6. What is the expected order of melting points, from highest to lowest, for compounds 1–4?

Question

Below is the image of this question AND it's correct answer. Could you please expand on WHY this would be the best choice?

Thanks alot!

http://i138.photobucket.com/albums/q271/Link3324/6.png

Answer 1

It all has to do with bond strength, the strongest 2 has hydrogen bonding which is particularly strong next number 1 has a double bond which take more engery to break over a single bond, then 4 since it has 4 carbons and more vanderwal forces over the 3 with 3 carbons and slightly less vanderwal forces

Answer 2

I dont know if this is helpful, but in my chem class this semester, we studied intermolecular forces. Compounds with greater intermolucular forces, which are responsible for the existence of serveral different phases of matter, have higher boiling points and melting points.

Types of IMFs and their typical energies: (from my chem book - Chemical Principles - The Quest for Insight)
ion-dipole interaction : 15kJ/mol - between ions and polar molecules
dipole-dipole interaction: 2kJ/mol - between stationary polar molecules
dipole-induced dipole interactions: 2kJ/mol - at least one molecule must be polar
London (dispersion): 2kJ/mol - all types of molecules (polar and non-polar)
hydrogen bonding: 20kJ/mol - molecules containing N,O,F; the link is a shared H atom.

So compounds with hydrogen bonds are going to have strong IMFs and high melting and boiling points. This to me, explains why choice 2 is first.

Differences in electronegativity also account for the strength in bonding and trends in boiling/melting points. If the difference in e-neg. is greater, the dipole strength is greater, which increases dipole-dipole IMFs, and therefore increases the boling and melting points.

Molecular Shape is also important. The boiling points in linear molecules is usually greater then that of spherical molecules.

Molecular Weight is also important, according to kinetic thoery, the higher the molecular mass of a compound the higher the boiling/melting point of a substance. (There is a noted exception with hydrogen bonding - hydrogen bonds with N,F, or O will have a higher boiling and melting point because of hydrogen bonding IMFs - for example H2O has a much higher boling point then H2S even though H2S has a greater molecular mass. This is because of the OH hydrogen bonding IMF in H2O.)

I would imagine that bond strength plays a role as well, since a double bond has a higher dissociation energy then a single bond, you would think the melting and boiling points of a compound with double bonds would be higher then that with single bonds. (This is pure speculation - not from my text.)

I hope this was somewhat helpful. I'm taking the very first chem class you can take - so what youre studying may be more advanced.

Best of Luck!
Cynthia