Acid Base solubility Question

[victorias]

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Can someone go over step by step how to approach this question?

Since it asks for solubility in HCl (strong Acid), we need to look for a base so it can't be B or C.
Choice D seems tempting

 

[Cawolf]

You have the correct line of thought, but you should recognize that aniline, choice A, is a weak base. It can accept a proton to form the cation anilinium, making it soluble in acid.

While the alcohol in choice D may be tempting, its pKa is around 17, while the protonated amine is around 35.

 

[victorias]

Why do we need a weak base? Since HCl is a strong acid, wouldn't a strong base be more reactive/soluble?
Since we are not given the pka values, I don't know how to approach this intuitively.

 

[Cawolf]

Sure a strong base would work also, but there is not one here.

You should have a basic idea of the basicity of common functional groups. Not numbers, but relative strengths.

 

[aldol16]

Since we are not given the pka values, I don't know how to approach this intuitively.

You don't need a pKa to figure out if aniline or benzyl alcohol is more basic. To a crude approximation, you can draw the conjugate acid and figure out which conjugate acid is more favorable. Have you ever seen a positive charge on an amine, making it NH3? How about on a hydroxide, making it OH2? Which one have you seen more and why do you think it's more favorable?

This neglects the fact that the nitrogen lone pair on aniline is actually in the correct geometry to delocalize into the benzene ring, so it won't be as basic as benzyl amine, for instance, but to a crude approximation, it works.

 

[JoeCaputo]

You should look at each protonatable group, and its pKa. If pH is below the pKa the group will receive a proton and vice a versa. But you have to be careful with which proton and which group, because gaining or losing a proton could make a molecule ionic or organic. You really should only need a basic estimate of these groups.

a.) pKa of RNH3+ is probably 9-11 (on the lower end due to electron withdrawing phenyl group), pKa of RNH2 is ~35. It will be positive in solutions below pH =10, neutral at 10<pH<35, and negative at pH>35
b.)pKa of an aldehyde: 17-20. I'll skip this one.
c.) pKa of COOH is ~4.7. If pH<4.7 it will be in the organic form, if pH>4.7 it will be in the ionic form
d.)pKa of ROH2+ is ~ -1.7, pKa of ROH is ~16. For pH below -1.7 it will be ionic positive, for -1.7<pH<16 it will be in the OH form, and at pH>16 it will be in the anionic form

pH of the 1M HCl solution is zero, so you figure out how many protons will be on each group, and the charge. So a) will be in the NH3+ form, b) will not receive a proton, c) will keep its proton as COOH

d) will not receive an additional proton at pH=0, but it would receive one if the pH was below -1.7

 

[David513]

Here's my crude, rude, dumb way of doing it. Enjoy!

The question asks...
Which is most soluble in a **** ton of HCl, i.e. which will pick up the **** ton of protons floating around easiest, i.e. which is most basic.

I use ARIO (Atom, Resonance, Induction, Orbital) for comparing acids/bases...
Atom: are the atoms different and if so what is the relative electronegativity? Nitrogen vs. oxygen vs. oxygen vs. oxygen ... nitrogen is the least electronegative, i.e. would be the best base, i.e. choice A earns a point. (We should probably stop here but that's no fun.)
Resonance: which (if any) can stabilize the conjugate acid best through resonance? None of the choices can do this. Moving on!
Induction: which experience positive induction? A, B and D. Choice C actually has NEGATIVE induction (bad for acting as a base); that's another 1 point for A, B and D, 0 points for C.
Orbital: A is sp3, B is sp2, C is sp3, D is sp3. That's another point for A, C and D but nada for B insofar as being a good base.

Grand total: A has 3 points, B has 1, C has 1, D has 2, noting that the order in which the points are earned actually affect their value, so the point choice A earned from "Atom" is worth more than the points any of the choices got from "Induction" or "Orbital."

Final answer: A!

Anyway, hope that wasn't too complicated! That's how I think about it 😉

 

[basophilic]

This is not at all a complicated problem. If we have an aqueous strong acid solution, the strongest base of the 4 will be the most soluble. The amine is the strongest base because Nitrogens are less electronegative than Oxygens and hence more likely to donate electrons to a proton.