Of 2-methyl-2-butanamine and ethypropylamine, which has the highest boiling point and why?

You have isomeric primary and secondary amines. Hydrogen bonding can occur for both substances. Ordinarily, we would predict that the primary amine would be most involatile, and have the higher boiling point, because it has the greater opportunity for intermolecular interaction thru hydrogen bonding.

On the other hand, as the chain length of an organic molecule grows, there is greater possibility for chain-chain interactions, and boiling point increases. For the #2^@# amine there is a longer chain, and this would tend to decrease volatility.

I will stick my neck out, and claim that #H_3C-CH_2CH_2NHCH_2CH_3# has the higher boiling point.

But a scientist relies on data. I may be right, I may be wrong. But I have to consult the data. I just had a look at my trusty Aldrich catalogue, and it lists the boiling point of ethylpropylamine as #89-91^@C#, and that of 2-methyl-2-butanamine as #76-78^@C#. I was right for once.

Evidently, hydrophobic interaction along the chain are greater than the effect of hydrogen bonding in this specific instance. Given that the amino function of the primary amine is sterically hindered, which would tend to reduce intermolecular ineraction, this can be rationalized.