Why Is Phenol More Acidic Than Cyclohexanol?

Phenol and cyclohexanol are organic compounds with a variety of important uses in industry, especially in the manufacture of plastics. Although phenol is only a weak acid, its structure makes it more acidic than cyclohexanol.

Types

Cyclohexanol belongs to a class of organic compounds called alcohols, defined as compounds that feature a hydroxyl group (OH) attached to a carbon atom. Phenol and other phenols also have a hydroxyl group, but in its case the hydroxyl group is connected to a carbon atom that is part of a phenyl ring. This difference is important in determining the acidity of each compound.

Function

When phenol donates a hydrogen ion to water, the electron pair remaining on the oxygen atom becomes delocalized, meaning that it becomes redistributed into the phenyl ring and cannot be assigned with certainty to any one pair of atoms. This effect is possible because of the π bonds in the phenyl ring (bonds that form between overlapping unhybridized p-orbitals). In cyclohexanol, on the other hand, all of the carbons in the ring structure share single bonds and there are no π bonds, so the negative charge on the oxygen does not become delocalized if the hydrogen is donated to water.

Effects

Since the negative charge becomes delocalized after phenol loses the hydrogen from its hydroxyl group, the conjugate base of phenol is much weaker than the conjugate base of cyclohexanol. As a result, the dissociation constant is larger for phenol than it is for cyclohexanol, so phenol is a stronger acid in water (meaning that it donates more hydrogen ions). Other molecules in the class of compounds called phenols are typically weak acids as well, although they are much weaker acids than carboxylic acids like acetic acid as a general rule.

References

• "Chemical Principles, the Quest for Insight, 4th Edition"; Peter Atkins and Loretta Jones; 2008.