How to assign a specific pH value to a structural formula?
Hey,
I have a chemistry exam tomorrow (Q11) and I don't really understand this task or how to assign the pH value to the molecules/structural formulas.
Thank you in advance for your answers!
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Well, the pH is defined by the concentration of H3O+ ions – or casually said how acidic or basic is a solution. It can now be seen from the text that something happens to the molecule as soon as a solution becomes increasingly basic. This means that protons are drawn off from the neutral phenolphthalein. Now you just have to think about how that happens.
The structure of phenolphthalein in neutral solution is B (if you know, if you don't, you get it out of the following arguments). This must now lose H+ somewhere at the first stage (pH 8.2). That's what it does at C, you can see at the OH groups that have no Hs anymore. Because the os now lacks an H, they shift their electron density into the rings (so that one O is now double-bound) and the ester group at the bottom has to "loss" (so that the double bond would not fit into the ring). This double bond is also the reason why the color is formed. The rings are now conjugated via this double bond.
If one now makes even more basic (ie there are a lot of OH^- in the solution) an OH^- can now act on the central C and connect there as an OH group, so to speak. The structure A is formed. The double bond is folded over again upwards through the ring to the oxygen. The loss of this double bond then also loses the color.
Thank you for the answer!!