Experiment 23:
THEORY OF COLOR IN ORGANIC
COMPOUNDS;
PREPARATION AND APPLICATION
OF ORGANIC DYES
Objectives:



To synthesize azo dyes with various aryl
amines.
To study the relationship between the
extent of conjugation and the color
observed.
To study how conjugation and color are
affected by pH.
AZO DYE SYNTHESIS



Arene diazonium salts are formed by the reaction
between an aryl amine and nitrous acid (HNO2).
Since nitrous acid is unstable, it is generated in situ
by mixing sodium nitrite (NaNO2) with cold, dilute
HCl.
Arene diazonium ions are weak electrophiles in
electrophilic aromatic substitution, and will only react
with strongly activating rings.
Ar
NH2 +
primary arylamine
Ar
N
N
NaNO2
+ 2 HCl
Cl
N
N
Cl-
+ 2 H2O + NaCl
diazonium salt
sodium nitrite
-
Ar
OH
+
Ar
N
OH
N
diazonium salt
PHENOL
(a strongly activated ring)
AZO DYE
GENERAL CHEMICAL
EQUATION
OH
Azo linkage
Cl
NH2
N
N
NaNO2
HCl
R
monoamine
OH
N
N
+
R
diazonium
salt
activator
R
azo dye
COMPOUNDS USED IN DYE
SYNTHESIS
amines
NH2
NH2
NH2
H2N
OCH 3
H3CO
NO2
p-nitroaniline
o-anisidine
dianisidine
OH
Na
OCH 3
SO3
O3S
R-salt
activator
Na
COLOR REQUIRES THE
ABSORPTION OF LIGHT

LUMO
hn

HOMO
Ground
State
Excited
State
Molecules absorb Visible or
UV light by promoting an
electron from the HOMO
to the LUMO.
The wavelength of light
absorbed is inversely
proportional to the energy
difference between these
orbitals: E = hc/l = hn
smaller energy gap = lower energy of light required = higher l light absorbed
ELECTROMAGNETIC SPECTRUM
Colored compounds have such extended conjugated
systems that their “UV” absorptions extend into the
visible region.
REQUIREMENT FOR COLOR
CH3(CH=CH)nCH3
n
1
3
5
7
l(nm)
180
250
345
400
Extended Conjugation
(>7 double bonds) is required for color
in organic compounds


For a substance to
have color, it must
absorb within the
400-700 nm region
of the spectrum.
Substances which
absorb near 350 nm
often ‘tail’ into the
visible region and
appear yellowish .
pH AND CONJUGATION



When base is added to a phenol, it
is converted to a phenoxide ion.
The phenoxide ion has lone electrons
which can contribute to the
resonance stabilization of the
aromatic ring.
This makes the energy gap between
the two orbitals smaller, and a
longer wavelength is absorbed,
resulting in a different color than is
observed for a neutral solution of a
dye.
OH
l=210.5 nm
O
l=235 nm
THE COLOR WHEEL
400-420 nm
violet
420-430 nm
indigo
430-490 nm
blue
red 650-750 nm
orange
580-650 nm
An object appears
as the complement of
the color absorbed
(opposite on color wheel)
green
490-560 nm
yellow 560-580 nm
DYES



Dyes are colored compounds which adhere
to the material they color.
All of the various forces of intermolecular
attraction may be involved in fixing a dye
to a fabric: ion-ion, ion-dipole, dipoledipole, London dispersion (van der Waals),
and H-bonding.
The interaction of intermolecular forces
between the fabric and the organic dye
are responsible for the intensity of the
color observed. The stronger the
interaction of IMF, the more intense the
color will appear.
Table 23.1
Experimental Results (Part A)
•Part A involves the preparation of a monoamine
dye and the investigation of color based on
conjugation of the dye at various pH levels.
p-nitroaniline
dye
Acidic
Basic
m-nitroaniline
dye
Acidic
Basic
pH

use pH Hydrion paper!
1
12
color OBSERVED

Color that you SEE!
Y
R

Report entire range of wavelengths!
560-580
650-700

Use color wheel, p. 196!
V
G

Report entire range of wavelengths!
400-430
490-560
approximate
l OBSERVED
(nm)
color ABSORBED
approximate
l ABSORBED
(nm)
Table 23.2
Experimental Results (Part B)
• Part B involves the preparation of a monoamine
dye and a diamine dye, and the comparison of
color based on conjugation.
o-anisidine
dye
color OBSERVED
approximate
l OBSERVED
(nm)
color ABSORBED
approximate
l ABSORBED
(nm)
Dianisidine
dye

Color that you SEE!

Report entire range of wavelengths!

Use color wheel, p. 196!

Report entire range of wavelengths!
SAFETY CONCERNS
CAUTION:
These dyes may be
harmful; avoid contact!
WASTE MANAGEMENT

All dye solutions are to be placed
in the container labeled “Organic
Waste (Dyes)”.
CLEANING



Be sure to clean all glassware with
soap, water, and brush, followed by a
rinse with wash acetone, before
returning to lab drawer!
Be sure all test tubes are clean and
free of dye solution before returning
to lab drawer!
DO NOT return any glassware to lab
drawer dirty or wet!
In-lab Question
(The following question should be answered in laboratory notebook.)

Show the reaction that occurs when pnitroaniline is coupled with “R-salt”. Include
structures for the diazonium salt, as well as
the resulting azo dye structure.
SO3
Na
NH 2
NaNO2
HCl
?
?
+
NO 2
diazonium salt structure
HO
SO3
"R-salt"
Na
In-lab Question
(The following question should be answered in laboratory notebook.)

N
Draw the product structure that would result
from the reaction shown below. Briefly
explain why the dye formed would appear a
different color than that of an acidic dye.
N
OH
NaOH
OCH 3
?