Question

Among the following compounds, the most acidic is:
A.p-nitrophenol
B.p-hydroxybenzoic acid
C.o-hydroxybenzoic acid
D.p-toluic acid

Answer 1

The acidic strength is measured by how easily any given compound gives hydrogen ion. This depends on various factors such as inductive effect, electronegativity, resonance etc. The acids have greater acidic strength than phenol.

Complete step by step solution:
Let us study each of the option:
Option A is p-nitrophenol in which hydrogen is attached with oxygen. It will form a phenoxide ion which will be stabilized by the nitro group.
Option B is p-hydroxybenzoic acid, here also hydrogen is attached with oxygen, of hydroxide group but because p-hydroxybenzoic acid is itself an acid and the hydrogen will release from more acidic hydrogen that is from carboxylic acid group. Hence, it will be more acidic than phenol. This is because of more resonance structure due to delocalization of electrons in the carboxylic acid group.
Option C is o-hydroxybenzoic acid where hydrogen is attached with oxygen of the hydroxide group, but the hydroxide group is present at the ortho position. Ortho substituted benzoic acids are always most acidic due to ortho effect. Since carboxylic acid is itself an bulky group and whenever any other species is present at ortho position then the plane distorts and equivalent resonance occurs which makes the molecule most acidic.
Option D is Toluene which is least acidic because there is no acidic hydrogen present.
Hence the correct option is C.

Note: Not all hydrogen is acidic. Only that hydrogen which is attached with more electronegative elements such as oxygen, fluorine, chlorine, nitrogen is acidic. The hydrogen attached with carbon is not acidic unless or until some stabilizing factor is there to balance the negative charge after removal of hydrogen.