"CHEM-WHIZ(ರಸ-ಬಳಗ)" MMC SIRSI

 ರಸಯೋಗಿ- ಎಂ. ಆರ್. ಎನ್.

The Unity (of structural transformation) in the Diversity (of color change)

WONDER WORLD OF INDICATORS - 

Any graduate student in chemistry will have the thrill of color change as soon as the reaction is over. This is achieved with the use of INDICATORS. Each indicator has a specific transformation of color. Thus an indicator qualitatively (litmus) or quantitatively (methyl orange) is useful and attractive by its shift of color, which is simply the shift of λ in the visible region.

Some indicators are extracted in natural products where as some of them are synthetic. There are acid-base indicators as well as redox indicators. Anyway ionisation as acid itself is an oxidation, since proton is kicked out of the molecule leading to electron rich ion.

Do we have unity in the diverse set of indicators, which do change colour under stipulated condition? Both acid-base indicators and redox indicators have a common mechanism.

Phenolphthalein is an acid, a weak acid. It is represented as HPh in any books. In reality, it is H₂Ph

H2Ph   ⥂     Ph- + 2H+

(Colorless)     (Pink color)

 

Mere addition of free electrons by the removal of a proton adds colour to the structure what a wonder! Invisible electron pairs induce colour changes in the same molecule with charge centres (ionic form).

Phenolphthalein is produced by the condensation between phenol and phthalic anhydride. Two molecules of phenol interact with a molecule of phthalic anhydride

 

 

 

Now watch the change:

        i.            Unionised acid has Benzenoid like structure, and a carbonyl group.

     ii.            Ionised structure has quinonoid structure and carbonyl group.

   iii.            Number of double bonds is same, but the phenolic pi bonds of phenol are converted to carbonyl pi bond in each phenolic groups.

 

The same technique, you will find in all the indicators.  Just verify the difference in absorption of wavelength between the Benzenoid form and the Quinonoid form, is the key factor and locks.

There are many keys in chemistry if you can find them in Unique way of ‘UNITY IN DIVERSITY’

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