Which among the following is the strongest acid?

Concept:

The acidity of compounds:

• The acidity of compounds is determined by their ability to donate hydrogen ions in solution.
• The greater the ease of donation or liberation of the hydrogen ions, the stronger is the acid.
• The acidic proton of the compound is generally attached to an electronegative atom
•  The strength of the acidity is greatly influenced by the substituents or groups attached to the ring and their position.

Factors influencing the acid strength-

• The stability of the conjugate base-
  • if the negative charge is resonance stabilized in the conjugate base, then the compound is more acidic compared to the compound whose conjugate base has the charge localized.
• Electronegative substituents or groups like F, Cl, Br, I increase the acidity via inductive electron withdrawal (-I).
  • The closer the substituent is to the carboxyl group, the greater is its effect.
  • The more the number of substituents, the stronger is the acid.
• Electron donating groups such as - OR, -Me, -Et, NH2 or –NR, OH, etc. decrease the acidity via the +R and +I effect.
• Electron withdrawing groups such as NO2,-CF3, -COOH, -CN increases the acidity via the –R effect.
• The order of strength of -R Effect is: -NO₂>-CN>-SO₂>-CHO>-CO>COOCOR>-COOR>-COOH>-CONH₂>-COO^-
• The order of strength of +R Effect is: O->-NH₂>-OR>-NHCOR>-OCOR>-F>-Cl>-Br>-I
• Hydrogen attached to sp2 Carbon is more acidic than hydrogen attached to sp3 carbon.
  • The acidity order is sp> sp2>sp3.

​Explanation:

• Phenol is acidic in nature because it liberates the acidic proton easily and the conjugate base is also resonance stabilized.

and :

• When the Chloride group is introduced at the para position, it Has a -I effect towards the ring. Also, the +R effects operate at the ortho and para positions.
• We know that -I of Cl is dominating and hence it helps in increasing the overall acidity of phenol.
• But, the -R effect of the nitro group dominates the -I of chlorine making nitro phenols more acidic than chlorophenols.

Comparison between

 and 

• In the case of ortho nitrophenol, the negative charge is delocalized by -R of the nitro group.

• But -R does not operate in the case of meta nitrophenol and thus it is less acidic than ortho nitrophenol.

Comparison between

 and :

• There exists an intramolecular H bonding in ortho nitrophenol which makes the release of proton much more difficult.

Hence the most acidic compound is .