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Exercise 7 Post Lab Organic Chemistry

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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.

[pic 10]

  • directly donates electron pairs to the ring
  • ortho and para directing
  • para position is more favored due to less steric effects
  • substitution on ortho only occurs when para position is blocked

        

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

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