Halochromism in Phenolphthalein

[gemma786]

It is known that when pH of solution is greater than 12 phph solution turns colorless. And the structural change which occurs is

I want to ask why OH^_ group attack sp² hybridized carbon atom attached to three different rings? How elecrophilic site is generated there? In one of its bonding orbitals it contains phenoxide ion bonded through para position which is supposed to be electron rich site. Wouldn't this bonding sway positive charge created by adjacent benezecarboxalate ion bonded to carbon through ortho postion?

 

[gemma786]

Come on guys! I am still waiting for your response.

 

[Mike H]

You have a central sp² carbon atom bound to three aromatic rings, and you can very easily draw at least one resonance structure for said molecule. Given that it is desperate to fill its octet, I don't find it too shocking to see that said sp² carbon atom would act as an electrophile to the nucleophile of a hydroxide ion in a basic solution, resulting in a sp³ carbon atom.

That's about all the analysis I can bring myself to do, as I still have nightmares about organic chemistry...

 

[gemma786]

Thank you sir for giving consideration to my post.
But I would like to ask is it really so bad to have sp² hybrid state that a nucleophile like hydroxide ion could approach you at a relatively low concentration of about 0.01M inspite of the fact that a powerful +R group like phenoxide ion through its resonating structures would satisfy its hunger for negative charge? Well we don't often see alkenes being so much acidic that it give away a proton to hydroxide ion ? May be that's a bad correspondence? Don't know! Need help!

 

[Mike H]

A pH indicator like phenolpthalein is usually added in very small amounts, as they themselves are - in general - weak acids or bases. This is done so they don't perturb the solution chemistry under investigation. While you might only have a hydroxide concentration in the milliMolar range, your indicator will be at a much smaller concentration so as to only minimally perturb the system.

 

[gemma786]

Now probably I got the point. Phenolpthalein is acidic because it is phenol-pthalein and as the concentration of hydroxide ion increases in the reaction mixture a less stronger base eventually appears i.e.In(OH)^3-.
Is it just as simple as this?

 

[Mike H]

That's correct - phenolpthalein is a weak acid. It's really that the concentration of the hydroxide ion can be considered to be in excess given the trace amounts of phenolpthalein needed to effect a color change. There simply isn't enough indicator to shift the acid-base equilibrium significantly considering how much hydroxide is present.

 

[gemma786]

Thank you sir.
I was previously obsessed by the fact that a nucleophile doesn't attack a site which has pi electrons around it because in alkene chemistry we often see an electrophile attacking at pi electrons. But now it seems that a nucleophile could do so if it has enough of energy to overcome the repulsion and then by collision providing sufficient energy to take substrate to transition state. This is probably the case with phenolpthalein but here hydroxide ions have to break conjugated systems which are forming chromophore.And probably even at low pH(8-12) when we see pink color in solution there are some hydroxide ions which could do so,but they don't produce any significant color change? As the concentration of hydroxide ions increases number of nucleophile possessing the energy to take substrate to transition state increases, color of mixture keeps on fading and eventually disappears as pH goes beyond 12.
Is it what you wanted me to infer earlier from acid base equilibrium?

 

[Mike H]

I didn’t want you to infer anything from my earlier comment about acid-base equilibrium. I thought I had clearly stated that your comment about the concentration of hydroxide concentration being “low” was most likely to be erroneous given the question, as one uses indicators in extremely small amounts. If that was unclear, I should have been more emphatic about the point. If one is examining the behavior of an acid with a base (such as sodium hydroxide), one adds a very small amount of an indicator so that the acid-base chemistry is predominantly due to the acid and base under investigation, not the indicator. If one is working with 10 milliMolar base, then one will probably only need an indicator at, say, 10 microMolar concentration, or thereabouts.

Insofar as the reaction mechanism – certainly, while nucleophilic additions to simple, unpolarized alkenes is generally not seen to my knowledge (simple nucleophilic additions are typically to carbonyl carbons, given the polarity of the C=O double bond), when you’ve got a large molecule like phenolphthalein that is resonance-stabilized and asymmetric about that central carbon (two phenol substituents and a benzoate substituent), the observation of a nucleophilic addition to its central sp² hybridized carbon does not surprise me. After all, it is hardly the same as a simple, unsubstituted alkene. And yes, breaking up the conjugated structure across the two phenol substitutents is certainly why the color fades under strongly basic conditions, barring some unforeseen subtlety I haven’t considered.

Sure, it’s certainly possible that hydroxide ions can and do add to phenolphthalein, generating that colorless structure, under less basic conditions. We are dealing with a bulk sample that is macroscopically observable, after all. But it would also be logical that it would not be very many, and that it is not enough to change the color very much given the state of the rest of the system. It is also likely that such a structure would be disfavored for whatever reason, and would find its way back to the pink structure.

If you feel the need to further pursue this inquiry, I am afraid I won’t be of much help.

 

[gemma786]

Thanks for whatever information you have given me.