Physical properties of organic molecules depend both on the nature of the functional group and the size and shape of the molecule. Boiling point and melting point are important physical properties that are widely used to characterise organic compounds.
How can boiling point be correlated with the structure of organic molecules?Boiling point depends on the strength of the attractive forces between the molecules in the liquid phase. The nature and strength of the attractive forces between organic molecules depends both on the presence of polar bonds in the molecule and the shape and size of the molecule.
The attractive forces are due to the interaction of
temporary dipoles on adjacent organic molecules (also known as London dispersion forces, van der Waal's forces)
These attractive forces are ALWAYS present. Their strength depends on the size and shape of the electron cloud of the molecule.
permanent dipoles on adjacent organic molecules
Molecules have a permanent dipole ONLY if they have polar bonds in addition to C-H.
hydrogen-bonding between adjacent organic molecules.
Hydrogen bonding is possible in organic molecules ONLY if N-H or O-H bonds are present.
In general
When comparing organic molecules that have the same functional group and an electron cloud of a different size or shape,
higher boiling compounds have molecules with longer, thinner electron clouds.
When comparing organic molecules with a similar size electron cloud,
higher boiling compounds have
more polar bonds in their functional group.
| nonpolar
hydrocarbon | | low polarity
haloalkane | | low polarity
ether | | moderately polar
ketone | | polar
alcohol | | very polar
carboxylic acid |
| boiling point | CH3CH2CH3
lowest | < | CH3Cl | < | CH3OCH3 | < | CH3CCH3
||
O | < | CH3CH2OH | < | CH3CO2H
highest |
| most polar bond | C-H | | C-Cl | | 2 C-O | | C=O | | OH | | C=O
OH |