylation

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Most primary alkyl halides cannot be used effectively, because their complexes with AlCl3 readily undergo rearrangement to form a secondary or tertiary carbocations(1st example in image). In such a case, a mixture of products is obtained(2nd example in image)

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Friedel-Crafts Alkylation with Primary Alkyl Halides

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When choosing an alkyl halide, the carbon atom connected to the halogen must be sp^3 hybridized. Vinyl and aryl carbocations are not sufficiently stable to be formed under Friedel-Crafts condtiions

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Friedel-Crafts Alkylation with Vinyl and Aryl Carbocations

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Installation of an alkyl group activates the ring toward further alkylation. Therefore, polyalkylations often occur. Assume alkylations performed favor monoalkylation unless otherwise specified.

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Polyalkylations in Friedel-Crafts Alkylation

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There are certain groups, such as a nitro group, that are incompatible with a Friedel-Crafts reaction.

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Nitro Groups Incompatability with Friedel-Crafts Alkylation

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A reaction that installs an acyl group on an aromatic ring

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Friedel-Crafts Acylation

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An acyl chloride is treated with a Lewis acid to form a cationic species, called an *acylium ion*(1st example in image). Acylium ions are resonance stabilized and are therefore not susceptible to rearrangement(2nd example in image)

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Acylium Ion

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In the presence of HCl and amalgamated zinc(zinc that has been treated so that its surface is an alloy, or mixture, of zinc and mercury), the carbonyl group is completely reduced and replaced with two hydrogen atoms

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Clemmensen Reduction

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When a Friedel-Crafts acylation is followed by a Clemmensen reduction, the net result is the installation of an alkyl group. This two step provides an alkylation process where rearrangements are avoided because of the stability of the acylium ion

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Friedel-Crafts Acylation Followed by Clemmensen Reduction

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Polyacylation is not observed, because introduction of an acyl group deactivates the ring toward further acylation

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Polyacylation is Not Observed

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Toluene undergoes nitration approximately 25 times faster than benzene. In other words, the methyl group is said to *activate* the aromatic ring

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Activating Groups

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Alkyl groups are generally electron donating via hyperconjugation. As a result, a methyl group on toluene donates the electron density to the ring, thereby stabilizing the positively charged sigma complex and lowering the energy of activation for bromination

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How Activating Groups Work

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The presence of a methyl group directs the incoming nitro group into the ortho and para positions. *All activators are ortho-para directors*

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ortho-para Director

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A nitro group is said to *deactivate* the ring toward electrophilic aromatic substitution. *Most deactivators are meta directors*

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Deactivating Groups

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A nitro group is inductively electron withdrawing, because a positively charged nitrogen atom is extremely electronegative. By removing electron density from the ring, the nitro group destabilizes the positively charged sigma complex and raises the energy of activation for its formation

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How Deactivating Groups Work

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Activators are ortho-para directors and deactivators are meta directors

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Activators and Deactivators

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Many of the *halogens*(including Cl, Br, and I) are *ortho-para directors* despite the fact that they are *deactivators*

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Halogens; The Exception

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Characterized by the presence of a lone pair immediately adjacent to the aromatic ring(1st example in image). All these groups exhibit a lone pair that is delocalized into the ring, producing resonance structures and donating electron density. This strongly activates the ring(2nd example in image)

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Strong Activators

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Exhibit a lone pair that is already delocalized outside of the ring. In the 1st three of these compounds in this image, there is a lone pair next tot he ring, but that lone pair is participating in resonance outside of the ring. This effect diminishes the capability of the lone pair to donate electron density into the ring.

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Moderate Activators

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The lone pair of an alkoxy group (OR) is not participating in resonance outside of the ring, but it still belongs to the class of moderate activators. Generally more activating than moderate activators but less activating than strong activators

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Alkoxy Group

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Alkyl groups are *weak activators* because they donate electron density by the relatively weak effect of hyperconjugation

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Weak Activators

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Many of the halogens(Cl, Br, or I) are observed to deactivate a benzene ring. The electronic effects of halogens are determined by the delicate competition between resonance and induction, with induction emerging as the dominant effect. As a result, *halogens are weak activators*

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Weak Deactivators

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Groups that exhibit a pi bond to an electronegative atom, where the pi bond is conjugated with the aromatic ring(1st example in image). Each of these groups withdraws electron density from the ring via resonance(2nd example in image). The positive charge in the resonance structures indicates the group is withdrawing electron density from the ring

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Moderate Deactivators

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There are only a few common substituents that are *strong deactivators*. The nitro group is a strong deactivator because of resonance and induction. A positively charged nitrogen atom is extremely electronegative, and CX3 has three electron-withdrawing halogens

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Strong Deactivators

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Do not confuse a CX3 group with a halogen(X)

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Do Not Confuse CX3 and X

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In general, activators are ortho-para directors, while deactivators are meta directors, but halogens are the exception(ortho-para directors and deactivators)

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Summary of Activators and Deactivators by Category

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In the case of the reaction in this image, the methyl group directs to the ortho positions(the para position is already occupied), and the nitro group directs to the positions that are meta to the nitro group. Both the methyl group and the nitro group directs to the same two locations

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Directing Effects with Reinforcement

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In the case of the reaction in this image, the directing effects of the various substituents compete with each other. In such cases, the more powerful activating group dominates the directing effects

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Directing Effects with NO Reinforcement

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In this case, there are two substituents on the ring: an OH group(strong activator) and a methyl group(weak activator). The strong activator dominates, so the incoming nitro group is installed at a position that is ortho to the strong activator(the para position is occupied)

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Example of Directing Effects with NO Reinforcement

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For most monosubstitued aromatic rings, the para products generally dominates over the ortho product as a results of steric considerations

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Steric Effects of Aromatic Reactions for Monosubstituted Rings

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For 1,4-disubstituted rings, steric effects again play a significant role. In this example, nitration is more likely to occurs that the site that is less sterically hindered(ortho the the methyl group)

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Steric Effects of Aromatic Reactions for 1,4-Disubstituted Rings

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For 1,3-disubstituted aromatic rings, it is extremely unlikely that substitution will occur at the position between the two substituents because this position is the most sterically hindered position

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Steric Effects of Aromatic Reactions for 1,3-Disubstituted Rings

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Direct bromination of tert-butylbenzene produces the para product as the major product, while the desired ortho product is the minor product. In such a situation, a *blocking group* can be used to direct the bromination toward the otho position.

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Blocking Groups

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Sulfonation is commonly used as a blocking group, because the sulfonation process is reversible

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Sulfonation and Desulfonation

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In the case of the 1st example in this image, the blocking group is first installed at the para position. Once the para position is occupied, the desired reaction is forced to occur at the ortho position. Finally the blocking group is removed(2nd example in image). Sulfonation provides a valuable blocking technique that enables us to achieve the desired transformation because it is reversible(3rd example in image)

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Example of Blocking Groups

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1. Nitration cannot be performed on a ring that contains an amino group(1st example in image)... 2. A Friedel-Crafts reaction(alkylation or acylation) cannot be accomplished on rings that are either moderately or strongly deactivated. The ring must be either activated or weakly deactivated in order for a Friedel-Crafts reaction to occur(2nd example in image)

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Limitations When Planning Synthesis

Chapter 19: Aromatic Substitution Reactions

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