# What has a higher boiling point alkane, alkene, or alkyne?

Mar 26, 2018

Well, we might think that the heaviest molecule would have the highest boiling point....

#### Explanation:

And why would we think so? Well, because the heavier molecule necessarily has more electrons, and thus more opportunity for dispersion forces. And so let us examine the normal boiling points of the alkane versus olefin versus acetylene series..

${\text{Ethane, ethylene, acetylene:"-89""^@C; -103.7^@C; -84}}^{\circ} C$...

${\text{Propane, propylene, propyne:"-42""^@C; -47.6^@C; -23.2}}^{\circ} C$...

${\text{Butane, 1-butene, 1-butyne:"0""^@C; -6.3^@C; 8.1}}^{\circ} C$...

And our facile expectation was indeed justified for the alkane, versus the corresponding olefin....but the acetylene was more involatile, even tho it was the lightest molecule in the series. I propose an extra contribution to the intermolecular force, i.e. bond polarity, the which should dominate for the acetylene derivative, i.e. there is more charge separation for acetylene, which expresses itself as an intermolecular force.

And please note that you do not have to remember these volatilities. You do have to be able to interpret them....