[Monium101]

From my understanding, melting points of alkane molecules tend to increase as their molecular weights are larger and can be influenced by their packing arrangement, if they are packed tightly, then more energy is required to be able melt a molecule. I was wondering why an odd number of carbons would result in a less compacted arrangment than an even number of carbons in a molecule, what is the reasoning behind that and how it is related to melting points or for that matter boiling points of organic compounds such as alkanes? Any help is greatly appreciated! 

[Enthalpy]

You mean straight alkanes, for the odd/even difference, don't you? You might want to check if there's any density difference between them.

Melting points are extremely complicated, possibly unaccessible to simple reasoning and only to heavy software estimation. Working theories don't exist, additive models (Joback and few more) fail with 100K error. It's presently a research topic, with some limited success, where software is to try many possible packings of molecules - remember the orientation and overlap of molecules in a crystal isn't known a priori, nor the periodicity, and even the molecule conformation may adapt to the best crystal. Worse: many organic compounds have allotropes.

Symmetry of the molecule is very important, beyond packing possibilities. Symmetric cubane, adamantane, 2,2,3,3-tetramethyl-butane... have very high melting points close to their boiling point. Symmetry favours the solid since a free "liquid" molecule has better chances to stick to the crystal; check the odd/even n-alkane story with this.

Ease of rotations and vibrations at the "liquid" molecule favours the liquid, since these degrees of freedom use to be hampered at the solid. One more argument for cubane and adamantane. Branched alkanes also rotate more easily and gem-branched less so, which would be compatible with tendencies in the melting point - but easy packing is a very convincing reason as well.

Mankind would need a good means to estimate melting points. Up to now, we have additive methods that fail, and very (very) few measurements - maybe too few to build a theory or software on them. Most Internet data is software estimation, off by >100K, without warning. The Yaws is a heavy book filled by such faulty software, completely wrong, beware. So if you want to discover or invent this means, welcome! Be it a theory, a method, a light or more probably a medium-heavy (=PC) software, I don't care.

[Enthalpy]

I just wanted to check the story of packing density, so here's an image for C₅H₁₂ to C₂₀H₄₂, which show no odd/even effect on the molar volumes to my eyes - but on the melting points yes.

Data is from a Comcast table. The straight line is a linear interpolation between C₅H₁₂ and C₂₀H₄₂, impressive match. The spreadsheet was made with the excellent free Gnumeric, though Excel displays it (imperfectly?); remove the mock txt extension and expand the zip file. The png image is extracted from the xls.

My conclusion (other opinions welcome!) is that:

• The odd/even effect does not result from the packing density.
  
• You may try a statistical effect of symmetry; not completely obvious.
• Maybe some effect of symmetry on rotation or vibration heat capacity of the liquid, like for ortho/para hydrogen at 20K, having a residual importance at 300K.

Good luck!

[Monium101]

Yeah, I was referring to the odd and even difference between the straight chain alkanes but also for other geometries as well but this is definitely helpful. thank you so much for your *delete me* 

[Enthalpy]

Ahum. I've written too early.  Only octadecane and bigger melt above 298K. Densities from C₅ to C₁₇ are from liquids hence not significant.

So here's new data from C₁₈ to C₃₀, and:

• I still see no even/odd effect on the density (hopefully for the solid this time)
• The data set has a break at C₂₁, which is not the 298K melting limit
• Even for the C₂₁ to C₃₀ subcurve, and if removing C₂₅ and C₂₆, I see no even/odd effect
• I don't understand the erratic data. It would take 2.5K to explain 1cm³/mol
• The even/odd effect on the melting point is small for these alkanes.
  Density data for solid C₅ to C₁₅ at one temperature would have been better...

[Enthalpy]

To check a hypothetical even-odd effect on the packing density, a set of homogenous measures would be useful: not necessarily accurate, but comparable, that is, made under the same circumstances.

Fortunately, the volume shrinkage when freezing is more significant than the density, and is also easier to measure: see the sketch. The bottle (replace by adequate jargon) is completely filled with the liquid n-alkane which is then frozen; completing the volume tells the shrinkage. Some 10^-3 repeatability suffices now instead of 10^-5. Melting double-checks the measure.

If pure alkanes behave like wax and paraffin wax do, they will make a cavity as depicted. Voids must be avoided, so cooling from the bottom is better. I'd have preferred the alkane stopping at the conical section, and completing the liquid and then solid volumes with some liquid not miscible and lighter than the alkane - but what liquid?

Measuring just above and below freezing is excellent, but two arbitrary temperatures common to all alkanes would be good.

Alkanes solid at room temperature are already interesting; smaller ones, like C₇ to C₁₅ would be more convincing, because the even-odd effect influences their melting point more.

Marc Schaefer, aka Enthalpy

[curiouscat]

The Yaws is a heavy book filled by such faulty software, completely wrong, beware.

Interesting. I always came to think of Yaws as one of the standard respected sources in the Canon. A la Perrys, Lees, Prausnitz etc.

Can you give examples of faulty correlations in Yaws? I mean all correlations are expected to have errors & outliers but is Yaws egregiously bad?

[Enthalpy]

I too did believe Yaws was a serious data source, so much that I bought a "Yaws handbook of physical properties for hydrocarbons and chemicals". Alas, most data is computed, not experimental.

I was interested in melting points of ramified alkanes, and all Mankind's knowledge seems to reduce to the measure of some 50 petrol components in the 1950's by Naca, plus a few individual compounds meanwhile. These are the values available from any book that gives no melting point for other ramified alkanes. Yaws gives more melting points, but these are estimated.

Unfortunately, the estimation of melting points fails grossly. Every detail, the number of isomers, the exact position of each branch... has a tremendous importance, making 30 to >100 Kelvin difference. Though, Yaws estimates melting points just by the number and length of branches, independently of their position. That's useless and misleading.

If you search the Yaws for dimethyldecane, you find only 195K whatever the positions. Or for tetramethyloctane, you get 164.5K for any positions. In fact, most branched alkanes around C₁₂ (there are many...) get a mp of 164.5K, 179.5K, 194.5K, 211.9K or 226.9K. First example I see, some would be much worse: for 2,4- and 2,5-dimethyldecane, Yaws gives one melting point but Nist -84°C and -62°C.

[orgopete]

From my understanding, melting points of alkane molecules tend to increase as their molecular weights are larger and can be influenced by their packing arrangement, if they are packed tightly, then more energy is required to be able melt a molecule. I was wondering why an odd number of carbons would result in a less compacted arrangment than an even number of carbons in a molecule, what is the reasoning behind that and how it is related to melting points or for that matter boiling points of organic compounds such as alkanes? Any help is greatly appreciated! 

Is this or could this be a question of logic? The melting points of methane, ethane, and propane are very low and similar. The mp of butane and pentane are -140C and -130C. The mp of hexane and heptane are -96C and -90C. After that they begin to flatten out. However, this seems to be enough. Why is butane higher than propane and why is pentane nearly the same as butane? I have assumed there are specific interactions that control physicochemical properties. If so, then I assumed the interactions of butane must be different than propane and pentane cannot achieve a greater number than butane. Draw them out and tell me what you find?