Melting Point?

[solstice]

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Ok It seems that every one says what they feel like.

Based on Examcracker and destroyer, Branching increase the MP.
based on Kaplan and The princeton review, Branching decrease the MP.
The irony is that the 5th edittion Murry comletely leaves out the MP for branched alkane 😡

I kind of trust examcrakers in everything, but all reviews have valid reasons that make sense.

Can anyone check their text and tell me who is correct, before I have to hit the lab and melt some fruit flyes and write my own review?

 

[jorasic]

I'm pretty sure branching INCREASES melting point.
I've studied this from at least 4 sources, my ochem proff, destroyer, topscore, ochem textbook by Bruice.

There might be other factors affecting that compound but for the individual property of branching itself, it will increase melting point.

 

[2PacClone23]

Barron's also says branching INCREASES melting point.


"Branching often has the reverse effect on the melting point. Usually the belting point increases because of an increase in symmetry, which aids in the formation of crystal lattices"



🙂

 

[icalz]

Think of a triglyceride with a glycerol head and three fatty acid tails....the longer (more linear, less branched) the fatty acid tails (and the more saturated with hydrogens it is) the higher the melting point (also the higher the energy in its bonds). If fatty acids began to branch then they would become more unsaturated and their melting point would decrease.

I never realized that there was an ongoing debate about this....my above response is relayed from my gen chem text and my gen chem prof. Good luck finding a resolution 🙂

 

[sixkiller]

Quote:
Originally Posted by NumbaOneStunna
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.

Originally Posted by vlct0ria
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.

 

[jrobb220]

check out chads ochem review 1, part 4.

http://www.coursesaver.com/Chad/O-chem_1_Part_4.flv

it helped me out. if you havn't watched any of the condensed ochem ones then you have to download this too.

http://www.videolan.org/vlc/

 

[solstice]

Thank you guys for your answers. I did not know there had been a discussion about this before. It is just annoying when you find different books says different things and they says it in such a hurry, with no detail explanation.

Oh by by book I mean test prep books. The one that you buy 3-4 times more expensive than a whole valid text book, and they do not even bother to correct their mistake since the first publish.