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AAMC QP - Solubility
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Caroline_6187
#1 Posted : Tuesday, June 15, 2021 8:15:27 PM
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Hi there,

Question was wondering whihc is more soluble in water: compound 1 (CH3CH2)2NH or compound 2 (CH3CH2)2CH2. I was between anwsers "compound 1" and "Neither has appreciable water solubility". I picked the later because I though the commpound 1 could make H-bonds, I thought the branching in both would make it not soluble in water. This was incorrect and compound 1 was more soluble.

When does branching come into play for solubility and does the possibility of H-bonds usually override the branching hindrance on solubility?

Thank you!

For reference, AAMC Chemistry Question pack question 18
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INSTR_Katrina_128
#2 Posted : Tuesday, June 15, 2021 8:39:58 PM
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Hi Caroline! Great question. 😀

The main thing to look for in a question that says "which one is MORE or LESS whatever" is the difference between the molecules.

In this case, the only difference is the N and C in the "hinge" of the molecule. Everything else is identical.

Like you spotted, hydrogen bonding is possible for the first compound because of the presence of the N with a lone pair.

But what about the chains of CH2's that it is attached to? Do those get in the way? In this case, no. They are not very long. Only two carbons long (similar to the carbon chain in ethanol), and the molecule is still soluble in water!

How long is too long? Around the four to five carbon chain length gets to be too long. Those are some carbonny tendrils. 😲
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