How do you account for the difference in melting points between (a) and (b) between (c ) and (d) and between these two differences ?
.

Explanation

The crystal forces in (a) and (d) are largely van der Walls. Compounds (a) and (c ) containing the polar `OH` groups from H-bonding
(b) in (a) there is intramolecular H-bonding whereas in (c ) it is intermolecular H-bonding which leads to increases in meliting point of (c ) as compared to (a) and (d)
(c) In the absence of strong intermolecular H-bonding the difference in m.pt as compared with the reference substance (b) should be small This is due to differences in the crystal structure or due to the van der Waals forces, which would be slightly larger for (b) than for (a) on account of the slightly increased molecular mass .