Correlating density and melting point in alkenes

[Miffymycat]

Quoted data for cis and trans 1,2 dichloroethenes shows cis has higher density but lower melting point than trans. How can this be explained? Packing arguments are clearly unable to rationalise these observations. Other 1,2 disubstituted ethenes show similar pattern. Thank you.

 

[BvU]

This homework ? What other property differs strongly ?

 

[Miffymycat]

Boiling points differ as expected - the more polar molecule (cis) is higher

 

[BvU]

Yes, dipole moment is the key.

 

[Miffymycat]

Dipole moment explains boiling point but not the contrary melting point and density trend!