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Chapter 15- Benzene and Aromaticity
Ashley Piekarski, Ph.D.
Aromatic Compounds
• What do you think the word “aroma%c” means? • Back in the 19th century, the word was used to describe fragrant
substances
• Current definiBon: disBnguished from alipha&c compounds by electronic configuraBon • What is an aliphatic compound?
Examples of aromaBc compounds
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Why do I care, Dr. P?
• ReacBvity of subsBtuted aromaBc compounds is Bed to their structure
• AromaBc compounds provide a sensiBve probe for studying relaBonship between structure and reacBvity
• Remember in organic chemistry we are always trying to define structure-‐property rela%onships to generalize a class of compounds
Synthesis of Aromatic Hydrocarbons- not pretty L
• From high temperature (1000°C) fracBonal disBllaBon of coal tar
• HeaBng petroleum at high temperature and pressure over a catalyst
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Nomenclature of Aromatic Compounds
• Many common names (toluene = methylbenzene; aniline = aminobenzene)
• You must learn them! They are in your textbook.
Nomenclature of Aromatic Compounds
• MonosubsBtuted benzenes systemaBc names as hydrocarbons with –benzene • C6H5Br = bromobenzene • C6H5NO2 = nitrobenzene, and C6H5CH2CH2CH3 is
propylbenzene
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The Phenyl Group
• When a benzene ring is a subsBtuent, the term phenyl is used (You may also see “Ph” or “φ” in place of “C6H5”)
• “Benzyl” refers to “C6H5CH2”
• What circumstances do you think a benzene ring is considered a subsBtuent?
Nomenclature of Disubstituted Benzenes
• RelaBve posiBons on a benzene ring • ortho- (o) on adjacent carbons (1,2-substitution) • meta- (m) separated by one carbon (1,3-substitution) • para- (p) separated by two carbons (1,4-substitution)
• Describes reacBon pa^erns (“occurs at the para posiBon”)
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Nomenclature of Disubstituted Benzenes
• RelaBve posiBons on a benzene ring • ortho- (o) on adjacent carbons (1,2-substitution) • meta- (m) separated by one carbon (1,3-substitution) • para- (p) separated by two carbons (1,4-substitution)
• Describes reacBon pa^erns (“occurs at the para posiBon”)
Naming Benzenes With More Than Two Substituents
• Choose numbers to get lowest possible values for subsBtuents
• List subsBtuents alphabeBcally with hyphenated numbers • Common names, such as “toluene” can serve as root name
(as in TNT)
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Learning check
• Give the correct IUPAC name for the molecule below:
CH3
NH2
S
Structure and Stability of Benzene: Molecular Orbital Theory
• Benzene reacts slowly with Br2 to give bromobenzene (where Br replaces H)
• What product would we get if we reacted bromine with 1-‐propene?
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Heats of Hydrogenation as Indicators of Stability
• The addiBon of H2 to C=C normally gives off about 118 kJ/mol
• Two conjugated double bonds in cyclohexadiene add 2 equivalents of H2 to give off 230 kJ/mol
• How much energy would you expect benzene to give off?
• Benzene has 3 unsaturaBon sites but gives off only 206 kJ/mol on reacBng with 3 H2 molecules
• Therefore it has about 150 kJ more “stability” than an isolated set of three double bonds
Heats of Hydrogenation as Indicators of Stability
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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 idenBcal à electrostaBc map
• Structure is planar, hexagonal • C–C–C bond angles 120° • What is the hybridizaBon of each carbon atom in benzene?
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 π electrons in the ring but reminds us of the delocalized structure
• We shall use one of the resonance structures to represent benzene for ease in keeping track of bonding changes in reacBons
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Molecular Orbital Description of Benzene
• The 6 p-‐orbitals combine to give • Three bonding orbitals with 6 π electrons, • Three antibonding with no electrons
• Orbitals with the same energy are degenerate
Aromaticity and the 4n + 2 Rule
• Huckel’s rule, based on calculaBons – a planar cyclic molecule with alternaBng double and single bonds has aromaBc stability if it has 4n+ 2 π electrons (n is 0, 1, 2, 3, 4)
• For n=1: 4n+2 = 6; benzene is stable and the electrons are delocalized
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Compounds With 4n π Electrons Are Not Aromatic
• Planar, cyclic molecules with 4 n π electrons are much less stable than expected (an%aroma%c)
• They will distort out of plane and behave like ordinary alkenes • 4-‐ and 8-‐electron compounds are not delocalized (single and
double bonds) • Cyclobutadiene is so unstable that it dimerizes by a self-‐Diels-‐
Alder reacBon at low temperature
Compounds With 4n π Electrons Are Not Aromatic
• Planar, cyclic molecules with 4 n π electrons are much less stable than expected (anBaromaBc)
• They will distort out of plane and behave like ordinary alkenes • 4-‐ and 8-‐electron compounds are not delocalized (single and double
bonds) • Cyclobutadiene is so unstable that it dimerizes by a self-‐Diels-‐Alder
reacBon at low temperature • Cyclooctatetraene has four double bonds, reacBng with Br2, KMnO4, and
HCl as if it were four alkenes
What do you noBce about the structure and electron density?
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Aromatic Ions
• The 4n + 2 rule applies to ions as well as neutral species
• Both the cyclopentadienyl anion and the cycloheptatrienyl ca&on are aromaBc
• The key feature of both is that they contain 6 π electrons in a ring of conBnuous p orbitals
Aromaticity of the Cyclopentadienyl Anion
• 1,3-‐Cyclopentadiene contains conjugated double bonds joined by a CH2 that blocks delocalizaBon
• Removal of H+ at the CH2 produces a cyclic 6-‐electron system, which is stable
• Removal of H-‐ or H• generate nonaromaBc 4 and 5 electron systems
• RelaBvely acidic (pKa = 16) because the anion is stable
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Cycloheptatriene
• Cycloheptatriene has 3 conjugated double bonds joined by a CH2
• Removal of “H-‐” leaves the caBon • The caBon has 6 electrons and is aromaBc
Aromatic Heterocycles: Pyridine and Pyrrole
• Heterocyclic compounds contain elements other than carbon in a ring, such as N, S, O, P
• AromaBc compounds can have elements other than carbon in the ring
• There are many heterocyclic aromaBc compounds and many are very common
• Cyclic compounds that contain only carbon are called carbocycles
• Nomenclature is specialized
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Pyridine and Pyrimidine
• A six-‐membered heterocycle with a nitrogen atom in its ring • π electron structure resembles benzene (6 electrons) • The nitrogen lone pair electrons are not part of the aromaBc system
(perpendicular orbital) • Pyridine is a relaBvely weak base compared to normal amines but
protonaBon does not affect aromaBcity
Pyrrole and Imidazole
• A five-‐membered heterocycle with one nitrogen • π electron system similar to that of cyclopentadienyl anion • Four sp2-‐hybridized carbons with 4 p orbitals perpendicular to the ring and
4 p electrons • Nitrogen atom is sp2-‐hybridized, and lone pair of electrons occupies a p
orbital (6 π electrons) • Since lone pair electrons are in the aromaBc ring, protonaBon destroys
aromaBcity, making pyrrole a very weak base
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Learning check
• Is thiophene aromaBc or anBaromaBc? Explain why or why not.
CH3
NH2
S
Why 4n +2?
• When electrons fill the various molecular orbitals, it takes two electrons (one pair) to fill the lowest-‐lying orbital and four electrons (two pairs) to fill each of n succeeding energy levels
• This is a total of 4n + 2
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Polycyclic Aromatic Compounds
• AromaBc compounds can have rings that share a set of carbon atoms (fused rings)
• Compounds from fused benzene or aromaBc heterocycle rings are themselves aromaBc
Naphthalene Orbitals
• Three resonance forms and delocalized electrons
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Learning check
• Azulene is an isomer of napththalene. Is azulene aromaBc? Draw a second resonance structure of azulene.
Characterization of Aromatic Compounds
• IR: AromaBc ring C–H stretching at 3030 cm-‐1 and peaks 1450 to 1600 cm-‐1
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Characterization of Aromatic Compounds
• UV: Peak near 205 nm and a less intense peak in 255-‐275 nm range
Characterization of Aromatic Compounds
• 1H NMR: AromaBc H’s strongly deshielded by ring and absorb between δ 6.5 and δ 8.0 • Peak pattern is characteristic of positions of
substituents
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Ring Currents
• AromaBc ring oriented perpendicular to a strong magneBc field, delocalized π electrons producing a small local magneBc field • Opposes applied field in middle of ring but reinforces
applied field outside of ring
13C NMR of Aromatic Compounds
• Carbons in aromaBc ring absorb at δ 110 to 140 • Shit is disBnct from alkane carbons but in same range as
alkene carbons
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Learning check
• Propose a structure for an aromaBc compound with the molecular formula, C8H9Br, that has the following 1H NMR spectra:
