(18) session 18 arenes (aromatic hydrocarbons)

3/27/2012
1
Session 18
Organic Chemistry, UNAM School of Medicine1
Hydrocarbons:Hydrocarbons:Hydrocarbons:Hydrocarbons:
Arenes (Aromatic Hydrocarbons)
Dr L.H.A. Prins (Ph.D.)
Dept. of Pharmacy
UNAM
Learning Outcomes
2
By the end of this session, the student should be able to:
Understand the importance of arenes
Differentiate arenes from other hydrocarbons
Describe the structure of arenes
Determine if a compound is aromatic or not
Name arenes according to IUPAC system
Describe the physical properties of arenes
Organic Chemistry, UNAM School of Medicine

3/27/2012
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Importance??Importance??Importance??Importance??
3
Several medicines are aromatic compounds:
Salicylic acid
Included inWillow bark.
Used for centuries to treat pain & fever.
4 Organic Chemistry, UNAM School of Medicine
Acetylsalicylic acid
(aspirin)
Disprin®, etc.
A French chemist, Charles Frederic
Gerhardt, was the first to prepare
acetylsalicylic acid in 1853.

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Ibuprofen
Brufen®, etc.
Aspirin & Ibuprofen = Non-steroidal Anti-Inflammatory Drugs (NSAID’s).
Discovered by Andrew Dunlop & other
UK colleagues. Patented in 1961.
ArenesArenesArenesArenes/Aromatic hydrocarbons/Aromatic hydrocarbons/Aromatic hydrocarbons/Aromatic hydrocarbons
6
Arenes are cyclic planar hydrocarbons characterised by
alternating single & double bonds between carbons. It refers to
benzene & its structural relatives.
Arenes – contain delocalised π e-
Delocalised e- = Do not belong to a single atom nor are
they confined to a bond between 2 atoms
= Shared by 3 or more atoms

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ArenesArenesArenesArenes/Aromatic hydrocarbons/Aromatic hydrocarbons/Aromatic hydrocarbons/Aromatic hydrocarbons
7
Localised e- = Belong to a single atom or are confined to a
bond between 2 atoms
:
Lone pair e- belong to N e- pair confined to bond
between 2 atoms
Structure of benzeneStructure of benzeneStructure of benzeneStructure of benzene
8
Benzene molecular formula: C6H6
Early assumptions of benzene structure - a cyclic 6-membered ring
with alternating & = bonds
Analytical techniques show – all 6 C-C bonds in benzene have same
length (1.39 Å)
Shorter than C-C bond (1.54 Å)
Longer than C-C = bond (1.33 Å)
Therefore: Benzene does not actually have alternating & =
bonds!

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9
Each C of benzene is sp2 hybridised & have three sp2 orbitals
that lie in a plane with angles of 120° (trigonal planar molecular
geometry) (Session 8)
Remaining p orbital is perpendicular to the plane (90° to plane)
10
Each C of benzene uses two sp2 orbitals to bond to two other C
atoms to form σ bonds
The third sp2 orbital of each benzene C overlaps the s orbital of a H
Remaining unhybridised p orbital of each benzene C overlaps the
other unhybridised p orbitals on both adjacent/neighbouring C’s
Result: Continuous “doughnut-shaped” e- cloud above & below the
plane of benzene ring
This is called “conjugation in a benzene ring”

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11
(a) Each C of benzene has a p orbital
(b) Overlap of the p orbitals forms a cloud of e- above & below the plane of benzene
ring
(c) Electrostatic potential map for benzene shows that all the C–C bonds have same e-
density
12
NB!!!! -The 2 “doughnut-shaped” rings above & below plane of the
molecule represent one molecular orbital/bonding orbital
The 2 delocalised e- - found anywhere within those rings.
The other 4 delocalised e- - live in 2 similar (but not identical) molecular
orbitals/bonding orbitals
Previous diagram shows 1 of the molecular orbitals containing 2 of the
delocalised e-
Never describe structure as p orbitals on C’s overlapping sideways to
produce a delocalised π bond (π bond – only accommodate 2 e-)
Instead – describe as a "π system" or delocalised e- system (containing
6e-)

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13
Therefore:The 6 π e- in benzene – shared by all 6 C atoms
: Not localised on a single atom or in a bond between 2
C atoms
:They are delocalised, i.e. roam freely within
“doughnut-shaped” clouds that lie above & below the C
ring
Correct benzene line-bond structure should actually be:
14
Disadvantages of using dashed lines or circle representation:
Does not indicate amount of π e- in molecule
Not useful when drawing reaction mechanisms
In fused rings – actually incorrect to use dashed lines/circles
- Suggests that Naphthalene has 12 π e-
- Actually only has 10 π e-

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15
Although π e- are delocalised in benzene, the structure
containing = bonds (localised e-) is preferred for benzene
representation
DeterminingDeterminingDeterminingDetermining aromaticityaromaticityaromaticityaromaticity
16
Not all cyclic molecules with alternating & = bonds are
necessarily aromatic!!!
For a compound to be aromatic, it must be:
Cyclic & planar + have an uninterrupted cloud of e- (each p orbital
must overlap with p orbitals on either side of it)
π e- cloud must contain an odd number of pairs of π e- (Hückel’s
Rule) (1, 3, 5, 7, 9, etc., pairs of π e-)
Is the following structure aromatic??

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Nomenclature of substituted benzenesNomenclature of substituted benzenesNomenclature of substituted benzenesNomenclature of substituted benzenes
17
Monosubstituted benzenes - systematically named in the same
manner as other hydrocarbons, with -benzene as the parent
name/suffix
Examples:
Alkyl-substituted benzenes
If alkyl substituent is ≤ than benzene ring (6 or fewer C’s) - named as
alkyl-substituted benzene
If alkyl substituent is > than benzene ring (7 or more C’s) - named as
phenyl-substituted alkane
Phenyl Benzyl
R = Carbon chain
• Examples:
2-phenylheptane
1
2
3
4
5
6
7
hexylbenzene
1
2
3
4
5
6

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19
Disubstituted benzenes - named using one of the prefixes
ortho- (o),meta- (m) or para- (p)
ortho-disubstituted benzene: 2 substituents in a 1,2 relationship on
the ring
meta-disubstituted benzene: 2 substituents in a 1,3 relationship on the
ring
para-disubstituted benzene: 2 substituents in a 1,4 relationship on the
ring
• Examples:
1
2
3
4
1 1
o-dimethylbenzene
(1,2-dimethylbenzene)
m-dimethylbenzene
p-dimethylbenzene

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21
Polysubstituted benzenes - benzenes with more than 2
substituents are named by choosing a point of attachment as C 1 &
numbering the substituents on the ring so that the 2nd substituent
has lowest possible #
If ambiguity still exists, number so that the 3rd or 4th substituent has
lowest possible #, until a point of difference is found
• Some common benzene names:
Methylbenzene
(Toluene)
NH2
Aminobenzene
(Aniline)
O H
Hydroxybenzene
(Phenol)
Benzenecarboxylic
acid
(Benzoic acid)
Benzenesulphonic
acid
Benzonitrile
Common names = Can be used as parent
names during nomenclature.

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Physical properties ofPhysical properties ofPhysical properties ofPhysical properties of arenesarenesarenesarenes
23
Boiling points:
In benzene - only attractions between neighbouring molecules are
Van derWaals dispersion forces (force between 2 instantaneously
induced dipoles)
No permanent dipole on benzene molecule
Benzene boils at 80°C - Higher than other hydrocarbons of similar
molecular size (example: pentane & hexane)
Presumably due to ease with which
temporary dipoles can be set up involving the
delocalised e-
24
Melting points:
Benzene - melts at 5.5°C, Methylbenzene – melts at -95°C
Molecules must pack efficiently in the solid if they are to make best
use of their intermolecular forces
Benzene is a tidy, symmetrical molecule & packs very efficiently
Methyl group sticking out in methylbenzene tends to disrupt the
closeness of the packing
If molecules aren't as closely packed, the intermolecular forces don't
work as well & melting point falls

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25
Water solubility:
Arenes - insoluble in H2O
Benzene - quite large compared with a H2O molecule
For benzene to dissolve - have to break lots of existing H bonds
between H2O molecules
- have to break the quite strong Van der
Waals dispersion forces between benzene
molecules
Not energetically favourable
Quiz: Arenes
Review of General Chemistry, UNAM School of Medicine26
Are the following compounds aromatic or not:
1. H
Are the following compounds ortho-, meta- or para- substituted:
3. h
Give the systematic (IUPAC) name for the following:
5. h
2.
4. h
6.

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Thank you
27
END
Review of General Chemistry, UNAM School of Medicine28
Quiz answers:
1. Aromatic (planar, cyclic & 9 pairs of π e-)
2. Non-Aromatic (planar, cyclic & 4 pairs of π e-)
3. Meta (1,3 relationship)
4. Ortho (1,2 relationship)
5. 2-bromo-1-chloro-4-fluorobenzene
(NOT o-bromo-p-fluoro-chlorobenzene or any other combination!! o, m,
p: Only used for disubstituted benzenes!!)
6. 4-bromo-aminobenzene/p-bromo-aminobenzene
(4-bromoaniline/p-bromoaniline)

(18) session 18 arenes (aromatic hydrocarbons)

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