Rank from highest to lowest melting point of the following molecules: ethane, 2-butane, trans-2-butane, and cis-2-butane.
please explain how you come to your answer.
I suspect you have made some typos here. Is there such a molecule as 2-butane? 2-butene, perhaps. And with the cis and trans isomers, the saturated butane has no cis and trans isomers, does it? Again, you may mean cis-2-butene and trans-2-butene.
To rank the molecules in terms of their melting points, we need to consider the strength of their intermolecular forces. The stronger the intermolecular forces, the higher the melting point.
1. Trans-2-butane and cis-2-butane: These molecules belong to the same compound, but their spatial arrangement differs. The trans-2-butane molecule has its two methyl groups on opposite sides of the double bond, while the cis-2-butane molecule has the two methyl groups on the same side of the double bond. The presence of the double bond gives these molecules stronger intermolecular forces (known as van der Waals forces) compared to ethane.
2. Ethane: Ethane is a simple hydrocarbon with a linear structure. It does not contain double bonds. Since it lacks any functional groups that can create dipole-dipole or van der Waals forces, its intermolecular forces are weaker than those of the butane isomers.
3. 2-Butane: 2-Butane is an isomer of butane where the two methyl groups attach to the second carbon atom. It has similar intermolecular forces to ethane, but the extra methyl group induces a slightly greater electron density on the molecule, leading to slightly stronger van der Waals forces.
Therefore, the ranking from highest to lowest melting point would be:
Trans-2-butane > cis-2-butane > 2-butane > ethane.
Please note that the differences in melting points between the isomers may not be significant since they share almost identical chemical structures.