Experiment 1: SYNTHESIS OF METHYL ORANGE
REACTION MECHANISM
Merrene Bright D. Judan (BSIP, 2015-00957)
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.
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.
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.
1. Preparation of diazonium salt of sulfanilic acid: Dissolve sulfanilic acid in basic solution.
2. Formation of nitrosonium ion: In order to obtain the nitrosonium ion (NO+ ), sodium nitrite has to be treated with HCl.
3. Formation of diazotized sulfanilic salt: nitrosonium ion + diazonium salt
4. 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.
Net reaction:
In summary:
Reference:
Altig, J. (n.d.). Diazo coupling: Synthesis of methyl orange. In Chem 334L: Organic chemistry laboratory. Retrieved from
http://infohost.nmt.edu/~jaltig/Diazo.pdf
Long Island University (n.d.). Experiment 17: Preparation of methyl orange. Retrieved from
http://myweb.brooklyn.liu.edu/swatson/Site/Laboratory_Manuals_files/Exp17.pdf