Chemistry 2213a Fall 2012 Western University
Topic 6. Benzene and Derivatives (Arenes)
Benzene, C H 6 d6scovered by Michael Faraday
in 1825, has 4 units of unsaturation. It is
remarkably stable due to resonance.
Due to resonance, all carbon-carbon bonds are
equivalent. All carbon and hydrogen atoms are in
the same plane due to sp hybridization.
The structure is best described
by molecular orbital theory. Th2
remaining p orbitals of the sp -
hybridized carbon atoms overlap
sideways to form a big molecular
orbital that has a torus-shaped
orbit on each side of the ring. Benzene and Derivatives 2
The carbon-carbon “double bonds” of the
Lewis structure are so stable that benzene H O no
and compounds containing benzene rings 2 reaction
don’t undergo the typical alkene reactions.
Is there a way to estimate how much stability resonance imparts to a benzene
ring? Yes, by measuring the heat (H) of hydrogenation reactions of benzenes
Consider the addition of H to one C=C in cyclohexene:
H 2 H = –120 kJ/mol
So, we expect benzene to be 3 × –120 = –360 kJ/mol, but the measured value
is H = –208 kJ/mol. (Because of the stability of benzene, this reaction
requires extreme conditions for hydrogenation to occur.)
3 H2 H = –209 kJ/mol Benzene and Derivatives 3
Benzene is therefore 360 –
209 = 151 kJ/mol more
stable than a cyclohexane
ring containing 3 C=C is
expected to be.
This increased stability due
to resonance is called
Aromatic is the term used to describe cyclic compounds that contain double
bonds but do not undergo the typical reactions of alkenes. Resonance
stabilization is a central feature of aromaticity.
For a compound to be aromatic, it must meet three criteria:
o Have a p orbital on each ring atom
o Be planar or nearly planar
o Have 2, 6, 10, 14, 18, etc. electrons Benzene and Derivatives 4
Rings with heteroa2oms (atoms other than C or H) can be aromatic. The
heteroatoms are sp -hybridized so that a lone pair can be placed in the
remaining p orbital, which forms a molecular orbital with the p orbitals of the
other atoms. (See “How To 9.1” in the text.)
furan pyrrole Benzene and Derivatives 5
compounds are very
Many aromatic compounds have pleasant smells,
hence the term aromatic. For example,
benzaldehyde (lab #5) smells like almond oil.
However, many of them are toxic and carcinogenic
(e.g. benzopyrene in cigarette smoke).
benzopyrene Benzene and Derivatives 6
Many simple derivatives of benzene have common names.
CH OH NH CHO
napthalene toluene phenol aniline benzaldehyde
COOH N CH
O 2 NO 2
benzoic acid pyridine TNT
When a benzene ring is named as a substituent, it is called a phenyl group. If
the benzene ring is attached to2a CH , the entire group is a benzyl group.
CH 2 CH3 CH 2 CH2 Cl
phenyl group phenylethane benzyl group benzyl chloride Benzene and Derivatives 7
A special naming convention is used for disubstituted benzene compounds.
o 1,2-substituted ortho (o) 1,3-substituted meta (m)
o 1,4-substituted para (p)
CH 3 OH NH 2
m-bromotoluene p-chlorophenol o-chloroaniline
If there are multiple
substituents, the compound
can be named similarly to Cl Br
cycloalkanes. If the
compound is named using a
common name, the H2N
substituent responsible for th1-chloro-4-ethyl- 4-bromo-3-methylaniline
common name is always
assigned position #1. 2-isopropylbenzene 5-amino-2-bromotoluene Benzene and Derivatives 8
D. Reactions of Aromatic Compounds
1. Ability to stabilize cations and anions
Although the aromatic ring is not very reactive, it is able to stabilize adjoining
cations and anions by resonance, as previously seen.
CR 2 CR 2 CR 2 CR 2
CR 2 CR 2 CR 2 CR 2 Benzene and Derivatives 9
2. Acidity of phenols
Resonance stabilization of the oxygen anion makes phenols much stronger
acids than other alcohols. Added electron- withdrawing groups on the ring can
make some phenols as acidic as carboxylic acids.
OH OH O
O2N NO 2
pKa = 15.9 10.0 4.0 4.8
3. Oxidation of benzylic carbon atoms
The bonds of aromatic rings are not easily oxidized (as alkanes are), but
carbon atoms directly attached to the benzene ring, termed benzylic carbons,
are oxidized to carboxylic acids if they are bonded to one or more H.
H 2rO 4r KMnO 4
CH COOH Benzene and Derivatives 10
Oxidation is not affected by other ring substituents. Tertiary (with respect to the
other carbons besides the ring) benzylic carbons don’t oxidize because they
don’t have any H.
H N excess H N
2 KMnO 4 2 COOH
(H3C)3C (H3C)3C COOH
4. Electrophilic aromatic substitution (EAS)
This is characteristic of all aromatic compounds
E is a strong electrophile, usually a cation. It can replace two or more H, but
we’ll only look at monosu