Density and packing ability

[Meredith92]

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Tbr says the greater the density of a compound the more tightly packed the compound is in a crystal lattice. I may be overthinking this but why is this true? Is it just assumed that the packing is tighter since there is more mass per unit volume?

Also ( separate question ) if something is more symmetric, tbr says it packs more easily... What effect does this have on melting point? Would something that packs easily have a higher melting point since it is a strong/ tight crystal? However this concept seems to go against saturated and unsaturated fats and melting points. Saturated fats don't pack well but have a higher melting point....

I think tbr contradicts itself because it had a meso compound ( symmetric) that had the lowest density out of a list of compounds and said that it packed the least tightly- wouldn't it being symmetric mean it packed the most tightly? ( this is question 26 on ch3 Orgo)

If anyone can help with this confusion that would be great thank you

 

[Shjanzey]

Tbr says the greater the density of a compound the more tightly packed the compound is in a crystal lattice. I may be overthinking this but why is this true? Is it just assumed that the packing is tighter since there is more mass per unit volume?

density = mass/volume , so you could look at it like this. Density is more mass packed into the same amount of volume...so better "packing" means higher density.

Also ( separate question ) if something is more symmetric, tbr says it packs more easily... What effect does this have on melting point? Would something that packs easily have a higher melting point since it is a strong/ tight crystal? However this concept seems to go against saturated and unsaturated fats and melting points. Saturated fats don't pack well but have a higher melting point....

In this case you are comparing an entire molecular structure to "density" which may not be a fair comparison. In large organic molecules you have to take into account the type of bonds present and the molecular interactions.

In the case of unsaturated fats, they have double bonds that can cause more separation, or in the case of "trans" fats, the double bond causes a linear conformation which leads to very efficient packing. If you have many bends in your fat molecule (cis) there will be more space between each molecule which mean it is probably a liquid at room temperature (or an oil).

I hope this helps.

 

[Meredith92]

In general though, are symmetric molecules better or worse at packing?
Could you summarize the rules that lead to a higher mp and a high bp?
I know that higher mw leads to increase in bp and mp because there are more intermolecular interactions... And branching decreases boiling point because less liquid interactions and branching increases mp because it's more compact and has better packing.

But when We start talking about symmetry/ amount of double bonds/ degree of saturation I get confused...