Question

Question: Phenols (ArOH) are relatively acidic, and the presence of a substituent group on the aromatic ring has a large effect. The${\mathbf{pk}}_{\mathbf{a}}$ of unsubstituted phenol, for example, is 9.89, while that of p-nitrophenol is 7.15. Draw resonance structures of the corresponding phenoxide anions and explain the data.

Short answer

Resonating structure of phenoxide ion is followed as:

Step-by-step solution

Step 1: Acidic compound

The compound is acidic in nature when its ${\mathbf{pk}}_{\mathbf{a}}$value is low and more acidic the compound will be and the gibs energy is more in negative shows show spontaneous is the compound that is more acidic.

Electron withdrawing group stabilise the molecule make it more acidic and electro releasing group de stabilise the molecule make it less acidic.

p-nitro phenol acidic nature

As the nitro is the electron withdrawing group which destabilize the positive charge and stabilizes the negatively charge intermediate and thus raising the value of the gibs energy and${\mathrm{pk}}_{\mathrm{a}}$ value thus it lowers the acidic strength as shown in the resonating structure below:

phenol acidic nature

As the phenol on losing the proton form phenoxide which is the electron donating group which stabilize the positive charge and de stabilize the negatively charge intermediate and thus decreasing the value of the gibs energy and${\mathrm{pk}}_{\mathrm{a}}$ value thus it increases the acidic strength as shown in the resonating structure below: