AAMC 9, Question #7

[Diocletian]

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Basically we're supposed to compare two substances, ethanol and MTBE (C4H9OCH3). The question asks, what kind of intermolecular interaction can ethanol undergo with water, which MTBE cannot?

a) van der Waals
b) dipole-dipole
c) Hydrogen bonding
d) Covalent bonding

The answer is apparently "C" because only ethanol has a hydrogen for the creation of an H-bond. I don't get why MTBE and water can't do H-bond. Even though MTBE has no hydrogens bonded to its oxygen, can't it still receive a hydrogen from the water and therefore create an H-bond?

 

[MedPR]

Basically we're supposed to compare two substances, ethanol and MTBE (C4H9OCH3). The question asks, what kind of intermolecular interaction can ethanol undergo with water, which MTBE cannot?

a) van der Waals
b) dipole-dipole
c) Hydrogen bonding
d) Covalent bonding

The answer is apparently "C" because only ethanol has a hydrogen for the creation of an H-bond. I don't get why MTBE and water can't do H-bond. Even though MTBE has no hydrogens bonded to its oxygen, can't it still receive a hydrogen from the water and therefore create an H-bond?

It's true that MTBE can technically h-bond with water because it can receive, but it cannot donate. Also, A, B, and D all don't make sense so by POE you should know it's C.

 

[ingramw1202]

Basically we're supposed to compare two substances, ethanol and MTBE (C4H9OCH3). The question asks, what kind of intermolecular interaction can ethanol undergo with water, which MTBE cannot?

a) van der Waals
b) dipole-dipole
c) Hydrogen bonding
d) Covalent bonding

The answer is apparently "C" because only ethanol has a hydrogen for the creation of an H-bond. I don't get why MTBE and water can't do H-bond. Even though MTBE has no hydrogens bonded to its oxygen, can't it still receive a hydrogen from the water and therefore create an H-bond?

I agree with MedPR. In my experience, in talking about hydrogen bonding, at least for the mcat, an electronegative atom must be directly bound to a hydrogen i.e. ethanol, water, HF, etc etc. Two compounds containing an electronegative atom bound to hydrogen can hydrogen bond together forming vast networks. But if you just had something such as diethyl ether and water, sure the positive hydrogen could bind to the negative oxygen, but the difference in electronegativity between oxygen and carbon is not as great as the difference between hydrogen and oxygen. The oxygen in diethyl ether has a smaller (magnitude) partial negative charge than the oxygen in water. So the hydrogen bond isn't really a hydrogen bond because its not as strong, which is a noteworthy feature of hydrogen bonds: pretty darn strong.

 

[MedPR]

FWIW, I remember staring at this question for way longer than I should have because my thought process was the same as yours. Eventually I ruled out the other 3 answers and went with H-bond.

 

[Diocletian]

Thanks all. I still think this question is pretty dirty and unfair, but I'll get over it. And at least I kind of understand now...

 

[ingramw1202]

Thanks all. I still think this question is pretty dirty and unfair, but I'll get over it. And at least I kind of understand now...

It's like they are purposely TRYING to trick us. Jerks. haha lol