Crack DAT Ochem Question

[StudentDoc1234]

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Hi guys,

So I have always been under the assumption that the highest melting point is the most branched molecule but the answer to this question states it's hexane because although the others are branched, hexane is their isomer. Can someone please rationalize for me why an isomer of a molecule would have the highest melting point regardless of branching? Thanks!

 

[tooth_hurty]

Branching decreases boiling point due to reduced surface area and intermolecular forces. So in this case, hexane is the least branched isomer and has the most surface area so the highest boiling point.

 

[TheGreatJoseon]

http://www.masterorganicchemistry.com/2010/07/09/chemical-tetris/

 

[FeralisExtremum]

Branching decreases boiling point due to reduced surface area and intermolecular forces. So in this case, hexane is the least branched isomer and has the most surface area so the highest boiling point.

This question is about melting point though, which branching can affect differently.

It's not just any branching that raises the melting point, but highly symmetrical branching. In this case there's symmetry, but not to a considerable degree. I wouldn't sweat this particular problem too much, chances are the real DAT will give you something more reasonable.

 

[StudentDoc1234]

This question is about melting point though, which branching can affect differently.

So as a rule of thumb, for the DAT, do you think it's fine to just go with the notion that increased branching increases melting point?

 

[FeralisExtremum]

Only if it's highly symmetrical, e.g.:

I would assume (but personally have no idea) that if the DAT wanted to test this concept, they'd use a relatively straightforward example like the image here, and not at the level in CrackDAT.

 

[StudentDoc1234]

Got it, thanks for the help!