Melting and Boiling Point Question

[NumbaOneStunna]

---

Members don't see this ad.

All of the information below is correct - right?
I was confused about melting point because this old thread: http://forums.studentdoctor.net/showthread.php?t=542327

Has different answers on how branching changes mp of alkanes! Also what does symmetry have to do with the mp of alkanes? If someone could explain this thoroughly that would be awesome! thanks🙂

For alkanes:

Anyway, so from what I've gathered, simply put based on destroyer's reasoning. Increasing the chain length obviously increases BP and MP. Increasing branching decreases the BP, but increases MP. Hydrogen bonding increases BP. Ionic compounds will have higher BP compared to hydrocarbons. That should be it. I'm glad this error is rectified.

For alkenes:

for alkenes, its a bit different..
trans alkenes pack better in sold state so their mp is higher, but boiling point is lower because theres no polarity to overcome. For cis alkenes, there is a net polarity so boiling point is higher here because it will takes more to overcome those bonds. Cis alkenes have lower melting point since they dont pack as well as trans

 

[vlct0ria]

All of the information below is correct - right?
I was confused about melting point because this old thread: http://forums.studentdoctor.net/showthread.php?t=542327

Has different answers on how branching changes mp of alkanes! Also what does symmetry have to do with the mp of alkanes? If someone could explain this thoroughly that would be awesome! thanks🙂

For alkanes:




For alkenes:

I agree with DrHoya that increased branching increases the MP...more branching means the structure is more compact, meaning it can fit more tightly into the solid state...thus it is less inclined to go from solid--> liquid, so the melting point is increased.

 

[TerpDentist]

Kaplan materials say that increased chain length increases bp, mp, and density. While increased branching DECREASES bp, mp, and density. I don't know where it is in the blue book but in the course materials they give it is on page 286 (the second page of the alkane chapter).

For alkenes, it only talks about cis vs trans where cis alkenes have a higher boiling point due to polarity (the dipoles of the trans cancel each other out), but trans alkenes have a higher melting point due to higher symmetry.

 

[NumbaOneStunna]

Kaplan materials say that increased chain length increases bp, mp, and density. While increased branching DECREASES bp, mp, and density. I don't know where it is in the blue book but in the course materials they give it is on page 286 (the second page of the alkane chapter).

For alkenes, it only talks about cis vs trans where cis alkenes have a higher boiling point due to polarity (the dipoles of the trans cancel each other out), but trans alkenes have a higher melting point due to higher symmetry.

That's what I'm confused about. Destroyer says increased branching increases alkane mp around question 172 while kaplan says the opposite. What's correct?

 

[razblo]

From looking through old threads, searching the internet, and looking at my textbook this is what I have written down.

MP
- Increased by hydrogen bonding
- Increased by branching
- Increased by larger chain
- Increased by trans stereochemistry

BP
- Decreased by branching
- Increased by hydrogen bonding
- Increased by larger chain
- Increased by cis stereochemistry

I'm not 100% sure on the MP hydrogen bonding one but I think that's what I read.

 

[NumbaOneStunna]

Good stuff Razblo, thanks.

 

[sixkiller]

That's what I'm confused about. Destroyer says increased branching increases alkane mp around question 172 while kaplan says the opposite. What's correct?

Depends on WHAT kind of branching is involved.....

If it is SYMMETRICAL branching then yes, mp of the alkane is raised.

An example of symmetrical branching is neopentane.

However, ASYMMETRICAL branching results in a decreased mp. An example of asymmetrical branching would be isopentane.

Unfortunately, most of the rules given so far are over simplified and need to be examined more carefully.

 

[NumbaOneStunna]

Depends on WHAT kind of branching is involved.....

If it is SYMMETRICAL branching then yes, mp of the alkane is raised.

An example of symmetrical branching is neopentane.

However, ASYMMETRICAL branching results in a decreased mp. An example of asymmetrical branching would be isopentane.

Unfortunately, most of the rules given so far are over simplified and need to be examined more carefully.

Thanks bud.

 

[sixkiller]

I agree with DrHoya that increased branching increases the MP...more branching means the structure is more compact, meaning it can fit more tightly into the solid state...thus it is less inclined to go from solid--> liquid, so the melting point is increased.

It has nothing to do with a "tighter" fitting solid state.

The reason involves entropy. When branching is symmetrical, a greater number of lattice configurations are available due to the symmetrical nature of the lattice constituents. *entropy in the strictest sense has to do with the number of possible configurations; ie ways in which energy can be distributed*

MP is the point at which the rate of melting equals the rate of freezing, or when the vapor pressure of the solid state equals the vapor pressure of the liquid state; both phases have equal chemical potentials.

The tendency to vaporize is due to the gain in entropy. In other words, the driving force is entropic, not enthalpic.

Because the more symmetrically branched compound's lattice is greater in absolute entropy, it's tendency to vaporize is lower since the change in entropy in going to the vapor phase is lower. This causes the solid state to reach equilibrium (point at which the liquid and solid phase have identical vapor pressure) with its liquid phase at a HIGHER temperature resulting in an elevated mp relative to the asymmetrically branched alkane.

 

[vlct0ria]

It has nothing to do with a "tighter" fitting solid state.

The reason involves entropy. When branching is symmetrical, a greater number of lattice configurations are available due to the symmetrical nature of the lattice constituents. *entropy in the strictest sense has to do with the number of possible configurations; ie ways in which energy can be distributed*

MP is the point at which the rate of melting equals the rate of freezing, or when the vapor pressure of the solid state equals the vapor pressure of the liquid state; both phases have equal chemical potentials.

The tendency to vaporize is due to the gain in entropy. In other words, the driving force is entropic, not enthalpic.

Because the more symmetrically branched compound's lattice is greater in absolute entropy, it's tendency to vaporize is lower since the change in entropy in going to the vapor phase is lower. This causes the solid state to reach equilibrium (point at which the liquid and solid phase have identical vapor pressure) with its liquid phase at a HIGHER temperature resulting in an elevated mp relative to the asymmetrically branched alkane.

Yeah I double-checked in my organic book after posting earlier and realized my mistake...I just forgot to edit it afterwards. Thanks for the clarification 🙂