Benzene and Aromaticity

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Benzene and Aromaticity

• It was thought for a long time that aromatic
  compound is a fragrant substance like
  benzaldehyde (from cherries, peaches and almond),
  toluene (from Tolu balsam), benzene (from coal
  distillate) and naphthalene (from moth balls).
• TODAY, the word aromatic refers to the class of
  compounds that contain six-membered benzene-like
  rings.

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CHARACTERISTICS

• Unusual Chemical stability compared with their
  non-aromatic isomers.
• Their Chemical behavior is quite different than
  the aliphatic compounds.

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Structure and Stabillity of BenzeneMO theory

• Benzene does not react like alkenes (1,2 or 1,4
  addition reaction), but reacts via substitution
  reaction instead.
• Heats of hydrogenation-
• Cyclohexene ?H -118 kJ/mol
• 1,3 cyclohexadiene ?H -230 kJ/mol
  (theoretically -236 kJ/mol)
• Benzene ?H -206 kJ/mol (theoretically -354
  kJ/mol)
• So, benzene is more stable than expected by
  about 150 kJ/mol
• All bonds in benzene are equal in length (139pm)
  which is intermediate between single bond (154pm)
  and double bond (134pm).
• Electrostatic potential map shows that the
  electron density in all six C-C bond is
  identical.
• Benzene is planar regular hexagon with all C-C-C
  bond angles are 120? with SP2 hybridization and
  each C-atom has p orbital perpendicular to the
  plane of the 6-membered ring.

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Benzene is more correctly be presented by
6-membered ring with a circle in the center
instead of the localized double bonds as the true
is that all the 6 p- orbitals electrons are
completely delocalized symmetrically over the
ring.
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Aromaticity and the Hückel (4n2) rule

• This rules completes the description of the
  aromaticity, the rule states that a molecule is
  aromatic ONLY if it has a planar, monocyclic
  system of conjugation and contains a total of
  4n2 ? electrons, where n is an integer (0, 1, 2,
  3, ..). Molecules with ONLY 4n ? electrons are
  NOT aromatic or called antiaromatic.

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• Cyclobutadiene
• It does not fulfill the 4n2 rule, the 4 ?
  electrons are localized into 2 double bonds
  rather than delocalized around the ring, it is
  also highly reactive even at -78 C? it dimerizes
  by Diels-Alder reaction as one molecule react as
  diene and the other as dienophile.
• Cyclooctatetraene
• It does not fulfill the 4n2 rule, the 8 ?
  electrons are localized into 4 double bonds
  rather than delocalized around the ring. In
  addition, the molecule is tub-shaped rather than
  planar. X-ray studies show that C-C single bonds
  are 147 pm and CC are 134 pm long.
• ? Why 1, 3, 5, 7, 9-cyclodecapentaene is
  antiaromatic??

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• Aromatic Ions
• The number of p- orbitals and the number of ?
  electrons are NOT necessarily be the same i.e.
  they could be different.
• Cyclopentadienyl anion has 5 p- orbitals and 6 ?
  electrons, cycloheptatrienyl cation has 7 p-
  orbitals and 6 ? electrons, cyclopropenyl cation
  has 3 p- orbitals and 2 ? electrons and all are
  aromatic.

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• Cyclopentadiene molecule is not aromatic as the
  -CH2- is SP3 hybridized and there is NO
  conjugation allowed.
• Cyclopentadienyl anion is aromatic as all -CH-
  groups are SP2 hybridized and anion is fully
  conjugated.
• Cyclopentadiene molecule can be converted into
  either a cation, a free radical or anion.
  Although, there are resonance structures in the
  three cases, but ONLY the anion is aromatic as it
  fulfills the Hückels rule.
• Cyclopentadienyl cation and free radicals are not
  stable as any aromatic species, but are highly
  reactive instead and difficult to prepare. On
  contrast, cyclopentadienyl anion is remarkably
  stable and is easily prepared.
• The same explanation is true for cycloheptatriene
  molecule as well as only the cationic form is
  aromatic according to Hückels rule.

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Aromatic Heterocycles

• Pyrrole and Pyridine Aromaticity necessitates
  that the molecular or ionic species to be cyclic,
  conjugated and hence planar, with 4n2 ?
  electrons. The aromatic molecule may be also
  contain hetero-atoms as well (O, N, S) and then
  called heterocyclic compound.
• The lone pair of electrons on (gtN-) atom in both
  pyridine and pyrimidine are in the plane of the
  6-membered ring and NOT counted of the 6 ?
  electrons.
• Pyrrole shares the (gtN-) lone pair of electrons
  to complete the 6 ? electrons.
• Imidazole resembles pyrrole in one (-NH-) group
  that shares with its lone pair of electrons,
  while the other (gtN-) has its lone pair of
  electrons available in the plane , NOT - to the
  plane of the ring.
• Thiophene has two lone pair of electrons, one of
  them share is - to the plane of the ring and
  completes the aromatic 6 ? electrons while the
  second lone pair is like the case of pyrrole
  ring.
• Furan is like thiophene, shares with one of the
  two lone pair of electrons to complete the
  aromatic 6 ? electrons.

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Why Hückel (4n2) rule?

• MO theory gives an account for the stabilization
  gained by the molecule by filling up the bonding
  ?-MO (see benzene MO) where the lowest bonding
  ?-MO fills up with 2 electrons and any succeeding
  ?-MO up to 4n2 ? electrons, any extra electrons
  will partially fill bonding ?-MO with unpaired
  ?-electrons OR fills up the anti-bonding ?-MO
  which doesnt offer the aromatic stability to the
  molecule/ion.

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Polycyclic Aromatic Compounds

• Hückel (4n2) rule applies ONLY to monocyclic
  compounds.
• Naphtalene fulfills the Hückel (4n2) rule with
  10 ?-electrons.
• Aromaticity of naphthalene is explained by its
  molecular stability as indicated by the heat of
  hydrogenation. In addition, it reacts with
  electrophilic reagents (like Br2) via
  substitution and NOT addition reactions.
• Various hetero polycyclic compounds are also
  aromatic e.g. Quinoline, Isoquinoline, Indole,
  Purine.
• Ring current of benzene ring creates a magnetic
  field that opposes the external applied and hence
  the ring protons become de-shielded and show NMR
  spectrum at 6.5-8 ?.
• 18 Annulene is aromatic monocyclic with huge
  ring which fulfills Hückel (4n2) rule with 18
  ?-electrons. The inside H are shielded and show
  NMR spectrum at -3.0 ? while outside H are highly
  de-shielded and show NMR spectrum at 9.3 ?.

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18 Annulene
Outside H atoms show NMR spectrum at 9.3
? Inside H atoms show NMR spectrum at -3.0 ?
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• Pyrene is a polycyclic aromatic hydrocarbon
  consisting of four fused benzene rings, resulting
  in a flat aromatic system.
• It could be hydrogenated with H2 without
  affecting the aromaticity of the whole molecule.

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Suggested problems

• 15.1, 15.3, 15.5, 15.7, 15.8, 15.10, 15.11,
  15.18, 15.25, 15.29, 15.31-15.36.
• http//bama.ua.edu/chem/courses/f2009/ch232/ch232
  -f09.html