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Name: Amy L.
Status: student
Age: 19
Location: N/A
Country: N/A
Date: 2000


Question:
Why is phenol, C6H5OH, more soluble in water than is naphthol,C10H7OH?


Replies:
Hello Amy :

If you look at a picture of a water molecule, you will see that it is not linear. The unbonded electrons on the oxygen atom forces the two small hydrogen atoms off to one side. This results in the formation of a permanent electric dipole. This means that, within an individual water molecule, the geometric center of the positive charges does not coincide with the geometric center of the negative charges. A line drawn from the geometric center of the negative charges to the geometric center of the positive charges defines the electric dipole. The presence of an electric dipole is the reason that water conducts electricity so well with dissolved ions.

(Do not drop an electric appliance into the bathtub!)

In addition to each water molecule having a permanent electric dipole, the negatively-charged oxygen atoms on one water molecule can form an electrostatic, hydrogen bond, with one of the positively-charged hydrogen atoms of an adjacent water molecule. The formation of this three-dimensional bonding network is why water has such a high boiling point for such a small molecule (and which makes life possible on Earth.....).

Now let us talk about phenol and naphthol. You have probably heard the saying that "like dissolves like". This means that compounds which are polar should dissolve into water, since water is very polar. The -OH group on both the phenol and naphthol molecules (called a hydroxl group) is indeed very polar, and would promote the dissolution of both compounds in water. However, look at the structures of phenol versus naphthol. The phenol has a single carbon ring attached to the -OH group, while the naphthol molecule has a double carbon ring attached to the -OH group. The carbon ring portions of both the phenol and naphthol molecules are non-polar, and therefore discourage dissolution in water. (Structures like this are called hydrophobic, or "water hating") Since both the phenol and the naphthol molecules have the same polar group (the -OH group), but in the case of naphthol, it is attached to a larger, non-polar, water hating, part than phenol. The result is that the naphthol molecule does not dissolve as well in water as does phenol.

Jim Rubin



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