Question About Chad's O-Chem

[Div2388]

Advertisement - Members don't see this ad

This question is in regards to the effect of branching on an alkane's melting point.

Chad says: Branching an alkane sandwiches the liquid phase, lowering the boiling point and RAISING the melting point.

His reasoning: With a more branched structure, a molecule will fit into an ionic crystal better.

KBB says: Branching an alkane LOWERS the melting point along with the boiling point.

KBB reasoning: "Branched molecules are more difficult to pack into a tight, three-dimensional structure. This difficulty is reflected in the lower melting points of branched alkanes."

So.... who's right?

 

[ridhi465]

I noticed that too but after going through the Organic Chem Odyssey, it should be Chad's explanation. In the Odyssey, he says that melting and boiling points don't always correlate and as a molecule gets more branched, they fit into the crystals better and hence raise the melting point.
It does make sense as I think Kaplan also mentioned that trans alkenes have a higher melting point due to symmetry, compared to cis alkenes.

 

[nyr201]

Hey thanks for the clarification, one more thing I get confused about is what they mean by symmetry, ie symmetrical carbon. what's this mean??

 

[ridhi465]

For example, the tert-butyl group has 3 methyl groups that allows it to be symmetrical and hence easily fit into the crystal lattice. The straight chain butane will not be this compressed to fit in so easily.