Chapter 21, Benzene and and the Concept of Aromaticity

Benzene - Kekulé In 1872, August Kekulé proposed the following structure for benzene. This structure, however, did not account for the unusual chemical reactivity of benzene.

Benzene - Resonance We often represent benzene as a hybrid of two equivalent Kekulé structures. Each makes an equal contribution to the hybrid and thus the C-C bonds are neither double nor single, but something in between.

Benzene - Resonance 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 planar regular hexagon. All C-C-C and H-C-C bond angles 120°.

The Pi System of Benzene (a) The carbon framework with the six 2p orbitals. (b) Overlap of the parallel 2p orbitals forms one torus above the plane of the ring and another below it this orbital represents the lowest-lying pi-bonding molecular orbital.

Benzene-Molecular Orbital Model The molecular orbital representation of the pi bonding in benzene.

Orbitals of the pi System of Benzene Number of nodal surfaces 3 2 1

Benzene - Resonance Resonance energy: The difference in energy between a resonance hybrid in which the electrons are delocalized and the most stable one of its hypothetical contributing structures 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.

Benzene- Resonance Energy Model Experimental data

Concept of Aromaticity The underlying criteria for aromaticity were recognized in the early 1930s by Erich Hückel, based on molecular orbital (MO) calculations. To be aromatic, a compound must Be cyclic. Have one p orbital on each atom of the ring. Be planar or nearly planar so that there is continuous or nearly continuous overlap of all p orbitals of the ring. Have a closed loop of (4n + 2) pi electrons in the cyclic arrangement of p orbitals.

Frost Circles Frost circle: A graphic method for determining the relative order of pi MOs in planar, fully conjugated monocyclic compounds. Inscribe in a circle a polygon of the same number of sides as the ring to be examined such that one of the vertices of the polygon is at the bottom of the circle. The relative energies of the MOs in the ring are given by where the vertices of the polygon touch the circle. Those MOs Below the horizontal line through the center of the ring are bonding MOs. on the horizontal line are nonbonding MOs. above the horizontal line are antibonding MOs.

Frost Circles Frost circles describing the MOs for monocyclic, planar, fully conjugated four-, five-, and six-membered rings.

Relationship of hexa-1,3,5-triene to benzene How does the linear triene relate to benzene? 13

Relationship of hexa-1,3,5-triene to benzene ? 14

Relationship of hexa-1,3,5-triene to benzene Look at orbitals 2 and 3. p3 ? p2 Bonding, stabilizing Curve around Antibonding, destabilizing 15

Aromatic Hydrocarbons Annulene: A cyclic hydrocarbon with a continuous alternation of single and double bonds. [14]Annulene is aromatic according to Hückel’s criteria. n = 3

Aromatic Hydrocarbons [18]Annulene is also aromatic. n = 4

Aromatic Hydrocarbons According to Hückel’s criteria, [10]annulene should be aromatic; it has been found, however, that it is not. Nonbonded interactions between the two hydrogens that point inward toward the center of the ring force the ring into a nonplanar conformation in which overlap of the ten 2p orbitals is no longer continuous.

Aromatic Hydrocarbons What is remarkable relative to [10]annulene is that if the two hydrogens facing inward toward the center of the ring are replaced by a methylene (CH2) group, the ring is able to assume a conformation close enough to planar that it becomes aromatic.

Antiaromatic Hydrocarbons Antiaromatic hydrocarbon: A monocyclic, planar, fully conjugated hydrocarbon with 4n pi electrons (4, 8, 12, 16, 20...). An antiaromatic hydrocarbon is especially unstable relative to an open-chain fully conjugated hydrocarbon of the same number of carbon atoms. Cyclobutadiene is antiaromatic. In the ground-state electron configuration of this molecule, two electrons fill the 1 bonding MO. The remaining two electrons lie in the 2 and 3 nonbonding MOs.

Cyclobutadiene The ground state of planar cyclobutadiene has two unpaired electrons, which make it highly unstable and reactive.

Cyclooctatetraene Cyclooctatetraene, with 8 pi electrons is not aromatic; it shows reactions typical of alkenes. X-ray studies show that the most stable conformation is a nonplanar “tub” conformation. Although overlap of 2p orbitals occurs to form pi bonds, there is only minimal overlap between sets of 2p orbitals because they are not parallel.

Cyclooctatetraene MO energy diagram for a planar conformation of cyclooctatetraene.

Heterocyclic Aromatics Heterocyclic compound: A compound that contains more than one kind of atom in a ring. In organic chemistry, the term refers to a ring with one or more atoms that differ from carbon. Pyridine and pyrimidine are heterocyclic analogs of benzene; each is aromatic.

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 system (the aromatic sextet). Resonance energy of pyridine is134 kJ (32 kcal)/mol.

Furan and Pyrrole The oxygen atom of furan is sp2 hybridized. one unshared pairs of electrons on oxygen lies in an unhybridized 2p orbital and is a part of the aromatic sextet. The other unshared pair lies in an sp2 hybrid orbital and is not a part of the aromatic system. The resonance energy of furan is 67 kJ (16 kcal)/mol.

Other Heterocyclics

Aromatic Hydrocarbon Ions Any neutral, monocyclic, unsaturated hydrocarbon with an odd number of carbons must have at least one CH2 group and, therefore, cannot be aromatic. Cyclopropene, for example, has the correct number of pi electrons to be aromatic, 4(0) + 2 = 2, but does not have a closed loop of 2p orbitals.

Cyclopropenyl Cation If, however, the CH2 group of cyclopropene is transformed into a CH+ group in which carbon is sp2 hybridized and has a vacant 2p orbital, the overlap of orbitals is continuous and the cation is aromatic.

Cyclopropenyl Cation When 3-chlorocyclopropene is treated with SbCl5, it forms a stable salt. This chemical behavior is to be contrasted with that of 5-chloro-1,3-cyclopentadiene, which cannot be made to form a stable salt.

Cyclopentadienyl Cation If planar cyclopentadienyl cation were to exist, it would have 4 pi electrons and be antiaromatic. Note that we can draw five equivalent contributing structures for the cyclopentadienyl cation. Yet this cation is not aromatic because it has only 4 pi electrons.

Cyclopentadienyl Anion, C5H5- To convert cyclopentadiene to an aromatic ion, it is necessary to convert the CH2 group to a CH group in which carbon becomes sp2 hybridized and has 2 electrons in its unhybridized 2p orbital. n = 1

Cyclopentadienyl Anion, C5H5- As seen in the Frost circle, the six pi electrons of cyclopentadienyl anion occupy the p1, p2, and p3 molecular orbitals, all of which are bonding.

Cyclopentadienyl Anion, C5H5- The pKa of cyclopentadiene is 16. In aqueous NaOH, it is in equilibrium with its sodium salt. It is converted completely to its anion by very strong bases such as NaNH2 , NaH, and LDA.

Cycloheptatrienyl Cation, C7H7+ Cycloheptatriene forms an aromatic cation by conversion of its CH2 group to a CH+ group with its sp2 carbon having a vacant 2p orbital.

Nomenclature Monosubstituted alkylbenzenes are named as derivatives of benzene. Many common names are retained.

Nomenclature Benzyl and phenyl groups

Disubstituted Benzenes Locate two groups by numbers or by 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.

Disubstituted Benzenes Where neither group imparts a special name, locate the groups and list them in alphabetical order.

Polysubstituted Derivatives If one group imparts a special name, name the molecule as a derivative of that compound. If no group imparts a special name, list them in alphabetical order, giving them the lowest set of numbers.

Phenols The functional group of a phenol is an -OH group bonded to a benzene ring.

Phenols Hexylresorcinol is a mild antiseptic and disinfectant. Eugenol is used as a dental antiseptic and analgesic. Urushiol is the main component of the oil of poison ivy.

Acidity of Phenols Phenols are significantly more acidic than alcohols.

Acidity of Phenols Separation of water-insoluble phenols from water-insoluble alcohols.

Acidity of Phenols (Resonance) The greater acidity of phenols compared with alcohols is due to the greater stability of the phenoxide ion relative to an alkoxide ion.

Phenol Subsitituents (Inductive Effect) Alkyl and halogen substituents effect acidities by inductive effects: Alkyl groups are electron-releasing. Halogens are electron-withdrawing.

Phenol Subsitituents(Resonance, Inductiion) Nitro groups increase the acidity of phenols by both an electron-withdrawing inductive effect and a resonance effect.

Acidity of Phenols Part of the acid-strengthening effect of -NO2 is due to its electron-withdrawing inductive effect. In addition, -NO2 substituents in the ortho and para positions help to delocalize the negative charge.

Acidity of Phenols Phenols are weak acids and react with strong bases to form water-soluble salts. Water-insoluble phenols dissolve in NaOH(aq).

Acidity of Phenols Most phenols do not react with weak bases such as NaHCO3; they do not dissolve in aqueous NaHCO3. Carbonic acid is a stronger acid than phenol. Therefore, the position of this equilibrium lies far to the left.

Synthesis: Alkyl-Aryl Ethers Alkyl-aryl ethers can be prepared by the Williamson ether synthesis: but only using phenoxide salts and haloalkanes. haloarenes cannot be used because they are unreactive to SN2 reactions.

Synthesis: Alkyl-Aryl Ethers

Synthesis: Kolbe Carboxylation Phenoxide ions react with carbon dioxide to give a carboxylate salt.

Mechanism: Kolbe Carboxylation The mechanism begins by nucleophilic addition of the phenoxide ion to a carbonyl group of CO2. Go back to aromatic structure

Synthesis: Quinones Because of the presence of the electron-donating -OH group, phenols are susceptible to oxidation by a variety of strong oxidizing agents.

Quinones

Quinones Readily reduced to hydroquinones.

Coenzyme Q Coenzyme Q is a carrier of electrons in the respiratory chain.

Vitamin K Both natural and synthetic vitamin K (menadione) are 1,4-naphthoquinones.

Benzylic Oxidation Benzene is unaffected by strong oxidizing agents such as H2CrO4 and KMnO4 Halogen and nitro substituents are also unaffected by these reagents. An alkyl group with at least one hydrogen on its benzylic carbon is oxidized to a carboxyl group.

Benzylic Oxidation If there is more than one alkyl group on the benzene ring, each is oxidized to a -COOH group.

Benzylic Chlorination Chlorination and bromination occur by a radical chain mechanism.

Mechanism: Benzylic Reactions Benzylic radicals (and cations also) are easily formed because of the resonance stabilization of these intermediates. The benzyl radical is a hybrid of five contributing structures.

Benzylic Halogenation Benzylic bromination is highly regioselective. Benzylic chlorination is less regioselective.

Hydrogenolysis Hydrogenolysis: Cleavage of a single bond by H2 Benzylic ethers are unique in that they are cleaved under conditions of catalytic hydrogenation.

Synthesis, Protecting Group: Benzyl Ethers The value of benzyl ethers is as protecting groups for the OH groups of alcohols and phenols. To carry out hydroboration/oxidation of this alkene, the phenolic -OH must first be protected; it is acidic enough to react with BH3 and destroy the reagent.