[appleapplebb]

This is a question on a homework set that I'm very confused about. I'm wondering how to design a phenolphthalein derivative that would change color at a lower pH than phenolphthalein (phenolphthalein turns pink at a pH of 8.2), simply by adding or modifying the substituent groups on the phenolphthalein backbone.

[wildfyr]

Well what is it that drives this this color changing reaction? What is the mechanism, and what might ease the forward reaction?

I actually work closely with such dyes, and while there is a fairly straightforward answer to this specific question, its quite interesting system.

[appleapplebb]

I was thinking that as phenolphthalein reacts with a base, it gets deprotonated. When it's fully deprotonated and becomes its conjugate base, that's when the color changes. So to make phenolphthalein change color at a lower pH, my guess is that there would need to be fewer OH groups on the backbone. I'm not sure if that's right though.

[wildfyr]

Youre sort of in the right direction. Think of it in terms of electron richness on the specific rings.

[zarhym]

I guess this will help you designing new molecules.

[appleapplebb]

So... since phenolphthalein is weakly acidic and gets deprotonated to ionize and change color, could I exchange the OH groups for COOH groups instead? that would make the molecule more acidic, since COOH is more acidic than OH so it would deprotonate more readily and ionize at a lower ph. Would this be reasonable?

[Babcock_Hall]

Perhaps, but that is a fairly drastic change.  I would be inclined to try something more modest.

[appleapplebb]

would it be simpler to just remove the H from the OH groups and make the oxygens double bonded to the rings instead? That would reduce the number of deprotonations needed to ionize phenolphthalein, so shouldn't that decrease the pH that it changes color too?

[wildfyr]

Think about maybe leaving all the current functional groups, since they are pretty instrumental to how it works.  what could you add to a specific ring at a different position that would have the desired effect.