Quinonoid Theory, Resonance Theory, Valance bond Theory

1. Dr. Dhaval Desai
Asst. Prof.

2. On the basis of this theory it could be possible to explain the colouring properties of the important compound
The ARMSTRONG theory can be consider as an extension of chromophore theory & hence this called “QUINONOID THEORY”
The Quinonoid ring is only the combination of the chromophore that is “=C=C” & “=C=O”
Thus, According to this theory the aromatic compound which can change in to quinonoid structure are coloured
This was based on the observation that benzene is colourless where as benzoquinone is yellow coloured
The Colour of the compound is due to the presence of qunOnoid type structure

3. O
O
O
O
Benzene O-benzoquinone P-benzoquinone

4. e.g.
(i)
Benzene is colourless where
as benzoquinone are coloured
(ii)
When Phenolphthalein is
present in benzoid from it is
colourless but it’s sodium salt
can be assumes quinonides
structure & hence it is
coloured in alkaline Medium.

5. C
C
O
O
OHOH
C
C
O
O
NaOONa
NaOH
Phenolphthaliene
Sodium salt of
Phenolphtheline
C
C
O
-
Na
+
O
OO
-
Na
+
P-Quinonoide
Structure

6. -: LIMITATION :-

7. NHO NHNH
Iminoquinone Diminoquinone

8. ( 2 )
There are large number of coloured but they can
not be represented by quinonoide structure
N=NCH3 C C CH3
O O
Diacetyl (Yellow) Azobenzene (yellow)

9. Velence bond theory
Or
Resonance theory

11. (i)
Chromophores are the group of
atoms , the p-electrone of which make
get transtermed from ground state to
excited state by the absorption of
radiation which increase resonance.

12. (ii)
Auxchrome are the group
which tend’es in increase resonance by
intecting the unshared pairs of electrone
nitrogen & oxygen atom of auxochromes
with the p- electrons of the aromatic
ring.

13. This increase inresonance
increase the intensity of absorption of
light and also swifts the absorption bend
to longer wavelength

14. Hence, this occurs the
depending of the colour from this it is
evidence that increase in resonance must
deepen the colour

15. (iii)
The diapolemoment change is a result of oxillation of electrone pair
the following order has been observered for the excitation of
different group
-N=O > =C=S > -N=N- > =C=O > -C N > =C=C= .
The coloured deepen in the opposite direction.

16. Wavelength
Hypso-
chromic
Hyper-
chromic
Batho-
chromic
Hypo-
chromic

17. (iv)
Reasonance theory the
chromophore is considered as an electron
withdrawing group and auxochrome as an
electron donating group

18. When they are conjugated
through double bond then electron move
from auxochrome to chromophore

19. This increase the resonance and
also couse the change in diapole moment
& hence intencity of colour is inceased

20. A Simple the resonance theory
is that benzene is colourless while nitro
benzene is pale yellow and p-nitro aniline
is dark yellow this can be explain as
under

21. CH
+
CH
-
( I ) ( II ) ( III )

22. It’s contribution is relatively
sammler to either ground state or exited state .
So, benzene absorbe light in U.V. region

23. Benzene is symmetrical & therefore
the absorption is very weak hence it is colourless.

24. In nitro benzene charge structure
contributed much more & therefore absorption is
strong which produce pale yellow colour.

25. N
+O
-
O
N
O
-
O
-
N
O
-
O
-
CH
+
N
+O
-
O
-
N
+OO
-

26. N
+ O
-
O
NH2
N
O
-
O
-
NH2
+

27. The dipole moment of nitrobenzene
is 0.21 D for aniline is 1.48 D and p-nitro aniline
6.1 D. it also prove that nitro aniline occur
relatively high constribution to the charge
structure of reasonce.

28. This is a liner relation between the
number of double bonds & increase in wave length
of absorption hence there is a strong
bethocromic swift & colour goes to deepening
with increase in double bond.