Chemistry question on boiling points (1 Viewer)

[HelloItzMe]

Why do aldehydes have higher boiling points than amines?!??


I’m really confused

 

[HelloItzMe]

Amines have an -NH2 bond, while all alcohols have an O-H bond (Alcohols can either be aldehydes or carboxylic acids). The electronegativity of oxygen is greater than that of nitrogen, so the polar forces are larger in the O-H bond. Therefore, the H bond formed in amines is weaker than that of the ones formed in all alcohols, which include aldehydes

Thank you so much I have an exam on this and I was panicking because I got so confused and all but yes thank you

 

[CM_Tutor]

Sorry, but the [now removed] answer is wrong. Aldehydes are compounds with a terminal carbonyl group (C=O) and a C-H bond.

For example, acetaldehyde is CH3-CHO, with the H and O atoms on the -CHO group both bonded the the C atom and not to each other. Aldehydes are definitely not alcohols where there is a hydroxyl (-O-H) group bound to a C atom.

Aldehydes are not alcohols and they are unable to form hydrogen bonds on their own, though they can accept H-bonds in the presence of an H-bond donor.

I am also dubious about the premise here. As a general rule, I would expect an amine to have a higher boiling point than an aldehyde of comparable molar mass because the amine can form H-bonds. However, they are very similar in some simple cases, like:

acetaldehyde, C2H4O, M = 44 g/mol, BP = 20 degC
ethylamine, C2H7N, M = 45.1 g/mol, BP = 16-20 degC

propylamine, C3H9N, M = 59.1 g/mol, BP = 47-51 degC
propionaldehyde / propanal, C3H6O, M = 58.1 g/mol, BP = 46-50 degC

I would not expect an exam question to ask for a qualitative rationalisation in circumstances where the quantitative data show such small differences. Further, actually understanding what is leading aldehydes to have higher than expected BPs requires knowledge of material that is beyond the HSC syllabus.

 

[Pedro123]

Sorry, but this answer is wrong. Aldehydes are compounds with a terminal carbonyl group (C=O) and a C-H bond.

For example, acetaldehyde is CH3-CHO, with the H and O atoms on the -CHO group both bonded the the C atom and not to each other. Aldehydes are definitely not alcohols where there is a hydroxyl (-O-H) group bound to a C atom.

Aldehydes are not alcohols and they are unable to form hydrogen bonds on their own, though they can accept H-bonds in the presence of an H-bond donor.

I am also dubious about the premise here. As a general rule, I would expect an amine to have a higher boiling point than an aldehyde of comparable molar mass because the amine can form H-bonds. However, they are very similar in some simple cases, like:

acetaldehyde, C2H4O, M = 44 g/mol, BP = 20 degC
ethylamine, C2H7N, M = 45.1 g/mol, BP = 16-20 degC

propylamine, C3H9N, M = 59.1 g/mol, BP = 47-51 degC
propionaldehyde / propanal, C3H6O, M = 58.1 g/mol, BP = 46-50 degC

I would not expect an exam question to ask for a qualitative rationalisation in circumstances where the quantitative data show such small differences. Further, actually understanding what is leading aldehydes to have higher than expected BPs requires knowledge of material that is beyond the HSC syllabus.

Apologies - I tried my best to answer the question and I simplified it greatly, which resulted in a wrong statement. In the context, I thought it would give as much of an answer as I could provide, as I agree it is a stupid question to ask in HSC chemistry. The post above is much more useful. I have removed my response as such.
Sorry again!

 

[CM_Tutor]

Apologies - I tried my best to answer the question and I simplified it greatly, which resulted in a wrong statement. In the context, I thought it would give as much of an answer as I could provide, as I agree it is a stupid question to ask in HSC chemistry. The post above is much more useful. I have removed my response as such.
Sorry again!

Pedro, there's no need to be sorry for giving an incorrect answer, and both you and other readers can learn from mistakes. It is good that you are open to being corrected. The value of a site like this comes from students working together to come up with answers / solutions, hopefully aided by corrections from other students or teachers / tutors / etc.