Question

Which among the given has lowest pKa value :-
1) chloroacetic acid
2) bromoacetic acid
3) nitroacetic acid
4) cyanoacetic acid

Please provide me answer with proper reasoning.I know that stronger acid has lower pKa value but how to determine which one is stronger ?

Answer 1

нєу тнєяє ιѕ αиѕωєя !!

ι нσρє уσυ нєℓρ !!

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Dear student, 

Low pKa means they are asking for stronger acid,

this will work on the principle of conjugate base, the more stronger the acid is, the weaker/stable  is its conjugate base.

here all are electron withdrawing groups which will pull and delocalize the electron of the conjugate base of substituted acetic acid and make it more stable.

 

The electron withdrawing tendecy/ base stabilising tendency/imparting more acidic character tendency are in this order:-

-NO2>-CN>-Cl>-Br

Answer 2

Among the given acids, the acid that has the lowest $\[pKa\]$value is 3) Nitroacetcic acid.

Explanation:

• The strength of an acid is determined by its $\[pKa\]$ value.
• The lower $\[pKa\]$ value of an acid represents that it is a strong acid.
• An acid is stronger if the conjugate base of the acid is stable.
• It can be determined by the presence of the electron-donating or electron-withdrawing groups on the acid.
• Electron-donating groups decrease the strength of the acid by increasing the electron density in the molecule.
• On the other hand, electron-withdrawing groups increase the strength of the acid by decreasing the electron density in the molecule.
• In the given acids, the electron-withdrawing groups are chloro, bromo, nitro and cyano.
• The electron-withdrawing tendency of these groups can be given as:

        $\[-N{O_2} > - CN > -Cl > -Br\]$

• As nitro ($\[-N{O_2}\]$) group is the most electron-withdrawing group, it decreases the electron density in the molecule and the release of $\[{H^ + }\]$ from the acid becomes easier.
• The conjugate base of nitro acetic acid is stabilised by the nitro group.
• Thus, nitro acetic acid is the most acidic and has the lowest $\[pKa\]$ value.
• Hence, the stronger acid can be determined on the basis of electron-withdrawing groups present in the acid.

#SPJ2