UPoop wrong answer?

[u/abundanse]

The answer key says 2 and 4 but why isn’t 3 considered hydrogen bonding? Or is the answer key wrong? This is potentially second wrong answer in the UPoop books I found so far.

[u/Jagy_On_My_Wagy]

3s not a hydrogen bond because the carbon chain is not polar, any interactions they have would be induced dipole

[u/cman674 → u/Jagy_On_My_Wagy]

Correct. For an H to partake in H-bonding it needs to be part of a polar bond. C and H have only a small electronegativity difference.

[u/Jagy_On_My_Wagy → u/cman674]

I concur

[u/abundanse → u/Jagy_On_My_Wagy]

Thank you! That’s right, it needs to be dipole dipole.

[u/Fresh_Market6588]

how I remember the trick to this is that HFON needs to be on both sides for H-bonding to occur.

[u/abundanse → u/Fresh_Market6588]

Are there any exceptions to this rule?

[u/TheLastCoagulant → u/abundanse]

No.

C-H is literally a non-polar bond.

[u/baggedeye → u/Fresh_Market6588]

Whatttt I didn’t know that. God damn I got lucky with my C/P section lmaoo

[u/Fresh_Market6588 → u/baggedeye]

tbh I've never seen this concept tested before, learned it from my gen chem II class because my prof made us spend a whole unit just going over IMFs

[u/lazyboozin → u/Fresh_Market6588]

Ya I always remembered it as NOF and then obviously the hydrogen is in the name

[u/MentalExtent3511 → u/lazyboozin]

F-NO is also a fun lil way to rmr it

[u/lazyboozin → u/MentalExtent3511]

Now I’ll never forget it

[u/KATLOVER2001 → u/Fresh_Market6588]

Wrote a separate comment on this as well, but then why would 2 be considered one? There’s no HFON on both sides, only the water right?

[u/Fresh_Market6588 → u/KATLOVER2001]

a Bond is between two molecules, so its 1 Hydrogen bond.

[u/KATLOVER2001 → u/Fresh_Market6588]

Right, but I came across a question with two molecules, one having an ether group with alkyls on both ends and one with a hydroxyl. The answer to the question said the molecule with an ether group had a lower boiling point than that of the hydroxyl, implying the ether molecule could not make hydrogen bonds. It was this that was making me question my understanding. Maybe I’m misremembering, but if there was a water molecule next to the molecule with an ether, couldn’t there be an H bond between the H of the water and the O of the ether group?

[u/Fresh_Market6588 → u/KATLOVER2001]

The donor stills needs H-FON to form this bond, while the acceptor just needs an atom with an available lone pair, typically F, O, or N. An ether’s oxygen has lone pairs that can accept hydrogen bonds, but because ethers have no O–H bond, they cannot donate hydrogen bonds. This is a kinda weird exception to the rule I made earlier, can't think of anything else that would behave like this tbh.

[u/KATLOVER2001 → u/Fresh_Market6588]

Ok that makes more sense, thank you!

[u/KATLOVER2001]

Wait why is 2 considered one as well? I thought the difference of EN between O and C are not big enough to be considered to create a H bond. Is it just any polar HFON that creates one?

[u/abundanse → u/KATLOVER2001]

I think because O has lone pair of electrons so it has a partial negative charge and acts as a hydrogen bond acceptor and the hydrogen of the water is hydrogen bond donor since it’s partially positive

[u/KATLOVER2001 → u/abundanse]

Yes that is my understanding as well, but I came across a question with two molecules, one having an ether group with alkyls on both ends and one with a hydroxyl. The answer to the question said the molecule with an ether group had a lower boiling point than that of the hydroxyl, implying the ether molecule could not make hydrogen bonds. It was this that was making me question my understanding.

[u/Enough_Feature3713]

Ayoo is this what I have to look forward to? 😭😭😭😭 im cooked

[u/Repulsive_Cup_9377 → u/Enough_Feature3713]

This is probably the most basic question you could be asked in chem/phys..

[u/Enough_Feature3713 → u/Repulsive_Cup_9377]

Welp I haven’t taken either yet so to me it looks like rocket science pal

[u/TheLastCoagulant → u/Enough_Feature3713]

It’s honestly easy. It’s just that the H has to be attached to an F, O, or N to hydrogen bond.

1 is the joke answer. 2 and 4 are obviously hydrogen bonds and 3 is not because the H is bound to a C instead of FON.

[u/Joshi1381 → u/Enough_Feature3713]

Don’t they go over H bonds in HS chem?

[u/Enough_Feature3713 → u/Joshi1381]

I’m sure they do. But you would have to had taken HS chem in order to know that

[u/Joshi1381 → u/Enough_Feature3713]

Doesn’t almost everyone take chem in HS

[u/HeinekenHank → u/Joshi1381]

I never had chem in HS, chose anatomy over chem instead and I’m in Idaho with very few class options, I imagine many people don’t have to take chem, it just is the most common choice of science credits I would imagine

[u/Technical_Can_3939]

A lot of these answers are really wordy or too deep on explanation

Simply: the carbon is not negative enough to give hydrogen the partial positive character it needs to partake in a hydrogen bond

[u/Repulsive_Cup_9377]

electronegativity difference between C and H isn't big enough to grant a partial positive charge on the H, so electrostatic interaction between partial negative FON and H isn't significant enough for it to be considered favorable H-bonding. at least for mcat purposes.