aromaticity in benzenoid and non benzenoid compounds ppt

aromaticity

1. Arom
aticity
Asst. Prof. Miss.Rupali A. Chaudhari.
Department of Chemistry
KCES’s Post Graduate College of Science Technology and
Research, Jalgaon

2. Aromaticity
hydrocarbons
aliphatic aromatic
alkanes alkenes alkynes

3. Aliphatic compounds: open-chain compounds and ring
compounds that are chemically similar to open-chain
compounds. Alkanes, alkenes, alkynes, dienes, alicyclics,
etc.
Aromatic compounds: unsaturated ring compounds that
are far more stable than they should be and resist the
addition reactions typical of unsaturated aliphatic
compounds. Benzene and related compounds.

4. Benzene. This aromatic hydrocarbon was first discovered in 1825 but
its structure was not generally agreed upon until 1946.
Facts about benzene:
a)Formula = C
6
H
6
b)Isomer number:
one monosubstituted isomer C
6H
5Y known
three disubstituted isomers C
6H
4Y
2 known
c)Benzene resists addition reaction, undergoes substitution reactions.
d)Heats of hydrogenation and combustion are far lower than they
should be.
e)From X-ray, all of the C—C bonds in benzene are the same length
and intermediate in length between single and double bonds.

5. a) Formula = C
6
H
6
Max. number of H’s for 6-carbons = 14. Benzene only has 6
hydrogens. Given one degree of unsaturation (double bond or
ring) for every two missing hydrogens less than the maximum,
benzene has 4 degrees of unsaturation; that is four
combinations of pi-bonds and rings.
CH
3
CC-CCCH
3
HCC-CC-CH
2
CH
3
HCCCH
2
CCCH
3
HCCCH
2
CH
2
CCH

6. CH
3
HCCCHCCH CH
2=CHCCCH=CH
2
CH
2
=CHCH=CHCCH CH
2
=C=CHCH
2
CCH
CH3
CH
2
=C=CHCCCH
3
CH
2
=C=CCCH
CH=CH
2
CH
2
=C=CHCH=C=CH
2
CH
2
=C
CCH
CH
3
CH=C=CHCCH

7. CH
2
H
2C
CH
2
=C=C=CH
2
=C=CH
2
CH
2 CH
2
CH
2
=CH
2HC
2=

8. =CH
2
Which of these structures is benzene?

9. KMnO
4 oxidationno reaction
Br
2/CCl
4 additionno reaction
HI additionno reaction
H
2/Ni reductionno reaction
Reagent Cyclohexene Benzene

10. Reactions of benzene:
1.Nitration
C
6H
6 + HNO
3, H
2SO
4  C
6H
5NO
2 + H
2O
2.Sulfonation
C
6
H
6
+ H
2
SO
4
, SO
3
 C
6
H
5
SO
3
H + H
2
O
3.Halogenation
C
6
H
6
+ X
2
, Fe  C
6
H
5
X + HX
4.Freidel-Crafts alkylation
C
6
H
6
+ RX, AlCl
3
 C
6
H
5
R + HX
substitutions

11. e) From X-ray, all of the C—C bonds in benzene are the same length and
intermediate in length between single and double bonds.
C—C single bonds  1.50 Å
C = C double bonds  1.34 Å
The bonds in benzene are all equal and 1.39 Å
but 1,3,5-cyclohexatriene has
three double bonds and three
single bonds!

12. Resonance!
We can draw more than one classic
structure that differ only in where the
electrons are. The two structures are
of the same energy, so resonance is
important. The molecule cannot be
adequately represented by classic
structures but must be thought of as a
hybrid of the contributing structures.
Additionally, the hybrid is more
stable than any of the contributing
structures (resonance stabilization
energy).

13. Facts about benzene:
a)Formula = C
6
H
6
b)Isomer number:
one monosubstituted isomer C
6H
5Y known
three disubstituted isomers C
6
H
4
Y
2
known
c)Benzene resists addition reaction, undergoes substitution reactions.
d)Heats of hydrogenation and combustion are far lower than they
should be.
e)From X-ray, all of the C—C bonds in benzene are the same length
and intermediate in length between single and double bonds.

14. Aliphatic hydrocarbons are open-chain and ring compounds
that react like open chain compounds:
saturated: alkanes and cycloalkanes (typical reaction =
substitution)
unsaturated: alkenes, alkynes, dienes, cycloalkenes (typical
reaction = addition).
Aromatic hydrocarbons are unsaturated ring compounds
that resist the typical addition reactions of aliphatic
unsaturated compounds, instead undergoing substitution
reactions. They are also much more stable than they should
be.

15. Can we predict which compounds will be aliphatic and
which ones will be aromatic like benzene? Yes.
In order to be aromatic, the compound must be:
1)cyclic with p-atomic orbitals on all members of the
ring.
and
2)have 4i + 2 electrons in the p-orbitals of the ring
(where i = 0, 1, 2, 3, …).
[ eg. = 2 or 6 or 10 or 14 or 18 or 22 or 26… ]

16. annulenes: monocyclic compounds with the formula:
-[-CH=CH-]
n
-
HC
HCCH
CH
4 pi electrons 6 pi electrons
8 pi electrons
10 pi electrons
12 pi electrons
aromatic
aromatic

17. aromatic
aromatic
•
•
:
:
4 e
-



4 e
-
5 e
-
does not have
p-orbitals on
all atoms
8 e
-
7 e
-
6 e
-
6 e
-

18. napthalene anthracene phenanthrene
Some other aromatic hydrocarbons: