[FiendishFrog]

I am just starting orgo, and we have been talking about melting points of organic compounds. I was trying to answer some questions in the book, and I feel like a complete idiot for not knowing. Any help would be appreciated.


1)   What typically determines physical properties like melting points?

2)   Why would a sample being tested shrink as the melting point is neared?

I think that for #1 it has to do with the molecular structure and the number of bonds in the compound, but I didn't know if there may be more to it than that or if I am totally wrong all together.
I am not sure about #2. Water loss maybe? Eh.

[Siddy]

1) Melting point (and boiling point) is determined by bond strength. That is carbon chain length, molecular weight, functional groups etc...
So you are right for number 1.

2) If something shrinks in volume the molecules are being drawn together, if this is from a increase in heat, the new length in secondary bonds is at a more stable length than at the lower temp. (Nothing to do with water content, note that many organic compounds melt bellow 0 degrees C, when water is crystalline.)
If the volume decreased from water loss, that would have to be at waters boiling point and you would be determining the melting point of an impure substance (ie a solution).

[pharmDoo]

Depending on how pure the substance being tested is, the purer the substance the higher the melting point and the Mp range is also narrower.

[Custos]

1) Melting point (and boiling point) is determined by bond strength. That is carbon chain length, molecular weight, functional groups etc...
So you are right for number 1.

To be precise, I don't think melting points have anything to do with internal bond strengths, but actually the interactions between molecules - like hydrogen bonds, pi stacking, crystal lattice energy and so on. The same compound can have two or more different melting points depending on the crystal lattice structure.

[Siddy]

^ yes, i was referring to to 2ndary bonding.

[AWK]

Depending on how pure the substance being tested is, the purer the substance the higher the melting point and the Mp range is also narrower.

Melting and boiling points are intrinsic properties of pure substances. The purer substance the closer its property to this point.