benzen and aromaticity

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  • Note: I put delta ahead of value (delta = “chemical shift” and is not a unit)
  • benzen and aromaticity

    1. 1. Benzene andAromaticity
    2. 2. Aromatic Compounds Aromatic was used to described some fragrantcompounds in early 19thcentury Not correct: later they are grouped by chemicalbehavior (unsaturated compounds that undergosubstitution rather than addition) Current: distinguished from aliphatic compounds byelectronic configuration
    3. 3. Why this Chapter? Reactivity of substituted aromatic compoundsis tied to their structure Aromatic compounds provide a sensitiveprobe for studying relationship betweenstructure and reactivity
    4. 4. 15.1 Sources and Names ofAromatic Hydrocarbons From high temperature distillation of coal tar Heating petroleum at high temperature and pressureover a catalyst
    5. 5. Naming Aromatic Compounds Many common names (toluene = methylbenzene;aniline = aminobenzene) Monosubstituted benzenes systematic names ashydrocarbons with –benzene C6H5Br = bromobenzene C6H5NO2 = nitrobenzene, and C6H5CH2CH2CH3 ispropylbenzene
    6. 6. The Phenyl Group When a benzene ring is a substituent, the termphenyl is used (for C6H5) You may also see “Ph” or “φ” in place of “C6H5” “Benzyl” refers to “C6H5CH2”
    7. 7. Disubstituted Benzenes Relative positions on a benzene ring ortho- (o) on adjacent carbons (1,2) meta- (m) separated by one carbon (1,3) para- (p) separated by two carbons (1,4) Describes reaction patterns (“occurs at the paraposition”)
    8. 8. Naming Benzenes With More ThanTwo Substituents Choose numbers to get lowest possible values List substituents alphabetically with hyphenatednumbers Common names, such as “toluene” can serve as rootname (as in TNT)
    9. 9. 15.2 Structure and Stability ofBenzene: Molecular Orbital Theory Benzene reacts slowly with Br2 to give bromobenzene(where Br replaces H) This is substitution rather than the rapid additionreaction common to compounds with C=C,suggesting that in benzene there is a higher barrier
    10. 10. Heats of Hydrogenation asIndicators of Stability The addition of H2 to C=C normally gives off about118 kJ/mol – 3 double bonds would give off356kJ/mol Two conjugated double bonds in cyclohexadieneadd 2 H2 to give off 230 kJ/mol Benzene has 3 unsaturation sites but gives off only206 kJ/mol on reacting with 3 H2 molecules Therefore it has about 150 kJ more “stability” than anisolated set of three double bonds (See Figure 15-2)
    11. 11. 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 C–C–C bond angles 120° Each C is sp2and has a p orbital perpendicular tothe plane of the six-membered ring
    12. 12. Drawing Benzene and Its Derivatives The two benzene resonance forms can berepresented by a single structure with a circle in thecenter to indicate the equivalence of the carbon–carbon bonds This does indicate the number of π electrons in thering but reminds us of the delocalized structure We shall use one of the resonance structures torepresent benzene for ease in keeping track ofbonding changes in reactions
    13. 13. Molecular Orbital Description ofBenzene 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
    14. 14. 15.3 Aromaticity and the Hückel4n+2 Rule Unusually stable - heat of hydrogenation 150 kJ/molless negative than a cyclic triene Planar hexagon: bond angles are 120°, carbon–carbon bond lengths 139 pm Undergoes substitution rather than electrophilicaddition Resonance hybrid with structure between two line-bond structures
    15. 15. Aromaticity and the 4n + 2 Rule Huckel’s rule, based on calculations – a planarcyclic molecule with alternating double and singlebonds has aromatic stability if it has 4n+ 2 πelectrons (n is 0,1,2,3,4) For n=1: 4n+2 = 6; benzene is stable and theelectrons are delocalized
    16. 16. Compounds With 4n π Electrons Are NotAromatic (May be Antiaromatic) Planar, cyclic molecules with 4 n π electrons are much less stablethan expected (antiaromatic) They will distort out of plane and behave like ordinary alkenes 4- and 8-electron compounds are not delocalized (single anddouble bonds) Cyclobutadiene is so unstable that it dimerizes by a self-Diels-Alder reaction at low temperature Cyclooctatetraene has four double bonds, reacting with Br2,KMnO4, and HCl as if it were four alkenes
    17. 17. 15.4 Aromatic Ions The 4n + 2 rule applies to ions as well as neutralspecies Both the cyclopentadienyl anion and thecycloheptatrienyl cation are aromatic The key feature of both is that they contain 6 πelectrons in a ring of continuous p orbitals
    18. 18. Aromaticity of the CyclopentadienylAnion 1,3-Cyclopentadienecontains conjugated doublebonds joined by a CH2 thatblocks delocalization Removal of H+at the CH2produces a cyclic 6-electronsystem, which is stable Removal of H-or H•generate nonaromatic 4 and5 electron systems Relatively acidic (pKa = 16)because the anion is stable
    19. 19. Cycloheptatriene Cycloheptatriene has 3 conjugated double bondsjoined by a CH2 Removal of “H-” leaves the cation The cation has 6 electrons and is aromatic
    20. 20. 15.5 Aromatic Heterocycles: Pyridineand Pyrrole Heterocyclic compounds contain elements otherthan carbon in a ring, such as N,S,O,P Aromatic compounds can have elements other thancarbon in the ring There are many heterocyclic aromatic compoundsand many are very common Cyclic compounds that contain only carbon arecalled carbocycles (not homocycles) Nomenclature is specialized
    21. 21. Pyridine 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 aromaticsystem (perpendicular orbital) Pyridine is a relatively weak base compared to normal amines butprotonation does not affect aromaticity
    22. 22. Pyrrole 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 thering and 4 p electrons Nitrogen atom is sp2-hybridized, and lone pair of electronsoccupies a p orbital (6 π electrons) Since lone pair electrons are in the aromatic ring, protonationdestroys aromaticity, making pyrrole a very weak base
    23. 23. 15.6 Why 4n +2? When electrons fill the various molecular orbitals, ittakes two electrons (one pair) to fill the lowest-lyingorbital and four electrons (two pairs) to fill each of nsucceeding energy levels This is a total of 4n + 2
    24. 24. Polycyclic Aromatic Compounds Aromatic compounds can have rings that share a setof carbon atoms (fused rings) Compounds from fused benzene or aromaticheterocycle rings are themselves aromatic
    25. 25. Naphthalene Orbitals Three resonance forms and delocalized electrons
    26. 26. 15.8 Spectroscopy of AromaticCompounds IR: Aromatic ring C–H stretching at 3030 cm−1andpeaks 1450 to 1600 cm−1(See Figure 15-13) UV: Peak near 205 nm and a less intense peak in255-275 nm range 1H NMR: Aromatic H’s strongly deshielded by ringand absorb between δ 6.5 and δ 8.0 Peak pattern is characteristic of positions ofsubstituents
    27. 27. Ring Currents Aromatic ring oriented perpendicular to a strongmagnetic field, delocalized π electrons producing asmall local magnetic field Opposes applied field in middle of ring butreinforces applied field outside of ring
    28. 28. 13C NMR of Aromatic Compounds Carbons in aromatic ring absorb at δ 110 to 140 Shift is distinct from alkane carbons but in samerange as alkene carbons

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