Direct Dyes and Acid dyesBy Dr. Robert D. Craig, Ph.D

N,N-dimethylaniline and HCl and anything anhydrous

• Please look!!!Caution: Conc. acids are corrosive!• 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!

Dyes and Dyeing Prelab Orgo 250- spring 2012-due next lab!!

• Question 1: Operating on the simple hypothesis that the intensity of a dye on a fiber will depend on the number of strongly polar or ionic groups in the fiber molecule, predict the relative intensities of Methyl Orange when it is used to dye a variety of different fibers.

• Such as that found in a multifiber Fabric

Preparation of Methyl Orange

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]

Dyes work by change the surface of a substrate

Direct dyes –usually have sulfur groups

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

Wool-molecular structure

Nylon

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:

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.

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.”

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.

Reaction Scheme

SO3H

NH2

SO3

NH3

SO3- +Na

NH2

SO3- +Na

NH2

SO3- +Na

N N+

Cl

SO3- +Na

N N+

Cl

N N N(CH3)2Na+ -O3S

N(CH3)2

+

-

+ Na2CO32 2 + CO2 + H2O

Sulfanilic Acid (zwitterion) Sodium Sulfanilate

HCl / NO2-

Sodium SulfanilateDiazonium Chloride

+

N,N-Dimethylaniline Methyl Orange

CH3COOH

Diazotization:

Diazotization: need to make this

Mechanism of Diazotization:

O N OH O N OH2+ O N+

NHO3S

H

H

N ON

+HO3S

H

H

N OOH2

NHO3S

H

N O

OH2

NHO3S N O NHO3S N OH

NHO3S N OH2+ N+

HO3S N

-H2OH3O+

nitrous acid nitroso ion

:

+

N-Nitrosoamine

H3O+

Diazohydroxide

H3O+

.. ..

Diazonium (electrophile)

Diazo Coupling:

• Diazo Coupling: N,N-dimethylaniline

Diazo Coupling:

N+

Na+ -O3S N H NCH3

CH3

N N N(CH3)2+Na+ -O3SH

OH2

N N N(CH3)2Na+ -O3S

..+

.. ..

.. .. ..

Methyl Orange (azo dye)

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.

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).

Show ring

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

Electron withdrawing-lower energy barrier-DE – and increase lmax

Electron donating-increase energy barrier-DE – and decrease lmax

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).

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.

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.

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.

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-)

Color Theory

• Anionic (or acid) dyes have –SO3- groups that

form ionic bonds, or salt linkages, with the – NH3

+ groups on the fibers.

Acid-Base Indicator Properties of Methyl Orange

N N N(CH3)2Na+ -O3S

H+

OH2

N N N(CH3)2Na+ -O3S

H

N N(CH3)2Na+ -O3S N

H

.. .. ..

....

+

Resonance

+..

Protonation (H3O+)Deprotonation (OH-)

Methyl Orange (Azo Dye) pH> 4.4 (yellow w/ max = 462 nm)

Helianthin (Protonated Dye) pH< 3.2 (red w/ max = 506 nm)

Acid-Base Indicator Properties of Methyl Orange

What you will do first!!

***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

Please be careful

Procedure

Pretreatment before Diazotization1. 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

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.

•

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.

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.

Isolation of Crude Dye

10. Isolation of Crude Dye11. 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.

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.

Dying Nylon Fabric

• Dying Nylon Fabric12. 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.