Questions on Melting Points, anyone care to explain?

[johnwandering]

I don't quite understand why HIGHER branching = HIGHER Melting Point.

Boiling Point is quite easy. Higher branching causes steric hindrance from IM forces and hence it takes less energy to rip the molecules apart.

Melting Point seems counter-intuitive. High branching causes steric hindreance from IM forces, and hence it is easier to be a compact solid?


What is going on here?
If 2 isotopes exist: one branched (MP= 20C) and one unbranched (MP= -10C)
And it is 10 degrees C,
the branched one will be a solid and the unbranched will be a liquid.

Why does it take less energy to separate the nonbranched molecules?

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[orgohacks]

I don't quite understand why HIGHER branching = HIGHER Melting Point.

Boiling Point is quite easy. Higher branching causes steric hindrance from IM forces and hence it takes less energy to rip the molecules apart.

Melting Point seems counter-intuitive. High branching causes steric hindreance from IM forces, and hence it is easier to be a compact solid?


What is going on here?
If 2 isotopes exist: one branched (MP= 20C) and one unbranched (MP= -10C)
And it is 10 degrees C,
the branched one will be a solid and the unbranched will be a liquid.

Why does it take less energy to separate the nonbranched molecules?

Melting point comes down to efficiency of packing in the crystal and ultimately symmetry. Think of each molecule as a tile. The higher the symmetry of the tile, the easier it will be to pack them. As far as your q is concerned, the example you've been given is an oversimplification. Sometimes branching can increase symmetry vs. linear, but often times it does not. For some concrete examples see here: http://masterorganicchemistry.wordpress.com/2010/07/09/chemical-tetris/