The Chemistry of Dyes Part I: The Synthesis of Indigo Dye

The Chemistry of Dyes
Part I: The Synthesis of Indigo Dye
Name:_______________
Period:____
PURPOSE: To synthesize indigo dye in preparation for understanding how and why it dyes fabrics
THEORY: Organic Chemistry is the study of the structure and reactions of the compounds of carbon. Many
of the most useful compounds produced by the chemical industry are organic. From gasoline to plastic wrap to
the building blocks of life, the products of organic chemistry can be found everywhere.
One of the more interesting areas of organic chemistry is the chemistry of dyes. Dyes are typically organic
compounds that absorb light in specific areas of the visible spectrum. Natural dyes have been used for
thousands of years and are typically obtained from various forms of plant life. Indeed, indigo is one of these
dyes as it can be obtained from various plants. The primary plant from which indigo was obtained was called
Indigofere Sumatrana, found primarily in India. Indeed, one of the primary reasons for such vigorous trading
between India and the West was due to the high demand for the rich color that indigo could provide to fabrics.
Indigo belongs to a class of dyes called vat dyes. Vat dyes are perhaps the oldest dyes known, and the
word vat is used to described how the dyes were obtained from plant or other material. After the plant was
cultivated, it was placed in a vessel called a vat to extract the dye. The vat was typically filled with water and
fermentation by bacteria within the plant would extract the dye as a soluble solid. The vat would then be
agitated (usually be men running in the vat!) causing the dye to precipitate out of solution in its characteristic
purple color.
We know that the chemistry of a molecule is based on its structure and distribution of electrons. It is these
qualities that make indigo an excellent dye. But what are the qualities of a good dye? For most people the dye
should produce a desired color, be even, meaning that it colors the material uniformly, and colorfast, meaning
that the dye should not wash out of the clothes (and perhaps stain other pieces of clothing in the wash). We can
apply our knowledge of chemistry to the first question now, and we will be able to answer the other two
questions by the time we actually dye a material with the indigo.
To consider the first question in more detail, let us examine the chemical structure of indigo dye:
What you see above is the way an organic chemist would draw the lewis structure. Note that the carbon atoms
are not explicitly shown. Instead, they are assumed to be at every point in the structure where an atom is not
listed. Note also that unshared pairs are not shown (you will add these later). What we will focus on in class is
the number of double bonds in the molecule, particularly in the areas of the molecule with the six membered
carbon rings. This structure is said to be highly conjugated, meaning that it has many alternating double bonds.
We know that light exists in the form of photons if a molecule absorbs light in a certain portion of the visible

spectrum. The large number of alternating double bonds allows the electrons in the molecule to absorb light in
the visible spectrum. Most dyes will have this highly conjugated structure.
Today we will synthesize indigo dye. Unfortunately the actual reaction is fairly complicated, and we will
not delve into the actual steps of the process. However, the overall reaction can be presented as follows:
The sodium hydroxide added acts as a catalyst for the reaction. After you collect the product, you will do some
simple stoichiometry to determine the limiting reactant and the percent yield of the reaction. One interesting
thing to think about: the product you make is insoluble in water. How then does it dye jeans? This is the
question we will answer in part II of this lab later this year.
MATERIALS:
Large Vial Organic Reaction Vial
Hot water setup with immersion heater
Ice water beaker
Two beral pipets
Copper Wire
10 ml graduated cylinder
Buchner Funnel and vacuum filtration setup
CHEMICALS
o-nitrobenzaldehyde
acetone
sodium hydroxide
Data Table I – Masses of Reactants and Products
Mass of o-nitrobenzaldehyde (g)
Volume of acetone added (ml)
Volume of sodium hydroxide added (ml)
Mass of Weighing boat
Mass of Weighing Boat + Indigo Dye
Cost of 10.0 g of o-nitrobenzaldehyde
OBSERVATIONS AFTER ACETONE IS ADDED TO SOLUTION:

OBSERVATIONS AFTER SODIUM HYDROXIDE IS ADDED TO SOLUTION:
OBSERVATIONS AFTER FILTERING AND WEIGHING:
CALCULATIONS:
1) Based on your pre-lab assignment the night before, rewrite the structural equation given above as a
standard balanced chemical equation showing the actual molecular formulas of each reactant and
product
2) The first thing we need to establish is what is the limiting reactant in this reaction. To do this we need
the number of moles of each reactant. First calculate the molar masses of acetone and o-
nitrobenzaldehyde.
Molar Mass of Acetone:
Molar Mass of o-nitrobenzaldehyde:
3) We now need the mass of acetone. Discover a way to calculate the mass from your data table and a
suitable literature reference. Explain how you calculated the mass (a calculation will suffice) and list
where you obtained your information to help you calculate this mass:

4) Now calculate the limiting reactant in this reaction. Clearly show all work:
5) Finally, calculate the theoretical and percent yield of your sample of indigo. Again, clearly show all
work:
6) Based on your percent yield, how many grams o-nitrobenzaldehyde would you need to start with if you
wanted to produce 1.0 pound of indigo? Explain with calculations
7) Your data table has the cost of 10.0 g of o-nitrobenzaldehyde. Based on this and your answer to 6,
would it be cheaper for you to make 1.0 pound of indigo or buy it from a supplier? (Of course you will
need to find a supplier of indigo on the web. Please include a website I can look up to verify the
supplier price)
CONCLUSION:

PROCEDURE: The synthesis of indigo
Day One
1) Form a lab group of two people and put on aprons, goggles, and rubber gloves!
2) Add approximately 350 ml of distilled water to a 400 ml beaker. Place an immersion heater in the
water, plug it in, and allow the water to come to a boil. DO NOT PULG IN THE HEATER UNTIL
IT HAS BEEN IMMERSED IN THE WATER.
3) At your station you will filled a preweighed bottle vial of o-nitrobenzaldehyde. Record the mass of the
reactant in the data table and remove the label from the vial.
4) Have a member of your group go to the reagent station with your 10 ml graduated cylinder and obtain
2.5 ml of acetone. Record the volume of the acetone in the graduated cylinder and add the acetone to the
reaction vial. Gently mix the solution to dissolve the o-nitrobenzaldehyde.
5) Rinse the graduated cylinder with distilled water and then return to the reagent station to obtain 4 ml of
sodium hydroxide. Add the sodium hydoxide to your reaction vial and quickly seal the vial with the
Teflon cap. Make sure the cap is tight and shake the reaction mixture again.
6) Wrap a small piece (approx. 6 inches) of copper wire around the cap to serve as a handle.
7) Place the sealed vial in the beaker containing the boiling water. If at any time, small bubbles start
coming out of the vial, the cap is not on tight enough. Remove the vial, allow it to cool, and retighten
the cap.
8) After 3-4 minutes, remove the vial from the hot water using the copper handle and place it on the
tabletop. Allow the mixture to cool for 3-4 minutes or until it is cool enough to touch. Vigorously
shake the vial. Tighten the cap again and place the vial back in the boiling water.
9) While the solution is boiling, prepare a Buchner funnel for vacuum filtration by adding a piece of filter
paper to the funnel and gently bathe the filter paper with water. In addition, create an ice water bath
using a Styrofoam cup filled with an ice / distilled water mixture
10) After 10-12 minutes, use the copper wire handle to remove the vial from the boiling water bath and
place on the counter for a few minutes to cool.
11) Place the vial in the cold water bath you created for a few minutes.
12) Carefully open the reaction vial. The contents of the vial are likely pressurized and some solution may
squirt out. This will not be a problem if you open the vial slowly.
13) Complete the final setup of your Buchner funnel (Ask if you need help) and gently pour your mixture
into the middle of the funnel. Rinse the vial with distilled water twice to make sure you have removed
all product. Then wash the product twice with distilled water from your ice bath.
14) Allow the vacuum filtration setup to run for 2-3 minutes. Then remove the filter paper containing your
product and place on a labeled watch glass to dry at least overnight.
Day Two:
15) Find the mass of an empty weighing boat
16) After the indigo is dry, transfer the product to a massed weighing boat. Obtain the mass of the product
and boat and place this mass in the data table.
17) Place your group’s indigo in the provided film canister. Label and give to the instructor to save for part
II of the lab later in the semester.