Chemistry Problem

Inky Pinky Ponky

Sonal, an industrial chemist, synthesised the xanthene derivative A as an attempt to create a new artificial textile dye. On treatment of A with phosphorus oxychloride, the compound B was obtained, which afforded compounds C, D, and E on treatment with hydrazine, 2-aminoimidazole/triethylamine and 2-methylimidazole/triethylamine respectively.

1.Give a plausible mechanism for the synthesis of A in the scheme given below.
2. Give the structures of B, C, D, and E.

Sonal observed that A is pink at pH 5.0 and turns colourless when the pH is increased to 8.0.
3.Give a plausible reason for the observed reversible colour change with varying pH.

Next, Sonal experimented with adding 10μM aqueous solutions of Cu²⁺ and Hg²⁺ salts to 1mM acetonitrile/water solution of C, D, and E (maintained at pH 5.0). On adding Cu²⁺ solution to the colourless acetonitrile/water solution of C and D, both the solutions turned pink within 1 minute. But adding Cu²⁺ to E did not lead to any colour change. However, addition of a Hg²⁺ salt to the solution of C and D led to a pink colour appearing only after stirring for 50 hours.

Puzzled by the observations, Sonal measured the absorption spectra of A, C, and D at pH 5.0. She found that A has an absorption maximum at λ_max = 498 nm, while C and D have absorption maxima at λ_max = 320 nm and 345 nm respectively. After 1 minute of adding Cu²⁺ to C and D, another peak appeared at λ = 498 nm.

4. Help Sonal explain why the neutral solution of C & D turns pink after the addition of Cu²⁺? Also comment on why Hg²⁺ takes more time for the colour change than Cu²⁺, when added to the aqueous solution of C & D.
5. Give a plausible reasoning to explain why the addition of Cu²⁺ to E does not lead to any colour change, while D turns pink.