MCAT Organic Question — Aqueous Solubility | Next Step Test Prep MCAT Organic Question — Aqueous Solubility | Next Step Test Prep

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Which of the following changes would have the largest effect on the aqueous solubility of sodium 3, 3-dimethyl-4-phenyl pentanoate?

 

A) Lowering the pH of the solution below the pKa of the carboxylic acid.

B) Replacing the phenyl substituent with a non-conjugated alkyl ring structure.

C) Increasing the length of the parent carbon chain by an additional carbon.

D) Moving the methyl groups so they are no positioned on the same carbon.

 

Explanation

 

The solubility of this molecule is largely determined by the charged ionic group, which is the conjugate base of a carboxylic acid. By lowering the pH below the pKa of the acid, the molecule will switch to the protonated uncharged form. This will significantly reduce the aqueous solubility of the molecule. Thus, choice (A) is the correct answer.

 

B: Hydrocarbon rings, whether alkyl or aryl have poor water solubility, so this change would not have the biggest impact on solubility.

 

C: While a longer carbon chain will be less water soluble, adding a single carbon will not have a bigger impact than switch from an ionic compound to a non-ionic one.

 

D: Spacing out the methyl groups could increase the overall surface area, thus decreasing aqueous solubility but only by a small amount.

 

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