CHEM - Comparing the b.p. and m.p. of cis/trans alkene
Organic Chemistry, Physical Properties of Alkene - IBDP | DSE | GCE | IAL | AP Chemistry
Explain why the boiling point of cis isomer is higher than the trans? 🤔
Remember, when it comes to boiling point, what we are focusing is always the intermolecular force between the molecules.
Referring to the above example, both isomers had the same molecular mass, and hence their dispersion/london force will be pretty much the same ⚖️
However, if we look at the cis isomer, it is more "polar" than the trans isomer.
The dipole moments in the cis isomer cannot cancel out each other, leaving a "net" dipole, giving polarity to the molecule
Looking at the trans isomer, the dipole moments cancel out each other, and thus with no net dipole behind.
As a matter of fact, given both isomers have similar dispersion/london force, the cis isomer has in addition the permanent-dipole attraction between themselves, giving them a higher boiling point.
But then why the melting point of the trans isomer is higher than the cis isomer?
The melting point of a molecule is determined by its intermolecular force and also the symmetry of the molecule
(*only for m.p. not b.p.)
Mathematically proven, trans is more "symmetrical" than cis (always). 😎
The more symmetric the molecule, the easier it is to pack closely and regularly, giving a high melting point. Just like the illustration below :
The left regular pentagon will have a higher melting point given its symmetry.
Hence in the exam, if they ask you to explain the difference in melting point of two molecules, apart from considering the bonding between them, don't forget to consider its symmetry!
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