Module 7 - Hydrogen Bonding Help!!! (1 Viewer)

[Nash__]

I have been trying to figure out the reason why amines have a lower boiling point than alcohols but amides have a higher boiling point than carboxylic acids.

Looking at hydrogen bond donors and acceptors, it appears that an alcohol has 2 acceptors (the two lone pairs on oxygen) and 1 donor (the oxygen with the hydrogen) thus it can form 3 H-bonds.

An amine will have 1 acceptor (the lone pair on the nitrogen) and 2 donors (the two hydrogen atoms on nitrogen) thus it can form 3 H-bonds. This makes sense as the H-N bond is weaker than H-O bond so amines have lower BP as same amount of bonds can be formed but they are weaker.

But, for the carboxylic acid and amide case this logic doesn't seem to apply. The carboxylic acid appears to have 4 acceptors (2 on oxygen in carbonyl and 2 on oxygen in hydroxyl) and 1 donor (hydrogen on oxygen) thus it can form 5 H-bond.

For amide it appears to have 3 acceptors (2 on oxygen in carbonyl and 1 on nitrogen) and 2 donors (2 hydrogen on oxygen) thus it can form 5 H-bond also. And I cannot reason why this would have a higher BP.

I assume im doing something wrong with the definition of H-bond donors and H-bond acceptors, can anyone clarify the concept in relation to alcohols vs amines and carboxylic acids vs amides??

Thanks

 

[jazz519]

Alcohols/Amines form same amount of hydrogen bonds. Alcohol OH hydrogen bonding is stronger because there is a greater electronegativity difference between an O and H versus a N and H, this means the bond is more polar and therefore leads to a stronger attraction force. What you're talking about in 3 being how many forms is not considering other factors that can influence whether the hydrogen bond can form such as the steric hinderance of the molecules, so often they won't be forming 3 per molecule but this is not in scope of HSC.

Carboxylic acids don't really form 5 hydrogen bonds they instead form something called a dimer, which has more hydrogen bonding than an alcohol

Amides form ladder hydrogen bonding which is even more extensive than a dimer therefore have highest BP

 

[Nash__]

Alcohols/Amines form same amount of hydrogen bonds. Alcohol OH hydrogen bonding is stronger because there is a greater electronegativity difference between an O and H versus a N and H, this means the bond is more polar and therefore leads to a stronger attraction force. What you're talking about in 3 being how many forms is not considering other factors that can influence whether the hydrogen bond can form such as the steric hinderance of the molecules, so often they won't be forming 3 per molecule but this is not in scope of HSC.

Carboxylic acids don't really form 5 hydrogen bonds they instead form something called a dimer, which has more hydrogen bonding than an alcohol

Amides form ladder hydrogen bonding which is even more extensive than a dimer therefore have highest BP

Thanks! Would there be any details that I could add to a response to show that amides are higher than carboxylic acids in addition to ladder bonding vs dimer? such as more sites for bonding, etc. I got marked down for not saying something like this last time.

 

[jazz519]

Thanks! Would there be any details that I could add to a response to show that amides are higher than carboxylic acids in addition to ladder bonding vs dimer? such as more sites for bonding, etc. I got marked down for not saying something like this last time.

Maybe drawing the diagrams showing the interactions, but other than that would ask your teacher for what they want you to add more