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, thegreater 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 asNO2,−CF3, -COOH, -CN increases the acidity via the –R effect.
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.
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
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
There exists an intramolecular H bonding in ortho nitrophenol which makes the release of proton much more difficult.