Exercise 7 Post Lab Organic Chemistry

EXERCISE 7

Diazo Coupling
Synthesis of Methyl Orange

Over-all reaction:

Preparation of diazonium salt

[pic 1]

Diazotization

[pic 2]

General Method:

Anhydrous Na2CO3

• deprotonates the –SO3H group
• prevents participation of the –SO3H group in the next steps of the synthesis

[pic 3]

NaNO2 NOT to be added directly to N,N-dimethylaniline

• formation of a side product

[pic 4]

Temperature kept at 0-5°C

• diazonium salts are very UNSTABLE and EXPLOSIVE
• temperature control is done to prevent its decomposition and/or hydrolysis to phenol

Reactions of Diazonium Salts

1. Replacement Reactions

[pic 5]

• Hydrolysis to Phenol

• SN1 type/SNAr (involves an aryl cation)

[pic 6]

• aryl cation forms despite its high energy because of great stability of N2
• driving force: release of N2

• Reaction with iodide

• involves free radical reaction

[pic 7]

1. Coupling Reaction

[pic 8]

Answers to Post-lab Questions:

1. Explain the importance of the –SO3- group in the synthesized dye in dyeing of fabrics.

• it makes the dye more soluble in water through ion-dipole interaction
• -SO3- is polar and becomes attracted to the –OH groups of fibers (e.g. in cellulose)

1. Could acetophenone, benzaldehyde, anisole and phenylacetate serve as coupling components in azo dye synthesis?

• consider the diazonium salt: [pic 9]

• weak electrophile (the pi electron system of the aromatic ring dilutes the positive charge on nitrogen; charge stabilization)
• would only couple with STRONGLY ACTIVATED aromatic compounds

Since coupling is an EAS reaction, there is a need for ELECTRON RELEASING substituents to activate the ring.

        Since acetophenone, benzaldehyde, phenylacetate and anisole contains electron withdrawing groups or weakly activating groups; NO COUPLING will occur