Why is acetic acid LESS volatile than methyl formate?
2 Answers
Ionic compounds usually have higher boiling points than covalent compounds.
Explanation:
“Boiling” is the observed phenomenon when a compound in a liquid phase converts enough thermal energy into kinetic energy to overcome the inter-molecular bonds of the compound and allow it to “vaporize” into a gas phase.
Although the subject compound have identical molecular weights and elements, the bonds forming the two compounds are quite different, forming an acid in the first example and a ester in the second one.
The ester is entirely covalently bonded, and only needs to overcome dispersion forces in its liquid phase in order to vaporize. The acid has both dispersion forces and ionic forces (much stronger) to overcome. Thus, it requires more heat (thermal energy) to vaporize (boil) than the ester.
As chemists, as physical scientists, we should seek out the data.
Explanation:
Acetic acid,
Clearly, the degree of INTERMOLECULAR force that operates between each TYPE of molecule is different. Acetic acid is a carboxylic acid for which hydrogen bonding would operate strongly. The acidic hydrogen would tend to interact with the electronegative oxygen on a NEIGHBOURING molecule; this force between molecules, this intermolecular force, tends to increase the boiling point. Such hydrogen bonding also operates in the very small water molecule, whose boiling point is even higher than the organic compounds.
On the other hand, in methyl formate, hydrogen is not bound to a strongly electronegative element. The molecule has no capacity to hydrogen-bond, and its boiling point is much reduced by comparison.
Note that the bonds between atoms within the molecules, the INTRAMOLECULAR force, are similar in each molecule. These are strong covalent bonds. It is the INTERMOLECULAR force that detemines boiling point.
