Here are the steps to dye nylon fabric with methyl orange: 1. Dissolve 0.5 g of methyl orange dye in 300 mL of tap water in a beaker. 2. Add 10 mL of 15% sodium sulfate solution and 5 drops of concentrated sulfuric acid to the dye solution. 3. Heat the dye bath to boiling. 4. Add the nylon fabric and stir for 5-10 minutes while boiling. 5. Remove the dyed nylon fabric and rinse well with water. Please let me know if you need any clarification or have additional questions!

2. Preparation of Methyl Orange
By Dr. Robert Craig, Ph.D.
Background Reading:
Small Scale Syntheses, pp 363 (azo dyes).
McMurry, J., Organic Chemistry, 7th Ed., pp 500-3
(UV-Vis spectroscopy)
and 941-5 (diazonium ions and their coupling
reactions) [6th Ed.: pp 483-6, 917-21]

3. Direct Dyes and Acid dyes

4. Dyes work by change the surface of a
substrate

5. Direct dyes –usually have sulfur groups

6. Bind to cellulose (fabrics)
Acetylated cellulose-or “Rayon”

7. Wool-molecular structure

8. Nylon

9. Direct dyes –usually have sulfur groups
• Most direct dyes are classified as azo dyes by
chemical structure. Direct dyes usually contain
sulfo groups and are readily soluble in water. A
typical example of a direct dye is direct light
blue:

10. a synthetic dye
• a synthetic dye and has an affinity to cellulosic
fibers. It is believed that direct dyes are
retained in the fibers owing to the presence of
hydrogen bonds and dispersed van der Waals
forces. Hydrogen bonds may form hydroxyl
groups of cellulose and OH groups, NH2
groups, and CONH groups of dyes.

11. Summary
Summary
• The first step is called “diazotization.” Sodium
sulfanilate reacts with sodium nitrite in
hydrochloric acid (i.e., nitroso cation) to form
an unstable “diazonium salt.”

12. Summary
• The second step is the “diazonium coupling
reaction.” The diazonium ion is used in situ,
and reacts with N,N-dimethylaniline to form
the acidic azo dye.
• The crude dye will then be isolated, and used
to create dyed nylon fabric.

13. Reaction Scheme
SO3 H SO3 - S O 3- + N a
2 + N a 2C O 3 2 + CO2 + H 2O
+
N H2 N H3
N H2
S u lf a n ilic A c id (z w itte rio n ) S o d iu m S u lf a n ila te
S O 3- + N a S O 3- + N a
H C l / N O 2- S o d iu m S u lf a n ila te
D ia z o n iu m C h lo rid e
+
N H2 N N Cl
S O 3- + N a
N( C H 3 ) 2
C H 3C O O H
Na+ -O3 S N N N( C H 3 ) 2
+
+
N N Cl
N ,N -D im e th y la n ilin e M e th y l O ra n g e

14. Diazotization:
Diazotization: need to make this

15. Mechanism of Diazotization:
H 3O + -H 2 O
+
O N OH O N O H2 + O N
n itro u s a c id n itro s o io n
H + N O H
O H2
+
HO3S N : HO3S N N O HO3S N N O
H
H H
N -N itro s o a m in e
H2 O
H 3O +
HO3S N N O HO3S N N OH
H 3O +
D ia zo h yd ro xid e
.. ..
+
HO3S N N O H2 + HO3S N N
D ia zo n iu m (e le c tro p h ile )

16. Diazo Coupling:
• Diazo Coupling: N,N-dimethylaniline

17. Diazo Coupling:
.. C H3
+
N a+ - O 3 S N N + H N
C H3
.. ..
N a+ - O 3 S N N N( C H 3 ) 2 +
H
O H2
.. .. ..
N a+ - O 3 S N N N( C H 3 ) 2
M e th yl O ra n g e (a zo d ye )

18. Color Theory
Color Theory
The first real attempt to interpret the color of
dyes in terms of chemical structure was made by
Otto Witt in 1876. He proposed that dyes
consist of conjugated P-bonded systems, called
chromophores, and salt-forming groups, called
auxochromes, which are polar substituents that
modify their colors.

19. Color Theory
• A chromophore is a simple, unsaturated, e-
withdrawing group attached to an aromatic
ring system. The extended conjugation due to
the chromophore decreases DE (and increases
lmax) of the P  P* transition, so that the lmax
is in the visible range. Examples include nitro
(-NO2), azo (-N=N-), carbonyl (C=O), and
nitrile (-CN).

20. Show ring
• Examples include nitro (-NO2), azo (-N=N-),
carbonyl (C=O), and nitrile (-CN).

21. Electron withdrawing-lower energy
barrier- E – and increase max

22. Electron donating-increase energy
barrier- E – and decrease max

23. Color Theory
• An auxochrome is an e- donating group (by P
resonance) attached to an aromatic ring
system. These also change DE and lmax.
Examples include hydroxyl (-OH) and amino
groups
(-NH2,
-NHR, and –NR2).

24. attaching a dye molecule
• The function of a dye is primarily to create a
particular color or hue in association with the
fabric or substrate. Both natural fibers (like
wool, silk, and cotton) and synthetic
substrates (like nylon) can be dyed. Dyeing is
generally carried out in aqueous solution, and
the process of attaching a dye molecule to the
fiber is called adsorption.

25. physical adsorption
• Dyes that are attracted to the textile by
physical forces (physical adsorption, or
physadsorption), and have a direct
attachment to the fiber (chemical
adsorption, or chemisorption), are called
direct dyes.

26. Acid dyes
• Acid dyes, like methyl orange and tartrazine,
are water-soluble anions (anionic dyes) that
form ionic bonds between the dye and the
fiber. Both wool and nylon are amide macro-
molecules that have terminal amino (-NH2)
and acid (-COOH) groups. These can form
ionic bonds with the dye molecules.

27. Color Theory
• As with amino acids, these groups often exist
as zwitterions, that is the molecule has both a
positively-charged group (-NH3+) and
negatively-charged group (-COO-)

28. Color Theory
• Anionic (or acid) dyes have –SO3- groups that
form ionic bonds, or salt linkages, with the –
NH3+ groups on the fibers.
•

29. Acid-Base Indicator Properties of
Methyl Orange
+
H O H2
.. .. ..
N a+ - O 3 S N N N( C H 3 ) 2
M e th yl O ra n g e (A zo D ye ) p H > 4 .4 (ye llo w w / m ax = 462 nm )
D e p ro to n a tio n (O H - ) P ro to n a tio n (H 3 O + )
H ..
..
N a+ - O 3 S N N N( C H 3 ) 2
+
H e lia n th in (P ro to n a te d D ye ) p H < 3 .2 (re d w / m ax = 506 nm )
R e so n a n ce
.. +
N a+ - O 3 S N N N( C H 3 ) 2
H

30. Acid-Base Indicator Properties of
Methyl Orange

31. What you will do first!!

32. ***Carol says-show portal
• Use 1.5 ml of 1 M Na2SO4 not 0.5 mL of 3 M
sodium sulfate
• But use the 15 mL of water and 5 drop of 3 M
sulfuric acid

33. Please be careful

34. Procedure
Pretreatment before Diazotization
1. Dissolve 1.2 g anhydrous Na2CO3 with 50 ml DI H2O
in a 125-ml Erlenmeyer flask using a stir bar.
2. Add 3.6 g anhydrous sulfanilic acid, and heat solution with
a hot water bath until dissolved. (It may still appear cloudy.)
3. Gravity filter if solids are present, then rinse paper with 3-5 ml
of hot water.
Discard filtered solids, and cool filtrate to room temperature

35. Diazotization Reaction
4. Add 1.5 g NaNO2 to filtrate, and stir until dissolved.
5. Add 5 ml conc. HCl to a 400-ml beaker containing
25 ml of (50% / 50%) ice water while stirring.
Caution: Conc. acids are corrosive!
6. Pour sulfanilate solution into 400-ml beaker with HCl
solution, and place beaker in an ice bath. The
diazonium salt will form as a white solid.
•

36. Coupling Reaction
7. Obtain 2.7 g of N,N-dimethylaniline (d = 0.96
g/ml) in a 10-ml graduated cylinder, and
pour directly into the 400-ml beaker. Wash
cylinder with 2.0-ml conc. acetic acid, and add
contents to 400-ml beaker. Stir for 15-20 min
at 20-25oC to ensure complete reaction.

37. Coupling Reaction
Caution: N,N-dimethylaniline is toxic and is
readily absorbed through the skin! Handle only
with gloves! It is also volatile, and needs to
remain completely in the hood until transfer is
complete!
8. Slowly add 30 ml of 10% NaOH. Check pH of
aqueous phase.
9. Add additional NaOH solution, if necessary, until
basic.

38. Isolation of Crude Dye
10. Isolation of Crude Dye
11. Heat to boiling with hot plate for 10-15
minutes. When most of dye is dissolved, add
10 g NaCl (salting out), then cool crude
reaction mixture in an ice bath.

39. Isolation of Crude Dye
• 10. Vacuum filter, and wash dye twice with 10
ml cold, saturated NaCl solution.
• Discard filtrate in sink (~110 ml aq salt waste).
• 11. Remove 0.5 g of dye, and dry remainder in
oven overnight to obtain yield.

40. Dying Nylon Fabric
• Dying Nylon Fabric
12. Place 0.5 g of dye in a beaker with 300 ml of tap
water.
• Add 10 ml of 15% Na2SO4 and 5 drops of conc.
H2SO4, then heat to boiling.
• Caution: Conc. acids are corrosive!
• 13. Add nylon fabric to bath and stir for 5-10
minutes.
• Then, remove dyed fabric and rinse well with
water.