Help for OC about Melting Point!

[potbelly]

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Hi all,

I am little bit confused about Melting point.
In Destroyer OC #152, it is asking for the highest melting point.
Choice a) is n-pentane and d) is neopentane.

The answer key is d) neopentane. If the question asked about highest boiling point, the answer would be a). RIght?

I have seen some examples that molecule with H-bond have high boiling point and high melting point as well. However, in above situation, it is not always true that boiling point is directly proportional to melting point.
When are melting point and boiling point in a inverse relationship? (for example, cis/ trans alkene)


Another question is...

Can you list these molecules from low to high of both melting point and boiling point?

-trans-2-butene
-cis-2-butene
-1-butanol
-t-butanol

Please help me! =) I just made up the last question ..
Thanks!

 

[mestmet31]

I think you are wrong..the more branching there is the lower the melting point. So in this case I would think that A would be correct.

 

[potbelly]

I think you are wrong..the more branching there is the lower the melting point. So in this case I would think that A would be correct.

I thought I was wrong but the solution says that " a molecule that is branched and possesses symmetry can fit into a crystal best, thus has the greatest melting point. Choice A, a molucule with no branching would have the highest boiling point however, a molecule such as choice A has the greatest amount of surface area, thus the greatest dispersion force. For melting point, symmetry is the main consideration thus D is correct."

U think Dr. Romano is wrong?

 

[zuma35]

Only if its perfectly symmetrical like neopentane does it increase the mp. otherwise, branching lowers the bp and mp.

 

[yuppers]

For the first neopentane has a higher mp because mp increases with symmetry and MW. Neopentane is very symmetryical allowing in to pack together very well. You look at it in this way:
n-pentane is ------ is symetric in 2 ways ---/--- or one way. But + is symetric is in 2 way up/down and left/right.

Cis usually leads to higher bp because there will be a dipole moment even in a hydrocarbon. As we know the dipole moment leads to a stronger intermolecular force holding them together.
Trans usually leads to a higher mp because the will pack together well. They are more symmetric.

bp all from lowest to highest
-trans-2-butene: 1
-cis-2-butene: 2
-1-butanol: 4
-t-butanol: 3

mp is tough and I honestly wouldnt worry about it.