BENZENE AND AROMATICITY

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BENZENE AND AROMATICITY

• Chapter 15

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The Term Aromatic

• Aromatic used to be used to describe a fragrant
  substance.
• Today the word aromatic refers to benzene and its
  structural relatives.
• Benzaldehyde, toluene, and benzene are all
  aromatic compounds.

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• The steroidal hormone estrone, the analgesic
  morphine, and the tranquilizer diazepam (Valium)
  are all examples of aromatic compounds.

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15.1 Sources of Aromatic Hydrocarbons

• Hydrocarbons come from two main sources coal and
  petroleum.
• Fractional distillation of coal yields benzene,
  toluene, xylene (dimethylbenzene), naphthalene,
  and a host of other aromatic compounds.
• Petroleum consists largely of alkanes with a few
  aromatic compounds.

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15.2 Naming Aromatic Compounds

• Aromatic substances have more nonsystematic names
  than any other class of organic compounds.
• Some widely used names are allowed by IUPAC
  rules. Methylbenzene is known as toluene,
  hydroxybenzene as phenol, aminobenzene as aniline.

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Common Names of Aromatic Compounds
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Monosubstituted Benzenes

• Monosubstituted benzenes are systematically named
  as other hydrocarbons, with benzene as the
  parent name.

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Alkyl-substituted Benzenes and Phenyl-substituted
Alkanes

• Alkyl-substituted benzenes are called arenes. If
  the alkyls substituent is smaller than the ring
  (six or fewer carbons), the arene is named as an
  alkyl-substituted benzene.
• If the alkyl substituent is larger than the ring
  (more than six carbons), the compound is named as
  a phenyl-substituted alkane.
• Phenyl is sometimes abbreviated as Ph or F.

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Phenyl and Benzyl Groups
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Disubstituted Benzenes

• Disubstituted benzenes use prefixes ortho-(o),
  meta(m), or para-(p). These prefixes are useful
  when discussing reactions.

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Benzenes with More Than Two Substituents

• Benzenes with more than two substituents are
  named by numbering the positions of each
  substituent, so that the lowest possible numbers
  are used.

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Structure and Stability of Benzene

• Benzene is more stable than typical alkenes.
• All C-C-C bond angles are 120 degrees, all six
  carbon atoms are sp2 hybridized, and each carbons
  has a p orbital perpendicular to the plane of the
  six-membered ring.

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Comparison of Heats of Hydrogenation
This provides a measure of benzenes unusual
stability its resonance energy is 29.6 kcal
mol-1.
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Bonds of Benzene Rings

• Each p bond overlaps equally well with both
  neighboring p orbitals six p electrons are
  delocalized around the ring.

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15.4 Molecular Orbital Description of Benzene

• Planar molecule with the shape of a regular
  hexagon.
• All C-C-C bond angles are 120 degrees
• All six carbons are sp2 hybridized.

• Each carbon atom has a p orbital perpendicular to
  the plane of the six-membered ring.
• All six carbon atoms and six p orbitals in
  benzene are equivalent.

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The six pi electrons are completely delocalized
around the ring.
This is why it is difficult to define when one p
orbital overlaps only one neighboring p orbital.
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Molecular Orbital of Benzene
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Bonding and Anti-bonding combinations

• Six benzene molecular orbitals result from the
  cyclic combination of six p atomic orbitals.
• Three low energy molecular orbitals are bonding
• Three high energy molecular orbitals are
  anti-bonding

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Lets review

• Benzene is a cyclic conjugated molecule
• It is unusually stable, with a heat of
  hydrogenation of -206 kJ/mol
• It is planar with the shape of a regular hexagon
  bond angles 120
• It undergoes substitution reactions that retain
  the cyclic conjugation
• It is a resonance hybrid

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So what makes a molecule aromatic?

• It must be cyclic
• It must be conjugated
• It must be flat so that the p orbital overlap can
  occur
• It must also have
• 4n 2 pi electrons

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15.5 The Huckel 4n 2 Rule

• To be aromatic, the molecule must also follow the
  Huckel 4n 2 rule

A molecule must have 4n 2 pi electrons where n
is an integer (0, 1, 2, 3, etc)
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• The 4n 2 rule does not apply just to neutral
  hydrocarbons.
• For example the cyclopentadienyl anion and
  cycloheptatrienyl cation are aromatic

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15.6A Cyclopentadienyl Anion

• Cyclopentadiene is not aromatic because it is not
  conjugated.
• The CH2 carbon in the ring is sp3 hybridized
  preventing cyclic conjugation
• Both the radical and carbocation are unstable

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15.6A Cyclopentadienyl Anion

• Removing one H and no electrons from
  cyclopentadiene leaves the anion which has six pi
  electrons (a Huckel number!)
• This anion now meets all the standards for
  aromaticity.

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15.6B Cycloheptatrienyl Cation

• Likewise, the cycloheptatrienyl cation has six pi
  electrons
• Its radical and anion have seven and eight pi
  electrons, respectively (not Huckel numbers)

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15.7 Aromatic Heterocycles Pyridine
Pyrrole

• Heterocycle a cyclic compound that contains an
  atom(s) in its ring other than carbon and can be
  aromatic. The heteroatom is often O or N.

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Pyridine is a 6 membered ring like Benzene with 6
p electrons perpendicular to the plane of the
ring (5 from the sp2 hybridized Cs and one from
the sp2 hybridized N). Nitrogens lone pair of
electrons are in the plane of the ring acting
like the H of the benzene C-H bond. A single
electron is in the P orbital perpendicular to the
ring resulting in 6 electrons in the cyclic pi
system. Pyridine is an aromatic heterocyclic
compound, a nitrogen analogue of benzene.
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Pyrrole is a five membered ring also with 6 p
electrons perpendicular to the plane of the ring.
However, 4 are from the sp2 hybridized Cs and
two are from the lone pair on the sp2 hybridized
N. In Pyrrole, the H of the N-H bond is in the
plane of the ring, just like the Hs of the C-H
bonds. This puts the two lone pair electrons in
the P orbital perpendicular to the ring.
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15.8 Why 4n 2?

• 2, 4, 6, 10, 14, 18 p electrons create aromatic
  stability because of molecular orbitals.
• Benzene has 6 overlapping atomic p orbitals
  therefore it also has 6 molecular orbitals.
• The six molecular orbitals are divided evenly
  between bonding and nonbonding orbitals.
• There is one lowest energy orbital, two
  degenerate (equal energy) pairs of orbitals and
  one highest energy orbital.

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• The single lowest energy molecular orbital holds
  two electrons.
• Each degenerate (equal energy) bonding orbital
  above it holds 2 electrons and since there is a
  pair of orbitals in each degenerate energy level,
  each degenerate orbital level (shell) holds 4
  electrons. The first degenerate energy level
  (n1) holds 4 electrons (2 pairs).

• n of filled pairs of degenerate orbitals
  (energy level).
• Anything but 4n2 would leave an partially
  unfilled
• orbital.

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Benzene has 6 molecular orbitals 3 filled
bonding orbitals (bottomlower energy) 1
filled lowest energy orbital, 2 filled
degenerate orbitals (n1) and 3 empty nonbonding
orbitals (tophigher energy).
AROMATIC Cyclobutadiene has 4 molecular
orbitals 1 filled lowest energy 2 half filled
degenerate orbitals (n0 because orbitals not
filled) 1 empty nonbonding orbital
ANTIAROMATIC Cyclooctatetraene has 8 molecular
orbitals 3 filled lowest energy bonding orbitals
(n1) 2 half filled orbitals (only fully filled
orbitals in an entire energy level
increase n) 3 empty nonbonding orbitals For a
total of 8 electrons. NON AROMATIC
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15.9 Polycyclic Aromatic Compounds

• Huckel Rule applies only to monocyclic compounds
  but the general concept of aromaticity can also
  be applied to polycyclic aromatic compounds.
• Examples of polycyclic aromatic compounds are
• Napthalene, anthracene and phenanthrene

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• All polycyclic aromatic hydrocarbons can be
  represented by multiple resonance forms yet the
  true structure is a hybrid of these resonance
  forms with delocalization of the pi electrons
  (see electrostatic potential map on right).

Napthalene has 10 pi electrons, 2n 2, (n4) and
is aromatic. Each ring, if looked at separately,
has six pi electrons, but because they share the
common central bond, the entire polycyclic
molecule has only 10 electrons making it aromatic.
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15.10 Spectroscopy

• Summary of Spectroscopic Information on Aromatic
  Compounds
• Kind of Absorption
• Spectroscopy Position Interpretation
• __________________________________________________
  __________
• Infrared cm-1 3030 Aryl C-H Stretch
• 1500 and 1600 Two absorptions due to ring
  motions 690-900 Intense C-H out-of-plane
  bending
• Ultraviolet (nm) 205 Intense absorption
• 255-275 Weak Absorption
• HNMR (d) 2.3-3.0 Benzylic Protons
• 6.5-8.0 Aryl Protons
• CNMR (d) 110-140 Aromatic Ring Carbons

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IR Aromatic Rings

• Low intensity C-H stretching absorption at 3030
  cm-1, just to the left of typical saturated C-H
  band.
• Series of C-C (ring bonds) at 1450 to 1600 cm-1.
  Four absorptions are often observed in a set of 2
  pairs. One pair near 1450 cm-1 and one pair
  near 1600-1650 cm-1. One band of each pair is
  usually stronger than the other, generally at or
  near 1500 cm-1 and at 1600 cm-1.
• Multiple weak absorptions in the 1660 2000 cm-1
  region
• Strong absorptions in the 690-900 cm-1 range.
  The exact position of the absorption can identify
  the substitution pattern of the ring.

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UV Spectroscopy

• UV Spec is applicable only to conjugated pi
  systems. Aromatic compounds show these
  characteristic ring absorptions
• a series of bands of fairly intense absorption
  near 205 nm.
• a less intense absorption in the range of 255 to
  275 nm.

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1HNMR

• Aromatic rings are easily identified by their
  strongly deshielded hydrogens that absorb between
  6.5 - 8.0 d.
• The difference between vinylic protons that
  absorb between 4.5 - 6.5 and aromatic (aryl)
  protons is ring current, an induced field created
  by the delocalized pi electrons circulating
  around the ring, creating a magnetic field of
  their own. Aryl protons therefore experience a
  greater magnetic field than that which is applied
  and come into resonance at a lower applied field.
• Benzyllic protons (H on C next to aromatic ring)
  absorb downfield from alkane Hs in the region of
  2.3 to 3.0d.

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13CNMR

• Aromatic (ring) carbons absorb in the range of
  110-140 d (the same range as alkene Cs). Thus,
  the presence of 13C absorptions are not
  conclusive evidence of an aromatic ring.
  Aromaticity must be confirmed by IR, UV or HNMR.