Aromatic Substitution 4/21/2017 Aromatic Substitution Chapter 22 WWU - Chemistry

Reading Assignment Sections 22.0 through 22.2 4/21/2017 Reading Assignment Sections 22.0 through 22.2 Sections 22.4 through 22.9 Sections 22.10 through 22.12 Section 22.17 Section 22.18 (chapter summary) Problems WWU -- Chemistry

Assigned Problems In-text problems End-of-chapter problems 4/21/2017 Assigned Problems In-text problems 22-1 through 22-2 22-4 through 22-13 22-23 through 22-29 End-of-chapter problems 1 through 19 WWU -- Chemistry

Substitution versus Addition 4/21/2017 Substitution versus Addition Benzene does not undergo addition reactions. To illustrate, consider the addition reaction of benzene and Br2: WWU -- Chemistry

Bromine Addition to Benzene 4/21/2017 Bromine Addition to Benzene As a result of the bromine addition, the benzene ring has lost its aromatic character -- it has lost its 150 kJ/mole of resonance energy. The reaction, as shown, is energetically unfavorable. WWU -- Chemistry

Bromine Substitution on Benzene 4/21/2017 Bromine Substitution on Benzene If, on the other hand, we allow a bromine atom to replace a hydrogen atom of benzene (i.e., a substitution reaction), the aromatic character of the benzene ring is retained. This type of process remains energetically favorable. WWU -- Chemistry

Bromine Substitution on Benzene 4/21/2017 Bromine Substitution on Benzene This is electrophilic aromatic substitution. WWU -- Chemistry

Electrophilic Aromatic Substitution 4/21/2017 Electrophilic Aromatic Substitution In general, the reactions of benzene follow the same pattern, as seen in the following image. A+, in this reaction, is used to represent any electrophile (Lewis acid). WWU -- Chemistry

Electrophilic Aromatic Substitution 4/21/2017 Electrophilic Aromatic Substitution WWU -- Chemistry

But first, a brief digression into nomenclature 4/21/2017 But first, a brief digression into nomenclature WWU -- Chemistry

IUPAC Nomenclature of Substituted Benzenes 4/21/2017 IUPAC Nomenclature of Substituted Benzenes WWU -- Chemistry

IUPAC Nomenclature of Benzene Derivatives 4/21/2017 IUPAC Nomenclature of Benzene Derivatives WWU -- Chemistry

Common Nomenclature in Substituted Benzenes 4/21/2017 Common Nomenclature in Substituted Benzenes WWU -- Chemistry

Common Nomenclature in Substituted Benzenes 4/21/2017 Common Nomenclature in Substituted Benzenes The ortho, meta, and para names can only be used when there are two substituents. If there are three or more substituents, you must use the IUPAC name (with numbers). WWU -- Chemistry

Electrophilic Aromatic Substitution 4/21/2017 Electrophilic Aromatic Substitution WWU -- Chemistry

Mechanism of Electrophilic Aromatic Substitution 4/21/2017 Mechanism of Electrophilic Aromatic Substitution Nearly all the substitution reactions of benzene follow the same mechanism. In the scheme that follows, A+ represents the electrophile (Lewis acid). WWU -- Chemistry

Mechanism of Electrophilic Aromatic Substitution (Step 1) 4/21/2017 Mechanism of Electrophilic Aromatic Substitution (Step 1) WWU -- Chemistry

Mechanism of Electrophilic Aromatic Substitution (Step 2) 4/21/2017 Mechanism of Electrophilic Aromatic Substitution (Step 2) WWU -- Chemistry

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4/21/2017 All that is required is for us to figure out how to generate the appropriate A+ for any substitution reaction. WWU -- Chemistry

Reaction with Acids Use H2SO4 What does this prove? 4/21/2017 Reaction with Acids Use H2SO4 What does this prove? The hydrogens of benzene are exchangeable. We can make “practical” use of this reaction in the following scenario: WWU -- Chemistry

Reaction with Acids Use D2SO4 4/21/2017 Reaction with Acids Use D2SO4 Can use this method to prepare benzene-d6 WWU -- Chemistry

Nitration of Benzene What does the H2SO4 do? 4/21/2017 Nitration of Benzene What does the H2SO4 do? It reacts with nitric acid to generate the electrophile (see next image) WWU -- Chemistry

Formation of Nitronium Ion 4/21/2017 Formation of Nitronium Ion Nitronium ion (NO2+) is the electrophile that attacks the benzene ring. WWU -- Chemistry

Halogenation of Benzene 4/21/2017 Halogenation of Benzene X can be either bromine or chlorine Other catalysts might include elemental iron (Fe0) or aluminum chloride (AlCl3) The ferric halide serves to generate the electrophile in this reaction. WWU -- Chemistry

Formation of the Halonium Ion 4/21/2017 Formation of the Halonium Ion X+ is a halonium ion: Cl+ = chloronium ion Br+ = bromonium ion The halonium ion is the electrophile in this reaction. WWU -- Chemistry

The Friedel-Crafts Alkylation Reaction 4/21/2017 The Friedel-Crafts Alkylation Reaction The product is an alkylbenzene The role of the anhydrous aluminum chloride is to generate a stable carbocation complex. The carbocation is the electrophile in this process. WWU -- Chemistry

Formation of a Stable Carbocation Complex 4/21/2017 Formation of a Stable Carbocation Complex WWU -- Chemistry

4/21/2017 Explain this: WWU -- Chemistry

The Friedel-Crafts Acylation Reaction 4/21/2017 The Friedel-Crafts Acylation Reaction The product is an acylbenzene (aromatic ketone) WWU -- Chemistry

The Friedel-Crafts Acylation Reaction 4/21/2017 The Friedel-Crafts Acylation Reaction The role of the anhydrous aluminum chloride is to generate a stable carbocation complex. The carbocation is the electrophile in this process. In this case, the carbocation is an acylium ion. WWU -- Chemistry

Formation of a Stable Acylium Ion Complex 4/21/2017 Formation of a Stable Acylium Ion Complex The acylium ion is the electrophile in this reaction Acylium ions do not rearrange The same result is obtained if one uses an acid anhydride instead of an acid chloride. WWU -- Chemistry

The Acylium Ion is Stabilized by Resonance 4/21/2017 The Acylium Ion is Stabilized by Resonance WWU -- Chemistry

Sulfonation of Benzene 4/21/2017 Sulfonation of Benzene The actual electrophile in this reaction is sulfur trioxide. WWU -- Chemistry

Why is Sulfur Trioxide Electrophilic? 4/21/2017 Why is Sulfur Trioxide Electrophilic? sulfur trioxide The resulting ion reacts with benzene WWU -- Chemistry

Sulfonation of Benzene 4/21/2017 Sulfonation of Benzene The protonated SO3 is the electrophile in this reaction. The reaction requires fuming sulfuric acid (i.e., sulfuric acid saturated with sulfur trioxide) This reaction is reversible (unlike the other examples). WWU -- Chemistry