CHEM 334L

Organic Chemistry Laboratory

Revision 2.0

Diazo Coupling

A Synthesis of Methyl Orange

In this experiment the azo dye methyl orange is prepared by a electrophilic substitution

with arenediazonium salts (diazo coupling).

p-Dimethylamino-Azobenzenesulfonic Acid (Methyl Orange)

Methyl orange is a pH indicator and due to its clear color change it is very often used in

titrations. Methyl orange changes color at the pH of a mid-strength acid and is usually

used in titrations for acids. Unlike a so called universal indicator, methyl orange does not

have a full spectrum of color change, but has a sharper end point.

Space-filling model of methyl

orange

http://en.wikipedia.org/wiki/File:

Methyl-orange-3D-vdW.png

Powder of methyl orange

http://en.wikipedia.org/wiki/File:

Methyl-orange-sample.jpg

Left: Methyl orange in an acidic

solution

Right: Methyl orange in an basic

solution

http://en.wikipedia.org/wiki/File:

Methyl_orange_02035.JPG

Methyl orange is prepared from sulfanilic acid and N,N-dimethylaniline. The first product

obtained from the coupling is the bright red acid form of methyl orange, called

helianthin. In base, helanthin is converted to the orange sodium salt, called methyl

orange.

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Dyes are used to give colors to substances, especially fabrics. Chromopohores, functional

groups that absorb light, give color to these dyes. The most common chromophores are

azo, nitro, and carbonyl groups. Auxochromes, functional groups that increase the

intensity of the color, are also important parts of dyes. The most common chromophores

are hydroxyl, amino, sulfonate, and carboxylate groups.

Azo dyes have a nitrogen to nitrogen double bond as their chromophore. These dyes are

created by taking a diazonium salt and adding it to a strongly activated aromatic system.

In this experiment, you will synthesize methyl orange, an azo dye, by a diazonium

coupling reaction with diazotized sulfanilic acid and N,N-dimethylaniline.

Preparation of the diazonium salt of sulfanilic acid

Although sulfanilic acid is insoluble in acid solutions, it is nevertheless necessary to carry

out the diazotization reaction in an acid (HNO2, nitrous acid) solution. This problem can

be circumvented by precipitating sulfanilic acid from a solution in which it is initially

soluble. The precipitate which is formed is a fine suspension and reacts instantly with

nitrous acid. The first step is to dissolve sulfanilic acid in basic solution.

Formation of nitrosonium ion

In order to obtain the nitrosonium ion (NO+), sodium nitrite has to be treated with

hydrochlorid acid. During the addition of the acid, the sulfanilic acid is precipitated out of

solution as a finely divided solid, which is immediately diazotized (see below).

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Formation of diazotized sulfanilic salt

Addition to N,N-dimethylaniline

The finely divided diazonium salt is allowed to react immediately with dimethylaniline in

the solution in which it was precipitated.

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Methyl orange is often used as an acid-base indicator. In solutions that are more basic

than pH 4.4, methyl orange exists almost entirely as the yellow negative ion. In solutions

that are more acidic then pH 3.2, it is protonated to form a red dipolar ion.

Due to these properties, methyl orange can be used as an indicator for titrations that have

their end points in the pH 3.2 to 4.4 region. The indicator is usually prepared as a 0.01%

solution in water. In higher concentrations in basic solution, of course, methyl orange

appears orange.

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Pre-Lab Questions

1. Look up the structure of Alizarin Yellow. How might this Azo Dye be prepared in a

Diazo coupling reaction? Alizarin Yellow is a pH indicator. What are the colors of

the acidic and basic form of this indicator?

2. Why does the dimethylaniline couple with the diazonium salt at the para position of

the ring?

3. The diazonium coupling reaction is an electrophilic aromatic substitution reaction.

Give a mechanism that clearly indicated this fact. Show all the important resonance

contributors.

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Procedure

Diazotized Sulfanilic Acid

1. Dissolve 0.29 g of anhydrous sodium carbonate in 12.5 mL of water. Use a

50 mL Erlenmeyer flask.

2. Add 1.0 g of sulfanilic acid monohydrate to the solution and heat it until it

dissolves. A small amount of suspended material may render the solution cloudy.

3. Gravity-filter the still hot solution and rinse the used filter paper with a little (0.5 -

1.0 mL) hot water.

4. Cool the filtrate to room temperature, add 0.375 g of sodium nitrite, and stir

until solution is complete.

5. Pour this mixture, while stirring, into a 100-mL beaker containing 8 mL of ice

water to which 1.25 mL of concentrated hydrochloric acid have been added; add

the HCl dropwise to maintain a temperature of 0-5oC. The diazonium salt of

sulfanilic acid should soon separate as a finely divided white precipitate. Keep

this suspension cooled in an ice bath until it is to be used.

Methyl Orange

6. In a test tube, mix together 0.7 mL of dimethylaniline and 0.5 mL of glacial acetic

acid.

7. Add this solution dropwise to the cooled suspension of diazotized sulfanilic acid

in the 100-mL beaker. Stir the mixture vigorously. In a few minutes, a red

precipitate of helianthin should form. Keep the mixture cooled in an ice bath for

about 15 minutes to ensure completion of the coupling reaction.

8. Add 7.5 mL of a 10% aqueous sodium hydroxide (NaOH) solution. Do this

slowly and with stirring, as you continue to cool the beaker in an ice bath. Check

with litmus or pH paper to make sure the solution is basic. If it is not, add more

base.

9. Heat the mixture to boiling with a Bunsen burner for 10 to 15 minutes to dissolve

most of the newly formed methyl orange. When all (or most of it) the dye is

dissolved, add 2.5 g of sodium chloride, and cool the mixture in an ice bath. The

methyl orange should recrystallize. Filter using a Buchner funnel.

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10. To purify the product, transfer the filter cake and paper to a large beaker

containing about 40 mL of boiling water. Maintain the solution at a gentle boil for

a few minutes, stirring it constantly.

Note: Not all the dye will dissolve, but the salts with which it is contaminated will

dissolve.

11. Remove the filter paper and allow the solution to cool to room temperature. Cool

the mixture in an ice bath, and when it is cold, collect the product by vacuum

filtration, using a Buchner funnel. Allow the product to dry, weight it, and

calculate the percentage yield.

Note: The product is a salt. Since salts do not generally have well-defined melting

points, the melting-point determination should not be attempted.

Spectroscopy

1. Obtain a NMR spectrum of the product. Consult with your laboratory instructor

about how to do this.

2. Assign all the NMR peaks in the spectrum.

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Post-Lab Questions

1. Give the products of diazo coupling of benzenediazonium chloride with each of the

following molecules:

(a) Methoxybenzene

(b) 1-Chloro-3-methoxybenzene

(c) 1-(dimethylamino)-4-(1,1-dimethylethyl)benzene

Hint: Diazo couplings are quite sensitive to steric effects.

2. Look up the structure of (a) Prontosil and (b) Congo Red.

Show the reagents that would be necessary for the synthesis by diazo coupling of

each of the two compounds.