Question

a) If a molecule can hydrogen bond, does it guarantee that it will have a higher boiling point than a molecule that cannot? Explain why not

b) If a molecule has a stronger dipole moment than another molecule, does it guarantee it will have a higher boiling point? Explain why not

Answer 1

(a) There is no guarantee if a molecule can hydrogen bond will have a higher boiling point than a molecule that cannot.

For example 1-propanol can hydrogen bond while 2-pentanone cannot have hydrogen bond.

1-propanol has a lower boiling point of 97.20°C than the 2-pentanone boiling point of 102.26°C. Even though 1-propanol has a hydrogen bond, its intermolecular forces are still weaker than 2-pentanone.

(b) Here also there is no guarantee.

For instance Propanoic acid has a dipole moment of 1.673 Debye while propanal has a dipole moment of 2.472 Debye.Eventhough propanal has a stronger dipole moment, it has a lower boiling point of 48.0°C than propanoic acid’s boiling point of 141.1°C. Propanoic acid has hydrogen bonds as its strongest intermolecular force while propanal has strongest intermolecular force is dipole-dipole, so even though propanal has a stronger dipole moment it still has a lower boiling point because its intermolecular forces are still weaker than those of propanoic acid.