boiling points

[bigballer27]

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so hydrogen generally increases the boiling boint, but how is BP affected by inter vs intra molecular hydrogen bonding?

for ex: would a para molecule (para-nitrophenol, for ex) have a higher bp than ortho nitrophenol because it only experiences intermolec h-bonds, while ortho has inter and intra? kinda confused as to how that works


also, are the trends generally the same for melting points?

on a side note, how is a C-O-H more polar than a C=O as it is explained on one of the aamc practice tests about a cyclohexenone vs a cyclohexenol (i dont wanna post the specific just in case ppl didnt take it yet)...but if u want to know, its #100 on AAMC 7 CBT

 

[PiBond]

Intramolecular H-bonding would decrease the boiling point. Think of it as there being less H-bonds available to bond with other molecules in solution because it's bonding with itself instead. Since there's less intermolecular forces it'll be easier to boil them off (there's less bonds to make it "stick" to its neighbors). Melting points, to my knowledge, would follow an entirely different train of thought.

 

[bigballer27]

ah ok thanks, what about the second question? isnt a C=O bond supposed to be more polar?

 

[PiBond]

the alcohol is actually more polar than the carbonyl and it has to do with it's ability to hydrogen bond (both donate and accept). the carbonyl oxygen can accept h-bonds but it has none to donate. kinda weird, i know

 

[loveoforganic]

There's more going on in your example than intramolecular h bonding. Nitro, as a strong Ewg, will increase the acidity of the phenolic proton. This effect is most dramatic from the ortho and para positions, due to resonance.

 

[bigballer27]

There's more going on in your example than intramolecular h bonding. Nitro, as a strong Ewg, will increase the acidity of the phenolic proton. This effect is most dramatic from the ortho and para positions, due to resonance.

ok so if it increases the acidity, will that affect the boiling point?

it seems plausible that the inter h bonding vs intra makes inter have a higher boiling point

in general though, i don't understand why H bonding creates a more polar molecule though. is it because the H-0----H creates a more (neg) oxygen?


btw loveoforganic you are too good at this stuff lol maybe i can just sneak u in for the orgo portions of my test hahaha

 

[loveoforganic]

I only respond to what I know!

Think of a hydrogen bond like a single velcro interaction. The more you have, the harder two things are to pull apart. So two extensively velcroed (hydrogen bonded) molecules are going to to want to go from the liquid phase to the gas phase as one unit (essentially MW of your compound times 2).

Alcohols aren't very protic to start with. When you increase their acidity, they're better able to H-bond.

 

[puravida85]

ok so if it increases the acidity, will that affect the boiling point?

it seems plausible that the inter h bonding vs intra makes inter have a higher boiling point

in general though, i don't understand why H bonding creates a more polar molecule though. is it because the H-0----H creates a more (neg) oxygen?


btw loveoforganic you are too good at this stuff lol maybe i can just sneak u in for the orgo portions of my test hahaha

In a situation where the acidity increases, it usually means that the proton has a higher ability to "break off" from the O in your situation (That's why we evaluate the strength of acids based on Ka's). Also meaning, the connected O (electronegative) is stealing more of the electron coverage from the H. If so, than H is lacking hardcore in (-) charges in its vicinity. That means that any other O from say "H2O" is highly attracted to the (+) charge of the proton. Thus making it a great bond. Remember, that if a compound has strong intermolecular forces, its vapor pressure decreases, and so it boiling point increases. I hope that helps visualize what's going on.