TBR OChem Q

[RiPSTONE]

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I don't understand this question and its answer explanation.

Charged compounds are found to be more soluble in water than neutral compounds. At which of the following pH values would a tertiary amine be MOST soluble in water?

A. 3
B. 5
C. 9
D. 11

Answer explanation:
An amine of any substitution, tertiary included, will exist in its protonated state in solutions with a pH value less than its pKa value. When it is protonated, it is cationic and thus more soluble in water than it is in its neutral state. This implies that as the pH of the solution is lowered, the amine becomes more soluble. The lowest pH of the answer choices is choice A, 3, so choice A is the best choice.

If anyone could clarify this for me that would be awesome. Thanks!

 

[Aerus]

Can you specify what exactly you are confused about?

 

[Alucard6]

So first off, you have two types of compounds: Charged and Uncharged (neutral). Water is a polar solvent, and by polar I mean that it has a charge to it because of the electronegative oxygen. The electronegative oxygen attracts the electrons from the hydrogens and thus gives the molecule an overall charge.

I am sure you have heard the phrase but it is key here: "Like dissolves like". So in this case, water is a charged species so it wants to be around other charged molecules. In this case, Amines are basic which means they like to be in high pH values. When the pH is high, they are stable and in a neutral form. However, as the pH drops, the solution becomes more acidic. For acidic solutions, solutions will get protonated because pH solutions are high in hydronium or hydrogen concentrations so they attach to molecules.

So if an amine, which likes HIGH pH, is put into a solution with LOW pH, it will become unstable. As it becomes unstable, it gets protonated and gets a charge. Now that the amine has a charge, it will be more soluable in water than before, when it was neutral. Again this is because water is a charged species and wants to be around other charged species, such as the amine in low pH values that has become protonated.

And FYI: There is a subarea in the MCAT section for where you can post these types of questions. You will get a better response there because people go there specifically to help people.

 

[RiPSTONE]

That was a very thorough explanation. Thanks for the heads up, still new to the community, and lovin' it.

 

[daking]

a base e.g an amine would be most soluble in the more acidic/lowest pH solution.