How to answer a question asking you to compare the
boiling points of two compounds?
Steps to answer
i.
Identify the intermolecular forces between them.
All of the covalent compounds will have the temporary diploe forces between the
molecules.
Is it polar ? Is it also going to have hydrogen bond? Or permanent dipole force?
ii.
iii.
Generally hydrogen bond is stronger than the permanent dipole force while
temporary dipole forces are the weakest among these 3.
Mention the relationship between the boiling points and the intermolecular forces
Example
Let’s see an example of comparing the boiling points of ethanol and butanol.
CH3CH2OH, CH3CH2CH2OH
Molecules with strong intermolecular forces need high temperature to break them. Hence
they will have higher boiling points.
They both have hydrogen bonds and the temporary dipole forces between the molecules.
The strength of the hydrogen bonds among them is similar due to OH functional group in
both of them. However the temporary dipole forces between the molecules are different.
##Temporary dipole forces depend on the size of electron cloud in the molecules. Larger the
molecule higher the temporary dipole force will be## As butanol is a larger molecule
compare to ethanol, despite of having similar hydrogen bond, butanol will have slight
stronger temporary dipole forces between the molecules. Therefor the boiling point of
butanol will be higher than that of ethanol.