2 bp questions

[rmm30]

1)why is the bp for a terminal alkene/alkyne lower than bp for internal alkene/alkyne?

2)why is the boiling point of water so much higher than NH3? 100C and
-33C respectively. They both have H bonds, and a molecular geometries conducive to generating a dipole?

 

[2PacClone23]

1)why is the bp for a terminal alkene/alkyne lower than bp for internal alkene/alkyne?

2)why is the boiling point of water so much higher than NH3? 100C and
-33C respectively. They both have H bonds, and a molecular geometries conducive to generating a dipole?

1) because terminal alkenes are more stable

2) water is just special lol

 

[tobeornottobe]

1. for the first one, its because terminal alkenes/alkynes are a little less stable than internal ones. If you think of it in terms of the more conjugated system is more stable, it might make more sense.
2. Because oxygen is more electronegative than nitrogen it has a higher affinity for hydrogen bonding even though its in a stable state with 2 hydrogens. This multiplies a few times over when water molecules are grouped hence you have a much greater bp.

hope this helps

 

[2PacClone23]

1. for the first one, its because terminal alkenes/alkynes are a little less stable than internal ones. If you think of it in terms of the more conjugated system is more stable, it might make more sense.
2. Because oxygen is more electronegative than nitrogen it has a higher affinity for hydrogen bonding even though its in a stable state with 2 hydrogens. This multiplies a few times over when water molecules are grouped hence you have a much greater bp.

hope this helps

wait i thought terminal were more stable

 

[RSD2014]

1)why is the bp for a terminal alkene/alkyne lower than bp for internal alkene/alkyne?

2)why is the boiling point of water so much higher than NH3? 100C and
-33C respectively. They both have H bonds, and a molecular geometries conducive to generating a dipole?

A molecule with a high boiling point means that it is stable and requires more heat to break the bonds.

• Internal Alkene could have 2 to 4 substituted groups.(Highly substituted Alkenes are more stable = higher b.p.)

• Terminal Alkene only has at most 2 substituted groups, so it is less stable than internal alkene; therefore, lower b.p.

I am not sure about Internal/Terminal Alkyne. Don't know if the same rules apply.

2) Water H-O-H has higher boiling point because Oxygen is more electronegative than Nitrogen and hold on to the electrons strongly, won't easily break away....stronger bond = higher b.p.

 

[RSD2014]

wait i thought terminal were more stable

You are incorrect. Terminal alkene is less substituted compared to internal Alkene. highly substituted alkene are more stable = stronger bonds = higher b.p.

 

[RSD2014]

1. for the first one, its because terminal alkenes/alkynes are a little less stable than internal ones. If you think of it in terms of the more conjugated system is more stable, it might make more sense.
2. Because oxygen is more electronegative than nitrogen it has a higher affinity for hydrogen bonding even though its in a stable state with 2 hydrogens. This multiplies a few times over when water molecules are grouped hence you have a much greater bp.

hope this helps

With alkene it is certain that more conjugated system is more stable.
how do you account for internal alkyne being more stable than terminal alkyne?

 

[DQLEUCD]

With alkene it is certain that more conjugated system is more stable.
how do you account for internal alkyne being more stable than terminal alkyne?

I believe stability due to the carbocation formation that is formed once a bond is broken.

 

[apb000]

These answers are mostly nonsense. The stability of a molecule tells you nothing about the intermolecular forces between molecules that determine the boiling point. I'm not sure why internal alkynes boil higher than terminal alkynes - you'd expect the polarity of the bond to the acetylenic hydrogen to make terminal alkynes boil HIGHER. But in any case, the answers that claim it has something to do with the stability of the pi bonds are wrong.
On the other hand, I can tell you that water boils higher than ammonia because the molecular dipole is much higher, for two reasons. The first is that oxygen is more electronegative than nitrogen, so the bond polarities in water are greater than those in ammonia and we would predict a higher molecular dipole as a result.
But the molecular dipole of ammonia is even less than we would expect for another reason. Imagine ammonia with the molecule sitting on a table with the lone pair pointing straight up. This molecule will spontaneously invert itself: keep the nitrogen in place and imagine the hydrogens popping up above it and the lone pair dropping down until it points straight down from the nitrogen. The molecule has inverted like an umbrella in a strong wind. (This is much like the Walden inversion of the SN2 reaction). Because of this inversion, and because it happens very, very rapidly, the ammonia molecule doesn't have a dipole moment - it points equally in opposite directions in the two forms, which average out.
This doesn't happen with water, which always has the bent geometry, and a permanent dipole pointing through the oxygen. Thus water boils a lot higher than ammonia.
( While both water and ammonia form hydrogen bonds, pure water has an equal number of hydrogen donors - i.e. hydrogens - and acceptors - i.e. lone pairs. Ammonia has only one acceptor for every three donors. I don't know if or how much this affects the boiling point, but it wouldn't surprise me if this fact were also part of the answer.)

 

[selfstudyman33]

dipole moment, dipole moment, dipole moment

 

[youngy]

i am with apb. Not sure if his rationales are 100% correct, but all other responses seem a little non-sensical.
Stability of alkenes are usually associated with low combustion energy, not with boiling points.

I doubt you will see somethign this detailed on the real exam.
From my experience, my exam was overly simple that it made me almost laugh.

 

[selfstudyman33]

I can't even google the answer. Seriously, terminal alkynes having a higher boiling point is the opposite of what I would expect, and there are no good reasons to rationalize whats actually observed (lower boiling of terminal alkynes)

Why can't the internet give me a satisfying answer on this one?!?

The answers here are very nonsensical. Stability of the molecule may be distantly related, but we were always taught that INTERMOLECULAR INTERACTIONS explained boiling point and melting point. Whats going on?