AAMC Free Sample (Scored): CP Q3

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Which structural change to Compound 1 would make it more water soluble?

  • A.Replacing benzene CH with N in the ring (Correct)
  • B.Replacing C=O with C=CH2
  • C.Replacing N-N=N with CH-CH=CH
  • D.Replacing NH with NCH3

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I'm looking at analyzing my questions and I did get this correct because I approached it as which one increases polarity.

My question is D's reasoning.

"Replacing NH with NCH3 removes a hydrogen bond donor, thus decreasing water solubility."

How does this make sense? shouldn't it increase its solubility?

I looked at this in many different ways:

  1. they said "removes a hydrogen bond donor"... Are they saying that the NH's Hydrogen atom can donate itself to create a bond with water? if so, wouldn't replacing the NH group into a NCH3 group increase the amount of hydrogens available to bond with other water molecules?
  2. According to the reasoning for (A) which states "Nitrogen in the benzene ring would have a lone pair that could accept a hydrogen bond from water, thus increasing the solubility of the compound.", shouldn't replacing NH for a NCH3 do NOTHING? since the lone-pair of nitrogen still exists?
  3. Polarity. (assuming H has a EN of 1) N-H has a difference of EN of 2.04 with the resultant polarity pointing towards N. but when replaced with NCH3 instead, C-H has a difference of 1.55 favouring C, and N-C has a difference of 1.49. Therefore, NH is more polar thus, decreasing water solubility...
  4. Someone on reddit also said it's because of the creation of H-bonds... where H bonds are created with FON and not with C. Therefore a reduction of H-bonds with water. resulting in a decrease in water solubility

Which is the correct reasoning????

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