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Naphthol: Structure, Polarity & Solubility

Instructor: Korry Barnes

Korry has a Ph.D. in organic chemistry and teaches college chemistry courses.

In this lesson, we'll discuss the structure, polarity, and solubility of an organic compound called naphthol. We'll also cover the main industrial uses of this compound.

What's that smell?

Have you ever walked into an elderly person's closet and immediately noticed a strange and pungent odor? It's a distinctive scent to which younger generations aren't accustomed, since the product emitting the scent has been phased out in recent years. What's responsible for the smell, you ask? It's called mothballs. In order to keep moths and other insects from damaging textiles, mothballs were placed in clothes to serve as a fumigant.

The main ingredient in traditional mothballs is a compound called naphthalene, a polyaromatic hydrocarbon. Polyaromatic refers to the fact that the compound is made up of more than one benzene ring and hydrocarbon means it's only made of carbon and hydrogen atoms. Today we'll learn about a derivative of naphthalene called naphthol. Naphthol is very similar to its parent compound, as it only differs from naphthalene by a hydroxyl (-OH) group. Let's look together at some important features of this molecule!

Structure

Naphthol is composed of a hydroxyl group bonded to a naphthalene ring. Naphthol can exist as one of two isomers (compounds with the same chemical formula, but different atom connectivity): 1-naphthol or 2-naphthol. The two isomers differ by which carbon in the naphthalene ring the hydroxyl group is bonded to.


Isomers of naphthol: 1-naphthol and 2-naphthol
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Another important structural feature of naphthol is that it's aromatic. Characteristic of aromatics, it has alternating double and single bonds throughout its rings.

Polarity

Since naphthol contains a hydroxyl group, it's a highly polar molecule, with the oxygen atom attracting electron density through the bonds toward itself. For a molecule to be polar, there has to be a difference in electronegativity (the ability of an atom to attract electrons to itself) between one or more atoms, and in the case of naphthol, oxygen is more electronegative than both carbon and hydrogen. For this reason, it can 'hog' more of the electron density in the form of the bonds, making it polar.


Polarity of naphthol
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Solubility

In terms of the solvents that naphthol is soluble in (things it will form solutions with), it's very versatile. Because of the presence of the hydroxyl group, it can hydrogen bond with other alcohol-based (polar) solvents like ethanol, methanol, and isopropanol. The ability to form these hydrogen bonds makes it readily soluble in these kinds of solvents. For example, consider the case of 2-naphthol forming hydrogen bonds with methanol molecules:


Hydrogen bonding capability of 2-naphthol with methanol
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Notice how the hydrogen atoms of methanol are attracted to the oxygen atom of 2-naphthol and the hydrogen of 2-naphthol is attracted to the oxygen of a methanol molecule. This is because the oxygen bears a partial negative charge (since it's more polarized) and that makes the partially positive hydrogens attracted to it.

Not only is naphthol soluble in alcohol-based solvents, but because of the presence of the aromatic ring, it's also soluble in a variety of non-polar solvents such as chloroform and dichloromethane.

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