Internal triple/double bonds

[apache]

Hey,

 

Does anyone know why internal db's/tb's boil at higher temperature? I would imagine terminal DB's would result in more polarity and intermolecular adhesion; hence, higher bp's.

 

Thanks

[AndyDude]

I would think that would be due to the degree of substitution. In case of alkenes the more substituted molecules are more stable. For ex: 2,3-dimethyl-2-butene both Cs of double bond are di-substituted while, say, 2-methyl-butene (a terminal alkene) has one C that is mono-substituted while other is di-substituted. The former is more stable that the latter.

[OneDay]

I read that it was the opposite. That alkenes have lower bp's than their counterpart alkane...because the bp is only dependant on van der waals. I'm confused!

[AndyDude]

OP's question was about the stability of internal multiple bonds vs terminal multiple bonds. Alkenes have lower boiling point than alkanes due to the kinks introduced by the multiple bonds which reduce the London dispersion forces. For smaller alkanes and alkenes the difference b/w bps is marginal due to similar London forces. Hope that helps.