As we will learn in Section 15.12, many antioxidants—compounds that prevent unwanted radical oxidation reactions from occurring—are phenols, compounds that contain an OH group bonded directly to a benzene ring.

1. Explain why homolysis of the O-H bond in phenol requires considerably less energy than homolysis of the O-H bond in ethanol (362 kJ/mol vs 438 kJ/mol).
2. Why is the C-O bond in phenol shorter than C-O bond in ethanol?

The cleavage of a bond in which both the atoms involved in bonding retain their shared electron is named homolysis.

The homolysis of the oxygen-hydrogen (O-H) bond in phenol requires less energy than the homolysis of the oxygen-hydrogen (O-H) bond in ethanol as the phenoxide radical is resonance stabilized.

Generation of phenoxide radical

The resonance structures of phenoxide radical are provided below.

Resonance structures of phenoxide radical

The carbon-oxygen (C-O) bond in phenol is shorter than the carbon-oxygen (C-O) bond in ethanol, as, in the case of phenol, the carbon atom involved in bonding with oxygen is ${\mathrm{sp}}^2$hybridized.

While in the case of ethanol, the carbon atom is ${\mathrm{sp}}^3$hybridized.

As the percent of s character of the atoms involved in bonding increases, the bond length decreases.

Carbon-oxygen bond in the given compounds