Isolated and Conjugated Dienes

Reactions of Isolated Dienes

The Mechanism

Double Bonds can have Different Reactivities

Reactions of Conjugated Dienes

1,2-Addition and 1,4-Addition

Reaction of a Conjugated Diene Mechanism for the Reaction of a Conjugated Diene

The Diels–Alder Reaction forms a Six-Membered Ring

The Mechanism

Faster if there is an Electron Withdrawing Group on the Dienophile

The Electron Withdrawing Group makes the Electrophile a better Electrophile

Another Diels–Alder Reaction

Alkynes can also be Dienophiles The cyclic product has two double bonds.

The Stereochemistry of the Diels–Alder Reaction The product will be a racemic mixture.

How to Determine the Reactants of a Diels–Alder Reaction

Benzene is an Aromatic Compound The delocalization energy of benzene is 36 kcal/mol.

Criteria for a Compound to be Aromatic It must have an uninterrupted cloud of π electrons.(cyclic, planar, every ring atom must have a p orbital). The π cloud must have an odd number of pairs of π electrons.

Criteria for Aromaticity Uninterrupted arrangement of p cloud Cyclic molecule Every atom in the ring must have a p orbital Molecule has to be planar Total number of p-electron pairs is odd (1,3,5…) Exceptionally stable molecules

Examples of Compounds that are not Aromatic Cyclobutadiene has an even number of pairs of π electrons. Cyclooctatetraene has an even number of pairs of π electrons and it is not planar.

Nonaromatic and Aromatic Compounds 2 pairs of p electrons 2 pairs of p electrons 3 pairs of p electrons planar nonplanar planar

Resonance Contributors and the Resonance Hybrid

Cyclopentadienyl anion Resonance contributors. Overall every C atom has the same amount of negative charge charge equally distributed

Benzene Benzene: planar, ring, all atoms have p orbitals, 3 pairs of p electrons.

Aromatic Compounds

Nomenclature of Monosubstituted Benzenes State the name of substituent and add the word “benzene”

Nomenclature Many trivial names in use

Nomenclature

How Benzene Reacts Aromatic compounds such as benzene undergo electrophilic aromatic substitution reactions.

Benzene Reacts with Electrophiles The π electrons above and below the ring make benzene a nucleophile. Benzene attacking an electrophile is like an alkene attacking an electrophile.

Benzene undergoes Substitution, not Addition Aromaticity is restored in the product from electrophilic substitution.

Benzene undergoes Substitution, not Addition The reaction of benzene with an electrophile forms the aromatic substitution product, not the nonaromatic addition product.

An Electrophilic Aromatic Substitution Reaction An electrophile (Y+) substitutes for H+.

The Mechanism for Electrophilic Aromatic Substitution

Generating the Electrophile: Halogenation Donating a lone pair to a Lewis acid weakens the Br—Br or Cl—Cl bond.

Halogenation of Benzene

Halogenation of Benzene 1st step is generating an electrophile: FeBr3 is a Lewis acid that coordinates with Br2 and polarizes it, generating an electrophilic Br+

Halogenation of Benzene

Generating the Electrophile: Nitration

Nitration of Benzene Nitration:

Nitration of Benzene Nitration:

Generating the Electrophile: Sulfonation

Sulfonation of Benzene

Sulfonation of Benzene

Generating the Electrophile: Friedel–Crafts Acylation and Alkylation

Friedel-Crafts Acylation of Benzene

Friedel-Crafts Acylation of Benzene

Friedel-Crafts Alkylation of Benzene

Friedel-Crafts Alkylation of Benzene

How Some Substituents on a Benzene Ring Can be Chemically Changed

How Some Substituents on a Benzene Ring Can be Chemically Changed

Nomenclature of Disubstituted Benzenes

The Effect of Substituents on Reactivity Monosubstituted benzenes undergo electrophilic aromatic substitution. Substituents influence reactivity. Substituents determine position of second electrophilic substitution. Substituents that increase electron density increase reactivity. Substituents that decrease electron density decrease reactivity.

The Effect of Substituents on Reactivity

Donating and Withdrawing Electrons by Resonance

Electron-donating Substituents Resonance contributors increase electron density in ortho and para positions. Overall electron density is bigger.

Donating and Withdrawing Electrons by Resonance

Electron-withdrawing Substituents Resonance contributors decrease electron density in ortho and para positions. Overall electron density is lower.

Substituent Effects Both types of substituents, electron Donating and electron Withdrawing, act on the same positions, namely ortho and para, however, in opposite directions.

Relative Reactivity of Substituted Benzenes Activating Deactivating

Figure: 07-03-29.1UN.T1 Title: Table 7.1 The Effects of Substituents on the Reactivity of a Benzene Ring Toward Electrophilic Substitution Caption: Those at the top of the table are the most activating substituents when compared to hydrogen. The bottom of the table contains the deactivating groups. Notes: Ortho/para directors are generally activating substituents (except the halogens) while meta directors are generally deactivating substituents.

Classification of Substituents Activating Substituents (ortho/para directing): prim. Amines sec. Amines tertiary amines phenolic OH phenol ethers amides phenolic esters alkyl substituents halides Strongly activating weakly activating deactivating

Classification of Substituents Deactivating Substituents (meta directing): aldehydes esters carboxylic acid nitrile sulfonic acid nitro ammonium weakly deactivating strongly deactivating

The Effect of Substituents on Orientation 1. All activating substituents direct an incoming electrophile to the ortho and para positions.

The Effect of Substituents on Orientation 2. The weakly deactivating halogens also direct an incoming electrophile to the ortho and para positions.

The Effect of Substituents on Orientation 3. All moderately deactiating and strongly deactivating substituents direct an incoming electrophile to the meta position.

Organizing what we know about the Reactions of Organic Compounds