Stereochemistry symmetry and melting point

[Monkeymaniac]

In one of the TBR passages is stated that "the compound can pack most easily when it is symmetric." But then it contradicts this by presenting a table which shows that a meso compound stereoisomer of tartaic acid has lower melting point than either form of the acid's enantiomers. WTF? EK also states that trans alkene has more symmetry, which helps lattice formation, than cis and thus has higher melting point, which makes sense.

So why does the meso compound of the acid have lower melting point? Thanks in advance.

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[rls303]

In one of the TBR passages is stated that "the compound can pack most easily when it is symmetric." But then it contradicts this by presenting a table which shows that a meso compound stereoisomer of tartaic acid has lower melting point than either form of the acid's enantiomers. WTF? EK also states that trans alkene has more symmetry, which helps lattice formation, than cis and thus has higher melting point, which makes sense.

So why does the meso compound of the acid have lower melting point? Thanks in advance.

I think because it will pack more easily, it will reduce the surface area in the solution of the compound, thus requiring less heat to melt the compound.

Someone please correct me if I'm wrong.

 

[Isoprop]

It's more difficult to predict mp on the basis of "packing" because there are so many exceptions that do exist. For the MCAT, such exceptions won't be tested so I wouldn't worry about it. Just keep the basic idea that the stronger the intermolecular forces, the higher the mp and bp.