Benzene and Its Derivatives

Benzene and Its Derivatives
CHAPTER NINE Benzene and Its Derivatives Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.
Benzene - Kekulé The first structure for benzene was proposed by August Kekulé in 1872. This structure, however, did not account for the unusual chemical reactivity of benzene. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Benzene – Orbital Overlap Model
The concepts of hybridization of atomic orbitals and the theory of resonance, developed in the 1930s, provided the first adequate description of benzene’s structure. The carbon skeleton is a regular hexagon, with all C-C-C and H-C-C bond angles 120°. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Benzene - Orbital Overlap Model
Figure 9.1 (a) The carbon framework; the six parallel 2p orbitals, each with one electron, are shown uncombined. (b) Overlap of the six 2p orbitals forms a continuous pi cloud, shown as one torus above the plane of the ring, the other below it. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Benzene - Resonance Model
We often represent benzene as a hybrid of two equivalent Kekulé structures. Each Kekulé structure makes an equal contribution to the hybrid. The C-C bonds are neither double nor single but something in between. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Benzene - Resonance Energy
Resonance energy: The difference in energy between a resonance hybrid and its most stable hypothetical contributing structure in which electrons are localized on particular atoms and in particular bonds. One way to estimate the resonance energy of benzene is to compare the heats of hydrogenation of benzene and cyclohexene. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Heats of hydrogenation for both cyclohexene and benzene are negative (heat is liberated). These results are shown graphically on the next slide. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Concept of Aromaticity
The criteria for aromaticity were recognized in the early 1930s by Erich Hückel. To be aromatic, a ring must: have one 2p orbital on each atom of the ring. be planar or nearly planar, so that overlap of all 2p orbitals of the ring is continuous or nearly continuous. have 2, 6, 10, 14, 18, and so forth pi electrons in the cyclic arrangement of 2p orbitals. Benzene meets these criteria It is cyclic, planar, has one 2p orbital on each atom of the ring, and has 6 pi electrons (an aromatic sextet) in the cyclic arrangement of its 2p orbitals. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Heterocyclic Aromatics
Heterocyclic compound: A compound that contains one or more atoms other than carbon in its ring. Heterocyclic aromatic compound: A heterocyclic compound whose ring is aromatic. Pyridine and pyrimidine are heterocyclic analogs of benzene; each is aromatic. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Pyridine The nitrogen atom of pyridine is sp2 hybridized. The unshared pair of electrons lies in an sp2 hybrid orbital and is not a part of the six pi electrons of the aromatic sextet. Pyridine has a resonance energy of 32 kcal (134 kJ)/mol, slightly less than that of benzene. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Nomenclature Disubstituted benzenes Locate substituents by numbering or Use the locators ortho (1,2-), meta (1,3-), and para (1,4-) Where one group imparts a special name, name the compound as a derivative of that molecule. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Nomenclature Polysubstituted benzenes If there are three or more substituents, number the atoms of the ring. If one group imparts a special name, it becomes the parent name. If no group imparts a special name, number to give the smallest set of numbers, and list alphabetically. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Polynuclear Aromatic Hydrocarbons
Polynuclear aromatic hydrocarbons (PAHs) Contain two or more fused aromatic rings. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Carcinogenic PAHs Benzo[a]pyrene is a carcinogen. Once absorbed, the body oxidizes it to a more soluble compound that can be excreted. The diol epoxide contains a reactive epoxide ring and can bind to DNA, thereby altering the structure of DNA and producing a cancer-causing mutation. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Benzylic Oxidation Benzene is unaffected by strong oxidizing agents such as H2CrO4 and KMnO4. Halogen and nitro substituents are unaffected by these reagents. An alkyl group with at least one hydrogen on the benzylic carbon is oxidized to a carboxyl group. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Benzylic Oxidation If there is more than one alkyl group, each is oxidized to a -COOH group. Terephthalic acid is one of the two monomers required for the synthesis of poly(ethylene terephthalate), PETE, a polymer that can be fabricated into Dacron polyester fibers and into Mylar films. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Reactions of Benzene The most characteristic reaction of aromatic compounds is substitution at a ring carbon. Shown are chlorination and nitration. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Electrophilic Aromatic Substitution
Electrophilic Aromatic Substitution (EAS): A reaction in which an electrophile, E+, substitutes for an H atom on an aromatic ring. We study several common types of electrophiles and for each: how the electrophile is generated. the mechanism by which each electrophile replaces hydrogen. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

All EAS reactions occur by a three-step mechanism. Step 1: Generation of the electrophile. Step 2: Reaction of the electrophile and a nucleophile to form a new covalent bond. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Chlorination and Bromination
Step 1: Formation of the electrophile (a chloronium ion or bromonium ion). Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Friedel-Crafts Alkylation
Friedel-Crafts alkylation forms a new C-C bond between an aromatic ring and an alkyl group. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Step 2: Reaction of an electrophile and a nucleophile to form a new covalent bond. Step 3: Take a proton away. Proton transfer regenerates the aromatic ring. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Friedel-Crafts Alkylations
There are two major limitations on F-C alkylations. It is practical only with stable carbocations, such as 2° and 3° carbocations. It fails on benzene rings bearing one or more of these strongly electron-withdrawing groups. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Friedel-Crafts Acylations
Treating an aromatic ring with an acid chloride in the presence of AlCl3. Acid (acyl) chloride: a derivative of a carboxylic acid in which the —OH is replaced by a chlorine. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Di- and Polysubstitution
Existing groups on a benzene ring influence further substitution in both orientation and rate. Orientation Certain substituents direct new substitution preferentially toward ortho-para positions, others direct preferentially toward meta positions. Rate Certain substituents are activating toward further substitution, others are deactivating toward further substitution. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Generalizations 1. Alkyl groups, phenyl groups, and substituents in which the atom bonded to the ring has an unshared pair of electrons are ortho-para directing; all other substituents are meta directing. 2. All ortho-para directing groups are activating toward further substitution; the exceptions to this generalization are the halogens, which are weakly deactivating. 3. All meta directing groups carry either a partial or full positive charge on the atom bonded to the ring. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Theory of Directing Effects
The rate of electrophilic aromatic substitution The rate of EAS is determined by the slowest step in the reaction. For almost every EAS, the rate-determining step is attack of the electrophile (E+) on the aromatic ring to give a resonance-stabilized cation intermediate. The more stable this cation intermediate, the faster the rate-determining step and the faster the overall reaction. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

For ortho-para directors, ortho-para attack forms a more stable cation than meta attack. Ortho-para products are formed faster than meta products. For meta directors, meta attack forms a more stable cation than does ortho-para attack. Meta products are formed faster than ortho-para products. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Activating-Deactivating
Any resonance effect such as those of -NH2, –OH, and –OR, which delocalizes the positive charge on the cation intermediate, lowers the activation energy for its formation and activates the ring toward further EAS. Any resonance or inductive effect such as those of –NO2, –C=O, -SO3H, –NR3+, –CCl3, and –CF3, which decreases electron density on the ring, deactivates the ring toward further EAS. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Halogens Halogens: the resonance and inductive effects operate in opposite directions. The inductive effect: halogens have an electron-withdrawing inductive effect; therefore, aryl halides react more slowly in EAS than benzene. The resonance effect: a halogen ortho or para to the site of electrophilic attack stabilizes the cation intermediate by delocalizing the positive charge; halogen, therefore, is ortho-para directing. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Acidity of Phenols The greater acidity of phenols compared with alcohols is the result of the greater stability of a phenoxide ion relative to an alkoxide ion. Copyright © 2016 John Wiley & Sons, Inc. All rights reserved.

Benzene and Its Derivatives

Similar presentations

Presentation on theme: "Benzene and Its Derivatives"— Presentation transcript:

Similar presentations

About project

Feedback

Log in

Auth with social network:

Benzene and Its Derivatives

Similar presentations

Presentation on theme: "Benzene and Its Derivatives"— Presentation transcript:

Similar presentations

About project

Feedback