boiling point

[Guest ikaj]

Why do internal alkenes and alkynes have higher boiling points than terminal alkenes and alkynes?

[Guest Kirsteen]

Hi there,

 

As far as I can recall, the answer may lie in the relative stability of the carbocation intermediates that alkenes and alkynes form during a reaction (and a reaction can involve the simple addition of heat, i.e., boiling).

 

A primary carbocation or vinylic cation (those that are formed from a terminal alkene and alkyne, respectively) shall be less stable than a secondary carbocation or vinylic cation (those formed from an internal alkene or alkyne, respectively) during a reaction. Molecules that are less stable tend to exhibit lower boiling points, hence, since terminal alkenes and alkynes form less stable reaction intermediates, they have a lower energy of activation and lower boiling points.

 

Cheers,

Kirsteen

[Guest ikaj]

thank you , kirsteen. I don't quite understand though what the carbocation stability has to do with boiling point. I thought boiling point depends on intermolecular forces (vanderwaals forces). Doesn't the carbocation stability and energy of activation etc. have more to do with the chemical properties than physical properties such as boiling point?

Could you furthur clarify that for me?

thnks

[Guest T dawg 2004]

If I recall correctly, internal alkenes tend to "fit" better together - which is why trans-alkenes have a higher bp than those of the cis-alkene configurations.

 

www.scientia.org/cadonlin...keprop.ASP

 

With terminal alkenes/alkynes, the advantage of the "better fit" is greatly decreased, since there isn't the chance of additional "better fitting" configurations (i.e. esp in large organic molecules)...

 

It is all about vanderwaals and attraction and "best fit"...