What is a phenoxide ion?

1 Answer

A phenoxide ion is the conjugate base of phenol. It looks like:



According to the Bronsted-Lowry theory of acids and bases, an acid is a molecule that donates #H^+# ions, and a base is a molecule that takes them up again.

Taking the example of

#NH_3 + H_2O -> NH_4^+ + HO^-#

then #NH_3# acts like a base and #H_2O# acts like an acid, based on the Bronsted-Lowry definitions above.

This makes #NH_4^+# into a conjugate acid (because it would, given the chance, give up hydrogen ions) and #HO^-# the conjugate base (since it would now accept hydrogen).

Phenoxide is a conjugate base, which means it is formed from an acid that has given up its hydrogen. This acid is a phenol molecule, which is a benzene ring with a hydroxyl group. The hydrogen of the hydroxyl leaves, and an #O^-# remains, forming the 'oxide ion' part of the phenoxide ion.

'Phenyl' is simply another systematic name for benzene, a ring of #C_6H_6# with delocalised electrons. 'Phenol' is the alcohol form of benzene, because it has an #-OH# or hydroxyl group.