[rleung]

Hi,

I was just thinking to myself the other day, and I realized that I never learned "officially" the trends for double and triple bonds on the melting and boiling points of various elements.  I always assumed that as you increase the number of double bonds in a molecule, your BP and MP would both increase since double and triple bonds are both stronger than single bonds.  For example, I assumed that ethene would have a higher BP and MP as compared to ethane. 

However, I also knew that in fatty acids, the greater number of double bonds you have, the more unsaturated the molecule is and the more liquidy the molecule is; hence, an unsaturated fatty acid would seemingly have a lower MP and BP as compared to its saturated counterpart. 

I couldn't seem to reconcile these seemingly contradictory results.  I went to chemfinder and confirmed that in fatty acids, the greater degree of unsaturation means LOWER mp and LOWER bp.

However, I get weird results when I look up pure hydrocarbon chains.  For example, ethane has MP of -183, while ethene has a MP of -169, a HIGHER mp.  However, ethane has a BP of -87, while ethene has a BP of -103, a LOWER bp.   Furthermore, acetylene has a MP of -80.75 (higher than ethene).

When I check butane, I get BP=-0.5 and MP=-138, while in butene, BP=-6.3 and MP=-185.  Butyne has a BP=8 and MP=-126.

So I am a bit confused....do double and triple bonds increase MP and BP or decrease MP and BP?  Or is there not a definite trend?

Ryan

[Dan]

I always assumed that as you increase the number of double bonds in a molecule, your BP and MP would both increase since double and triple bonds are both stronger than single bonds.

Do these bonds break when the substance melts/boils?

BP/MP is due to intramolecular forces, like van der Waals, dipole-dipole interactions and H bonding. It is generally difficult to predict because there are alot of interplaying factors - and it gets messy once you get to larger molecules. It's all about shape, polarisability, electric dipoles etc.

[rleung]

OH YES, the covalently-bonded molecules do not break!

[latent_lamp]

Atkins explains what you are asking very well.

I think just on the basis of bonds it would be difficult to determine trends in m.p and b.p. I will try to explain my reasoning:

Phenol has higher m.p. than o-nitrophenol, which has lower m.p. than 2-bromo-6-nitrophenol

Now the degree of saturation for o-nitrophenol is same as 2-bromo-6-nitrophenol and also H defciency is the same. Yet the difference is about 20^oC So you have to ask the question what is the
big issue here?

Now the trends in the aforementioned sequence deal with intramolecular forces and it can be easily proven as well. The above example is similar along the lines with your example, however the answer for the above is very simple: intramolecular H bonding.

Dan mentioned some really good considerations; but you can add to it by considering the more physical properties as well. How the molecules crystallize (or how well they do so) also affects the m.p. For instance if I had a sample of triphenylmethanol, well after recrystaliizing it, the m.p. will change by I think 8 degrees. Crystal structure is something that is more physical than chemical, and I am not referring to bond arrangements, but rather how a substances sublimes/solidifies at the molecular level.

Also, steric hindrance, angular strain on molecues and other considerations are taken into account prior to establishing a pattern.

You will find a lot of established theories on trends within functional groups, so there are principles already stated that consider m.p. trends in alkenes relative to other alkenes (i.e. simple hydrocarbon alkenes versus halogenated and the likes). It becomes much more difficult when you try to establish correlation between functional groups since there will be too many exceptions and exclusions to any establishing principle.

Now if you have different constitutional isomers, and you want to explain difference in m.p. The first thing I would do is do H-NMR and IR nad use those to explain any issues with m.p. This particularly true when you are looking at just benzene and the various positioning of substituent groups.
For instance its easier to proove intramolecular H bonding in 0-nitrophenol just by doing H-NMR.

[rleung]

Thank you.