The melting points of isomeric xylenes are in the order:
(a) p-xylene > o-xylene > m-xylene
(b) p-xylene > m-xylene > o-xylene
(c) o-xylene > m-xylene > p-xylene
(d) p-xylene > p-xylene > m-xylene
The Correct Answer and Explanation is :
The correct order of melting points for the isomeric xylenes is:
(b) p-xylene > m-xylene > o-xylene
This sequence is due to the distinct molecular structures and symmetries of the three isomers: ortho-xylene (o-xylene), meta-xylene (m-xylene), and para-xylene (p-xylene).
Melting Points of Xylene Isomers:
- p-Xylene (para-xylene): 13–14°C
- m-Xylene (meta-xylene): -47.4°C
- o-Xylene (ortho-xylene): -25.2°C
These values indicate that p-xylene has the highest melting point, followed by m-xylene, with o-xylene having the lowest.
Explanation:
The melting point of a compound is influenced by the efficiency of molecular packing in its solid-state crystal lattice. More symmetrical molecules tend to pack more efficiently, resulting in stronger intermolecular interactions and, consequently, higher melting points.
- p-Xylene: The para isomer is highly symmetrical, with two methyl groups positioned opposite each other on the benzene ring. This symmetry allows for efficient packing in the crystal lattice, leading to a higher melting point.
- m-Xylene: The meta isomer has two methyl groups separated by one carbon atom on the benzene ring. This arrangement is less symmetrical than that of p-xylene, resulting in less efficient packing and a lower melting point.
- o-Xylene: The ortho isomer has two adjacent methyl groups on the benzene ring. The proximity of these groups creates steric hindrance, disrupting efficient packing in the crystal lattice and leading to the lowest melting point among the three isomers.
In summary, the differences in melting points among the xylene isomers can be attributed to their molecular symmetries and the resulting packing efficiencies in the solid state. The symmetrical structure of p-xylene facilitates efficient packing, resulting in the highest melting point, while the steric hindrance in o-xylene leads to the lowest melting point.