The boiling point of substances that exist as molecules depends on the strength of the attractive forces between the molecules in the liquid phase.
induced temporary dipole (on mouseover)
NO dependence on orientation
permanent dipole-dipole
must have correct orientation
hydrogen-bonding
must have correct orientation
The size of the electron cloud should always be considered when rationalising (or predicting) boiling points.
For molecules which have the same type and number of polar bonds,
the molecule with the largest electron cloud will have the highest boiling point.
This is because the total attractive force due to temporary dipoles is highest for molecules with large electron clouds.
Because temporary dipoles do NOT depend on orientation there are many more such forces operating in a collection of molecules.
For molecules which have the same size of electron cloud,
induced temporary dipole (on mouseover)
NO dependence on orientation
permanent dipole-dipole
must have correct orientation
hydrogen-bonding
must have correct orientation
The size of the electron cloud should always be considered when rationalising (or predicting) boiling points.
For molecules which have the same type and number of polar bonds,
the molecule with the largest electron cloud will have the highest boiling point.
This is because the total attractive force due to temporary dipoles is highest for molecules with large electron clouds.
Because temporary dipoles do NOT depend on orientation there are many more such forces operating in a collection of molecules.
the molecule with the most polar bonds will have the highest boiling point.
The examples below show that the relative boiling points of molecules with similar electron clouds and a different numbers of polar bonds.
The examples below show that the relative boiling points of molecules with similar electron clouds and a different numbers of polar bonds.
| boiling point: (electron number) |  (34) | < |
| attractive forces | C-H induced temporary dipole |
 (32) | < | CH3CH2CH2OH (34) |
| C=O induced temporary dipole permanent dipole | OH induced temporary dipole hydrogen bond |