More important to BP

[pineappletree]

---

Members don't see this ad.

I posted a topic here wondering about a statement in one of BR's passages that Molecular mass is the most important determinant of boiling point
I said that I thought that intermolecular forces were, but I was informed that mass was the single most important determinant



However, I was wondering about a statement in the orgo BR book. It says that in page 46,
"the intermolecular forces are the primary consideration when approximating physical properties. When forces are not enough to determine the physical properties such as boiling and melting point, then structural features such as molecular mass and molecular rigidity become the determining factors."

So what's going on? This seems to indicate that I was right... however I am confused since on the BR passages did say that molecular mass was the most important determinant.

 

[lesset01]

MW affects the IMFs btwn molecules of the same type. This means if you are comparing only straight chain alkanes to each other, you already know that they have the same IMFs. But with increasing MW, you have increasing polarizability which means that the IMFs for large molecules are stronger, therefore increasing the BP.

Btwn molecules of different types you need to take the different IMFs for each respective molecule into account. So if you are comparing butane, pentane, and methyl alcohol, you know that methyl alcohol has the highest BP b/c it has H bonding. Both butane and pentane have only VDW forces, but they are stronger for pentane than for butane b/c its heavier and more polarizable.