Question

The correct order of strength of acidity of the following compounds is

Answer 1

Order of acidity:

The correct order of acidity of the given compounds is: $\left(\mathrm{iv}\right)>\left(\mathrm{iii}\right)>\left(\mathrm{i}\right)>\left(\mathrm{ii}\right)$

• The electron-donating group decreases acidity, while electron-withdrawing groups increase the acidity.
• Thus, the presence of electron-withdrawing groups, such as nitro, increases the acidic strength of Phenol, whereas the presence of electron releasing groups, such as Alkyl, lessens it.
• $-\mathrm{I}$ effect increases acidity.
• This is due to the fact that the conjugate base of the acid, $\mathrm{RCOO}-$, is stabilised by delocalization of the generated negative charge if $\mathrm{R}$group is electron-withdrawing.
• If $\mathrm{R}$ group had donated electrons, the conjugate base would be destabilized due to inter-electronic repulsions.

• $+\mathrm{M}$ decreases acidity.
• When electrons or pi electrons are transported from one group to another, increasing the electron density of the conjugated system, this is referred to as the ($+\mathrm{M}$) effect or positive mesomeric effect.
• Because ${\mathrm{NO}}_2$ has a $-\mathrm{R}$ effect, it raises acidity, whereas ${\mathrm{CH}}_3$ has a $+\mathrm{I}$ effect, which lowers acidity.

Thus, the correct order of acidity of the given compounds is: $\left(\mathrm{iv}\right)>\left(\mathrm{iii}\right)>\left(\mathrm{i}\right)>\left(\mathrm{ii}\right)$