Boiling and melting point of trans- and cis-but-2-ene

The boiling point of ***trans-but-2-ene*** is lower than that of its ***cis*** isomer but the melting point of the former is higher than the later. Why is it not following the same order? Is there any factor of structural symmetry coming playing a role here?




Yes . You are right that structural symmetry comes into play . 


Boiling point depends upon intermolecular interactions which over here is more in cis due to its net dipole moment . The dipole moment enables electronic interactions which hold molecules together . [This](http://www.masterorganicchemistry.com/2010/10/25/3-trends-that-affect-boiling-points/) shows some general factors of boiling point . Also the below link to the google book covers the structural aspect of a molecule towards b.p on pg47.


For melting points , let me quote Joseph Hornback from Chapter 2.6 [Organic Chemistry](http://www.worldcat.org/title/organic-chemistry/oclc/424076558) pg46 ([google books](https://books.google.com/books?isbn=0534389511)) : 



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> The shape of a compound is very important and has a dramatic effect on its melting point . More symmetrical molecules pack into the crystal lattice better , allowing closer approach and larger attractive forces , resulting in higher melting points . 
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Melting points of alkenes depends on the packaging of the molecules. Alkenes have similar melting points to that of alkanes, however, in cis isomers molecules are package in a U-bending shape, therefore, will display a lower melting points to that of the trans isomers.([source](http://chemwiki.ucdavis.edu/Organic_Chemistry/Hydrocarbons/Alkenes/Properties_of_Alkenes/Physical_Properties_of_Alkenes))


Trans 2-Butene has more symmetry than its cis isomer which results in better packing and hence higher melting point . 


For further reading I suggest you visit 'Molecular Shape' over [here](http://www.brynmawr.edu/chemistry/Chem/mnerzsto/Labs/Forces_and_Factors.pdf).


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