I've noticed the molar mass of diethyl ether is considerably higher than that of ethyl alcohol.. yet, diethyl ether has a much lower boiling point. Why is this?

Ethyl alcohol has the VERY polar OH groups on the molecule and therefore exhibits some form of hydrogen bonding. Diethyl Ether is completely non-polar and only has weak Van-der-Waals forces holding the molecules together. As a result, Diethyl ether has a MUCH lower boiling point than ethyl alcohol does.

jdurg is correct..

Mass doesn't necessarily contribute to a high/low boiling point

Polar bonds are difficult to "break" and therefore molecules with those bonds have higher boiling points..

 

Exactly. Take a look at water. With a mass of only 18 it should not be a liquid. (Ethane, Propane, Butane, Carbon Dioxide, etc. all have higher masses yet are gasses at room temperature). However it has VERY strong hydrogen bonding which causes its BP to be much higher than expected.

A good rule of thumb for a quick estimate of boiling points, and I hope I explain this well, is to consider every:

 

carbon as 1

chlorides as 2.5

ethers as 1

primary amines 2.5

secondary amines 1.5

tertiary amines 1

alcohols 4

aldehydes 2.5

ketones 2.5

nitriles 4

acids (O + OH) 5.5

Esters (O + O) 2.5

Anhydrides 4

Acid Chlorides (O + CL) 3.5

Amides (O + NH2) 10

 

Diethyl ether has four carbons and an ether. That’s 4 + 1 = 5

 

Ethyl alcohol has two carbons and an alcohol group so 2 + 4 = 6

 

Therefore you would predict that ethyl alcohol has a higher boiling point than diethyl ether.

 

It’s a crude yardstick at best but works in most chem labs.

there's actually a formula in lange's handbook. there's a constant for every radical, etc. it's crazy but brilliant