Chapter 15 Benzene and Aromaticity
Introduction • In early 19th century, the termaromatic was used to describe some fragrant compounds • Not correct: later they are grouped by chemical behavior (unsaturated compounds that undergo substitution rather than addition). coal distillate cherries, peaches and almonds Tolu balsam
Currently, the termaromatic is used to refer to the class of compounds related structurally to benzene • They are distinguished from aliphaticcompounds by electronic configuration steroidal hormone analgesic tranquilizer
1.Sources of Aromatic Hydrocarbons • There are two main sources of simple aromatic hydrocarbons: • coal • petroleum
High temperature distillation of coal tar • Coal is a mixture of benzene-like rings joined together. Under high temperature, it produces coal tar which, upon fractional distillation, yields:
Heating petroleum at high temperature under high pressure over a catalyst • Petroleum consists mainly of alkanes which, at high temperature under pressure over a catalyst, convert into aromatic compounds.
2.Naming Aromatic Compounds • Aromatic compounds are named according to the system devised by the International Union of Pure and Applied Chemistry (IUPAC).
Aromatic compounds have many common names that have been accepted by IUPAC: • Toluene= methylbenzene • Phenol = hydroxybenzene • Aniline = aminobenzene
Monosubstituted benzenes • Monosubstituted benzenes, like hydrocarbons, are systematically named with –benzene as the parent name C6H5Br C6H5NO2C6H5CH2CH2CH3
Arenes • Arenes are alkyl-substituted benzenes • If # Csubstituent < or = 6, then the arene is named as an alkyl-substituted benzene • If # Csubstituent > 6, then the arene is named as a phenyl-substituted alkane
Aryl groups • “Phenyl”refers to C6H5 • It is used when a benzene ring is a substituent • “Ph” or“f”can also bein place of “C6H5” • “Benzyl” refers to “C6H5CH2”
Disubstituted benzenes • Relative positions on a disubstituted benzene ring: • ortho- (o) on adjacent carbons (1,2 disubstituted) • meta- (m)separated by one carbon (1,3 disubstituted) • para- (p)separated by two carbons (1,4 disubstituted)
The ortho- (o), meta- (m), and para- (p) nomenclature is useful to describe reaction patterns Example: “Reaction of toluene with Br2 occurs at the para position”
Multisubstituted benzenes • Multisubstituted benzenes (more than two substituents) are named as follows: • Choose the sequence when the substituents have the lowest possible number • List substituents alphabetically with hyphenated numbers • Use common names, such as “toluene”, as parent name (as in TNT)
Use common names, such as “toluene”, as parent name • The principal substituent is assumed to be on C1 • See Table 15.1
Practice Problem: Tell whether the following compounds are ortho-, meta-, or para-disubstituted (a) Meta (b) Para (c) Ortho
Practice Problem: Give IUPAC names for the following compounds • (a) m-Bromochlorobenzene (b) (3-Methylbutyl)benzene • (c) p-Bromoaniline (d) 2,5-Dichlorotoluene • (e) 1-Ethyl-2,4-dinitrobenzene (f) 1,2,3,5-Tetramethylbenzene
Practice Problem: Draw structures corresponding to the following IUPAC names: • p-Bromochlorobenzene • p-Bromotoluene • m-Chloroaniline • 1-Chloro-3,5-dimethylbenzene
3.Structure and Stability of Benzene • Benzene is very stable • It undergoes substitution rather than the rapid addition reaction common to compounds with C=C, suggesting that in benzene there is a higher barrier • Example:Benzene reacts slowly with Br2 to give bromobenzene (where Br replaces H)
Heats of Hydrogenation as Indicators of Stability • The addition of H2 to C=C normally gives off about 118 kJ/mol – 3 double bonds would give off 356 kJ/mol • Two conjugated double bonds in cyclohexadiene add 2 H2
Benzene has 3 unsaturations but gives off only 206 kJ/mol on reacting with 3 H2 molecules • Therefore it has about 150 kJ/mol more “stability” than an isolated set of three double bonds
Benzene has 150 kJ/mol more “stability” than expected for “cyclohexatriene”
Benzene’s Unusual Structure • All its C-C bonds are the same length: 139 pm — between single (154 pm) and double (134 pm) bonds • Electron density in all six C-C bonds is identical
Structure is planar, hexagonal • All C–C–C bond angles are 120° • Each C is sp2-hybridized and has a p orbital perpendicular to the plane of the six-membered ring
Drawing Benzene and Its Derivatives • The two benzene resonance forms can be represented by a single structure with a circle in the center to indicate the equivalence of the carbon–carbon bonds • This does not indicate the number of electrons in the ring but shows the delocalized structure • One of the resonance structures will be used to represent benzene for ease in keeping track of bonding changes in reactions
4.Molecular Orbital Description ofBenzene • In benzene, 6 p orbitals combine to form 6 p molecular orbitals (MO): • 3 bonding orbitals with 6 electrons • 3 antibonding orbitals
3 bonding, low-energy MO: y1, y2, and y3 • 3 antibonding high-energy MO:y4*, y5*, and y6*
Orbitals with the same energy are degenerate • 2 bonding orbitals, y2 and y3 • 2 antibonding orbitals, y4* and y5*
y3 andy4* have no p electron density on 2 carbons because of a node passing through these atoms
Practice Problem: Pyridine is flat, hexagonal with bond angles of 120°. It undergoes electrophilic substitution rather than addition and generally behaves like benzene. Draw the p orbitals of pyridine.
Recall: Key Ideas on Benzene • Benzene is a cyclic conjugated molecule • Benzene is unusually stable - DHhydrogenation = 150 kJ/mol less negative than a cyclic triene • Benzene is planar hexagon: bond angles are 120°; carbon–carbon bond lengths, 139 pm • Benzene undergoes substitution rather than electrophilic addition • Benzene is a resonance hybrid with structure between two line-bond structures • Benzene has 6 p electrons, delocalized over the ring
5.Aromaticity and the Hückel 4n+2 Rule The Hückel 4n + 2 rule: • was devised by Eric Hückel in 1931 • states that planar, monocyclic conjugated systems with a total of 4n + 2 p electrons where n is an integer (n = 0, 1, 2, 3,…) are aromatic
Aromatic compounds with 4n + 2 p electrons • Benzene • It has 6 p electrons: 4n + 2 = 6, thus n = 1 • It is aromatic: it is stable and the electrons are delocalized
Compounds with 4n electrons are NOT aromatic (May be Antiaromatic) • Planar, cyclic conjugated molecules with 4n electrons are antiaromatic • They are much less stable than expected • They will distort out of plane and behave like ordinary alkenes Which of the above is antiaromatic?
Cyclobutadiene • It has 4 p electrons: 4n + 2 = 4, thus n = ½ (not an integer) • It is antiaromatic: The p electrons are localized into two double bonds localized p electrons
Cyclobutadiene • It has 4 p electrons: 4n + 2 = 4, thus n = ½ (not an integer) • It is antiaromatic: The p electrons are localized into two double bonds • It is so unstable that it dimerizes by a self-Diels-Alder reaction at lowtemperature dienophile diene
Cyclooctatetraene • It has 8 p electrons: 4n + 2 = 8, thus n = 3/2 (not an integer) • It is nonaromatic: • the p electrons are localized into four double bonds • it is tub-shaped not planar • it has four double bonds, reacting with Br2, KMnO4, and HCl as if it were four alkenes p orbitals not parallel for overlap
Practice Problem: To be aromatic, a molecule must have 4n + 2 p electrons and must have cyclic conjugation. Is cyclodecapentaene aromatic? • It has 10 p electrons: 4n + 2 = 10, thus n = 2 (an integer) • It is not planar due to steric strain, thus the neighboring p • orbitals are not properly aligned for overlap. It is not • conjugated. Thus it is not aromatic.
6.Aromatic Ions • The Hückel 4n + 2 rule applies to ions as well as to neutral species: • To be aromatic, a molecule must be planar, cyclic conjugated system with 4n + 2 p electrons • Example:Both the cyclopentadienyl anion and the cycloheptatrienyl cation are aromatic.
Example: Both the cyclopentadienyl anion and the cycloheptatrienyl cation are aromatic. The key feature of both is that they contain 6 electrons in a ring of continuous p orbitals
Aromaticity of the Cyclopentadienyl Anion Not fully conjugated and not aromatic Unstable and nonaromatic Stable and aromatic
Cyclopentadiene is relatively acidic (pKa = 16) because its conjugate base, the aromatic cyclopentadienyl anion, is so stable. • Other hydrocarbons have pKa > 45
Aromaticity of the Cyclopentadienyl Anion • 1,3-Cyclopentadiene contains conjugated double bonds joined by a CH2 that blocks delocalization • Removal of H+ at the CH2 produces a cyclic 6-electron system, which is stable • Removal of H- or H• generate nonaromatic 4 and 5 electron systems • Relatively acidic (pKa = 16) because the anion is stable
Aromaticity of the Cycloheptatrienyl Cation Not fully conjugated and not aromatic Stable and aromatic Unstable and nonaromatic
The cycloheptatrienyl cation (six p electrons) is aromatic and very stable • Reaction of cycloheptatriene with Br2 yields cycloheptatrienylium bromide, an ionic substance containing the cycloheptatrienyl cation
Aromaticity of the Cycloheptatrienyl Cation • Cycloheptatriene has 3 conjugated double bonds joined by a CH2 • Removal of H- at the CH2 produces the cycloheptatrienyl cation • The cation is a cyclic 6-electron system, which is stable and is aromatic • Removal of H+ or H• generate nonaromatic 7 and 8 electron systems
Practice Problem: Draw the five resonance structures of the cyclopentadienyl anion. Are all carbon-carbon bonds equivalent? How many absorption lines would be in the 1H and 13C NMR spectra of the anion?
Practice Problem: Cyclooctatetraene readily reacts with potassium metal to form the stable the cyclooctatetraene dianion, C2H82-. Why does the reaction occur so easily? What is the geometry of the dianion?
7.Aromatic Heterocycles: Pyridine and Pyrrole • A heterocycle is a cyclic compound that contains an atom or atoms other than carbon in its ring, such as N, O, S, P • There are many heterocyclic aromatic compounds and many are very common • Cyclic compounds that contain only carbon are called carbocycles (not homocycles) • Nomenclature is specialized • Example:Pyridine and Pyrrole