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Phenol and Naphthol Solubility
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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Update: June 2012
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