How to determine the order of boiling points and solubilities for (chloro-)alkanes, ethers, aldehydes, and alcohols? What rules can be applied to make these choices?

Take for example the following question:

Which one of the following compounds has the highest solubility in water?
Which one of the following compounds has the lowest boiling point?

  • ethane, $\ce{CH3CH3}$
  • dimethylether, $\ce{CH3-O-CH3}$
  • acetaldehyde, $\ce{CH3CHO}$
  • methanol, $\ce{CH3OH}$
  • chloromethane, $\ce{CH3Cl}$
  • methane, $\ce{CH4}$

Solubility in water

The two most important (and somewhat related) factors are the polarity of the molecule and the presence of hydrogen bonding groups. As a rule of thumb, the higher the polarity the more soluble a molecule will be in water (which is highly polar as well). Also, the more hydrogen bonding groups, the more soluble in water.

With these two rules of thumb you should be able to order the compounds you listed. I'll leave it to you to actually do it.

Boiling point

The website masterorganicchemistry.com[1] gives a fairly good summary of 3 important factors that determine the boiling point of organic compounds. They are:

  1. The relative strength of the four intermolecular forces is: ionic > hydrogen bonding > dipole dipole > van der Waals dispersion forces. The influence of each of these attractive forces will depend on the functional groups present, but generally stronger forces means a higher boiling point.
  2. Boiling points increase as the number of carbons is increased.
  3. Branching decreases boiling point.

With these 3 'rules' you should be able to figure out the ordering of your listed compounds.

[1]: Master Organic Chemistry: 3 Trends That Affect Boiling Points or via the Internet Archive


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