Question

Gas and attraction questions

CH3OCH3 - This is a large molecule. How do you determine if the main attractive force is london or dipole-dipole? The answer is dipole in the book.

Which has the greater attraction?
d) HBr or HCl - The answer in the book is HBr since it's more polarizable. However HCl is a more polar since the difference in electronegativity is greater. How do you determine which force wins? Especially when you don't have a table of specific electronegativity (I know the diagonal trends)
e) C6H13OH or C3H7OC3H7
The answer is C6H13OH since it forms hydrogen bonds. But Continued...

Answer 1

But the second molecule also has H and O together at H7O. How come it doesn't or has weak hydrogen bonds?

Answer 2

CH3OCH3 is dimethyl ether. Compared to other functional groups, ethers are quite nonpolar due to the alkyl chains present. In this case though, there are two methyl groups attached to the central atom, so the alkyl chains are very short. The molecule adopts a bent conformation (according to VSEPR, if you know it), so the lone pairs on oxygen are not distributed evenly within the molecule and a dipole to forms.

As for the HCl and HBr, my guess would be it has to do with atomic radius. Because Br is a period below Cl, its atomic radius is greater. This means the H bonded to it is held farther from the nucleus, and so can be removed more easily (more polarizable). It is for this reason that HBr is a strong acid, since it readily loses H (HCl is a strong acid too for the same reason, though HF is not).

As for the last question, note that C6H13OH is an alcohol (hexanol), while C3H7OC3H7 is another ether (dipropyl ether). An alcohol by definition has an -OH group, meaning it can form hydrogen bonds. In an ether, the oxygen is bound to two carbon atoms but no hydrogen atoms, so it cannot form hydrogen bonds.

More input from other people is welcome :)