Orgo questions about hydrogen bonding and solubility

[JFK90787]

OK two questions I'm having a hard time finding clarification for, maybe these will help others:

1. Does the ability to H-bond = solubility in water? I.E. Something like a phenol has 6 carbons, and I always learned that anything with >5 carbons is supposed to kill solubility, but it is still able to H-bond. So does the ability to H-bond make phenol soluble despite having 6 carbons? What about something a little larger, like 12 carbons; would this still be soluble because of the ability to form H-bonds?

2. Are non-polar things like O2 and CO2 able to be H-bond acceptors? I thought O2 couldn't be a H-bond acceptor because it was non-polar (also the same reason why it can't dissolve in blood and we need hemoglobin), but in the section of my review book where they talk about hydrogen bonding, they demonstrate it by using an O2 molecule as an acceptor. They have another example with a CO2 as an acceptor, which is also non-polar. I'm very confused right now.

Sorry if these are dumb questions but I just spent about 20 minutes googling with no luck

[Insulinshock]

Originally Posted by JFK90787

OK two questions I'm having a hard time finding clarification for, maybe these will help others:

1. Does the ability to H-bond = solubility in water? I.E. Something like a phenol has 6 carbons, and I always learned that anything with >5 carbons is supposed to kill solubility, but it is still able to H-bond. So does the ability to H-bond make phenol soluble despite having 6 carbons? What about something a little larger, like 12 carbons; would this still be soluble because of the ability to form H-bonds?

2. Are non-polar things like O2 and CO2 able to be H-bond acceptors? I thought O2 couldn't be a H-bond acceptor because it was non-polar (also the same reason why it can't dissolve in blood and we need hemoglobin), but in the section of my review book where they talk about hydrogen bonding, they demonstrate it by using an O2 molecule as an acceptor. They have another example with a CO2 as an acceptor, which is also non-polar. I'm very confused right now.

Sorry if these are dumb questions but I just spent about 20 minutes googling with no luck

Im just using what I recall from my classes, but I'll give it a shot:

1. Unless a molecule is strongly hydrophilic (like H2O) or strongly hydrophobic (like hexane) I typically characterize solubility in terms of relative amounts. For instance, if you have a cycloheptane, cycloheptanol and a cycloheptadiol all three would be considered not soluble when compared to something like ethanol. However, cycloheptadiol is considerably more soluble than cycloheptane; it's all relative. Especially in bigger compunds you can never (at least I cant) give exact definitions of how soluble something is, you can only compare it to other molecules.

2. I think you might be reading the question wrong... CO2 and O2 don't typically undergo hydrogen bonds.. Molecules such as H2O and amino groups most readily do, but not CO2 or O2 because there is no inherent polarity in the molecule. HOWEVER, if you're talking about electron acceptors that is a different story. CO2 and O2 are both electron acceptors (O2 in the final stage of the ETS, and I'm pretty sure CO2 is too but I can't think of any examples.)

Hope this helps. Again, this was just all memory recall so it might not be 100% accurate.

[JFK90787]

Originally Posted by Insulinshock

2. I think you might be reading the question wrong... CO2 and O2 don't typically undergo hydrogen bonds.. Molecules such as H2O and amino groups most readily do, but not CO2 or O2 because there is no inherent polarity in the molecule. HOWEVER, if you're talking about electron acceptors that is a different story. CO2 and O2 are both electron acceptors (O2 in the final stage of the ETS, and I'm pretty sure CO2 is too but I can't think of any examples.)

This is what my review book said:

Edit: The O on the first H2O should have a negative dipole. whoops



I'm very confused because I was always told a H bond acceptor had to be on a polar molecule, and O2 and CO2 are non-polar. Anyone know is my review book wrong here or am I?

[Evergrey]

Hmm.. I don't really see how CO2 and O2 would form H-bonds. The net polarity is 0 due to molecular symmetry, and even though the electrostatic potential map would still show a partial charge separation, the electrons there wouldn't really want to "latch" onto a partial positive charge since they're already pretty happy sitting in their symmetric poles.

[Insulinshock]

Originally Posted by JFK90787

This is what my review book said:

Edit: The O on the first H2O should have a negative dipole. whoops



I'm very confused because I was always told a H bond acceptor had to be on a polar molecule, and O2 and CO2 are non-polar. Anyone know is my review book wrong here or am I?

Hm... I have a pretty good memory and I've can't recall ever seeing molecular oxygen undergoing a hydrogen bond like that.

[wanderer100]

The O in O2 does not have a partial negative charge on it.

[JFK90787]

Thanks for the replies everyone, I also think my review book was wrong here.

[rama kandra]

What review book is this?

In theory, yes, these are likely examples of hydrogen bonding but could have used more reasonable examples. These molecules are not going to be in solution together and exhibit H bonding like you would find in water/ethanol.

All you should take away from H bonding is this: hydrogen bonded to a strong electronegative atom (FON, for Flurine, Oxygen and Nitrogen) will create a strong interaction with another electronegative atom (FON). Thats it, you have seen the H with the dashed lines going to oxygen among water molecules? That sums it up pretty well, and H bonds are used to keep DNA helices together, and for secondary and tertiary structures in protein.

These examples seem off putting because they arnt exactly common - but they seem to just make a similar (albeit bad) point about the bonds.

[ElenaV]

Hydrogen bond acceptors can be any N or O with a LONE PAIR of electrons. Water can make hydrogen bonds with CO2 for sure. You can think of it this way, even though it's a linear molecule with = there's still some bond bending and stretching, which creates a change in dipole moment, if you want to justify the polarity rule for hydrogen bonding.

[BennieBlanco]

Originally Posted by ElenaV

Hydrogen bond acceptors can be any N or O with a LONE PAIR of electrons. Water can make hydrogen bonds with CO2 for sure. You can think of it this way, even though it's a linear molecule with = there's still some bond bending and stretching, which creates a change in dipole moment, if you want to justify the polarity rule for hydrogen bonding.

Water + CO2 won't Hbond, it will beyond carbonic acid.