Organic Chemistry

Organic Chemistry William H. Brown & Christopher S. Foote

Aromatics I Chapter 20

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

Benzene - MO 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°

Benzene - MO Model

Benzene - MO Model • Combination of six 2p atomic orbitals gives six molecular orbitals • three bonding MOs and • three antibonding MOs • In the ground state, the six pi electrons of benzene occupy the three bonding MOs

Benzene MO Model • the stability of benzene results from the fact that these three bonding MOs are much lower in energy than the six uncombined 2p atomic orbitals

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 • Resonance energy:the difference in energy between a resonance hybrid and the most stable 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

Concept of Aromaticity • The underlying criteria for aromaticity were recognized in the early 1930s by Erich Hückel, based on 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 • Inscribe a polygon of the same number of sides as the ring to be examined such that one of the vertices is at the bottom of the ring • The relative energies of the MOs in the ring are given by where the vertices 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 • Following are Frost circles describing the MOs for monocyclic, planar, fully conjugated four-, five-, and six-membered rings

Aromatic Hydrocarbons • Annulene:a cyclic hydrocarbon with a continuous alternation of single and double bonds • [10]Annulene: according to Hückel’s criteria, this unsaturated hydrocarbon should be aromatic • it is cyclic • it has one 2p orbital on each carbon of the ring, • it has 4(2) + 2 = 10 pi electrons • It is not aromatic, however, because it is not planar

[10]Annulene • 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

[14]Annulene • this unsaturated hydrocarbon meets the Hückel criteria and is aromatic

[18]Annulene • This unsaturated hydrocarbon is also aromatic

Antiaromatic Hydrocbn • 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 • planar cyclobutadiene has two unpaired electrons, which make it highly unstable and reactive

Cyclobutadiene • cyclobutadiene is trapped within the cage of a larger molecule called a hemicarcerand (to rotate this molecule, see the accompanying CD)

Cyclooctatetraene • Planar cyclooctatetraene, if it existed, would be antiaromatic; it would have two unpaired electrons in 4 and 5 nonbonding MOs

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 are other than 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 • pyridine has a resonance energy of 134 kJ (32 kcal)/mol, slightly less than that of benzene

Furan • 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 Hydrcbn 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 C+ • if planar cyclopentadienyl cation existed, 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 • 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

Cyclopentadienyl Anion • 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 C+ • 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 the 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 Derivs • 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

NMR Spectroscopy • Hydrogens bonded to a benzene ring appear in the region  6.5 to 8.5 • Aryl hydrogens absorb further downfield than vinylic hydrogens, which is accounted for by an induced ring current • the induced ring current has an associated magnetic field which opposes the applied field in the middle of the ring but reinforces it on the outside of the ring • thus, hydrogens on the benzene ring come into resonance at a lower applied field; that is, at a larger chemical shift than vinylic hydrogens

NMR Spectroscopy • There are, of course, no hydrogens on the inside of a benzene ring. But there are in larger annulenes, for example [18]annulene

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, compounds that also contain the -OH group

Acidity of Phenols • the greater acidity of phenols compared with alcohols is due to the greater stability of the phenoxide ion relative to an alkoxide ion

Acidity of Phenols • Alkyl and halogen substituents effect acidities by inductive effects • alkyl groups are electron-releasing • halogens are electron-withdrawing

Acidities of Phenols • nitro groups increase the acidity of phenols by both an electron-withdrawing inductive effect and a resonance effect

Acidities 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

Alkyl-Aryl Ethers • Alkyl-aryl ethers can be prepared by the Williamson ether synthesis • but only using phenoxide salts and alkyl halides • aryl halides are unreactive to SN2 reactions • The following two examples illustrate • the use of a phase-transfer catalyst • the use of dimethyl sulfate as a methylating agent

Alkyl-Aryl Ethers

Organic Chemistry