Report for: Synthesis Of An Azo Dye For The Dyeing Of Materials

the report is typed report for this experiment should follow the following guidelines: 1. The entire report should be typed with 1.5 line spacing, including reaction schemes
for the reactions, sample calculations, and organic chemical structures. a. Learn how to make subscripts and superscripts with your word processing
program. b. Learn to use MarvinSketch. The synthesis scheme(s) MUST be your own
work. Do not share a scheme with someone else. You should install the
program on your personal computer by Downloading Marvin Beans at: If this isn’t possible, the
software is available on a few computers in the chemistry department. c. The font size should be 12, and the pages should have 1” margins. 2. The title page should show the title of the experiment, your name and lab section, the
names of your partners, the dates the experiment was conducted, and the course name
and number. 3. The pages should be numbered starting on page 2. The numbers should appear at the
top right or the bottom center of the page. 4. Be sure to use correct grammar, including complete sentences. Use a “neutral voice”:
avoid the first person, except when very awkward. Since you will be discussing an
experiment you have already done, all narrative should be in the past tense. 5. Always use your own words! Do NOT work on this report with your partners or any
other classmate. Likewise, do not copy material out of the laboratory manual or any
other source. 6. Use appropriate headings to separate the sections of the report. The separate sections
should include the following: Introduction: A brief statement about the experiment. Provide an introduction to dyes. Describe the reaction sequence in words (but remember this is not the place for
the procedure), and then give complete balanced equation(s) for all reactions
which occur, using your own compounds as the starting compound. Be sure to
use a chemical drawing program NOT molecular formulas. Be sure your
introduction is a fluid introduction to your experiment. Procedure: Present the complete procedure YOU used in the synthesis of your azo dye and
the following analysis and applications. Include observations and also discuss
any problems you encountered. This should be in paragraph format NOT an
outline. Results and Conclusions: Use boxed, clearly-labeled tables with grid-lines to present all of the results of
your synthesis and the data obtained. Be sure to include appearance of the
product, appearance of your dyed cloth (picture would be ideal), appearance of
your dyed (hopefully) crystals, and the absorption maximums for each solution.
Include all of your group’s UV-Vis spectra, a picture or scan of your fabric, and a
picture or scan of your crystals in the body of your text. Treat these images as
figures in your text.
Then provide a brief discussion of your results. Be sure to include a discussion of
the correlation between fabric results and the chemical nature of each fabric. Be
sure to discuss your acid-base results and their relationship to the suspected
structure of your dye. Also, incorporate the acid-base properties of your dye into
the discussion of fabric results. Finally, discuss what colors the acidic and basic
forms of your dye absorb? References: References should follow the format:
Author, Title of publication, Publisher, Date, Page numbers
For denoting specific references (for example, a passage being quoted), use a
superscript or number in parentheses after the particular passage, and then number
your references accordingly and in sequence at the end of the report.
List all sources, including the lab text.

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Experiment 11
Synthesis Of An Azo Dye For The Dyeing Of Materials
Required Reading
Zubrick: Chapter 4 and 5
Dyes and Color
In this lab you are going to synthesize an azo dye. You may not have thought about it
before but; what is it about a dye that makes it colored? Dyes are organic molecules that
selectively absorb wavelengths of light within the visible range of the electromagnetic spectrum
(400-800 nm). The human eye responds to wavelengths within this range. The white light we
receive from the sun contains all the wavelengths within the visible range. When an object
absorbs a particular wavelength, we see the wavelengths that are left over, and the object appears
colored. Filtering orange light out of “white” light, for example, results in a blue-green (cyan)
hue. The hue resulting from the removal of a color from white light is the latter’s
complementary color.
Complementary Colors
Color absorbed
Wavelength absorbed (nm)
Color observed
Cyan (green-blue)
So now what determines the wavelength absorbed? The color in dyes is the consequence
of the presence of a chromophore. Chromophores in dyes are generally large systems of
conjugated double bonds (alternating double single bonds). It is this delocalized electron system
that absorbs the energy from the light. For example, if the electrons in the dye require only a
small amount of energy to be rearranged into a new energy state, then the substance absorbs a
low energy wavelength (l). The longer the wavelength the lower the energy (E = hc/l). If you
look at the table above, you will see that the longest wavelength is associated with the absorption
of red light. If the incident light is white and red light is absorbed, then the light reflected is
perceived as green (the complementary color of red). If a lot of energy is required for the
electrons to be promoted to a higher energy state, then it absorbs only short wavelength light,
since short wavelengths correspond to high energy. If it absorbs blue light, then the light it
reflects is perceived as yellow. In general, the more conjugation (more double bonds) you have
in a dye the less energy it takes to excite the electrons.
But there is more to it than that. While the chromophore is the color-producing portion
of the dye molecule there are other factors. Dyes also contain auxochromes, which are a group
of atoms attached to a chromophore that modify the ability of that chromophore to absorb light.
In general, auxochromes influence the intensity of the dye; but they can also provide a site by
which the dye can chemically bond to the fabric.
Examples of chemical groups that are chromophores and auxochromes are shown below.
Aryl groups
Double bonds
Azo groups
Alkyl groups
hydroxyl groups
amino groups
sulfonate groups
In this lab you are going to synthesize a dye that contains both aryl and azo functionalities (see
above). To look at the impact of conjugation on the wavelength absorbed, let’s look at two
commercially available azo dyes.
Sudan II
?max=493 nm
Sudan IV
?max=520 nm
Notice that Sudan IV has a more extensive system of conjugation and thus absorbs a longer,
lower energy wavelength.
Life would be simple if we could just look at conjugation and predict the color of our
dye, but the color we see is also dependent upon other factors such as: the auxochrome, what the
dye is bound to in the fabric, the pH, – the list goes on. As part of this lab you will look at the
effect of a change in the pH on the color of the dye, and sometimes it is very dramatic. This is
generally due to a change in the charge of the dye molecule or a change in the level of
conjugation. Adding or subtracting auxochromes can also effect the electron delocalization and
thus change the color. Therefore, much of the work done with dyes can be considered “trial and
error”. You just try something and see what color you get. In essence, this is what you are going
to do in lab.
In this experiment you will use various organic amines to synthesize your azo dye. Once
the dye is synthesized you will observe the effect of pH on the color, use your dye to color a
variety of fabrics, and you will try to incorporate your dye into a crystal of potassium dihydrogen
phosphate (KDP). You will also quantify the type (wavelength) and amount (molar absorptivity)
of light absorbed by your dye.
Azo Dye Synthesis
Azo dyes, which were developed in the mid 1800s, are one of the most common dye
materials. They contain the basic structure of Ar-N=N-Ar. Their color is due to the high level
of conjugation that extends through the N=N bond to the aryl unit.
Azo dyes are synthesized via the following reaction. A primary amine (R-NH2) is
converted to a diazonium salt and this is reacted with another aryl unit.
primary amine
diazonium salt
activated aromatic
azo dye
The aromatic ring can be substituted with different functional groups (auxochromes) and
these substituents, due to their conjugation with the azo system, will affect the color of the dye.
In this experiment you and your group will be assigned a dye to synthesize from the various
amines and activated aryl compounds shown on the next page. Different combinations will lead
to different colors.
Designing your Dye: Your group will be assigned two compounds from the list on the
following page. The first will be a primary amine (R–NH2). This is the compound you
will react with sodium nitrite in the first step of the reaction. The second compound will be
the activated aromatic used in the second step of the reaction. Each member of the group
will perform their own synthesis. In other words, each member of your group will
synthesize the same compound. This will allow your group to have enough dye material to
complete the experiment as a group (Parts B-D).
Your dye has a visible color indicating that it absorbs light in the visible region (400nm –
800nm) of the electromagnetic spectrum. Many of the dyes being synthesized in this experiment
have acidic and/or basic functionalities in their structure (–SO3H, –OH, and –NH2). The addition
of an acid or base to a solution of these dyes will alter the number of electrons available for
conjugation, and hence may alter the color and/or color intensity of the dye. You will take a UVVis spectra of your dye in acidic and basic solutions and determine the wavelength of its
maximum absorbance in each solution. You will also determine the intensity of that absorbance
as reflected by its molar absorptivity (e).
6-amino-2-naphthalenesulfonic acid
mw 223
sulfanilic acid,
sodium salt
mw 195
8-amino-1,5-naphthalenedisulfonic acid
(mono sodium salt)
mw 325
mw 127
mw 172
mw 121
1-amino-8-naphthol2,4-disulfonic acid
mono sodium salt
mw 341
2-naphthol-6-sulfonic acid
sodium salt
mw 246
4-aminonaphthalene sulfonic acid
mw 223
mw 138
2-naphthol-3,6-disulfonic acid
disodium salt
mw 348
1-nitroso-2-naphthol-3,6-disulfonic acid
disodium salt
mw 377
Warning: Amines are, in general, toxic compounds; avoid skin contact.
Dyeing Crystals
Virtually every civilization has developed or appropriated technologies for dyeing animal
or vegetable fibers. Human beings desire the sensation of color.
On the other hand, we do not often practice the dyeing of crystals. The reason for this
distinction seems plain: fibrous materials such as wool, silk, cotton, wood pulp, or papyrus have
large surface area to mass ratios compared with polyhedral crystals. But, the surface area of a
growing crystal, the sum of the surface areas after the accretion of each new ionic or molecular
layer, is enormous. For instance, the surface area of a 1 cm3 cube is only 6 cm3 but the surface
area computed as a sum following the addition of each new 10 Å layer to a 10 Å3 seed is 109 cm3.
As a matter of fact, one can find in the crystallographic literature of the last 150 years,
odd examples of simple transparent crystals that have been stained by dyes during growth from
solution. When dyes express different affinities for the facets of a growing crystal they produce
striking patterns of color that are often determined by the host crystal’s symmetry. Since many
faces are pair wise related to one another in centrosymmetric crystals, one frequently observes a
“bow-tie” pattern. Shown below is a typical example, the mineral gypsum (CaSO4 • 2H2O)
stained by a dye called eosin. For obvious reasons, these crystals have often been called
hourglass inclusions.
Dye-colored crystal polyhedra naturally attracted the attention of scientists of past
generations. But, they were more than artificial gems. Since neither the presumed shape nor the
constitution of the dye molecules were similar to the host molecules or ions, dyed crystals
seemed to violate everything that we assume about crystal growth as a method of purification.
How did a dye molecule adopt an oriented position within an otherwise close packed single
crystal? How was the crystal lattice able to accommodate these seemingly obtrusive impurities?
Researchers have shown that molecules in solution – even molecules that are very
different from the crystal molecules and ions – can arrange themselves on a growing crystal
surface so that they make specific non-covalent bonds. If these interactions are strong enough,
and crystal growth is fast enough, the crystal can actually grow around and entomb the impurity.
However, this only works for molecules with particular structures that match structural features
of the growing crystals.
In part C of this lab, you will attempt to grow dyed crystals of KH2PO4 (potassium
dihydrogen phosphate) with azo dyes that you have synthesized. These dyes tend to selectively
stain the pyramid faces (called {101} in crystallographic language). See below. We will then
collectively determine which azo dyes worked and which did not. From this information we will
hope to develop an understanding of the structural features of the dyes that strongly bond to the
KH2PO4 surfaces. We don’t know the answer. THIS IS RESEARCH!
Dyeing Fabric
In the next part of the experiment (part D) you will use your synthesized dye to color a
special swatch of fabric. The swatch is woven such that it contains bands of some of the more
common fibers used in making clothing. The fibers included are both natural fibers and
synthetic. Here you will explore how the interaction of the dye with the fiber affects the color.
The fibers contained in the swatch are shown below.
Fabric type (starting a end with black thread)
Arnel (bright)
Bleached cotton
Creslan 61
Dacron 54
Dacron 64
Nylon 6.6
Orlon 75
Spun silk
It is usually possible to tell which end is wool, because it is fuzzy. Note the color of each fabric
after the fabric has been rinsed.
Try to make some correlations between the dying results and the chemical nature of each
fabric. Consult your lecture text for basic structures. Cotton is pure cellulose, and acetate rayon
is cellulose in which a few of the -OH groups have been acetylated. Nylon is a polyamide made
by polymerizing adipic acid and 1,6-diaminohexane, and polymerizing ethylene glycol and
terephthalic acid makes Polyester. Acrylic is a polymer of acrylonitrile (propenenitrile). Finally,
Wool is a polypeptide (a polymer of amino acid residues), with its chains crosslinked with
disulfide bridges.
Part A. Synthesis of Azo Dye
Step 1–Formation of the diazonium salt: Add 10 mL of water and 2 mL of
concentrated phosphoric acid to a 50 mL Erlenmeyer flask with spin bar. Cool the solution
in an ice-water bath and stir. To make the ice-water bath, fill a large (big enough to hold the
50 mL Erlenmeyer flask) beaker with ice then add water to the beaker. Be careful! The
flasks have a habit of turning over in the beaker and ruining your experiment. Add 1 mmole
of the amine to be converted into a diazonium salt. Let the solution stir at or below 5o C for
10 minutes.
While the above solution is cooling, prepare a solution of 0.069 g of sodium nitrite in
1 mL of water in a test tube. Chill this solution in your ice bath for about 5 minutes. Add the
sodium nitrite solution to the Erlenmeyer flask (a color change may occur). Let this solution
stir at approximately 5o C for 10 minutes.
After 10 minutes, test the reaction solution for excess sodium nitrite using starchIodide paper*. If the paper immediately shows a dark blue/black spot then this means that all
of the amine has been converted to the diazonium salt. If the paper does not turn black then
add another 3-4 drops of sodium nitrite solution and let stir for another 5 minutes.
*Dip a stir rod into the reaction vial and then touch the capillary onto a piece of
Starch Iodide paper and let the solution bleed onto it.
Once the solution tests positive on the starch iodide paper you are ready to add the
activated aromatic compound, which is the second component of your dye.
Step 2–Addition of the activated aromatic compound: Add 1 mmole of the
aromatic amine or phenol to the diazonium salt solution. Stir at approximately 5o C for 5
minutes (There may be a color change). At the end of 5 minutes let the solution stir at room
temperature for 30 minutes. The solution will slowly turn color over time. Place 0.5, 1, or
1.5 mL (vary this amongst your group) of the solution in a small test tube (test tube #1) for
use in the crystallization of KDP (Part B). Place another 1 mL of the solution in another
small test tube (test tube #2) for the dyeing of the fabric (Part D). Then evaporate the solvent
in a small beaker using a hot plate set to 100°C until about 0.5mL – 1.0mL of solvent
remains. Filter the liquid and collect the solid dye material.
Part B. Incorporation of Dye into Crystals of Potassium Dihydrogen Phosphate (KDP)
In this section you will attempt to dye a crystal of KDP with your synthesized azo
dye. It is unknown whether your dye will incorporate (color) a KDP crystal. So, observe
very carefully.
Crystallization is an art. Crystal growth is effected by an array of subtle factors;
concentration of the solution, amount of heat used to dissolve, rate of cooling, rate of
evaporation, and stillness of the crystal solution all can make a critical difference in the shape
and beauty of the crystals you grow.
For this lab it is important that you get your KDP to dissolve by heating at low
temperature. It is also important that once you have dissolved your KDP and added the
solution to the crystallizing dish that you do not move it–it is important that the solution
remains still (and to be on the safe side still your mind as well)
Incorporation of your dye. Label your crystallizing dish with your name and
section. Eventually you will pour your crystallization solution into this dish (Once this has
been done it cannot be moved).
Preset the heat knob of your stirring hot plate to 90°C and let it warm up.
Weigh 17 grams of potassium dihydrogen phosphate (KDP), and transfer into a 250
mL beaker. Add 50 mL of distilled water and a magnetic stirrer.
Cover the beaker with a watch glass and place the beaker on the hot plate. Stir the
solution with the heat set at 90°C 0until all solid has dissolved (if the KDP does not dissolve
after 10 minutes you may turn up the heat a little).
Once the KDP has dissolved add 0.5, 1, or 1.5 mL (vary this amount amongst your
group members) of the reaction mixture from test tube 1 of Part A. Stir this solution for 1
minute (be ready to do the next step immediately after the minute of stirring).
While the solution is still warm, carefully pour it (but not the stir bar) into a
crystallizing dish, immediately cover the dish, and place it in the designated area. You will
recover the crystals using a Büchner funnel during the next lab meeting.
Incorporation of Chicago Sky Blue or Amaranth Red Dye. In a 250 mL beaker
labeled with your name, dissolve KDP in the same manner as described above. Once
dissolved add 3 mL of 1mg/mL solution of Chicago Sky blue dye solution or 4 mL of
1mg/mL of Amaranth Red instead of your dye. Once the dye has mixed for one minute,
carefully pour the solution (not the stir bar!) into a crystallizing dish, cover the solution with
a watch glass, and quickly and carefully place this solution in the space designated for crystal
Amaranth Red
Chicago Sky Blue 6B
Part C. Visible Spectrum (and Beer’s Law) and Indicator Action
Allow your product to dry until the next lab period so you can weigh it accurately.
Dissolve 5.0 mg of the product in 100.0 mL water (measure carefully with a graduated
cylinder). Take 25.0 mL of this solution and add a couple of drops of 6 M HCl to it (until the
solution is red to litmus). Take another 25.0 mL sample of the dye solution and add a couple
of drops of 6M NaOH to it (until it is blue to litmus). These three samples are now ready to
analyze with the scanning UV/Vis spectrophotometer. Follow the instructions with the
instrument to obtain a spectrum of each sample, from 300 to 800 nm. What color(s) is/are
absorbed by each solution? Compare these colors with the colors of the solutions. It may be
useful to place one drop of each solution on a piece of filter paper to assist the noting of
solution color (you may need to spot more than once in order to see the color).
Calculate the molar absorptivity (e) for each solution at the wavelength of its
maximum absorbance (your volume and mass measurements were not super-precise here, but
you can still get rough values for e). A reminder from General Chemistry: Beer’s Law gives
the relationship between absorbance and concentration:
A = elc
In this equation, A is the absorbance, e is the molar absorptivity, l is the thickness of
the sample (1.00 cm for the sample cells we will be using), and c is the molarity of the dye
compound in the solution.
Part D. Dyeing of Fabric
Combine the 1 mL from test tube #2 from Part A with 10-15 mL of distilled water.
Obtain a piece of multibanded fabric and submerge the fabric into the dye solution and gently
heat (~50°C) on a hot plate for 10-15 minutes (cover with a watch glass to avoid
evaporation). If you do not have enough product to prepare this solution, borrow some
product from one of your group members.
Once you are done heating, rinse the fabric with water. In some cases the dye color
and intensity will change once the fabric has dried out. Attach the fabric to your report.
Warning: Be careful not to touch your dye solution or the unrinsed cloth without
using gloves or tweezers. Your azo dye may be toxic and/or a skin irritant. It is okay to
handle the cloth after it has been rinsed.
Final Report
Your final typed report for this experiment should follow the following guidelines:
1. The entire report should be typed with 1.5 line spacing, including reaction schemes
for the reactions, sample calculations, and organic chemical structures.
a. Learn how to make subscripts and superscripts with your word processing
b. Learn to use MarvinSketch. The synthesis scheme(s) MUST be your own
work. Do not share a scheme with someone else. You should install the
program on your personal computer by Downloading Marvin Beans at: If this isn’t possible, the
software is available on a few computers in the chemistry department.
c. The font size should be 12, and th …
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