Blueprint MCAT Formerly Next Step
Return to Blog Homepage

MCAT Organic Question — Aqueous Solubility

  • by John
  • Dec 12, 2014
  • MCAT Blog, MCAT Organic, MCAT Question of the Day

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.

 

Want more MCAT practice?

We’ve got options for every schedule and learning style!

From the best online MCAT course created by top instructors with 524+ MCAT scores to the most representative full-length practice exams and private tutoring, we can custom tailor your MCAT prep to your goals!

Not sure which option is right for you? Schedule a free MCAT consultation with an MCAT Advisor using the form below. No obligation, just expert advice.

Schedule My Free Consultation

Submit a Comment