Molar Mass vs. Boiling Point

[Phospholipid]

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?

[jdurg]

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.

[Cyanide]

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..

 

[jdurg]

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.

[Technologist]

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.

[budullewraagh]

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