Question

Rationalize why p-aminobenzoic acid has the highest mp, trans-stilbene has an intermediate mp, and o-toluic acid has a lower mp. Be specific.

Answer 1

Meting point is temperature at which solid phase of substance changes to liquid phase. Melting point is a solid state property . Molecular symmetry play a important role in determination of melting point of any substance. Molecular symmetry determine the packing fraction of the molecule in crystal and packing fraction is fraction of space occupied by atoms more is packing fraction , more will be density and more will be melting point. So we can say High molecular symmetry associated with high melting point.Other factor like Intermolecular interaction play minor role in Melting point determination.Steric Crowding result in decrease in packing coefficient in crystal and low packing fraction of molecules in crystal result in lower symmetry hence melting point will decrease due to lower symmetry.

p-aminobenzoic acid has high melting point because one factor is High molecular symmtery in para disubstituted benzene and other factor is intermolecular H-bonding is present in p-aminobenzoic acid .

trans-stilbene has second highest melting point because in trans goemetry both phenyl rings are coplanar and packing of molecules will be more compact in trans stilbene. Molecular symmetry will present in crystal of trans-stilbene .

o-toluic acid has lowest melting point because the factor ( molecular symmetry ) is not present in o-toluic acid . Steric Crowding in o-toluic acid due to ortho methyl group result in decrease in packing coefficient in crystal hence melting point will decrease due low symmetry.