Question

How can we rank the following compounds in order of increasing melting point? And why? 1-CH3CH2CH2CH2CH2CH3 2-CH3CH2CH2CH3 3-CH3CH2CH2CH2CH2CHO 4-(CH3) 4C 5-CH3CH2CH(CH3) 2?

Answer 1

To rank the compounds in order of increasing melting point, we need to consider several factors such as molecular size, molecular shape, and intermolecular forces. The stronger the intermolecular forces between molecules, the higher the melting point.

Here's the ranking of the compounds you provided in increasing order of melting point:

1. 1-CH3CH2CH2CH2CH2CH3 (hexane)
2. 2-CH3CH2CH2CH3 (butane)
3. 5-CH3CH2CH(CH3)2 (2,2-dimethylbutane)
4. 3-CH3CH2CH2CH2CH2CHO (pentanal)
5. 4-(CH3)4C (tetramethylbutane)

Explanation:

1. Hexane (1-CH3CH2CH2CH2CH2CH3) has the weakest intermolecular forces among the compounds listed. It is a straight-chain hydrocarbon, and the only intermolecular force it experiences is London dispersion forces. Since it has the fewest carbon atoms and a linear structure, its intermolecular forces are relatively weak, resulting in a lower melting point compared to the other compounds.

2. Butane (2-CH3CH2CH2CH3) is also a straight-chain hydrocarbon but has more carbon atoms than hexane. It experiences slightly stronger London dispersion forces due to the increased number of electrons and surface area, leading to a higher melting point than hexane.

3. 2,2-Dimethylbutane (5-CH3CH2CH(CH3)2) has a branched structure with two methyl groups attached to the third carbon atom. The branching reduces the surface area available for intermolecular interactions, weakening the London dispersion forces. As a result, its melting point is lower than that of butane.

4. Pentanal (3-CH3CH2CH2CH2CH2CHO) contains a carbonyl group, which introduces dipole-dipole interactions in addition to London dispersion forces. The presence of the polar carbonyl group increases the strength of intermolecular forces, resulting in a higher melting point compared to the previous compounds.

5. Tetramethylbutane (4-(CH3)4C) is highly branched and has four methyl groups attached to a central carbon atom. This structure reduces the surface area available for intermolecular interactions and weakens the intermolecular forces, resulting in the lowest melting point among the listed compounds.

It's important to note that this ranking is based on general trends and assumptions. Other factors, such as molecular symmetry, crystal packing, and impurities, can also influence the melting point of compounds.